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MAGILL�S ENCYCLOPEDIA OF SCIENCE

P L A N T L I F E

MAGILL�S ENCYCLOPEDIA OF SCIENCE

P L A N T L I F E

Volume 3Microbial Nutrition and Metabolism–Sustainable Agriculture

EditorBryan D. Ness, Ph.D.Pacific Union College, Department of Biology

Project EditorChristina J. Moose

Salem Press, Inc.Pasadena, CaliforniaHackensack, New Jersey

Copyright © 2003, by Salem Press, Inc.All rights in this book are reserved. No part of this work may be used or reproduced in any manner what-

soever or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording,or any information storage and retrieval system, without written permission from the copyright ownerexcept in the case of brief quotations embodied in critical articles and reviews. For information address thepublisher, Salem Press, Inc., P.O. Box 50062, Pasadena, California 91115.

Some of the updated and revised essays in this work originally appeared in Magill’s Survey of Science: LifeScience (1991), Magill’s Survey of Science: Life Science, Supplement (1998), Natural Resources (1998), Encyclopediaof Genetics (1999), Encyclopedia of Environmental Issues (2000), World Geography (2001), and Earth Science (2001).

∞ The paper used in these volumes conforms to the American National Standard for Permanence ofPaper for Printed Library Materials, Z39.48-1992 (R1997).

Library of Congress Cataloging-in-Publication Data

Magill’s encyclopedia of science : plant life / edited by Bryan D. Ness.

p. cm.Includes bibliographical references (p. ).

ISBN 1-58765-084-3 (set : alk. paper) — ISBN 1-58765-085-1 (vol. 1 : alk. paper) —ISBN 1-58765-086-X (vol. 2 : alk. paper) — ISBN 1-58765-087-8 (vol. 3 : alk. paper) —ISBN 1-58765-088-6 (vol. 4 : alk. paper)

1. Botany—Encyclopedias. I. Ness, Bryan D.QK7 .M34 2002580′.3—dc21

2002013319

First Printing

printed in the united states of america

Editor in Chief: Dawn P. DawsonManaging Editor: Christina J. Moose

Manuscript Editor: Elizabeth Ferry SlocumAssistant Editor: Andrea E. MillerResearch Supervisor: Jeffry Jensen

Acquisitions Editor: Mark Rehn

Photograph Editor: Philip BaderProduction Editor: Joyce I. BucheaPage Design and Graphics: James HutsonLayout: William ZimmermanIllustrator: Kimberly L. Dawson Kurnizki

TABLE OF CONTENTS

List of Illustrations, Charts, and Tables . . . . xlixAlphabetical List of Contents . . . . . . . . . . liii

Microbial nutrition and metabolism . . . . . . 663Microbodies . . . . . . . . . . . . . . . . . . . . 666Microscopy . . . . . . . . . . . . . . . . . . . . 668Mitochondria . . . . . . . . . . . . . . . . . . . 672Mitochondrial DNA . . . . . . . . . . . . . . . 675Mitosis and meiosis . . . . . . . . . . . . . . . 677Mitosporic fungi . . . . . . . . . . . . . . . . . 681Model organisms . . . . . . . . . . . . . . . . . 682Molecular systematics . . . . . . . . . . . . . . 686Monocots vs. dicots . . . . . . . . . . . . . . . 688Monocotyledones . . . . . . . . . . . . . . . . . . 690Monoculture . . . . . . . . . . . . . . . . . . . 692Mosses . . . . . . . . . . . . . . . . . . . . . . . 694Multiple-use approach . . . . . . . . . . . . . . 697Mushrooms . . . . . . . . . . . . . . . . . . . . 699Mycorrhizae. . . . . . . . . . . . . . . . . . . . 702

Nastic movements . . . . . . . . . . . . . . . . 704Nitrogen cycle . . . . . . . . . . . . . . . . . . 706Nitrogen fixation . . . . . . . . . . . . . . . . . 709Nonrandom mating . . . . . . . . . . . . . . . 712North American agriculture. . . . . . . . . . . 714North American flora . . . . . . . . . . . . . . 718Nuclear envelope . . . . . . . . . . . . . . . . . 722Nucleic acids . . . . . . . . . . . . . . . . . . . 723Nucleolus . . . . . . . . . . . . . . . . . . . . . 727Nucleoplasm . . . . . . . . . . . . . . . . . . . 729Nucleus . . . . . . . . . . . . . . . . . . . . . . 730Nutrient cycling . . . . . . . . . . . . . . . . . 732Nutrients . . . . . . . . . . . . . . . . . . . . . 734Nutrition in agriculture . . . . . . . . . . . . . 737

Oil bodies . . . . . . . . . . . . . . . . . . . . . 740Old-growth forests . . . . . . . . . . . . . . . . 741Oomycetes. . . . . . . . . . . . . . . . . . . . . 743Orchids . . . . . . . . . . . . . . . . . . . . . . 745

Organic gardening and farming . . . . . . . . 747Osmosis, simple diffusion, and

facilitated diffusion . . . . . . . . . . . . . . 751Oxidative phosphorylation . . . . . . . . . . . 755Ozone layer and ozone hole debate . . . . . . 757

Pacific Island agriculture . . . . . . . . . . . . 761Pacific Island flora . . . . . . . . . . . . . . . . 765Paclitaxel . . . . . . . . . . . . . . . . . . . . . 768Paleobotany . . . . . . . . . . . . . . . . . . . . 770Paleoecology . . . . . . . . . . . . . . . . . . . 774Parasitic plants . . . . . . . . . . . . . . . . . . 777Peat. . . . . . . . . . . . . . . . . . . . . . . . . 779Peroxisomes . . . . . . . . . . . . . . . . . . . . 782Pesticides . . . . . . . . . . . . . . . . . . . . . 783Petrified wood . . . . . . . . . . . . . . . . . . 787Pheromones . . . . . . . . . . . . . . . . . . . . 790Phosphorus cycle . . . . . . . . . . . . . . . . . 791Photoperiodism. . . . . . . . . . . . . . . . . . 794Photorespiration . . . . . . . . . . . . . . . . . 796Photosynthesis . . . . . . . . . . . . . . . . . . 800Photosynthetic light absorption. . . . . . . . . 804Photosynthetic light reactions. . . . . . . . . . 807Phytoplankton . . . . . . . . . . . . . . . . . . 809Pigments in plants . . . . . . . . . . . . . . . . 812Plant biotechnology . . . . . . . . . . . . . . . 815Plant cells: molecular level . . . . . . . . . . . 821Plant domestication and breeding . . . . . . . 826Plant fibers . . . . . . . . . . . . . . . . . . . . 829Plant life spans . . . . . . . . . . . . . . . . . . 831Plant science. . . . . . . . . . . . . . . . . . . . 835Plant tissues . . . . . . . . . . . . . . . . . . . . 839Plantae . . . . . . . . . . . . . . . . . . . . . . . 843Plants with potential . . . . . . . . . . . . . . . 849Plasma membranes. . . . . . . . . . . . . . . . 851Plasmodial slime molds . . . . . . . . . . . . . 854Poisonous and noxious plants . . . . . . . . . 857Pollination. . . . . . . . . . . . . . . . . . . . . 862Polyploidy and aneuploidy . . . . . . . . . . . 865

xlvii

Population genetics . . . . . . . . . . . . . . . 868Prokaryotes . . . . . . . . . . . . . . . . . . . . 870Proteins and amino acids . . . . . . . . . . . . 873Protista . . . . . . . . . . . . . . . . . . . . . . . 875Psilotophytes . . . . . . . . . . . . . . . . . . . 879

Rain-forest biomes . . . . . . . . . . . . . . . . 883Rain forests and the atmosphere . . . . . . . . 885Rangeland . . . . . . . . . . . . . . . . . . . . . 889Red algae . . . . . . . . . . . . . . . . . . . . . 892Reforestation . . . . . . . . . . . . . . . . . . . 894Reproduction in plants. . . . . . . . . . . . . . 897Reproductive isolating

mechanisms . . . . . . . . . . . . . . . . . . 899Resistance to plant diseases . . . . . . . . . . . 901Respiration . . . . . . . . . . . . . . . . . . . . 904Rhyniophyta . . . . . . . . . . . . . . . . . . . . 907Ribosomes . . . . . . . . . . . . . . . . . . . . . 909Rice. . . . . . . . . . . . . . . . . . . . . . . . . 912RNA . . . . . . . . . . . . . . . . . . . . . . . . 914Root uptake systems . . . . . . . . . . . . . . . 917Roots . . . . . . . . . . . . . . . . . . . . . . . . 920Rubber . . . . . . . . . . . . . . . . . . . . . . . 925Rusts . . . . . . . . . . . . . . . . . . . . . . . . 929

Savannas and deciduous tropical forests . . . 932Seedless vascular plants . . . . . . . . . . . . . 934Seeds . . . . . . . . . . . . . . . . . . . . . . . . 937Selection . . . . . . . . . . . . . . . . . . . . . . 941Serpentine endemism . . . . . . . . . . . . . . 943Shoots . . . . . . . . . . . . . . . . . . . . . . . 944Slash-and-burn agriculture . . . . . . . . . . . 946Soil . . . . . . . . . . . . . . . . . . . . . . . . . 949Soil conservation . . . . . . . . . . . . . . . . . 955Soil contamination . . . . . . . . . . . . . . . . 957Soil degradation . . . . . . . . . . . . . . . . . 959Soil management . . . . . . . . . . . . . . . . . 962Soil salinization . . . . . . . . . . . . . . . . . . 963South American agriculture . . . . . . . . . . . 965South American flora . . . . . . . . . . . . . . 969Species and speciation . . . . . . . . . . . . . . 973Spermatophyta . . . . . . . . . . . . . . . . . . . 976Spices . . . . . . . . . . . . . . . . . . . . . . . 977Stems. . . . . . . . . . . . . . . . . . . . . . . . 980Strip farming . . . . . . . . . . . . . . . . . . . 983Stromatolites . . . . . . . . . . . . . . . . . . . 985Succession . . . . . . . . . . . . . . . . . . . . . 988Sugars . . . . . . . . . . . . . . . . . . . . . . . 991Sustainable agriculture. . . . . . . . . . . . . . 993

xlviii

Magill’s Encyclopedia of Science: Plant Life

PUBLISHER’S NOTE

Magill’s Encyclopedia of Science: Plant Life is de-signed to meet the needs of college and high schoolstudents as well as nonspecialists seeking generalinformation about botany and related sciences. Thedefinition of “plant life” is quite broad, covering therange from molecular to macro topics: the basics ofcell structure and function, genetic and photosyn-thetic processes, evolution, systematics and classi-fication, ecology and environmental issues, andthose forms of life—archaea, bacteria, algae, andfungi—that, in addition to plants, are traditionallystudied in introductory botany courses. A numberof practical and issue-oriented topics are covered aswell, from agricultural, economic, medicinal, andcultural uses of plants to biomes, plant-related en-vironmental issues, and the flora of major regionsof the world. (Readers should note that, althoughcultural and medicinal uses of plants are occasion-ally addressed, this encyclopedia is intended forbroad information and educational purposes.Those interested in the use of plants to achievenutritive or medicinal benefits should consult aphysician.)

Altogether, the four volumes of Plant Life survey379 topics, alphabetically arranged from Acid pre-cipitation to Zygomycetes. For this publication, 196essays have been newly acquired, and 183 essaysare previously published essays whose contentswere reviewed and deemed important to include ascore topics. The latter group originally appeared inthe following Salem publications: Magill’s Survey ofScience: Life Science (1991), Magill’s Survey of Science:Life Science, Supplement (1998), Natural Resources(1998), Encyclopedia of Genetics (1999), Encyclope-dia of Environmental Issues (2000), World Geography(2001), and Earth Science (2001). All of these previ-ously published essays have been thoroughly scru-tinized and updated by the set’s editors. In additionto updating the text, the editors have added newbibliographies at the ends of all articles.

New appendices, providing essential researchtools for students, have been acquired as well:

• a “Biographical List of Botanists” with briefdescriptions of the contributions of 134 fa-mous naturalists, botanists, and other plantscientists

• a Plant Classification table

• a Plant Names appendix, alphabetized bycommon name with scientific equivalents

• another Plant Names appendix, alphabetizedby scientific name with common equivalents

• a “Time Line” of advancements in plant sci-ence (a discursive textual history is also pro-vided in the encyclopedia-proper)

• a Glossary of 1,160 terms

• a Bibliography, organized by category of re-search

• a list of authoritative Web sites with theirsponsors, URLs, and descriptions

Every essay is signed by the botanist, biologist,or other expert who wrote it; where essays havebeen revised or updated, the name of the updaterappears as well. In the tradition of Magill reference,each essay is offered in a standard format that al-lows readers to predict the location of core informa-tion and to skim for topics of interest: The title ofeach article lists the topic as it is most likely to belooked up by students; the “Category” line indi-cates pertinent scientific subdiscipline(s) or area(s)of research; and a capsule “Definition” of the topicfollows. Numerous subheads guide the reader

vii

through the text; moreover, key concepts are itali-cized throughout. These features are designed tohelp students navigate the text and identify pas-sages of interest in context. At the end of each essayis an annotated list of “Sources for Further Study”:print resources, accessible through most libraries,for additional information. (Web sites are reservedfor their own appendix at the end of volume 4.) A“See also” section closes every essay and refersreaders to related essays in the set, thereby linkingtopics that, together, form a larger picture. For ex-ample, since all components of the plant cell arecovered in detail in separate entries (from the Cellwall through Vacuoles), the “See also” sections forthese dozen or so essays list all other essays cover-ing parts of the cell as well as any other topics of in-terest.

Approximately 150 charts, sidebars, maps, ta-bles, diagrams, graphs, and labeled line drawingsoffer the essential visual content so important tostudents of the sciences, illustrating such core con-cepts as the parts of a plant cell, the replicationof DNA, the phases of mitosis and meiosis, theworld’s most important crops by region, the partsof a flower, major types of inflorescence, or differentclassifications of fruits and their characteristics. Inaddition, nearly 200 black-and-white photographsappear throughout the text and are captioned tooffer examples of the important phyla of plants,parts of plants, biomes of plants, and processes ofplants: from bromeliads to horsetails to wheat; fromArctic tundra to rain forests; from anthers to stemsto roots; from carnivorous plants to tropisms.

Reference aids are carefully designed to alloweasy access to the information in a variety of modes:The front matter to each of the four volumes in-

cludes the volume’s contents, followed by a full“Alphabetical List of Contents” (of all the volumes).All four volumes include a “List of Illustrations,Charts, and Tables,” alphabetized by key term, toallow readers to locate pages with (for example)a picture of the apparatus used in the Miller-UreyExperiment, a chart demonstrating the genetic off-spring of Mendel’s Pea Plants, a map showing theworld’s major zones of Desertification, a cross-section of Flower Parts, or a sampling of the manytypes of Leaf Margins. At the end of volume 4 isa “Categorized Index” of the essays, organizedby scientific subdiscipline; a “Biographical Index,”which provides both a list of famous personagesand access to discussions in which they figureprominently; and a comprehensive “Subject Index”including not only the personages but also the coreconcepts, topics, and terms discussed throughoutthese volumes.

Reference works such as Magill’s Encyclopediaof Science: Plant Life would not be possible withoutthe help of experts in botany, ecology, environmen-tal, cellular, biological, and other life sciences; thenames of these individuals, along with their aca-demic affiliations, appear in the front matter tovolume 1. We are particularly grateful to the pro-ject’s editor, Bryan Ness, Ph.D., Professor of Biol-ogy at Pacific Union College in Angwin, California.Dr. Ness was tireless in helping to ensure thorough,accurate, and up-to-date coverage of the content,which reflects the most current scientific knowl-edge. He guided the use of commonly acceptedterminology when describing plant life processes,helping to make Magill’s Encyclopedia of Science:Plant Life easy for readers to use for reference tocomplement the standard biology texts.

viii


CONTRIBUTOR LIST

About the Editor: Bryan Ness is a full professor in the Department of Biology at Pacific Union College, afour-year liberal arts college located atop Howell Mountain in the Napa Valley, about ninety miles north ofSan Francisco. He received his Ph.D. in Botany from Washington State University. His doctoral work focusedon molecular plant systematics and evolution. In addition to authoring or coauthoring a number of scientificpapers, he has contributed to The Jepson Manual: Higher Plants of California, The Flora of North America, amultivolume guide to the higher plants of North America, and more than a dozen articles for various SalemPress publications, including Aging, Encyclopedia of Genetics, Magill’s Encyclopedia of Science: Animal Life,Magill’s Medical Guide, and World Geography. For four years he managed The Botany Site, a popular Internetsite on botany. He is currently working on a book about plant myths and misunderstandings.

Stephen R. AddisonUniversity of Central Arkansas

Richard AdlerUniversity of Michigan, Dearborn

Steve K. AlexanderUniversity of Mary Hardin-Baylor

Michael C. AmspokerWestminster College

Michele ArduengoIndependent Scholar

Richard W. ArnsethScience Applications International

J. Craig BaileyUniversity of North Carolina,

Wilmington

Anita Baker-BlockerApplied Meteorological Services

Iona C. BaldridgeLubbock Christian University

Richard BeckwittFramingham State College

Cindy BenningtonStetson University

Alvin K. BensonUtah Valley State College

Margaret F. BoorsteinC. W. Post College of Long Island

University

P. E. BostickKennesaw State College

J. Bradshaw-RouseIndependent Scholar

Thomas M. BrennanDickinson College

Alan BrownLivingston University

Kenneth H. BrownNorthwestern Oklahoma State

University

Bruce BruntonJames Madison University

Pat CalieEastern Kentucky University

James J. CampanellaMontclair State University

William J. CampbellLouisiana Tech University

Steven D. CareyUniversity of Mobile

Roger V. CarlsonJet Propulsion Laboratory

Robert E. CarverUniversity of Georgia

Richard W. Cheney, Jr.Christopher Newport University

John C. ClauszCarroll College

Miriam ColellaLehman College

William B. CookMidwestern State University

J. A. CooperIndependent Scholar

Alan D. CopseyCentral University of Iowa

Joyce A. CorbanWright State University

Mark S. CoyneUniversity of Kentucky

Stephen S. DaggettAvila College

William A. DandoIndiana State University

ix

James T. DawsonPittsburg State University

Albert B. DickasUniversity of Wisconsin

Gordon Neal DiemADVANCE Education and

Development Institute

David M. DiggsCentral Missouri State University

John P. DiVincenzoMiddle Tennessee State University

Gary E. DolphIndiana University, Kokomo

Allan P. DrewSUNY, College of Environmental

Science and Forestry

Frank N. EgertonUniversity of Wisconsin, Parkside

Jessica O. EllisonClarkson University

Cheryld L. EmmonsAlfred University

Frederick B. EssigUniversity of South Florida

Danilo D. FernandoSUNY, College of Environmental

Science and Forestry

Mary C. FieldsMedical University of South Carolina

Randy FirstmanCollege of the Sequoias

Roberto GarzaSan Antonio College

Ray P. GerberSaint Joseph’s College

Soraya GhayourmaneshIndependent Scholar

Carol Ann GillespieGrove City College

Nancy M. GordonIndependent Scholar

D. R. GossettLouisiana State University,

Shreveport

Hans G. GraetzerSouth Dakota State University

Jerry E. GreenMiami University

Joyce M. HardinHendrix College

Linda HartUniversity of Wisconsin, Madison

Thomas E. HemmerlyMiddle Tennessee State University

Jerald D. HendrixKennesaw State College

John S. HeywoodSouthwest Missouri State University

Jane F. HillIndependent Scholar

Joseph W. HintonIndependent Scholar

Carl W. HoagstromOhio Northern University

Virginia L. HodgesNortheast State Technical Community

College

David Wason Hollar, Jr.Rockingham Community College

Howard L. HosickWashington State University

Kelly HowardIndependent Scholar

John L. HowlandBowdoin College

M. E. S. HudspethNorthern Illinois University

Samuel F. HuffmanUniversity of Wisconsin, River Falls

Diane White HusicEast Stroudsburg University

Domingo M. JarielLouisiana State University, Eunice

Karen N. KählerIndependent Scholar

Sophien KamounOhio State University

Manjit S. KangLouisiana State University

Susan J. KarcherPurdue University

Jon E. KeeleyOccidental College

Leigh Husband KimmelIndependent Scholar

Samuel V. A. KisseadooHampton University

Kenneth M. KlemowWilkes University

Jeffrey A. KnightMount Holyoke College

Denise KnotwellIndependent Scholar

James KnotwellWayne State College

Lisa A. LambertChatham College

Craig R. LandgrenMiddlebury College

x


John C. LandoltShepherd College

David M. LawrenceJohn Tyler Community College

Mary Lee S. LedbetterCollege of the Holy Cross

Donald H. LesUniversity of Connecticut

Larry J. LittlefieldOklahoma State University

John F. LogueUniversity of South Carolina, Sumter

Alina C. LopoUniversity of California, Riverside

Yiqi LuoUniversity of Oklahoma

Fai MaUniversity of California, Berkeley

Jinshuang MaArnold Arboretum of Harvard

University Herbaria

Zhong MaPennsylvania State University

Dana P. McDermottIndependent Scholar

Paul MaddenHardin-Simmons University

Lois N. MagnerPurdue University

Lawrence K. MagrathUniversity of Science and Arts of

Oklahoma

Nancy Farm MännikköIndependent Scholar

Sergei A. MarkovMarshall University

John S. MechamTexas Tech University

Roger D. MeicenheimerMiami University, Ohio

Ulrich MelcherOklahoma State University

Iain MillerWright State University

Jeannie P. MillerTexas A&M University

Randall L. MilsteinOregon State University

Eli C. MinkoffBates College

Richard F. ModlinUniversity of Alabama, Huntsville

Thomas J. MontagnoSimmons College

Thomas C. MoonCalifornia University of Pennsylvania

Randy MooreWright State University

Christina J. MooseIndependent Scholar

J. J. MuchovejFlorida A&M University

M. MustoeIndependent Scholar

Jennifer Leigh MykaBrescia University

Mysore NarayananMiami University

Bryan NessPacific Union College

Brian J. NichelsonU.S. Air Force Academy

Margaret A. OlneyColorado College

Oghenekome U. OnokpiseFlorida A&M University

Oluwatoyin O. OsunsanyaMuskingum College

Henry R. OwenEastern Illinois University

Robert J. ParadowskiRochester Institute of Technology

Bimal K. PaulKansas State University

Robert W. PaulSt. Mary’s College of Maryland

Kenneth A. PidcockWilkes College

Rex D. PieperNew Mexico State University

George R. PlitnikFrostburg State University

Bernard Possidente, Jr.Skidmore College

Carol S. RadfordMaryville University, St. Louis

V. RaghavanOhio State University

Ronald J. RavenState University of New York

at Buffalo

Darrell L. RayUniversity of Tennessee, Martin

Judith O. RebachUniversity of Maryland,

Eastern Shore

David D. ReedMichigan Technological University

xi

Contributor List

Mariana Louise RhoadesSt. John Fisher College

Connie RizzoPace University

Harry RoyRensselaer Polytechnic Institute

David W. RudgeWestern Michigan University

Neil E. SalisburyUniversity of Oklahoma

Helen SalmonUniversity of Guelph

Lisa M. SardiniaPacific University

Elizabeth D. SchaferIndependent Scholar

David M. SchlomCalifornia State University,

Chico

Matthew M. SchmidtSUNY, Empire State College

Harold J. SchreierUniversity of Maryland

John Richard SchrockEmporia State University

Guofan ShaoPurdue University

Jon P. ShoemakerUniversity of Kentucky

John P. ShontzGrand Valley State University

Nancy N. ShontzGrand Valley State University

Beryl B. SimpsonUniversity of Texas

Sanford S. SingerUniversity of Dayton

Susan R. SingerCarleton College

Robert A. SinnottArizona Agribusiness and Equine

Center

Elizabeth SlocumIndependent Scholar

Dwight G. SmithConnecticut State University

Roger SmithIndependent Scholar

Douglas E. SoltisUniversity of Florida

Pamela S. SoltisIndependent Scholar

F. Christopher SowersWilkes Community College

Alistair SponselImperial College

Valerie M. SponselUniversity of Texas, San Antonio

Steven L. StephensonFairmont State College

Dion StewartAdams State College

Toby R. StewartIndependent Scholar

Ray StrossState University of New York at Albany

Susan Moyle StudlarWest Virginia University

Ray SumnerLong Beach City College

Marshall D. SundbergEmporia State University

Frederick M. SurowiecIndependent Scholar

Paris SvoronosQueen’s College, City University

of New York

Charles L. VigueUniversity of New Haven

James WaddellUniversity of Minnesota, Waseca

William J. WassermanSeattle Central Community College

Robert J. WellsSociety for Technical Communication

Yujia WengNorthwest Plant Breeding Company

P. Gary WhiteWestern Carolina University

Thomas A. WikleOklahoma State University

Donald Andrew WileyAnne Arundel Community College

Robert R. WiseUniversity of Wisconsin, Oshkosh

Stephen L. WolfeUniversity of California, Davis

Ming Y. ZhengGordon College

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LIST OF ILLUSTRATIONS, CHARTS, AND TABLES

Volume-pageAcid Precipitation pH Scale (bar graph). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-3Africa, Desertification of (map) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-301African Countries with More than 15 Percent Arable Land, Leading Agricultural Crops of (table) . . . I-15African Rain Forests (Sidebar) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-884Africa’s Agricultural Products (map) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-13Air Pollutant Emissions by Pollutant and Source, 1998 (table) . . . . . . . . . . . . . . . . . . . . . . . . I-45Algae, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-49Algae, Types of (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-48Angiosperm, Life Cycle of an (drawing). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-66Angiosperms and Continental Drift (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-63Antibiotic Resistance, The Growth of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-122Ants and Acacias (sidebar). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-254Asian Countries with More than 15 Percent Arable Land, Leading Agricultural Crops of (table) . . . . I-93Australia, Selected Agricultural Products of (map) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-103

Bacillus thuringiensis and Bacillus popilliae as Microbial Biocontrol Agents (table) . . . . . . . . . . . . . I-155Bacterial Diseases of Plants, Some (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-115Biomes and Their Features (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-148Biomes of the World (map) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-151Biomes (percentages), Terrestrial (pie chart) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-152Brown Algae (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-165

Cacao: The Chocolate Bean (sidebar) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-211Carbon Cycle, The (flow chart) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-184Carnivorous Plants (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-194Cell, Parts of a Plant (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-822Cell, Parts of the Plant (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-286Central America, Selected Agricultural Products of (map) . . . . . . . . . . . . . . . . . . . . . . . . . I-208Central American Countries, Leading Agricultural Crops of (table) . . . . . . . . . . . . . . . . . . . . I-209Chromosome, Schematic of a (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-229Conifer Leaves (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-270Conifers, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-271Corn-Producing Countries, 1994, Leading (bar graph) . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-275Crop Yields, Declining with Successive Harvests on Unfertilized Tropical Soils (bar graph). . . . . . III-948Crops and Places of Original Cultivation, Major (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-26Crops of African Countries with More than 15 Percent Arable Land (table) . . . . . . . . . . . . . . . . I-15Crops of Asian Countries with More than 15 Percent Arable Land (table) . . . . . . . . . . . . . . . . . I-93Crops of Central American Countries (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-209Crops of European Countries with More than 20 Percent Arable Land (table) . . . . . . . . . . . . . . II-387Crops of Pacific Island Nations (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-763Cycads, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-283

xiii

Darwin and the Beagle, Charles (sidebar/map) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-402Deforestation, Results of (flow chart) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-295Deforestation by Country, 1990-1995, Percentage of Annual (map). . . . . . . . . . . . . . . . . . . . . I-294Desertification of Africa (map) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-301Diatoms (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-308Dicot (Eudicot) Families, Common (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-74Dinoflagellates (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-312Diseases, Symptoms of Plant (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-314DNA, The Structure of (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-323DNA Replication, Stages in (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-330Drought, Impacts of (sidebar). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-336

Endangered Plant Species, A Sampling of the World’s (table) . . . . . . . . . . . . . . . . . . . . . . . II-352Endocytosis (drawing). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-356Endomembrane System, The (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-359Eudicot (Dicot) Families Common in North America (table) . . . . . . . . . . . . . . . . . . . . . . . . II-376Eukarya, Classification of Domain (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV-1106Eukaryotic Cell, Parts of a (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-383Europe, Selected Agricultural Products of (map) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-386European Countries with More than 20 Percent Arable Land, Leading Agricultural

Crops of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-387Evolution of Plants (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-411Exocytosis (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-357

Ferns, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-420Flower, Parts of a (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-429Flower Shapes: Perianth Forms (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-433Flower Structure, Variations in (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-430Forest Areas by Region (pie chart) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-455Forest Loss in Selected Developing Countries, 1980-1990 (bar graph) . . . . . . . . . . . . . . . . . . . I-296Fruit Types (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-465Fruit Types and Characteristics (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-466Fungi: Phyla and Characteristics (table). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-472

Garden Plants and Places of Original Cultivation (table) . . . . . . . . . . . . . . . . . . . . . . . . . . II-475Genetically Modified Crop Plants Unregulated by the U.S. Department of Agriculture (table) . . . . II-496Germination of a Seed (drawing). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-510Ginkgophyta, Classification of Phylum (table). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-513Gnetophytes, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-518Grasses of the United States, Common (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-523Greenhouse Effect (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-535Greenhouse Gas Emissions, 1990-1999, U.S. (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-536Growth Habits (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-542Gymnosperms, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-544

Hormones and Their Functions, Plant (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-575Hornworts, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-579

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Horsetails, Classification of (table). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-581Hydrologic Cycle (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-594

Inflorescences, Some Common (drawing). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-601Invasive Plants: Backyard Solutions (sidebar) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-606

Land Used for Agriculture by Country, 1980-1994, Increases in (map) . . . . . . . . . . . . . . . . . . . I-38Leading Peat-Producing Countries, 1994 (bar graph) . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-780Leaf, Parts of a (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-617Leaf Arrangements, Common (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-620Leaf Bases (drawing). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-626Leaf Lobing and Division, Common Patterns of (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . II-623Leaf Margins (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-624Leaf Shapes, Common (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-628Leaf Tips (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-625Leaf Tissue, Cross-Section of (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-618Leaves, Types of (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-622Liverworts, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-641Lumber Consumption, U.S. (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV-1062Lycophytes, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-646

Meiosis: Selected Phases (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-679Mendel’s Pea-Plant Experiments, Results of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-501Mendel’s Pea Plants (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-500Miller-Urey Experiment (drawing). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-405Mitochondrion, Structure of a (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-673Mitosis (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-678Monocot Families, Common (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-73Monocot Families Common in North America (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . III-690Monocots vs. Dicots, Characteristics of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-688Mosses, Classification of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-695

Nastic Movement (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-704Nitrogen Cycle (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-707No-Tillage States, 1994-1997 (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-369North America, Selected Agricultural Products of (map) . . . . . . . . . . . . . . . . . . . . . . . . . III-715Nutrients, Plant (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-735

Osmosis, Process of (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-752Ovule, Parts of an (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-70Ozone Hole, 1980-2000, Average Size of the (graph) . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-758

Pacific Island Nations, Leading Agricultural Crops of (table) . . . . . . . . . . . . . . . . . . . . . . . III-763Persons Chronically Undernourished in Developing Countries by Region,

Number of (bar graph) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-39Phosphorus Cycle, The (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-792

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Photoperiodism (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-794Photosynthesis (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-801Plant, Parts of a (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-840Plantae, Phyla of Kingdom (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-844Poisonous Plants and Fungi, Common (table). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-858Pollination (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-864Population Growth, 1950-2020, World and Urban (bar graph) . . . . . . . . . . . . . . . . . . . . . . . II-587Prokaryotic Cell, A (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-871Protista, Phyla of (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-876

Rain Forests, African (Sidebar) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-884Rice-Producing Countries, 1994 (bar graph) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-913Root, Parts of a (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-921Root Systems, Two (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-923Roots, Water Uptake by (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-637Rubber, End Uses of Natural (pie chart) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-928

Seed, Germination of a (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-510Seedless Vascular Plants: Phyla and Characteristics (table) . . . . . . . . . . . . . . . . . . . . . . . . III-935Seeds: Eudicots vs. Monocots (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-938Seeds of Dissent (sidebar) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-497Shoot, Parts of the (drawing). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-945Silvicultural Methods, Environmental Effects of Select (flow chart) . . . . . . . . . . . . . . . . . . . . II-452Soap and Water (sidebar). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-123Soil Horizons (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-950Soil Limits to Agriculture, by Percentage of Total Land Area (pie chart) . . . . . . . . . . . . . . . . . III-961Soil Orders in the U.S. Classification System, The Twelve (table) . . . . . . . . . . . . . . . . . . . . . III-952South America, Selected Agricultural Products of (map) . . . . . . . . . . . . . . . . . . . . . . . . . III-966Stem Structure (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV-1018

Time Line of Plant Biotechnology (table). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-816Tissues, Plant (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-842Transformation of Sunlight into Biochemical Energy (flow chart) . . . . . . . . . . . . . . . . . . . . . II-365Transport Across Cell Membranes, Mechanisms for (table) . . . . . . . . . . . . . . . . . . . . . . . . III-852Tropical Soils, Declining Crop Yields with Successive Harvests on Unfertilized (bar graph). . . . . . III-948Tropisms (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV-1028

Water Through a Plant, The Path of (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV-1049Water Uptake by Roots (drawing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-637Welwitschia: The Strangest Gymnosperm (sidebar). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II-545Wheat-Producing Countries, 1994 (bar graph) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV-1055Wheat Stem Rust: Five Stages (table) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III-931World Food Production (bar graph) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-40

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ALPHABETICAL LIST OF CONTENTS

Volume 1

Acid precipitation . . . . . . . . . . . . . . . . . . 1Active transport . . . . . . . . . . . . . . . . . . . 4Adaptations . . . . . . . . . . . . . . . . . . . . . 7Adaptive radiation. . . . . . . . . . . . . . . . . 10African agriculture. . . . . . . . . . . . . . . . . 12African flora . . . . . . . . . . . . . . . . . . . . 17Agricultural crops: experimental. . . . . . . . . 20Agricultural revolution . . . . . . . . . . . . . . 23Agriculture: history and overview. . . . . . . . 24Agriculture: marine . . . . . . . . . . . . . . . . 29Agriculture: modern problems . . . . . . . . . . 31Agriculture: traditional . . . . . . . . . . . . . . 34Agriculture: world food supplies . . . . . . . . 37Agronomy . . . . . . . . . . . . . . . . . . . . . 42Air pollution . . . . . . . . . . . . . . . . . . . . 43Algae . . . . . . . . . . . . . . . . . . . . . . . . 47Allelopathy . . . . . . . . . . . . . . . . . . . . . 51Alternative grains . . . . . . . . . . . . . . . . . 52Anaerobes and heterotrophs . . . . . . . . . . . 54Anaerobic photosynthesis . . . . . . . . . . . . 56Angiosperm cells and tissues. . . . . . . . . . . 58Angiosperm evolution . . . . . . . . . . . . . . 61Angiosperm life cycle . . . . . . . . . . . . . . . 65Angiosperm plant formation . . . . . . . . . . . 69Angiosperms . . . . . . . . . . . . . . . . . . . . 72Animal-plant interactions. . . . . . . . . . . . . 78Antarctic flora . . . . . . . . . . . . . . . . . . . 81Aquatic plants . . . . . . . . . . . . . . . . . . . 82Archaea . . . . . . . . . . . . . . . . . . . . . . . 84Arctic tundra . . . . . . . . . . . . . . . . . . . . 88Ascomycetes . . . . . . . . . . . . . . . . . . . . 90Asian agriculture. . . . . . . . . . . . . . . . . . 92Asian flora . . . . . . . . . . . . . . . . . . . . . 96ATP and other energetic molecules. . . . . . . 100Australian agriculture . . . . . . . . . . . . . . 103Australian flora . . . . . . . . . . . . . . . . . . 105Autoradiography . . . . . . . . . . . . . . . . . 109

Bacteria . . . . . . . . . . . . . . . . . . . . . . 112Bacterial genetics . . . . . . . . . . . . . . . . . 119Bacterial resistance and super bacteria . . . . . 121Bacteriophages . . . . . . . . . . . . . . . . . . 125Basidiomycetes . . . . . . . . . . . . . . . . . . . 127Basidiosporic fungi . . . . . . . . . . . . . . . . 129Biochemical coevolution in

angiosperms. . . . . . . . . . . . . . . . . . 131Biofertilizers. . . . . . . . . . . . . . . . . . . . 134Biological invasions . . . . . . . . . . . . . . . 136Biological weapons . . . . . . . . . . . . . . . . 139Bioluminescence . . . . . . . . . . . . . . . . . 142Biomass related to energy . . . . . . . . . . . . 144Biomes: definitions and determinants . . . . . 147Biomes: types . . . . . . . . . . . . . . . . . . . 150Biopesticides . . . . . . . . . . . . . . . . . . . 154Biosphere concept . . . . . . . . . . . . . . . . 157Biotechnology. . . . . . . . . . . . . . . . . . . 158Botany . . . . . . . . . . . . . . . . . . . . . . . 161Bromeliaceae . . . . . . . . . . . . . . . . . . . . 162Brown algae . . . . . . . . . . . . . . . . . . . . 164Bryophytes . . . . . . . . . . . . . . . . . . . . 166Bulbs and rhizomes . . . . . . . . . . . . . . . 169

C4 and CAM photosynthesis . . . . . . . . . . 172Cacti and succulents . . . . . . . . . . . . . . . 175Calvin cycle . . . . . . . . . . . . . . . . . . . . 178Carbohydrates . . . . . . . . . . . . . . . . . . 180Carbon cycle . . . . . . . . . . . . . . . . . . . 183Carbon 13/carbon 12 ratios . . . . . . . . . . . 186Caribbean agriculture . . . . . . . . . . . . . . 188Caribbean flora . . . . . . . . . . . . . . . . . . 190Carnivorous plants . . . . . . . . . . . . . . . . 192Cell cycle . . . . . . . . . . . . . . . . . . . . . 195Cell theory. . . . . . . . . . . . . . . . . . . . . 197Cell-to-cell communication . . . . . . . . . . . 199Cell wall . . . . . . . . . . . . . . . . . . . . . . 201

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Cells and diffusion . . . . . . . . . . . . . . . . 203Cellular slime molds . . . . . . . . . . . . . . . 205Central American agriculture . . . . . . . . . . 207Central American flora. . . . . . . . . . . . . . 210Charophyceae . . . . . . . . . . . . . . . . . . . . 211Chemotaxis . . . . . . . . . . . . . . . . . . . . 213Chlorophyceae . . . . . . . . . . . . . . . . . . . 216Chloroplast DNA. . . . . . . . . . . . . . . . . 218Chloroplasts and other plastids. . . . . . . . . 220Chromatin . . . . . . . . . . . . . . . . . . . . . 223Chromatography . . . . . . . . . . . . . . . . . 225Chromosomes. . . . . . . . . . . . . . . . . . . 228Chrysophytes . . . . . . . . . . . . . . . . . . . 230Chytrids . . . . . . . . . . . . . . . . . . . . . . 233Circadian rhythms . . . . . . . . . . . . . . . . 236Cladistics . . . . . . . . . . . . . . . . . . . . . 238Climate and resources . . . . . . . . . . . . . . 241Clines . . . . . . . . . . . . . . . . . . . . . . . 243Cloning of plants . . . . . . . . . . . . . . . . . 245Coal . . . . . . . . . . . . . . . . . . . . . . . . 247Coevolution . . . . . . . . . . . . . . . . . . . . 252Community-ecosystem interactions . . . . . . 255Community structure and stability. . . . . . . 257Competition . . . . . . . . . . . . . . . . . . . . 260

Complementation and allelism:the cis-trans test . . . . . . . . . . . . . . . . 263

Compositae . . . . . . . . . . . . . . . . . . . . . 265Composting . . . . . . . . . . . . . . . . . . . . 267Conifers . . . . . . . . . . . . . . . . . . . . . . 270Corn . . . . . . . . . . . . . . . . . . . . . . . . 273Cryptomonads . . . . . . . . . . . . . . . . . . 276Culturally significant plants. . . . . . . . . . . 278Cycads and palms . . . . . . . . . . . . . . . . 281Cytoplasm. . . . . . . . . . . . . . . . . . . . . 285Cytoskeleton . . . . . . . . . . . . . . . . . . . 289Cytosol. . . . . . . . . . . . . . . . . . . . . . . 291

Deforestation . . . . . . . . . . . . . . . . . . . 293Dendrochronology . . . . . . . . . . . . . . . . 297Desertification . . . . . . . . . . . . . . . . . . 300Deserts . . . . . . . . . . . . . . . . . . . . . . . 303Deuteromycetes. . . . . . . . . . . . . . . . . . 306Diatoms . . . . . . . . . . . . . . . . . . . . . . 307Dinoflagellates . . . . . . . . . . . . . . . . . . 311Diseases and disorders. . . . . . . . . . . . . . 313DNA: historical overview . . . . . . . . . . . . 316DNA in plants. . . . . . . . . . . . . . . . . . . 322DNA: recombinant technology . . . . . . . . . 326

Volume 2

DNA replication . . . . . . . . . . . . . . . . . 329Dormancy . . . . . . . . . . . . . . . . . . . . . 331Drought . . . . . . . . . . . . . . . . . . . . . . 334

Ecology: concept . . . . . . . . . . . . . . . . . 337Ecology: history. . . . . . . . . . . . . . . . . . 339Ecosystems: overview . . . . . . . . . . . . . . 341Ecosystems: studies . . . . . . . . . . . . . . . 344Electrophoresis . . . . . . . . . . . . . . . . . . 348Endangered species . . . . . . . . . . . . . . . 350Endocytosis and exocytosis . . . . . . . . . . . 355Endomembrane system and

Golgi complex . . . . . . . . . . . . . . . . . 358Endophytes . . . . . . . . . . . . . . . . . . . . 361Endoplasmic reticulum . . . . . . . . . . . . . 363Energy flow in plant cells . . . . . . . . . . . . 364Environmental biotechnology. . . . . . . . . . 367Erosion and erosion control . . . . . . . . . . . 369

Estrogens from plants . . . . . . . . . . . . . . 371Ethanol. . . . . . . . . . . . . . . . . . . . . . . 374Eudicots . . . . . . . . . . . . . . . . . . . . . . 375Euglenoids . . . . . . . . . . . . . . . . . . . . 378Eukarya. . . . . . . . . . . . . . . . . . . . . . . 380Eukaryotic cells . . . . . . . . . . . . . . . . . . 382European agriculture. . . . . . . . . . . . . . . 385European flora . . . . . . . . . . . . . . . . . . 389Eutrophication . . . . . . . . . . . . . . . . . . 393Evolution: convergent and

divergent . . . . . . . . . . . . . . . . . . . . 396Evolution: gradualism vs.

punctuated equilibrium . . . . . . . . . . . 398Evolution: historical perspective . . . . . . . . 400Evolution of cells . . . . . . . . . . . . . . . . . 404Evolution of plants . . . . . . . . . . . . . . . . 409Exergonic and endergonic reactions . . . . . . 412Extranuclear inheritance. . . . . . . . . . . . . 414

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Farmland . . . . . . . . . . . . . . . . . . . . . 417Ferns . . . . . . . . . . . . . . . . . . . . . . . . 419Fertilizers . . . . . . . . . . . . . . . . . . . . . 423Flagella and cilia . . . . . . . . . . . . . . . . . 426Flower structure . . . . . . . . . . . . . . . . . 428Flower types . . . . . . . . . . . . . . . . . . . 432Flowering regulation . . . . . . . . . . . . . . . 435Fluorescent staining of cytoskeletal

elements . . . . . . . . . . . . . . . . . . . . 438Food chain. . . . . . . . . . . . . . . . . . . . . 440Forest and range policy . . . . . . . . . . . . . 443Forest fires. . . . . . . . . . . . . . . . . . . . . 447Forest management . . . . . . . . . . . . . . . 450Forests . . . . . . . . . . . . . . . . . . . . . . . 454Fossil plants . . . . . . . . . . . . . . . . . . . . 458Fruit crops . . . . . . . . . . . . . . . . . . . . . 461Fruit: structure and types . . . . . . . . . . . . 464Fungi . . . . . . . . . . . . . . . . . . . . . . . . 469

Garden plants: flowering . . . . . . . . . . . . 474Garden plants: shrubs . . . . . . . . . . . . . . 478Gas exchange in plants. . . . . . . . . . . . . . 481Gene flow . . . . . . . . . . . . . . . . . . . . . 483Gene regulation. . . . . . . . . . . . . . . . . . 486Genetic code. . . . . . . . . . . . . . . . . . . . 488Genetic drift . . . . . . . . . . . . . . . . . . . . 490Genetic equilibrium: linkage . . . . . . . . . . 491Genetically modified bacteria . . . . . . . . . . 494Genetically modified foods . . . . . . . . . . . 496Genetics: Mendelian . . . . . . . . . . . . . . . 499Genetics: mutations . . . . . . . . . . . . . . . 503Genetics: post-Mendelian . . . . . . . . . . . . 505Germination and seedling development. . . . 509Ginkgos . . . . . . . . . . . . . . . . . . . . . . 512Glycolysis and fermentation . . . . . . . . . . 515Gnetophytes. . . . . . . . . . . . . . . . . . . . 517Grains . . . . . . . . . . . . . . . . . . . . . . . 520Grasses and bamboos . . . . . . . . . . . . . . 522Grasslands. . . . . . . . . . . . . . . . . . . . . 525Grazing and overgrazing . . . . . . . . . . . . 527Green algae . . . . . . . . . . . . . . . . . . . . 530Green Revolution . . . . . . . . . . . . . . . . . 532Greenhouse effect. . . . . . . . . . . . . . . . . 534Growth and growth control . . . . . . . . . . . 537Growth habits. . . . . . . . . . . . . . . . . . . 541Gymnosperms . . . . . . . . . . . . . . . . . . 544

Halophytes . . . . . . . . . . . . . . . . . . . . 548Haptophytes . . . . . . . . . . . . . . . . . . . 551Hardy-Weinberg theorem . . . . . . . . . . . . 554Heliotropism . . . . . . . . . . . . . . . . . . . 557Herbicides . . . . . . . . . . . . . . . . . . . . . 558Herbs. . . . . . . . . . . . . . . . . . . . . . . . 561Heterokonts . . . . . . . . . . . . . . . . . . . . 564High-yield crops . . . . . . . . . . . . . . . . . 566History of plant science . . . . . . . . . . . . . 569Hormones . . . . . . . . . . . . . . . . . . . . . 575Hornworts. . . . . . . . . . . . . . . . . . . . . 578Horsetails . . . . . . . . . . . . . . . . . . . . . 581Horticulture . . . . . . . . . . . . . . . . . . . . 583Human population growth . . . . . . . . . . . 586Hybrid zones . . . . . . . . . . . . . . . . . . . 589Hybridization . . . . . . . . . . . . . . . . . . . 591Hydrologic cycle . . . . . . . . . . . . . . . . . 593Hydroponics . . . . . . . . . . . . . . . . . . . 597

Inflorescences . . . . . . . . . . . . . . . . . . . 600Integrated pest management . . . . . . . . . . 602Invasive plants . . . . . . . . . . . . . . . . . . 604Irrigation . . . . . . . . . . . . . . . . . . . . . 607

Krebs cycle . . . . . . . . . . . . . . . . . . . . 610

Leaf abscission . . . . . . . . . . . . . . . . . . 613Leaf anatomy . . . . . . . . . . . . . . . . . . . 616Leaf arrangements . . . . . . . . . . . . . . . . 619Leaf lobing and division . . . . . . . . . . . . . 621Leaf margins, tips, and bases . . . . . . . . . . 624Leaf shapes . . . . . . . . . . . . . . . . . . . . 627Legumes . . . . . . . . . . . . . . . . . . . . . . 629Lichens. . . . . . . . . . . . . . . . . . . . . . . 632Lipids . . . . . . . . . . . . . . . . . . . . . . . 634Liquid transport systems . . . . . . . . . . . . 636Liverworts. . . . . . . . . . . . . . . . . . . . . 640Logging and clear-cutting . . . . . . . . . . . . 643Lycophytes . . . . . . . . . . . . . . . . . . . . 645

Marine plants . . . . . . . . . . . . . . . . . . . 649Medicinal plants . . . . . . . . . . . . . . . . . 652Mediterranean scrub . . . . . . . . . . . . . . . 655Membrane structure . . . . . . . . . . . . . . . 658Metabolites: primary vs.

secondary . . . . . . . . . . . . . . . . . . . 659

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Alphabetical List of Contents

Volume 3

Microbial nutrition and metabolism . . . . . . 663Microbodies . . . . . . . . . . . . . . . . . . . . 666Microscopy . . . . . . . . . . . . . . . . . . . . 668Mitochondria . . . . . . . . . . . . . . . . . . . 672Mitochondrial DNA . . . . . . . . . . . . . . . 675Mitosis and meiosis . . . . . . . . . . . . . . . 677Mitosporic fungi . . . . . . . . . . . . . . . . . 681Model organisms . . . . . . . . . . . . . . . . . 682Molecular systematics . . . . . . . . . . . . . . 686Monocots vs. dicots . . . . . . . . . . . . . . . 688Monocotyledones . . . . . . . . . . . . . . . . . . 690Monoculture . . . . . . . . . . . . . . . . . . . 692Mosses . . . . . . . . . . . . . . . . . . . . . . . 694Multiple-use approach . . . . . . . . . . . . . . 697Mushrooms . . . . . . . . . . . . . . . . . . . . 699Mycorrhizae. . . . . . . . . . . . . . . . . . . . 702

Nastic movements . . . . . . . . . . . . . . . . 704Nitrogen cycle . . . . . . . . . . . . . . . . . . 706Nitrogen fixation . . . . . . . . . . . . . . . . . 709Nonrandom mating . . . . . . . . . . . . . . . 712North American agriculture. . . . . . . . . . . 714North American flora . . . . . . . . . . . . . . 718Nuclear envelope . . . . . . . . . . . . . . . . . 722Nucleic acids . . . . . . . . . . . . . . . . . . . 723Nucleolus . . . . . . . . . . . . . . . . . . . . . 727Nucleoplasm . . . . . . . . . . . . . . . . . . . 729Nucleus . . . . . . . . . . . . . . . . . . . . . . 730Nutrient cycling . . . . . . . . . . . . . . . . . 732Nutrients . . . . . . . . . . . . . . . . . . . . . 734Nutrition in agriculture . . . . . . . . . . . . . 737

Oil bodies . . . . . . . . . . . . . . . . . . . . . 740Old-growth forests . . . . . . . . . . . . . . . . 741Oomycetes. . . . . . . . . . . . . . . . . . . . . 743Orchids . . . . . . . . . . . . . . . . . . . . . . 745Organic gardening and farming . . . . . . . . 747Osmosis, simple diffusion, and

facilitated diffusion . . . . . . . . . . . . . . 751Oxidative phosphorylation . . . . . . . . . . . 755Ozone layer and ozone hole debate . . . . . . 757

Pacific Island agriculture . . . . . . . . . . . . 761Pacific Island flora . . . . . . . . . . . . . . . . 765

Paclitaxel . . . . . . . . . . . . . . . . . . . . . 768Paleobotany . . . . . . . . . . . . . . . . . . . . 770Paleoecology . . . . . . . . . . . . . . . . . . . 774Parasitic plants . . . . . . . . . . . . . . . . . . 777Peat. . . . . . . . . . . . . . . . . . . . . . . . . 779Peroxisomes . . . . . . . . . . . . . . . . . . . . 782Pesticides . . . . . . . . . . . . . . . . . . . . . 783Petrified wood . . . . . . . . . . . . . . . . . . 787Pheromones . . . . . . . . . . . . . . . . . . . . 790Phosphorus cycle . . . . . . . . . . . . . . . . . 791Photoperiodism. . . . . . . . . . . . . . . . . . 794Photorespiration . . . . . . . . . . . . . . . . . 796Photosynthesis . . . . . . . . . . . . . . . . . . 800Photosynthetic light absorption. . . . . . . . . 804Photosynthetic light reactions. . . . . . . . . . 807Phytoplankton . . . . . . . . . . . . . . . . . . 809Pigments in plants . . . . . . . . . . . . . . . . 812Plant biotechnology . . . . . . . . . . . . . . . 815Plant cells: molecular level . . . . . . . . . . . 821Plant domestication and breeding . . . . . . . 826Plant fibers . . . . . . . . . . . . . . . . . . . . 829Plant life spans . . . . . . . . . . . . . . . . . . 831Plant science. . . . . . . . . . . . . . . . . . . . 835Plant tissues . . . . . . . . . . . . . . . . . . . . 839Plantae . . . . . . . . . . . . . . . . . . . . . . . 843Plants with potential . . . . . . . . . . . . . . . 849Plasma membranes. . . . . . . . . . . . . . . . 851Plasmodial slime molds . . . . . . . . . . . . . 854Poisonous and noxious plants . . . . . . . . . 857Pollination. . . . . . . . . . . . . . . . . . . . . 862Polyploidy and aneuploidy . . . . . . . . . . . 865Population genetics . . . . . . . . . . . . . . . 868Prokaryotes . . . . . . . . . . . . . . . . . . . . 870Proteins and amino acids . . . . . . . . . . . . 873Protista . . . . . . . . . . . . . . . . . . . . . . . 875Psilotophytes . . . . . . . . . . . . . . . . . . . 879

Rain-forest biomes . . . . . . . . . . . . . . . . 883Rain forests and the atmosphere . . . . . . . . 885Rangeland . . . . . . . . . . . . . . . . . . . . . 889Red algae . . . . . . . . . . . . . . . . . . . . . 892Reforestation . . . . . . . . . . . . . . . . . . . 894Reproduction in plants. . . . . . . . . . . . . . 897Reproductive isolating mechanisms . . . . . . 899

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Resistance to plant diseases . . . . . . . . . . . 901Respiration . . . . . . . . . . . . . . . . . . . . 904Rhyniophyta . . . . . . . . . . . . . . . . . . . . 907Ribosomes . . . . . . . . . . . . . . . . . . . . . 909Rice. . . . . . . . . . . . . . . . . . . . . . . . . 912RNA . . . . . . . . . . . . . . . . . . . . . . . . 914Root uptake systems . . . . . . . . . . . . . . . 917Roots . . . . . . . . . . . . . . . . . . . . . . . . 920Rubber . . . . . . . . . . . . . . . . . . . . . . . 925Rusts . . . . . . . . . . . . . . . . . . . . . . . . 929

Savannas and deciduous tropical forests . . . 932Seedless vascular plants . . . . . . . . . . . . . 934Seeds . . . . . . . . . . . . . . . . . . . . . . . . 937Selection . . . . . . . . . . . . . . . . . . . . . . 941Serpentine endemism . . . . . . . . . . . . . . 943Shoots . . . . . . . . . . . . . . . . . . . . . . . 944Slash-and-burn agriculture . . . . . . . . . . . 946

Soil . . . . . . . . . . . . . . . . . . . . . . . . . 949Soil conservation . . . . . . . . . . . . . . . . . 955Soil contamination . . . . . . . . . . . . . . . . 957Soil degradation . . . . . . . . . . . . . . . . . 959Soil management . . . . . . . . . . . . . . . . . 962Soil salinization . . . . . . . . . . . . . . . . . . 963South American agriculture . . . . . . . . . . . 965South American flora . . . . . . . . . . . . . . 969Species and speciation . . . . . . . . . . . . . . 973Spermatophyta . . . . . . . . . . . . . . . . . . . 976Spices . . . . . . . . . . . . . . . . . . . . . . . 977Stems. . . . . . . . . . . . . . . . . . . . . . . . 980Strip farming . . . . . . . . . . . . . . . . . . . 983Stromatolites . . . . . . . . . . . . . . . . . . . 985Succession . . . . . . . . . . . . . . . . . . . . . 988Sugars . . . . . . . . . . . . . . . . . . . . . . . 991Sustainable agriculture. . . . . . . . . . . . . . 993

Volume 4

Sustainable forestry . . . . . . . . . . . . . . . 997Systematics and taxonomy . . . . . . . . . . . 999Systematics: overview . . . . . . . . . . . . . 1004

Taiga . . . . . . . . . . . . . . . . . . . . . . . 1007Textiles and fabrics . . . . . . . . . . . . . . . 1009Thigmomorphogenesis . . . . . . . . . . . . . 1013Timber industry . . . . . . . . . . . . . . . . . 1015Tracheobionta . . . . . . . . . . . . . . . . . . . 1017Trimerophytophyta . . . . . . . . . . . . . . . . 1021Trophic levels and ecological niches . . . . . 1023Tropisms . . . . . . . . . . . . . . . . . . . . . 1027Tundra and high-altitude biomes . . . . . . . 1030

Ulvophyceae. . . . . . . . . . . . . . . . . . . . 1033Ustomycetes. . . . . . . . . . . . . . . . . . . . 1035

Vacuoles . . . . . . . . . . . . . . . . . . . . . 1037Vegetable crops . . . . . . . . . . . . . . . . . 1039Vesicle-mediated transport. . . . . . . . . . . 1043Viruses and viroids . . . . . . . . . . . . . . . 1045

Water and solute movement in plants . . . . 1048Wetlands . . . . . . . . . . . . . . . . . . . . . 1051

Wheat. . . . . . . . . . . . . . . . . . . . . . . 1054Wood . . . . . . . . . . . . . . . . . . . . . . . 1056Wood and charcoal as fuel resources . . . . . 1058Wood and timber . . . . . . . . . . . . . . . . 1060

Yeasts . . . . . . . . . . . . . . . . . . . . . . . 1065

Zosterophyllophyta . . . . . . . . . . . . . . . . 1067Zygomycetes. . . . . . . . . . . . . . . . . . . 1069

Biographical List of Botanists . . . . . . . . . 1073Plant Classification . . . . . . . . . . . . . . . 1105Plant Names: Common-to-

Scientific . . . . . . . . . . . . . . . . . . . . 1115Plant Names: Scientific-to-

Common . . . . . . . . . . . . . . . . . . . 1130Time Line. . . . . . . . . . . . . . . . . . . . . 1199Glossary . . . . . . . . . . . . . . . . . . . . . 1207Bibliography . . . . . . . . . . . . . . . . . . . 1244Web Sites . . . . . . . . . . . . . . . . . . . . . 1254

Biographical Index. . . . . . . . . . . . . . . . . IIICategorized Index . . . . . . . . . . . . . . . . VIIIndex . . . . . . . . . . . . . . . . . . . . . . . XVII

xxi

Alphabetical List of Contents

MICROBIAL NUTRITION AND METABOLISM

Categories: Algae; bacteria; fungi; microorganisms; nutrients and nutrition; Protista

The diverse metabolic activities of microorganisms make them a critical component of all the earth’s ecosystems and asource of many useful products for human industry.

Microorganisms—bacteria, fungi, algae, andprotists—are found in every environment on

the earth that supports life. Microorganisms havebeen found in hot springs where temperatures ex-ceed 80 degrees Celsius as well as in rocks of Ant-arctic deserts. To ensure survival in a variety of hab-itats, microorganisms have developed a fascinatingvariety of strategies for survival. The study of mi-crobial ecology involves consideration of the mech-anisms employed by microorganisms to obtain nu-trients and energy from their environment.

Nutritional ModesTo maintain life processes and grow, all cellular

organisms require both a source of carbon (the prin-cipal element in all organic molecules) and a sourceof energy to perform the work necessary to trans-form carbon into all the molecular components ofcytoplasm. Among plants and animals, two mainnutritional modes have evolved to meet these re-quirements. All plants are photoautotrophs, fixingcarbon from inorganic carbon and obtaining energyfrom light. All animals are chemoheterotrophs, meet-ing their carbon needs by taking preformed organicmolecules from the environment and extracting en-ergy from chemical transformation of the same or-ganic molecules.

Both of these nutritional modes, photoauto-trophy and chemoheterotrophy, are found amongmicroorganisms; for example, all algae are photo-autotrophs, while all fungi are chemoheterotrophs.In addition, certain specialized bacteria exhibit amode of nutrition, chemoautotrophy, found in nohigher organisms. Like photoautotrophs, chemo-autotrophs are able to use carbon dioxide for allof their carbon requirements; however, they donot use light as an energy source. Instead, chemo-autotrophic bacteria capture energy from inorganic

chemical reactions, such as the oxidation of ammo-nia. Chemoautotrophic bacteria are highly special-ized and can be found in unusual environments.The most spectacular display of chemoautotrophicenergy metabolism is exhibited at the hydrother-mal vents found in certain locations on the oceanfloor. There, where sunlight cannot penetrate, che-moautotrophic bacteria serve as the producers for arich and diverse ecosystem.

An appreciation of the metabolic diversity dis-played by microorganisms enhances understand-ing of the ways in which matter and energy aretransformed in the biosphere. Consideration of mi-crobial contributions to the flow of carbon, nitro-gen, and other elements is critical to defining thebalance of ecosystems and the effects of changes inenvironmental chemistry and species composition.Microorganisms are, by definition, unseen, andmany people become aware of them only in theirnegative manifestations as agents of disease andspoilage. In fact, however, the diverse metabolic ac-tivities of microorganisms make them a criticalcomponent of all the earth’s ecosystems and a sourceof many useful products for human industry.

Cellulose DigestionEven among chemoheterotrophs, microorgan-

isms possess metabolic capabilities unknown inhigher organisms. These include the ability of somebacteria and fungi to digest cellulose, a linear poly-mer of glucose that is the principal molecular con-stituent of paper. Sixty percent of the dry mass ofgreen plants is in the form of cellulose, although noanimal that eats the plants is directly able to obtaincarbon or energy from cellulose. Microorganismsthat digest cellulose do so by secreting exoenzymes,proteins that cause cellulose to be broken into sim-pler molecular units that are absorbed by the micro-

Microbial nutrition and metabolism • 663

organism. Cellulose-digesting microorganisms arefound in most terrestrial ecosystems and in the di-gestive tracts of animals, such as cattle and ter-mites, that depend on cellulose-rich plant materialas a nutrient source. By breaking down celluloseand other complex organic polymers, microorgan-isms make a significant contribution to the cyclingof carbon in ecosystems.

Nitrogen FixationDigestion of complex organic polymers is only

one of the ways in which microorganisms contrib-ute to the cycling of elements in the environmentsthey inhabit. Microorganisms also perform chemi-cal transformations involving nitrogen, which isfound in all cellular proteins and nucleic acids.Plants incorporate nitrogen from the soil in theform of nitrate or ammonium ions, and animals ob-tain nitrogen from the same organic compoundsthey use as carbon and energy sources.

When dead plant and animal tissue is decom-posed by chemoheterotrophic microorganisms, thenitrogen is released as ammonia. A group ofchemoautotrophic bacteria, the nitrifying bacteria,obtain their metabolic energy from the conversionof ammonia to nitrate; in this way, the nitrifiers con-vert the nitrogen released during decomposition toa form readily used by plants, thus contributing tosoil fertility. Asecond group of bacteria converts ni-trate to atmospheric nitrogen gas, which cannot beused by plants; these bacteria are called denitrifiersbecause (in contrast to the nitrifiers) their metabolicactivities cause a net loss of nitrogen from the soil.

Nitrogen lost from the environment by deni-trification is replaced by ammonia released duringdecomposition and by the metabolic activity ofnitrogen-fixing bacteria, so called because they “fix”nitrogen gas from the atmosphere in the form ofammonia. Nitrogen fixation requires a great quan-tity of energy, and nitrogen-fixing bacteria are often

664 • Microbial nutrition and metabolism

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found in symbiotic association with plants, espe-cially legumes. The bacteria provide nitrogen in ausable form to the plant, while the plant providescarbon and energy in the form of organic com-pounds to the chemoheterotrophic nitrogen-fixingbacteria. The presence of nitrogen-fixing bacteria isoften indicated by the formation of characteristicnodules on the roots of plants involved in the asso-ciations. Free-living nitrogen fixers are also known,and these may play a significant role in the nitrogenbalance of aquatic ecosystems.

Respiration and FermentationChemoheterotrophic microorganisms are found

both in aerobic environments, where oxygen is avail-able, and in anaerobic environments, where oxygenis lacking. The availability of molecular oxygenmay determine the type of energy metabolismemployed by a microorganism. Where oxygen isavailable, many microorganisms obtain energy byrespiration. In respiration, electrons removed fromorganic nutrient sources are transferred through acomplex sequence of reactions to molecular oxy-gen, forming water and carbon dioxide. In the pro-cess, energy is made available to the organisms. Inthe absence of oxygen, some microorganisms areable to carry out a form of anaerobic respiration us-ing nitrate or sulfate in place of oxygen. Deni-trification is an example of anaerobic respiration.

Other anaerobic microorganisms employ fer-mentation. In fermentation, electrons removed fromorganic nutrient sources are transferred to organicmolecules, forming fermentation products, such asalcohols and organic acids, which may be used asnutrient sources by other chemoheterotrophs. Anumber of bacteria, the facultative anaerobes, are ca-pable of performing either aerobic respiration orfermentation, depending on the availability of oxy-gen. These bacteria are able to achieve optimumgrowth in environments, such as soils, where theavailability of oxygen may vary over time.

Effects and UsesThe contributions of microorganisms to the

chemical transformations which characterize anecosystem are many. Along with higher plants,photoautotrophic and chemoautotrophic microor-ganisms capture inorganic carbon dioxide and, us-ing energy from sunlight or chemical reactions,synthesize organic molecules, which are used byanimals and by chemoheterotrophic microorgan-

isms as sources of carbon and energy. Through theprocesses of respiration and fermentation, chemo-heterotrophs return inorganic carbon dioxide to theenvironment.

Much of this recycling of carbon from organicmolecules to carbon dioxide depends on the activi-ties of microbial decomposers, which are able tobreak down organic polymers, such as cellulose.Nitrogen, released from organic molecules bychemoheterotrophs in the form of ammonia, maybe made available to chemoheterotrophs in theform of nitrate by nitrifying bacteria. Nitrate whichis lost from an ecosystem through the activities ofdenitrifiers may be returned by nitrogen-fixing mi-croorganisms.

Although the nature of the microbial world hasbeen known only since about the turn of the twenti-eth century, the metabolic activities of microorgan-isms have been exploited throughout human his-tory. The manufacture of alcoholic beverages,cheeses, vinegars, and linen depends on the meta-bolic activities of microorganisms. Farmers em-ployed practices designed to optimize the avail-ability of nitrogen to plants for centuries before therole of microorganisms in nitrogen cycling wasunderstood. Composting and other decompositionprocesses, including sewage treatment, are conse-quences of the metabolic activities of mixed popu-lations of microorganisms.

The number of organic compounds used as nu-trients by one or another chemoheterotrophic mi-croorganism is extraordinary. Some soil bacteriahave been shown to use more than one hundred dif-ferent organic molecules as their sources of carbonand energy. By using selective enrichment tech-niques, it has been possible to isolate microor-ganisms capable of degrading pesticides, complexpetroleum by-products, and other toxic chemicalspreviously assumed to be resistant to natural de-composition processes. Through application of ap-propriate engineering technologies, these microor-ganisms may play a part in solutions to toxic wastedisposal issues.

Kenneth A. Pidcock

See also: Algae; Anaerobes and heterotrophs;Archaea; Bacteria; Biofertilizers; Biopesticides; Bio-technology; Carbon cycle; Chemotaxis; Fungi; Le-gumes; Nitrogen cycle; Nitrogen fixation; Nutri-ents; Phosphorus cycle; Photosynthesis; Protista;Respiration.

Microbial nutrition and metabolism • 665

Sources for Further StudyCaldwell, Daniel R. Microbial Physiology and Metabolism. Dubuque, Iowa: Wm. C. Brown,

1995. For undergraduate and beginning graduate students. Coverage includes sub-cellular structures of microbes, physiological implications of nutrition catabolic metabo-lism, small molecules, genetics, and regulation.

Creager, Joan G., Jacquelyn G. Black, and Vee E. Davison. Microbiology: Principles and Applica-tions. Englewood Cliffs, N.J.: Prentice Hall, 1990. An accessible and beautifully illustratedtextbook of microbiology. Chapters 26, “Environmental Microbiology,” and 27, “AppliedMicrobiology,” summarize the importance of microorganisms in ecosystems and theiruses in industry. Includes references.

Gottschalk, Gerhard. Bacterial Metabolism. 2d ed. New York: Springer-Verlag, 1986. Writtenboth as an advanced college textbook and as a microbiologist’s desk reference, this mono-graph provides clear, concise outlines of all the major metabolic strategies employed bybacteria. The first place to look for the specifics of microbial metabolism. Includes refer-ences.

Kornberg, Arthur. For the Love of Enzymes: The Odyssey of a Biochemist. Cambridge, Mass.:Harvard University Press, 1991. Kornberg, a leading researcher in the field of bio-chemistry, provides a lucid introduction to the field in the form of an autobiographicalaccount of his own work. Discusses both the workings of cells and the workings of sci-ence.

Margulis, Lynn, and Dorian Sagan. Microcosmos: Four Billion Years of Evolution from Our Mi-crobial Ancestors. New York: Summit Books, 1997. An entertaining account of the evolu-tion of life on earth that focuses on the roles played by microorganisms in developing allthe basic metabolic pathways and on the continuing significance of microorganisms tothe biosphere. One of the few books describing the history of life which gives microorgan-isms the attention they deserve.

White, David. The Physiology and Biochemistry of Prokaryotes. 2d ed. New York: Oxford Uni-versity Press, 2000. Covers microbial and bacterial metabolism and physiology of Archaeaand prokaryotes. Includes bibliographical references, index.

MICROBODIES

Categories: Anatomy; cellular biology; transport mechanisms

Microbodies, found in cells, are spherical, membrane-bound organelles that play a part in photorespiration and the con-version of fats into sucrose.

Peroxisomes and glyoxysomes are the two majortypes of microbodies in plant cells. Their vesicles

(“packages”) vary in size from 0.3 to 1.5 microme-ters in diameter and are self-replicating. Newmicrobodies are formed by incorporation of re-quired proteins and lipids from the cytoplasm andsubsequent splitting when they reach a certain size.Although structurally similar, their roles, and thustheir contents, are different.

Functions of PeroxisomesPeroxisomes are present in leaves, and their oxi-

dative enzymes are involved in the breakdown ofhydrogen peroxide and, more important, in photo-respiration. Photorespiration occurs when carbondioxide levels in the leaves drop and oxygen levelsincrease, a typical phenomenon on hot, sunny dayswhen a plant is experiencing some level of waterstress. Under these conditions, the enzyme (Ru-

666 • Microbodies

bisco) that normally catalyzes the attachment ofcarbon dioxide to ribulose bisphosphate (RuBP) be-gins to have a higher affinity for oxygen than forcarbon dioxide. When oxygen is used instead ofcarbon dioxide, RuBP is split into two molecules,phosphoglycolate and 3-phosphoglycerate (PGA).PGA can be used in another part of the Calvin cycle,but phosphoglycolate must be extensively pro-cessed to be useful.

Phosphoglycolate is hydrolyzed and convertedto glycolate in the chloroplast. Glycolate is thentransported out of the chloroplast and into nearbyperoxisomes. Peroxisomal oxidase converts glyco-late to glyoxylate, and hydrogen peroxide is pro-duced as a by-product. Because hydrogen peroxideis toxic, it is quickly converted by a catalase to waterand oxygen. Glyoxylate goes through several moresteps which involve reactions in the mitochondriaand then back again in the peroxisomes. Even-tually, glycerate is formed in the peroxisomes. Gly-cerate is then transported out of the peroxisomesand into a chloroplast, where it is converted toPGA, which can reenter the Calvin cycle.

Functions of GlyoxysomesGlyoxysomes are found in the cells of fat-rich

seeds. Fats are synthesized and stored as oil bodies,sometimes called spherosomes. Spherosomes aresurrounded by a single layer of lipids instead of alipid bilayer and are therefore not organelles in thestrict sense. Glyoxysomes are responsible for con-verting fats and fatty acids into sucrose. The fatused by glyoxysomes comes from spherosomes.

Glyoxysomes are considered to be a type ofperoxisome. In some plants, small glyoxysomes arefound in the cotyledons of developing seeds. Dur-ing germination and seedling development, theymature into fully functional glyoxysomes. Theyfunction until the fats are completely digested intosucrose. The energy from sucrose is required todrive early seedling development before photosyn-thesis begins. Large fat molecules are difficult totransport into the plant embryo; they must be con-verted to the more mobile sucrose molecules.

Breakdown of fats is a collaborative effort be-tween enzyme-containing glyoxysomes and fat-containing spherosomes. Direct contact betweenspherosomes and glyoxysomes must occur. Fatsfrom spherosomes leak out close to the membraneof glyoxysomes. Most of the activity of lipase en-zymes does not take place in spherosomes butrather in or near the membrane of glyoxysomes. Li-pases in glyoxysomes hydrolyze the ester bonds offats and release the three fatty acids and one glyc-erol from each fat molecule. Glycerol is converted,at the cost of adenosine triphosphate (ATP), toglycerol phosphate, which is then oxidized by nico-tinamide adenine dinucleotide (NAD+) to dihy-droxyacetone phosphate, most of which is con-verted to glucose.

Domingo M. Jariel

See also: ATP and other energetic molecules; Cal-vin cycle; Cytosol; Lipids; Membrane structure; Oilbodies; Peroxisomes; Vacuoles; Vesicle-mediatedtransport.

Sources for Further StudyGunning, Brian E. S., and Martin W. Steer. Plant Cell Biology: Structure and Function. Sudbury,

Mass.: Jones and Bartlett, 2000. Text integrates microscopy with plant cell and molecularbiology. Includes more than four hundred micrographs and four pages of full-colorplates.

Hay, R. K., and Alastair H. Fitter. Environmental Physiology of Plants. 3d ed. New York: Aca-demic Press, 2001. Provides a synthesis of modern ecological and physiological thinking.Examines molecular approaches which can be harnessed to solve problems in physiol-ogy. Illustrations include color plates.

Hopkins, William G. G. Introduction to Plant Physiology. 2d ed. New York: John Wiley & Sons,1998. Uses interactions between the plant and the environment as a foundation for devel-oping plant physiology principles. Discusses the role of plants on specific ecosystems andglobal ecology and provides information on the cell, chemical background, plant growthregulators and biochemistry. Each chapter is illustrated with relevant graphs, figures, orphotographs.

Microbodies • 667

MICROSCOPY

Categories: Cellular biology; methods and techniques

In biology and botany, light microscopy is used for the observation of large numbers of cells or for the location of singlecells. Scanning electron microscopy is used in the examination of the surface profiles of cells. Transmission electron mi-croscopy is used to probe the interiors of cells to elucidate their structures. The scanning tunneling microscope can beused to examine still smaller structures. Each of the techniques described has been used extensively in the biological sci-ences.

Light MicroscopesThe first microscopes used a beam of light to

form an image and were probably invented in theNetherlands, where the devices were used in themanufacture of spectacles and cloth. Dutch micros-copist Antoni van Leeuwenhoek (1632-1723) im-proved upon the cloth merchants’ microscopes andused his version to study small objects from nature,such as single-celled organisms and red blood cells.English scientist Robert Hooke (1635-1703), usinghis own simple microscope, discovered “cells” in aslice of cork.

The human eye by itself is able to resolve imagesof about 100 micrometers (0.1 millimeter). Thismeans that two objects (such as lines or dots) lessthan 100 micrometers apart will appear to blur intoone object. The highest resolution available in lightmicroscopes will improve upon the human eye fivehundred times, allowing it to distinguish objectsthat are 0.2 micrometer (200 nanometers) apart.Resolution by a light microscope is limited becausethe shortest wavelength of visible light itself isabout 0.4 micrometer (400 nanometers). This limitsthe effective magnification of a light microscope toabout 2,000 times. At this magnification most bacte-ria are readily visible, as are a variety of organelleswithin plant cells, such as vacuoles, nuclei (in-cluding chromosomes during cell division), chloro-plasts, and mitochondria. Smaller structures, suchas ribosomes and microtubules (as well as othercomponents of the cytoskeleton), are not visible us-ing a light microscope.

Electron MicroscopesThe electron microscope was invented in 1931 by

Ernst Ruska, who won the Nobel Prize in Physics

for this effort in 1986. Since the time of its invention,several different types of electron microscope havebeen developed. Electron microscopy uses a beamof electrons instead of a beam of light to form an im-age. Light is a form of electromagnetic radiation, aprocess that transfers energy as a wave withouttransferring matter. An electromagnetic wave canbe visualized in terms of a wave traveling on thesurface of a pool of water: The undulating patternof peaks and troughs constitutes the wave. In thecase of electromagnetic radiation, the undulationsthat constitute the wave occur in electric and mag-netic fields that are perpendicular to each other.

Waves and WavelengthsIt is usual to consider light as a wave, or a ray,

and to consider electrons as particles. According toquantum physics, however, waves and particles aretwo aspects of the same phenomenon. Light can beconsidered as a stream of particles, and a stream ofelectrons can be considered as a wave. This behav-ior is usually referred to as wave-particle duality.

One property of a wave is its wavelength. Thewavelength of a wave is the distance between adja-cent peaks (or adjacent troughs) in the waveform.Wavelength is important because when a wave in-teracts with matter, any structures that are smallerthan one-quarter of the wavelength are “invisible”to the wave. In approximate terms, the smallest ob-ject that a wave can be used to image is equal in sizeto the wavelength of the wave used to form the im-age. The wavelength of visible light is between ap-proximately 400 nanometers and 700 nanometers (ananometer is one one-billionth of a meter).

A stream of electrons has a smaller wavelengththan a beam of light, and electrons can therefore be

668 • Microscopy

used to form images of small objects—such as cells.If distinct images of separate objects are formed, theimages are said to be resolved. Because the wave-length of an electron is much smaller than thewavelength of light, the electron microscope can re-solve images of objects that are a million timessmaller than the objects seen in traditional opticalmicroscopes. The resolving power of a scanningtunneling microscope (STM) is sufficient to renderit capable of determining the positions of individ-ual atoms in the surface layer of a material.

There are several different types of electron mi-croscope. The basic types include the transmission

electron microscope, the scanning electron micro-scope, and the scanning tunneling microscope. Thespecimen in an electron microscope is usually ob-served in a vacuum in order to prevent scattering ofthe electrons by air molecules; the need for a vac-uum presents the greatest difficulty in the applica-tion of electron microscopy to biological systems.

Transmission Electron MicroscopesThe simplest electron microscope is the transmis-

sion electron microscope (TEM). Electrons are pro-duced by an electron gun and are accelerated by apotential difference (voltage). The electrons fromthe electron gun pass through a condenser lens andare then used to illuminate the specimen. The elec-trons which pass through the specimen are then al-lowed to pass through an electron lens objective.The objective magnifies the image, and then a sec-ond electron lens, which plays the role of the eye-piece in the standard microscope, is used to focusthe image for observation. The “lenses” used inelectron microscopes are not lenses in the usualsense; instead, they are electric and magnetic fields,and they are accordingly referred to as electrostaticor magnetic lenses.

The image can then be formed on a photographicplate or observed on a fluorescent screen, or theelectrons can be collected by a charge-sensitive de-vice to produce an image on a cathode-ray tube.Higher magnification can be achieved by usingmore lenses.

The sample thickness will affect the resolvingability of the TEM. Usually, at least in the case of bi-ological samples, the sample should be no morethan ten times thicker than the structures that are tobe analyzed. The resolving power of the TEM issuch that it can observe structures that are slightlylarger than atoms, but since the development ofother systems, it has become less used, even thoughits resolution often exceeds that of the scanningelectron microscope. Typical resolutions are in thesubnanometer range, with magnifications of up to500,000 times.

The interpretation of the electron micrographsproduced by a TEM is sometimes difficult. Themajor source of difficulty is that the image is pro-duced by transmitted radiation. The eye is accus-tomed to interpreting images that are produced byreflection. In the absence of a sample, a TEM beamwould saturate a film plate used to record an image.The sample prevents some of the electrons from

Microscopy • 669P

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The human eye by itself is able to resolve images of about100 micrometers. This means that two objects (such asdots) less than 100 micrometers apart will appear to blurinto one object. The highest resolution available in a lightmicroscope (shown here) will improve upon the humaneye five hundred times, allowing it to distinguish objectsthat are 0.2 micrometer (200 nanometers) apart. The re-solving power of a scanning tunneling microscope is suf-ficient to determine the positions of individual atoms inthe surface layer of a material.

reaching the film plate; the image produced by aTEM is somewhat similar to a negative produced ina normal camera. Much of the difficulty can be re-moved by photographing the micrograph and con-verting it to a positive image—there are, however,some residual interpretation difficulties caused byshadows.

High-Voltage and Scanning ElectronMicroscopes

The high-voltage electron microscope (HVEM) is avariant of the TEM. The conventional TEM worksbest on particles less than 0.5 nanometer thick; elec-trons of higher speed can be produced by increas-ing the voltage used to accelerate them, and thickersamples can then be analyzed. The wavelength ofthe electrons decreases as their speed (hence, theirkinetic energy) increases. These short-wavelengthelectrons are less likely to collide with atoms as theypass through the specimen, and they are thereforeable to render a sharper image of a thicker sample.

The scanning electron micrscope (SEM) works on adifferent principle. Electrons are again producedand accelerated by an electron gun, but in an SEMthe beam is focused by electron lenses and used toscan a sample. The scanning will result in two dif-ferent electron beams being emitted by the sample,a primary beam of backscattered electrons pro-duced by reflection and a secondary beam of elec-trons emitted by the atoms of the sample. By scan-ning the entire sample and collecting the primaryand secondary electrons, the operator can producean image of a sample on a cathode-ray tube.

Scanning Tunneling Electron MicroscopesThe scanning tunneling electron microscope (STM)

works on a completely different principle. It wasdeveloped by Gerd Binnig and Heinrich Rohrer atan International Business Machines (IBM) researchlaboratory in the 1980’s. Binnig and Rohrer sharedthe 1986 Nobel Prize in Physics with Ernst Ruskafor their work. The STM uses quantum tunneling.Quantum tunneling is the penetration of a barrierby a particle that, when analyzed by classical phys-ics, does not have enough energy to pass throughthe barrier. Quantum mechanics predicts that thereis a finite probability of a particle passing through abarrier, even if it lacks the energy to pass over it. Thenumber of particles passing through the barrierwill vary with the barrier thickness and the particleenergy.

This principle is used in the STM by allowingelectrons to tunnel across a vacuum, from a sty-lus to a sample. The quantum tunneling of elec-trons sets up a “tunneling current” that increasesas the vacuum gap between the stylus and the sam-ple decreases. The variation in tunneling currentcan be used to map the surface of the sample asthe stylus moves across its surface. The STM is ca-pable of detecting structures 0.1 nanometer in sizein the direction parallel to the motion of the scan-ning stylus. This means that atoms can be easily de-tected. The performance in the vertical directionis even more impressive: The STM can detect ir-regularities on the order of 0.01 nanometer. Thus,the STM, with its higher resolving power than theSEM or TEM, has little difficulty in the imaging ofatoms.

Preparing SamplesThe principal accommodations that must be

made in the examination of biological samples us-ing electron microscopy occur in the preparation ofthe sample. A commonly used method is the con-struction of replicas, made by the vacuum deposi-tion of thin layers of carbon, metals, or alloys on thesurface of the sample. These films provide a replicaof the surface, which can be scanned. Another use-ful technique, which is the standard technique ofsample preparation used in optical microscopy, isthe sectioning of samples and their impregnationwith stains. The stains that are useful in electron mi-croscopy are usually chemical compounds of heavymetals. These are effective stains because theystrongly scatter electrons.

Many methods of sample preparation have beendeveloped to enable electron microscopy to bemore widely used on biological samples. Freeze-fracture and freeze-etch are methods of sample prep-aration that have been widely used. Freeze-fractureinvolves the freezing and splitting of a water-containing sample. Freeze-etch is a second step, inwhich ice is allowed to sublime (vaporize withoutforming a liquid) before the sample is analyzed inan electron microscope. Both techniques are used toexamine the internal structure of materials withoutsubjecting them to chemical changes. Freeze-fracture and freeze-etch allow the interior layers ofwater-containing samples to be investigated with-out the straining and chemical preparations that areneeded to render internal structures visible in astandard light microscope. In particular, these tech-

670 • Microscopy

niques allow the observation of cell walls and cellmembranes in a state which is as close as possible tothe living state.

UsesWhile light microscopy remains important in the

identification, location, and observation of cellsboth singly and in groups, the electron microscopehas revolutionized scientists’ understanding of themicroscopic world in the biological, medical, andphysical sciences. Although it is not possible to ob-serve living materials using electron microscopy,freeze-fracture and freeze-etch have allowed theobservation of biological materials in an almostnatural state. Correlative microscopy, an increasinglypopular method, involves the examination of asample using both light microscopy and electronmicroscopy; this method allows the acquisition of avariety of views of the same structure, and it re-moves the ambiguities that may result from viewsproduced by a single microscope.

The transmission electron microscope (TEM) wasthe first electron microscope to be developed, and itis the most common type. The TEM has more incommon with light microscopes than it does withany other electron microscope. It also shares thechief disadvantage of the optical microscope—thatis, it gives little impression of the vertical scale ofthe specimen under observation. This means thatthe structures present in the specimen are imaged,but the subtleties of the surface of the specimen arelost. Furthermore, the TEM imposes severe con-straints on the type of specimens which can be ana-lyzed. The sample must be thin enough to permitthe beam of electrons to pass through it, and it mustbe resilient enough to resist being damaged by theimaging electrons.

Most biological materials are too thick to be ob-served under the TEM, so it is necessary to prepareultra-thin sections of samples prior to their analysis.Both plant and animal samples have been exam-ined using the TEM, and their analysis has led tothe discovery of a variety of internal structures. Theinternal structure of the mitochondria, which hadpreviously been discovered with light microscopy,has been probed. The TEM has also been used to

examine the interiors of the nuclei of cells. The ex-amination of the cell nuclei has enabled the inves-tigation of chromosome organization and genestructure. This examination of the microstructureof cells has contributed to the development of mo-lecular biology and genetic engineering.

The TEM has also been used to examine bacteriaand viruses. The TEM detected the presence of nu-cleic acids in bacteria and produced the first imagesof viruses; most viruses are too small to be resolvedin light microscopes. In plants, the TEM has beenused in the study of chloroplasts and the walls ofcells. The observation of chloroplasts led to the dis-covery of the internal membranes called thylakoids,which absorb light for the process of photosynthe-sis. The discovery of the thylakoids has enhancedthe understanding of photosynthesis.

The scanning electron microscope (SEM) hasbeen widely used in the biological sciences. Physi-cal laws impose no constraints on the size of thesample to be examined—they are, instead, imposedby the available sample chambers. The SEM is usu-ally used in the magnification range of 10 to 100,000times. When compared with the light microscope,the main advantage of the SEM is that it is able toproduce three-dimensional images. These imagesare possible because the entire sample can be ob-served in focus at the same time, and the sample canbe observed from a variety of angles.

The SEM has allowed images to be formed of al-gae, bacteria, spores, molds, and fungi. These im-ages have enabled the structure and function ofthese samples to be determined. The xylem andphloem cells that transport water through thestems of plants have been examined in the SEM,thus allowing the water transportation process tobe better understood. The SEM also has applica-tions in exploring the pathology of cells. Manystructures formed by living cells are better under-stood because sample preparation techniques suchas freeze-fracture and freeze-etch have allowed theexamination of lifelike samples under the SEM.

Stephen R. Addison, updated by Bryan Ness

See also: Cell theory; Fluorescent staining of cyto-skeletal elements.

Sources for Further StudyBozzola, John J., and Lonnie D. Russell. Electron Microscopy: Principles and Techniques for Biol-

ogists. 2d ed. Boston: Jones and Bartlett, 1999. Covers the theory of scanning and transmis-sion electron microscopes, specimen preparation for both, digital imaging and image

Microscopy • 671

analysis, laboratory safety, and interpretation of images and provides an atlas of ultra-structure.

Harris, Robin, ed. Electron Microscopy in Biology: A Practical Approach. New York: IRL Press,1991. Part of the Practical Approach series; includes bibliographical references and index.

Hey, Tony, and Patrick Walters. The Quantum Universe. New York: Cambridge UniversityPress, 1988. This work, written for the layperson, is devoted to quantum physics. Allphysical processes necessary for a good understanding of electron microscopy are cov-ered, and there is a section devoted to microscopy. The main ideas are developed pictori-ally rather than through the use of mathematical equations.

Koehler, James K. Advanced Techniques in Biological Electron Microscopy. 3 vols. New York:Springer-Verlag, 1973-1986. As the title suggests, this is an advanced work, intended forthe reader who wishes to learn to use the electron microscope. An invaluable reference forthose who wish to know how samples are prepared for use in an electron microscope.

Shipman, James T., Jerry D. Wilson, and Arron W. Todd. An Introduction to Physical Science.6th ed. Boston: Houghton Mifflin, 2000. For readers who lack an understanding of wave-particle duality and quantum physics. It is an elementary college-level text requiring nomathematical prerequisites. Recommended for those who wish to develop a thoroughunderstanding of the advantages of electron microscopy.

Watt, Ian M. The Principles and Practice of Electron Microscopy. 2d ed. New York: CambridgeUniversity Press, 1997. An easily understood survey of the entire field of electron micros-copy. This book is a good resource for those who need more information about the field.Many references for further study are provided.

MITOCHONDRIA

Categories: Anatomy; cellular biology

An organelle of eukaryotic cells, a mitochondrion is bounded by a double membrane. It is the major source of adenosinetriphosphate (ATP), which is derived from the breakdown of organic molecules and contains the enzymes used in theKrebs cycle and the electron transport system.

With the exception of a few metabolically inerttypes, such as the red blood cells of many

higher animals, eukaryotic cells of animals, plants,fungi, and protozoa contain mitochondria. Mostcells contain several hundred. The efficiency of mi-tochondria in adenosine triphosphate (ATP) produc-tion provides the energy source that powers all thevaried activities of eukaryotic cells. For these rea-sons, mitochondria have been aptly termed the“powerhouses” of the cell.

StructureIn most cells, mitochondria appear as spherical,

elongated bodies about 0.5 micrometer in diameterand 1 to 2 micrometers in length. Their size isroughly the same as that of bacterial cells. In some

cells mitochondria may measure several microme-ters in diameter, with lengths up to 10 micrometers.The name “mitochondrion” comes from Greekwords meaning “thread” (mitos) and “granule”(chondros). Mitochondria appear in either elon-gated, threadlike forms or more spherical, granularshapes. Under the light microscope, mitochondriacan be seen to grow, branch, divide, and fuse to-gether.

Mitochondria are surrounded by two separatemembranes. The outer membrane is a continuous,unbroken membrane that completely covers thesurface of the organelle. It is smooth in appearance.The inner membrane is also continuous and unbro-ken. It is convoluted, with infoldings called cristaethat greatly increase its surface area.

672 • Mitochondria

The outer and inner membranes separate themitochondrial interior into two distinct compart-ments. The intermembrane space is located betweenthe outer and inner membranes. The innermostcompartment, enclosed by both membranes, is themitochondrial matrix. Each membrane and compart-ment carries out specific functions important to theproduction of ATP.

The outer membrane is more permeable than theinner membrane and allows molecules up to thesize of small proteins to pass freely from the sur-rounding cytoplasm into the intermembrane space.Larger proteins are prevented from escaping intothe surrounding cytoplasm, and certain cytoplas-mic proteins, such as potentially destructive en-zymes, are prevented from entering.

Electron TransportThe innermost compartment, the matrix, con-

tains a battery of enzymes that catalyze the oxida-tion of fuel substances of many types, includingsimple sugars, fats, amino acids, and other organicacids. The primary goal of oxidation is the removalof high-energy electrons and the use of them to per-form chemical work.

High-energy electrons are used in the membranethat immediately surrounds the matrix, the innermembrane. This membrane contains a group of

proteins that work as electron-driven protons (hydrogen ions orH+) pumps. The proteins, calledelectron transport carriers, acceptelectrons at a higher energy leveland release them at a lower level.The energy released by the elec-trons causes the shape of the car-rier proteins to change, which al-lows them to transport protonsacross the inner membrane. Thevarious electron transport carri-ers accept and release electrons atdifferent energy levels, allowingthem to act in a series called theelectron transport chain. The elec-trons released by one carrierhave sufficient energy to powerthe pumping activity of the nextone in line. When operating atpeak efficiency, a mitochondrionof average size conducts about100,000 electrons through the

electron transport chain per second. After passingthrough several carriers, most of the energy of theelectrons has been tapped off.

As a consequence of proton pumping, protons be-come depleted in the matrix and become more con-centrated in the intermembrane space. This createsan electrochemical gradient between the intermem-brane space and the matrix. It is called an electro-chemical gradient because there is a concentrationgradient across the inner membrane and becauseprotons have an electrical charge, so there is also anelectrical potential (a charge difference) across themembrane. Electrochemical gradients represent aform of stored energy capable of doing work and inthis case is used to drive the synthesis of ATP.

At the very end of the electron transport chain,the electrons have so little energy that the last car-rier molecule donates these low-energy electrons tooxygen that is already in the matrix. The oxygen eu-karyotes need to survive thus has its primary bio-logical role as the final acceptor of spent electronsreleased by the electron transport chain. When anoxygen molecule receives four electrons, it thenpicks up two protons from the matrix, and water(H2O) is produced. The protons used in this processcause an additional reduction in proton concentra-tion in the matrix, increasing the electrochemicalgradient across the inner membrane.

Mitochondria • 673

MatrixCristaIntermembrane spaceInner membraneOuter membrane

Structure of a Mitochondrion

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Mitochondria are special cellular organelles where respiration—the release ofenergy from organic molecules as ATP—occurs. Mitochondria are sur-rounded by two separate membranes. The outer membrane is a continuous,unbroken membrane that completely covers the surface of the organelle. Theinner membrane is also continuous and unbroken but is convoluted, withinfoldings called cristae that greatly increase the inner membrane’s surfacearea.

ATP SynthesisA protein complex embedded in the inner mem-

brane uses the proton gradient to make ATP. Thiscomplex, called ATP synthase, is a molecular “ma-chine” that acts as a hydrogen-driven ATP synthe-sizer. It takes ADP (adenosine diphosphate) fromthe matrix and combines it with inorganic phos-phate (Pi) to make ATP.

The proteins of the inner mitochondrial mem-brane thus work in two coordinated groups. One,the electron transport chain, uses the energy of elec-trons removed in oxidative reactions to pump pro-tons hydrogens from the matrix to the intermem-brane space. The second group, the ATP synthases,uses the proton gradient created by the electrontransport chain as an energy source to make ATPfrom ADP and Pi. ADP comes from the matrix itselfand from other regions of the cell in which ATP isused to facilitate cellular activities such as growthand movement. Much of the ATP produced in thisway is then transported out of the mitochondria foruse in other parts of the cell.

mRNA and DNAWhen examined with an electron microscope, a

number of structures are visible in mitochondrialmatrix, including granules of various sizes, fibrils,and crystals. Among the granules are ribosomes.These structures, like their counterparts in the cyto-plasm, are capable of protein assembly, using direc-

tions encoded in messenger ribonucleic acid (mRNA)molecules as a guide. The mitochondrial ribosomesare more closely related in structure and function toprokaryotic ribosomes than to ribosomes in the cy-toplasm.

Also in the mitochondrial matrix are moleculesof deoxyribonucleic acid (DNA). Mitochondrial DNA(mtDNA) stores the information required for syn-thesis of some of the proteins needed for mitochon-drial functions. Unlike nuclear DNA, which is lin-ear, mtDNA is circular, like bacterial DNA. Thepresence of bacterialike DNA and ribosomes insidemitochondria has given rise to the endosymbiotic the-ory, which proposes that mitochondria may haveevolved from bacteria that invaded the cytoplasmof other prokaryotic cells and established a symbi-otic relationship. Over long periods of time, thesebacteria are believed to have lost their ability to liveindependently and gradually became transformedinto mitochondria. The evolutionary advantage tothe host provided by the bacterial invaders mayhave been greater efficiency in ATP production.

Stephen L. Wolfe, updated by Bryan Ness

See also: ATP and other energetic molecules; Chlo-roplast DNA; Chloroplasts and other plastids; Cyto-plasm; DNA in plants; DNA replication; Extranu-clear inheritance; Krebs cycle; Membrane structure;Mitochondrial DNA; Oxidative phosphorylation;RNA.

Sources for Further StudyAlberts, Bruce, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson.

Molecular Biology of the Cell. 4th ed. New York: Garland, 2002. The chapter “Energy Con-version: Mitochondria and Chloroplasts” describes the structure of mitochondria, oxida-tive reactions taking place inside them, and the mechanisms synthesizing ATP. Discussesthe DNA of mitochondria and its hereditary functions. The text is clearly written at thecollege level and is illustrated by numerous diagrams and photographs. Includes an ex-tensive bibliography at the end of the chapter.

Darnell, James, Harvey Lodish, and David Baltimore. Molecular Cell Biology. 3d ed. NewYork: Scientific American, 1995. An excellent textbook written at the college level. Thechapter “Energy Conversion: The Formation of ATP in Chloroplasts, Mitochondria, andBacteria” describes all aspects of ATP production in these organelles and in bacteria.Many diagrams and photographs are included.

Tzagoloff, Alexander. Mitochondria. New York: Plenum Press, 1982. This book presents asurvey of experimental information in all phases of mitochondrial structure and functionand the history of mitochondrial research. Although written at a technical level for a sci-entific audience, much of the information in this book is accessible to a lay reader. Thebook is copiously illustrated with photographs of mitochondria, diagrams of mitochon-drial reactions, and many informative tables.

674 • Mitochondria

MITOCHONDRIAL DNA

Categories: Cellular biology; genetics

Plant cells have three sets of DNA to code for proteins: one set in the chromosomes of the nucleus, another in the chloro-plasts, and a third genome in mitochondria. The mitochondrial genomes of higher plants are larger than those of animalsand form a complex series of linear and circular molecules of different sizes.

Mitochondria play an essential role in the gen-eration of energy in eukaryotic cells. Mito-

chondria are the organelles that are the main“chemical factories” of the cell where cellular aero-bic respiration—using the Krebs (citric acid) cycleand respiratory electron transport to produceNADH (nicotinamide adenine dinucleotide) andATP (adenosine triphosphate)—occurs. In the lightmicroscope, mitochondria look like short rods orthin filaments about 0.5 to 2 microns long. Amitochondrion is made up of a smooth outer mem-brane and an inner membrane that is folded intotubular shapes called cristae. Many aerobic respira-tion reactions are catalyzed by enzymes that arebound to mitochondrial membranes. Other reac-tions occur in the space between the inner and outermitochondrial membranes. Cells may contain sev-eral hundred mitochondria. Cells that are dividingand cells that are metabolically active need largeramounts of ATP and usually have large numbers ofmitochondria.

Size and StructureAll eukaryotic cells except some primitive pro-

tozoans contain mitochondria. All mitochondriacontain their own DNA (genomes). There are typi-cally between twenty and one hundred copiesof the mitochondrial genome per mitochondrion.The mitochondria of multicellular animals containgenomes of 14 to 20 kilobases (kb), present as sin-gle circles. The mitochondrial DNA of some organ-isms, such as some protozoa, algae, and fungi, isorganized in linear molecules with ends of chro-mosomes (telomeres) much like nuclear chromo-somes.

In contrast, the mitochondrial DNA of higherplants is larger and more complex—from 200 to2,500 kb—and is present in many different mole-

cules. The size and organization of the mitochon-drial genome vary widely from one plant species toanother. Electron micrographs of mitochondrialDNA show linear and circular DNAs of a variety ofsizes and complex, branched molecules that arelarger than the size of the genome.

Cloning the mitochondrial DNA and comparingthe sequences of the clones show that the entirecomplexity of a plant mitochondrial genome can berepresented as a “master circle.” Also, it has beenlearned that sequences are repeated on the masterchromosome. The repeated sequences differ for dif-ferent plant species. A series of recombinationevents between these identical repeated sequencesresults in a series of rearrangements of mitochon-drial DNA and forms the complex, multiple mole-cules of varying sizes that are the physical structureof the plant mitochondrial genome.

Adding to the complexity of mitochondrialDNAs in higher plants is the fact that some plants,such as corn, contain extrachromosomal mitochon-drial nucleic acids. Plasmid-like DNAs (circulardouble-stranded molecules) and double-strandedand single-stranded RNAs have been found insome corn strains.

Genes Encoded by Mitochondrial DNAIn addition to containing their own genomes,

mitochondria contain enzymes for DNA replica-tion and transcription, and ribosomes and transferRNAs for protein synthesis. (Transfer RNA, ortRNA, carries the building blocks of proteins, calledamino acids, to the ribosome, where they are as-sembled according to the instructions found inmessenger RNA.) The ribosomes of mitochondriaare different from those of chloroplasts and the cy-toplasm, using a slightly different genetic code (asequence of three bases that codes for a particular

Mitochondrial DNA • 675

amino acid). Mitochondrial genomes code for all ofthe ribosomal RNAs found in mitochondria and formost of the tRNAs. Mitochondria make only asmall number of proteins that are needed for elec-tron transport and ATP production. The other pro-teins needed in mitochondria are coded by nuclearDNA, translated in the cytoplasm of the cell, thentransported into the mitochondria. Plant mitochon-dria do not encode a full set of tRNAs, and some areimported from the cytoplasm.

Even though the mitochondrial genome ofhigher plants is much larger than that of animals,the plant mitochondrial genome codes for only afew more genes. The mitochondrial genome ofArabidopsis has been sequenced and contains thirty-two protein-coding genes, twenty-two tRNA genes,and three ribosomal RNA genes.

Exchange of DNAMitochondrial DNA from plants also differs

from that of animals in that mitochondrial DNAscontain segments of DNA that originally were innuclear and chloroplast DNAs. There appear tohave been exchanges of DNAs between all three ofthe higher plant genomes. There is evidence thatmitochondrial genes have been transferred to thenucleus and some mitochondrial tRNAs appear tobe of chloroplast origin. Changes in nuclear geneshave been shown to lead to changes in the copy

number of the different mitochondrial DNA con-figurations.

RNA EditingMitochondria and chloroplasts contain the bio-

chemical machinery to alter the sequence of the fi-nal messenger RNA (mRNA) product in a processcalled RNA editing. The most common editing ischanging a cytosine to a uracil (two of the basesfound on the “rungs” of DNAmolecules and whichare responsible for determining the nucleotide se-quences that form the genetic code).

Inheritance of Mitochondrial DNAGiven the complex branched network of plant

mitochondrial DNA, it is difficult to see how the in-heritance of a complete genome is ensured. It is stillnot clearly understood how this complex networkof DNAs is passed to daughter cells in a way that as-sures that all of the genetic information is main-tained.

Susan J. Karcher

See also: Chloroplast DNA; Chloroplasts and otherplastids; Chromosomes; Cytoplasm; DNA in plants;DNA replication; Extranuclear Inheritance; Generegulation; Genetic code; Genetics: post-Mendelian;Mitochondria; Nucleus; Oxidative phosphoryla-tion; RNA.

Sources for Further StudyKung, Shain-Dow, and Shang-Fa Yang, eds. Discoveries in Plant Biology. Vol. 2. River Edge,

N.J.: World Scientific, 1998. Contains a chapter on the discovery of plant mitochondrialgenomes. For advanced students. Illustrations, references.

Lea, Peter J., and Richard C. Leegood, eds. Plant Biochemistry and Molecular Biology. WestSussex, England: John Wiley, 1993. A good overview that contains a chapter on ge-nome organization, protein synthesis, and processing in plants. Illustrations, references,index.

Mauseth, James D. Botany: An Introduction to Plant Biology. 2d ed. Boston: Jones and Bartlett,1998. This introductory textbook includes a well-explained summary of mitochondrialDNA. Illustrations, problems sets, glossary, index.

Moore, Randy, W. Dennis Clark, Kingsley R. Stern, and Darrell Vodopich. Botany. Dubuque,Iowa: Wm. C. Brown, 1995. Another introductory textbook with a basic overview. Illus-trations, questions, suggested readings, glossary, index.

Reece, Jane B., and Neil A. Campbell. Biology. 6th ed. Menlo Park, Calif.: Benjamin Cum-mings, 2002. Introductory textbook. Illustrations, problems sets, glossary, index.

676 • Mitochondrial DNA

MITOSIS AND MEIOSIS

Categories: Cellular biology; reproduction

Mitosis is the process of cell division in multicellular eukaryotic organisms. Meiosis is the process of cell division thatproduces haploid gametes in sexually reproducing eukaryotic organisms.

Organisms must be able to grow and reproduce.Prokaryotes, such as bacteria, duplicate de-

oxyribonucleic acid (DNA) and divide by splittingin two, a process called binary fission. Cells of eu-karyotes, including those of animals, plants, fungi,and protists, divide by one of two methods: mitosisor meiosis. Mitosis produces two cells, called daugh-ter cells, with the same number of chromosomes asthe parent cell, and is used to produce new somatic(body) cells in multicellular eukaryotes or new in-dividuals in single-celled eukaryotes. In sexuallyreproducing organisms, cells that produce gametes(eggs or sperm) divide by meiosis, producing fourcells, each with half the number of chromosomespossessed by the parent cell.

Chromosome ReplicationAll eukaryotic organisms are composed of cells

containing chromosomes in the nucleus. Chromo-somes are made of DNA and proteins. Most cellshave two complete sets of chromosomes, which oc-cur in pairs. The two chromosomes that make up apair are homologous, and contain all the same loci(genes controlling the production of a specific typeof product). These chromosome pairs are usually re-ferred to as homologous pairs. An individual chromo-some from a homologous pair is sometimes called ahomolog. For example, typical lily cells containtwelve pairs of homologous chromosomes, for a to-tal of twenty-four chromosomes. Cells that have twohomologous chromosomes of each type are calleddiploid. Some cells, such as eggs and sperm, containhalf the normal number of chromosomes (only oneof each homolog) and are called haploid. Lily eggand sperm cells each contain twelve chromosomes.

DNA must replicate before mitosis or meiosiscan occur. If daughter cells are to receive a full set ofgenetic information, a duplicate copy of DNA mustbe available. Before DNA replication occurs, each

chromosome consists of a single long strand ofDNA called a chromatid. After DNA replication,each chromosome consists of two chromatids,called sister chromatids. The original chromatidacts as a template for making the second chromatid;the two are therefore identical. Sister chromatidsare attached at a special region of the chromosomecalled the centromere. When mitosis or meiosisstarts, each chromosome in the cell consists of twosister chromatids.

Mitosis and meiosis produce daughter cells withdifferent characteristics. When a diploid cell under-goes mitosis, two identical diploid daughter cellsare produced. When a diploid cell undergoes meio-sis, four unique haploid daughter cells are pro-duced. It is important for gametes to be haploid, sothat when an egg and sperm fuse, the diploid con-dition of the mature organism is restored.

Cellular Life CyclesMitosis and meiosis occur in the nuclear region

of the cell, where all the cell’s chromosomes arefound. Nuclear control mechanisms begin cell divi-sion at the appropriate time. Some cells rarely di-vide by mitosis in adult organisms, while other cellsdivide constantly, replacing old cells with new. Mei-osis occurs in the nuclei of cells that produce ga-metes. These specialized cells occur in reproductiveorgans, such as flower parts in higher plants.

Cells, like organisms, are governed by life cycles.The life cycle of a cell is called the cell cycle. Cellsspend most of their time in interphase. Interphase isdivided into three stages: first gap (G1), synthesis (S),and second gap (G2). During G1, the cell performs itsnormal functions and often grows in size. Duringthe S stage, DNA replicates in preparation for celldivision. During the G2 stage, the cell makes materi-als needed to produce the mitotic apparatus and fordivision of the cytoplasmic components of the cell.

Mitosis and meiosis • 677

At the end of interphase, the cell is ready to divide.Although each chromosome now consists of twosister chromatids, this is not apparent when viewedthrough a microscope. This is because all the chro-mosomes are in a highly relaxed state and simplyappear as a diffuse material called chromatin.

MitosisMitosis consists of five stages: prophase, promet-

aphase, metaphase, anaphase, and telophase. Althoughcertain events identify each stage, mitosis is a con-tinuous process, and each stage gradually passesinto the next. Identification of the precise state istherefore difficult at times.

During prophase, the chromatin becomes moretightly coiled and condenses into chromosomesthat are clearly visible under a microscope, the nu-cleolus disappears, and the spindle apparatus beginsto form in the cytoplasm. In prometaphase, thenuclear envelope breaks down, and the spindle ap-paratus is now able to invade the nuclear region.Some of the spindle fibers attach themselves to aregion near the centromere of each chromosomecalled the kinetochore. The spindle apparatus is themost obvious structure of the mitotic apparatus.The nuclear region of the cell has opposite poles,like the North and South Poles of the earth. Spindlefibers reach from pole to pole, penetrating the entirenuclear region.

During metaphase, the cell’s chromosomes alignin a region called the metaphase plate, with the sisterchromatids oriented toward opposite poles. Themetaphase plate traverses the cell, much like theequator passes through the center of the earth. Sisterchromatids separate during anaphase. The sisterchromatids of each chromosome split apart, and thespindle fibers pull each sister chromatid (now a sep-arate chromosome) from each pair toward oppositepoles, much as a rope-tow pulls a skier up a moun-tain. Telophase begins as sister chromatids reach op-posite poles. Once the chromatids have reached op-posite poles, the spindle apparatus falls apart, andthe nuclear membrane re-forms. Mitosis is complete.

MeiosisMeiosis is a more complex process than mitosis

and is divided into two major stages: meiosis I andmeiosis II. As in mitosis, interphase precedes meio-sis. Meiosis I consists of prophase I, metaphase I,anaphase I, and telophase I. Meiosis II consists ofprophase II, metaphase II, anaphase II, and telo-

678 • Mitosis and meiosis

(6) Telophase

(5) Anaphase

(4) Metaphase

(3) Late prophase/prometaphase

(2) Mid-prophase

(1) Early prophase

Nuclear envelope

Phragmoplast

Nucleolus

Chromosome

Daughter chromosomes

Spindle fiber

Pole

Centromere withkinetochores

Chromatids

Chromosome

Nuclear envelope

Nucleolus

Nucleoplasm

Mitosis

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phase II. In some cells, an interphase IIoccurs between meiosis I and meiosis II,but no DNA replication occurs.

During prophase I, the chromo-somes condense, the nuclear envelopfalls apart, and the spindle apparatusbegins to form. Homologous chromo-somes come together to form tetrads(a tetrad consists of four chromatids,two sister chromatids for each chromo-some). The arms of the sister chroma-tids of one homolog touch the arms ofsister chromatids of the other homolog,the contact points being called chias-mata. Each chiasma represents a placewhere the arms have the same loci, so-called homologous regions. During thisintimate contact, the chromosomes un-dergo crossover, in which the chromo-somes break at the chiasmata and swaphomologous pieces. This process re-sults in recombination (the shuffling oflinked alleles, the different forms ofgenes, into new combinations), whichresults in increased variability in theoffspring and the appearance of char-acter combinations not present in ei-ther parent.

Tetrads align on the metaphase plateduring metaphase I, and one spindlefiber attaches to the kinetochore of eachchromosome. In anaphase I, instead ofthe sister chromatids separating, theyremain attached at their centromeres,and the homologous chromosomesseparate, each homolog from a tetradmoving toward opposite poles. Telo-phase I begins as the homologs reachopposite poles, and similar to telophaseof mitosis, the spindle apparatus fallsapart, and a nuclear envelope re-formsaround each of the two haploid nuclei.Because the number of chromosomesin each of the telophase I nucleus is halfthe number in the parent nucleus, mei-osis I is sometimes called the reductiondivision.

Meiosis II is essentially the same asmitosis, dividing the two haploid nu-clei formed in meiosis I. Prophase II,metaphase II, anaphase II, and telo-

Mitosis and meiosis • 679

(8) Late telophase II(7) Anaphase II

(6) Metaphase II(5) Anaphase I

(4) Metaphase I(3) Late prophase I

(2) Prophase I(1) Early prophase I

Meiosis: Selected Phases

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phase II are essentially identical to the stages of mi-tosis. Meiosis II begins with two haploid cells andends with four haploid daughter cells.

Nuclear Division and CytokinesisMitosis and meiosis result in the division of the

nucleus. Nuclear division is nearly always coordi-nated with division of the cytoplasm. Cleaving ofthe cytoplasm to form new cells is called cytokinesis.Cytokinesis begins toward the middle or end of nu-clear division and involves not just the division ofthe cytoplasm but also the organelles. In plants, af-ter nuclear division ends, a new cell wall must beformed between the daughter nuclei. The new cellwall begins when vesicles filled with cell wall mate-rial congregate where the metaphase plate was lo-cated, producing a structure called the cell plate.When the cell plate is fully formed, cytokinesis iscomplete. Following cytokinesis, the cell returns tointerphase. Mitotic daughter cells enlarge, repro-duce organelles, and resume regular activities. Fol-lowing meiosis, gametes may be modified or trans-ported in the reproductive system.

Alternation of GenerationsMeiotic daughter cells continue development

only if they fuse during fertilization. Mitosis and

meiosis alternate during the life cycles of sexuallyreproducing organisms. The life-cycle stage follow-ing mitosis is diploid, and the stage following meio-sis is haploid. This process is called alternation ofgenerations. In plants, the diploid state is referred toas the sporophyte generation, and the haploid stage asthe gametophyte generation. In nonvascular plants,the gametophyte generation dominates the life cy-cle. In other words, the plants normally seen on theforest floor are made of haploid cells. The sporo-phytes, which have diploid cells, are small and at-tached to the body of the gametophyte. In vascularplants, sporophytes are the large, multicellular in-dividuals (such as trees and ferns), whereas game-tophytes are very small and either are embedded inthe sporophyte or are free-living, as are ferns. Thegenetic variation introduced by sexual reproduc-tion has a significant impact on the ability of speciesto survive and adapt to the environment. Alterna-tion of generations allows sexual reproduction tooccur, without changing the chromosome numbercharacterizing the species.

Joyce A. Corban and Randy Moore

See also: Cell cycle; Chromosomes; DNA replica-tion; Gene regulation; Genetic equilibrium: link-age; Genetics: Mendelian; Genetics: mutations; Ge-netics: post-Mendelian; RNA.

Sources for Further StudyAlberts, Bruce, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson.

Molecular Biology of the Cell. 4th ed. New York: Garland, 2002. The chapter “How Cells AreStudied” gives extensive information regarding study methods in cell biology. Light andelectron microscopy are discussed as well as staining techniques and tissue culture.

Audesirk, Teresa, Gerald Audesirk, and Bruce E. Myers. Biology: Life on Earth. 6th ed. UpperSaddle River, N.J.: Prentice Hall, 2001. The chapter “Cellular Reproduction and the LifeCycles of Organisms” is a brief overview of mitosis, meiosis, and the cell cycle. Includesexcellent discussion of alternation of generations.

Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. San Francisco: Benjamin Cummings,2002. The chapter “Reproduction of Cells” provides extensive information regarding mi-tosis and the cell cycle. The phases of mitosis, the mitotic spindle, cytokinesis, controlmechanisms, and abnormal cell division are discussed in detail. The chapter “Meiosisand Sexual Life Cycles” addresses the stages of meiosis, sexual life cycles, and a compari-son of mitosis and meiosis. This text is intended for use in introductory biology and isvery readable and informative.

John, Bernard. Meiosis. New York: Cambridge University Press, 1990. Review and discus-sion of meiosis, the antithesis of fertilization. Discusses the scheduling, mechanisms, bio-chemistry, and the genetic control of the events in meiosis.

Keeton, William T., and James L. Gould. Biological Science. 5th ed. New York: W. W. Norton,1993. The chapter “Cellular Reproduction” discusses in detail the stages of mitosis andmeiosis. Excellent diagrams allow visualization of cell division.

680 • Mitosis and meiosis

MITOSPORIC FUNGI

Categories: Fungi; microorganisms; taxonomic groups

Mitosporic fungi—also known as Deuteromycota, Deuteromycotina, fungi imperfecti, and deuteromycetes—arefungi that are unable to produce sexual spores and are therefore placed in a separate phylum.

The term “mitosporic” is a combination of thewords “mitosis” and “sporic.” Mitosis is the

process of asexual cell division, which results in thedaughter cells having the same genetic makeup asthe mother cell. “Sporic” is used to denote the cre-ation of spores. Therefore, the fungi in Deutero-mycota do not produce sexual spores. The group-ings in this phylum are artificial.

ConidiumThe asexual spore produced by mitosporic fungi

is the conidium. Conidia are produced from cellscalled condiogenic cells without the combination ofnuclei; therefore the conidia are a product of mito-sis. The conidial phase is a repetitive one. Thou-sands of conidia can be produced under adequateenvironmental conditions. These conidia will thencontinue the repetitive cycle.

ParasexualityThe genetic content of most fungi is dikaryotic.

Each fungal cell has two individual and geneti-cally distinct haploid nuclei. This state is originallyformed by the combination of haploid mycelia.During parasexuality, the formation of diploid nu-clei occurs, and then these revert back to the hap-loid state. Parasexuality permits the exchange ofgenetic information between the two haploid nu-clei. The resultant haploid nuclei may be geneticallymodified. This is of great importance with plantpathogens, as parasexuality may help reduce ge-netic resistance of the host. Parasexuality is not con-sidered to be a meiotic state, as no resultant sexualstructure (such as an ascus or basidium) is formed.

Teleomorphs vs. AnamorphsThe teleomorph is the form of a fungus that oc-

curs when sexual reproduction has occurred. Theseforms are found in the Ascomycota and the Basid-

iomycota. The anamorph is the form of a fungus thatoccurs when no sexual recombination has occurred.Anamorphs are important, as they are often a repet-itive phase of the fungus and are able to producespores without the additional requirement of sex-ual recombination.

When a specific teleomorph has been identifiedby scientists, the anamorphic phase is referred toas a subset of the teleomorph. For example, theanamorph fungus that produces blight of rice isknown as Pyricularia oryzae. The teleomorph of thisfungus is know as Magnaporthe grisea. The propername for the anamorph is then the Pyriculariaanamorph (or state) of Magnaporthe grisea.

ClassificationClassification of the fungi in this group is based

on the presence of conidia, kind of conidia (shape,color, size), and whether or not the conidia are pro-duced in fungal structures called conidiomata.Conidiomata may have the generalized shape of aflask made of fungal tissue (a pycnidium); a pincushion (sporodochium); or a mass of conidio-phores located under either the epidermis or cuticleof a plant host (aecervulus).

There are three classes in this grouping. Thehyphomycetes contain the fungi that produceconidia and conidiophores on hyphae or groups ofhyphae. The agonomycetes do not produce conidia.The coelomycetes contain the fungi that produceconidia in distinct conidiomata.

Classification is dependent on the fungus meet-ing the following requirements. First, there mustbe the absence or presumed absence of the teleo-morph. Second, there may be the absence or pre-sumed absence of any meiotic or mitotic reproduc-tive structure (as in agonomycetes). Finally, theremay be the production of conidia from mitosis.There must not be any sexual structures.

Mitosporic fungi • 681

ChallengesOne of the challenges of the classification of

mitosporic fungi is that these are form-genera offungi that include both fungi that do not haveteleomorphic states and fungi that do. For example,the teleomorph genus Cochliobolus has anamorphsin both the anamorphic form-genera Bipolaris andCurvularia. At the same time, there are several form-species in both anamorphic genera Bipolaris andCurvularia that have no teleomorph stage.

The reason the teleomorph can be found in somespecies and not in others is not known. Some iso-lates of anamorph species are distinct mating typesand can be crossed with the other distinct matingtype. When distinct mating types occur, it is possi-ble that over time the mating types could be sepa-

rated or that one mating type could be destroyed.Over time, without having the pressure to sexuallyreproduce, it is possible that the ability to sexuallyreproduce may have declined. Also, it is possiblethat the proper environmental conditions for sex-ual reproduction may not be known. Whether thesefungi are best covered in the form-phylum mito-sporic fungi or the form-phylum Deuteromycota is aquestion that will continue to be discussed. Becauseonly fungi that form sexual stages are recognized aslegitimate species, the distinct species and generaof the anamorphic fungi will have to be covered inthis artificial phylum.

J. J. Muchovej

See also: Ascomycetes; Basidiomycetes; Basidio-sporic fungi; Deuteromycetes; Fungi.

Sources for Further StudyAlexopoulos, Constantine J., and Charles E. Mims. Introductory Mycology. New York: John

Wiley, 1995. The text gives detail into the biology, anatomy, and physiology of fungi. In-cludes illustrations, bibliographic references, and index.

Manseth, James D. Botany: An Introduction to Plant Biology. 2d ed. Sudbury, Mass.: Jones andBartlett, 1998. General botany text with section on fungi, their importance, and their biol-ogy. Includes illustrations, bibliographic references, and index.

Raven, Peter H., Roy F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Introductory college-level text. Chapter 15, “Fungi,” coverstheir importance in the environment and their biology. Includes photographs, drawings,appendixes, and index.

MODEL ORGANISMS

Category: Methods and techniques

Practitioners in various areas of experimental biology recognize particular organisms as standard objects of studywithin their specialties. These model organisms are species that are used to exemplify a given category of organisms. Forexample, the laboratory rat is a common model used to study mammals, and Arabidopsis thaliana is commonly used asa model to study plants.

The characteristics of well-chosen model organ-isms, such as their genetic makeup or their

development, make them suited to be the subjectsof biological research. They are often developedspecifically for laboratory study of pure-breedingstrains that can be relied upon to provide a consis-tent medium for experimentation or examination.Biologists planning an experiment might specify a

particular breed of rat, for example, whose traits arewell defined and that are familiar to their peers.When a given model organism has become thestandard in a particular field, it can become a kindof common currency that facilitates exchangeamong scientists.

Under these circumstances, a great deal ofknowledge about individual model organisms may

682 • Model organisms

be accumulated rapidly. The crucial assumption,based on the theory of evolution, that underlies theuse of specific organisms as models is that speciessharing a common ancestor will have fundamentalsimilarities of physiology and biochemistry. Amongubiquitous model organisms are the bacteriumEscherichia coli, the yeast Saccharomyces cerevisiae,the roundworm Caenorhabditis elegans, the fruit flyDrosophila melanogaster, and the plant Arabidopsisthaliana.

Common FeaturesAbove all else, model organisms must be practi-

cal to observe and use in experiments. They must beeasy to breed or propagate and resilient enough towithstand manipulation. For knowledge gained inthe study of model organisms to be applicable on alarger scale, the organisms must be representativeof the taxonomic group in question. Clearly, the ap-plicability of studies performed on a particularmodel organism varies, depending on the nature ofthe inquiry. For example, the yeast S. cerevisiae isbroadly representative of the fungi as a whole, butits study may also provide insights into specificmolecular processes common to all eukaryotes, in-cluding humans. Model organisms are often cho-sen because they are among the simplest examplesof the group being studied. They may have a partic-ularly small genome, a short life cycle, or even asmall size that makes them convenient organismswith which to work. They may also lend them-selves very well to the study of specific features. Forexample, fruit flies are commonly used in the studyof genetics because they have a small genome fromwhich it is easy to induce and detect mutations.

Well-chosen model organisms, those that pos-sess some or all of the aforementioned characteris-tics, have been valuable tools for scientific research.Given these characteristics, it is easy to identify twotypes of scientific inquiry that are well served bythe use of model organisms. There are studies inwhich a category of organisms is investigated bystudying one of its simplest members and those inwhich a particular feature or biological process is il-luminated by examining an organism in which it isespecially accessible. Thus, the mouse is frequentlyused as a model for all mammals, and the greenalga Chlamydomonas is used to study photosyn-thesis.

If a specific organism becomes the consensusmodel for a given category, the situation lends itself

well to a speedy advancement of knowledge. Thefact that many clusters of researchers choose to fo-cus on the same model promotes collaboration andthe more rapid accumulation of a body of knowl-edge about the organism, enhancing the likelihoodof broader insights or theoretical advances. Havinga research subject organism in common facilitatescommunication among researchers and leads to theformation of standard terminology. The wide-spread study of a single organism promotes the de-velopment and propagation of effective techniquesfor its use and allows for the introduction of stan-dard experimental practices. Many observers haveargued that the very success of science as a collabo-rative activity relies on scientists having some con-sensus about the tools and objects of their researchand the terminology with which they describe it.

Although there are many advantages of a modelorganism becoming widespread in a particularfield, there are some limitations to what can beachieved by the study of model organisms. Theremust always be a question of the applicability toother species of knowledge gained from the studyof a model organism. A poor choice of an organismfor a model can hinder the production of scientificknowledge just as much as research on a validmodel can be beneficial. There is also the risk thatfocusing a discipline on one or a few models mayinhibit understanding of diversity. As the botanistDina Mandoli has pointed out, “flowering plantshave an estimated 300,000 species . . . no one plant,not even Arabidopsis thaliana, can encompass thisenormous diversity at the whole plant, physiologic,chemical, genetic, or molecular level.” It is im-portant, therefore, that research be carried out onenough model organisms to produce adequatebreadth of knowledge. To that end, there are dozensof model plants in use representing a cross-sectionof the kingdom, of which Arabidopsis has been themost widely and successfully employed.

ArabidopsisArabidopsis is a genus of the mustard family that

is closely related to food plants such as canola, cab-bage, cauliflower, broccoli, radish, and turnip. Fur-thermore, although Arabidopsis is not used in agri-culture, it is assumed that its study can lead tobetter knowledge of crop plants such as corn andsoybeans because of evolutionary similaritiesamong the genomes of all angiosperms. In the1980’s Arabidopsis thaliana (thale cress) became the

Model organisms • 683

primary model organism used in botany. Manycharacteristics lend it to such use, including itssmall size—plants are a few inches tall when ma-ture—and its short life cycle of less than six weeks.The short life cycle allows researchers to see the ef-fects of experimentation across successive genera-tions in a relatively short span of time. A. thalianaalso has a small genome and the least amount ofDNA (deoxyribonucleic acid) per haploid cell ofany known flowering plant. The haploid comple-ment of A. thaliana is 117 million base pairs, com-pared to 1.6 billion in tobacco. As a result, it is com-paratively easy to trace effects of experimentationto specific genes. It is valuable in the laboratory be-cause of its prolific seed production and the avail-ability of numerous mutations. It may be efficientlytransformed with the bacterium Agrobacteriumtumefaciens, which is used as a vector for the intro-duction of foreign DNA to the plant genome.

Arabidopsis was publicly recognized for its po-tential as a model organism in the 1960’s. In 1985 itwas first promoted as a model for molecular genetic

research, and the first molecular map of one ofthe five Arabidopsis chromosomes was pub-lished in 1988. In 1990 the Arabidopsis GenomeProject was begun (in part because of supportby the codiscoverer of DNA James Watson).Thanks to a multinational effort, by the year2000 the Arabidopsis gene sequence was fullydecoded. The sequencing project was itselfacclaimed as a model, because the research-ers strove to be systematic and comprehen-sive in their investigation of the genome. It isnow known that the Arabidopsis has slightlymore than twenty-five thousand genes, mak-ing it comparable in its genetic complexity toa fruit fly.

Prior to the widespread use of Arabidopsis,many prominent scientists claimed that prog-ress in botanical research was hindered by thestudy of too many organisms at once. SinceArabidopsis became a principal subject of re-search, botanical knowledge has advancedmarkedly. Researchers concentrating on Arabi-dopsis have helped to unify the studies of clas-sical and molecular genetics, plant develop-ment, plant physiology, and plant pathology.These advances have led to a more funda-mental understanding of many processes ofplant growth and development at a molecularlevel.

Some specific areas in which Arabidopsis researchhas produced important advances are light percep-tion, floral induction, flower development, and re-sponse to pathogenic and environmental stresses.For example, the functions of individual phyto-chromes, which are photoreceptors involved inmany aspects of plant growth and development,were elucidated in Arabidopsis. Likewise, the firsthormone receptor isolated in plants, that for ethyl-ene, was discovered as a result of using Arabidopsismutants. The next goal for the community ofArabidopsis scientists is to assign functions to all ofthe plant’s genes by the year 2010.

Chlorella and ChlamydomonasChlorella pyrenoidosa and Chlamydomonas rein-

hardtii are unicellular green algae that have beenused extensively as model organisms. They havemany features in common with other model or-ganisms, including short and simple life cycles andeasily isolated mutants. Although not strictly mem-bers of the plant kingdom, they have been impor-

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Among plants, the most ubiquitous model organism is theplant Arabidopsis thaliana, thale cress.

tant tools for botanically related research becausethey are photosynthetic eukaryotic organisms. Theytherefore offer less complex subjects through whichto study many processes that are central to plantlife. There are no unicellular members of the plantkingdom, so study of many important botani-cal processes may be more easily undertaken onChlorella or Chlamydomonas than on any plant.

In the mid-twentieth century Melvin Calvinused Chlorella in his Nobel Prize-winning research,which elucidated the cycle involved in photosyn-thetic carbon fixation which now bears his name,the Calvin cycle. This is a perfect example of modelorganisms’ value in research. It is often easier towork out a mechanism in a simple organism andsee whether it operates the same way in complexorganisms (the understanding of which may be theultimate purpose of the research) than to attemptthe investigation on a complex organism in the firstplace. Once the Calvin cycle had been explained inChlorella, it was shown to be ubiquitous in the chlo-roplasts of higher plants.

Chlamydomonas is the green alga most commonlyused as a model organism in contemporary re-search. A Chlamydomonas genome project is underway with efforts taking place in the United Statesand Japan. Among the topics of research in whichChlamydomonas is the model organism of choice,one of the most compelling is that of chloroplastbiogenesis and inheritance. Chlamydomonas is often

referred to as the “green yeast,” and like the yeastS. cerevisiae, it is an important eukaryotic modelsystem. For studying certain aspects of cell biologyto which yeast is not applicable, Chlamydomonas ischosen in preference. Such areas include cell motil-ity caused by flagella, phototaxis (phototaxy), pho-tosynthesis, and the study of centrioles, basal bod-ies, and chloroplasts.

Other Prominent Model OrganismsFor areas other than those just mentioned, yeast

is the most commonly used simple eukaryoticmodel organism. In 1996 S. cerevisiae became thefirst eukaryote to have its genome fully sequenced;its size is approximately one-tenth of that ofArabidopsis, or twelve million base pairs. Aroundthe same time, the genome project was completedfor the preeminent prokaryotic model organism,E. coli. This bacterium has become crucial not onlyas a focus of experiment but also as a biotechno-logical workhorse. Genes can be cloned by theirinsertion into E. coli, and gene products can there-fore be mass-produced in large-scale fermentationsof the bacteria.

Alistair Sponsel

See also: Calvin cycle; Cell cycle; Chromatin; Chro-mosomes; Eukaryotic cells; Green algae; History ofplant science; Mitochondrial DNA; Plant biotech-nology; Thigmomorphogenesis; Yeasts.

Sources for Further StudyBowman, John, ed. Arabidopsis: An Atlas of Morphology and Development. New York: Springer

Verlag, 1994. Documents the morphology and development of Arabidopsis thaliana, cover-ing its embryogenesis, vegetative growth, root growth, reproductive structures, andhost-pathogen interactions. Extensively illustrated.

Creager, Angela N. H. The Life of a Virus: Tobacco Mosaic Virus as an Experimental Model, 1930-1965. Chicago: Chicago University Press, 2002. This is a detailed history of the wide-ranging experimental use of the plant virus TMV as a model organism. Creager’s accountdemonstrates how the use of such a model system allows specific biological knowledgegenerated within a single research laboratory to “travel” and become generally appli-cable.

Kohler, Robert E. Lords of the Fly. Chicago: Chicago University Press, 1994. A historical treat-ment of the fruit fly as a model organism in T. H. Morgan’s “Drosophila school” at Colum-bia University. Kohler contends that model organisms such as the fly essentially becomelaboratory instruments.

Meyerowitz, Elliot M., ed. Arabidopsis. Cold Spring Harbor, N.Y.: Cold Spring Harbor Labo-ratory, 1994. Monograph provides a comprehensive account of what is known about theplant, presented in the context of general plant biology.

Model organisms • 685

MOLECULAR SYSTEMATICS

Categories: Cellular biology; classification and systematics; disciplines; evolution

Molecular systematics is the discipline of classifying organisms based on variations in protein and DNA in order tomake fine taxonomic categorizations not solely dependent on morphology.

T axonomy, sometimes called systematics, is thestudy of categorizing organisms into logically

related groupings. Historically, the way to performtaxonomy was to examine physical characteristicsof organisms and classify species according to themost commonly held traits. Unfortunately, thismethod of systematizing plants and animals as-sumed that because they have common physicaltraits, they have common ancestry. A gross form ofthis miscategorization might take place, for exam-ple, if one suggested that since both mushroomsand ivy can grow on the sides of trees, they areclosely related. The two species certainly have com-mon physical traits but only vaguely resemble eachother.

It is such a realization that motivated systema-tists to begin using molecular differences to com-pare species and populations. Molecular systemat-ics uses variations in protein and deoxyribonucleicacid (DNA) molecules to determine how similar, ordissimilar, sets of organisms are. These moleculardifferences provide a much more accurate taxo-nomic picture.

Systematics and EvolutionThe real power of molecular systematics is that

it allows the examination of how species havechanged over evolutionary time, as well as of the re-lationships between species that have no commonphysical characteristics. Molecular changes can beused to explore phylogenetics (how populations arerelated evolutionarily and genetically). It has beensuggested that the amount of change that takesplace in DNA over time can act as a molecular clock,gauging how much evolutionary time has passed.The clock is set by first examining geological andhistorical records to determine how long two spe-cies have been physically separated. By examina-tion of the number of molecular changes that have

occurred between those species over that knowntime, a time frame of change can be established.Genes are thought to evolve and mutate at a con-stant, predictable rate, giving rise to this evolution-ary clock hypothesis.

There are three major domains of life: prokary-otes (modern bacteria), Archaebacteria (descendantsof ancient bacteria), and eukaryotes (cellular organ-isms with nuclei and organelles). All these organ-isms share a common ancestry of hundreds of mil-lions of years. All species over time are connected toone another through a web of interlacing DNA asthey reproduce, separate to become new species,and reproduce again. All organisms carry their an-cestors’ genetic information with them as a bundlein each cell, and the more closely related organismsare to one another, the more similar the contents ofthat bundle will stay over time. Humans share com-mon genes, unchanged over millennia, with allother organisms—from the bacterium Escherichiacoli to barley to gophers. The more important thejob of a gene, the less it changes over time; thisconcept is called conservation. Conservation is theforce that keeps a biological or genetic link betweenevery species on earth.

Protein-Level AnalysisProteins were the earliest biomolecules used to

study phylogenetics. Initially, protein differencescould be studied only at the grossest levels. It wasfound that populations of organisms could be dis-tinguished based on possessing different alleles(genetic sites) that made proteins possessing thesame function but with different chemical struc-tures. These enzymes were called isozymes. Iso-zymes can be separated and compared for size byemploying a technique called gel electrophoresis.Gel electrophoresis uses a slab of gelatin-like me-dium and an electric field to separate molecules on

686 • Molecular systematics

the basis of size and electric charge. The geneticsimilarity of two different species can be deter-mined based on common molecular weight of theisozymes.

Proteins are composed of strings of the twentyamino acids common to all life on earth. It is possi-ble to ascertain the amino acid sequence of a pro-tein. If the amino acid sequence of the same proteinis ascertained among several different species, thatsequence should be more similar between closelyrelated species than more distantly related species.These differences allow taxonomists to gauge simi-larity of populations.

Antibodies are biomolecules that are able to rec-ognize and bind very specifically to other mole-cules. Biologists employ antibodies that specificallyrecognize molecules at the surface of cells to test re-lationships between species. Antibodies that recog-nize cell-surface molecules on one species shouldrecognize those same molecules in closely relatedspecies, but not from distantly related species, al-lowing a researcher to gauge similarity betweenspecies.

DNA-Level AnalysisThe most common method used to establish tax-

onomic relationships is to compare DNAsequencesbetween species. DNA is the double-stranded,polymeric molecule that encodes the proteins thatdirect the inner workings of all cells. The DNAmol-ecule is structure like a ladder, with rungs formedby pairings of of four molecules, the bases guanine(G), adenine (A), thymine (T), and and cytosine (C).These bases, arranged in unique order, are read byspecial enzymes and encode messages that aretranslated into proteins. Sequences encoding forthe same protein can change between species. In

taxonomy, DNA sequences are obtained from sev-eral populations of organisms. Analysis of these se-quences allows one to obtain a picture of how dif-ferent populations have changed over time. ThisDNA sequencing may be used to compare many dif-ferent types of DNA: regions that encode for genes,do not encode for genes, reside in chloroplast DNA,or reside in mitochondrial DNA.

Another common method of DNA phylogeneticanalysis is called restriction mapping. In this method,DNA from different species is subjected to enzy-matic treatment from proteins called endonucleases.These endonucleases have the ability to cleaveDNA into fragments. Where the enzymes cleavethe DNAis determined by the DNAsequence itself.The size and pattern of the fragments created bythis treatment should be more similar in relatedspecies than in unrelated species.

A fairly new method of DNA analysis examinesrepetitive DNAs, called microsatellite sequences, thatare found in all eukaryotic organisms. Microsat-ellite sequences are short arrangements of bases,such as GATC, repeated over and over. The numberof repeats at a particular genetic location is usuallymore similar in related species than in unrelatedones. The differences in these repeated sequencesare called “simple sequence polymorphisms” andare detected by a special enzymatic reaction calledthe polymerase chain reaction. Once detected, thefragments are separated and compared for size bymeans of gel electrophoresis.

James J. Campanella

See also: Cladistics; DNA in plants; Electrophore-sis; Evolution of plants; Genetics: mutations; Nu-cleic acids; Proteins and amino acids; Systematicsand taxonomy; Systematics: overview.

Sources for Further StudyAvise, John. Molecular Markers, Natural History, and Evolution. New York: Chapman and Hall,

1994. Examines relationships between genetic changes, taxonomy, and evolution.Freeman, Scott, and Jon Herron. Evolutionary Analysis. New York: Prentice Hall, 2000.

Covers the connections between evolution and taxonomy.Nei, Masatoshi, and Suhir Kumar. Molecular Evolution and Phylogenetics. New York: Oxford

University Press, 2000. Covers the mathematics of phylogenetic analysis.Page, Roderic, and Edward Holmes. Molecular Evolution: A Phylogenetic Approach. Malden,

Mass.: Blackwell Science, 1998. Concentrates on molecular evolution in populations andhow species arise.

Molecular systematics • 687

MONOCOTS VS. DICOTS

Categories: Angiosperms; Plantae; reproduction and life cycles

Within the angiosperms (flowering plants), two classes have been traditionally recognized by botanists: monocots anddicots. The terms connote differences between these groups’ seed embryos. The recently introduced class, eudicots, liter-ally meaning “true dicots,” is increasingly used in place of the term “dicots.”

Of the large number of plant species which cur-rently inhabit the earth, most (about one-

quarter million) are angiosperms, plants that repro-duce by means of flowers. The famous naturalistCharles Darwin called angiosperms “an abomina-ble mystery.” Even today, much remains to belearned about the ancestry of angiosperms andwhen and where angiosperms first appeared. It isnow believed that the first extant angiosperm is asingle species of the genus Amborella. This plant,found on an island in the South Pacific, is a shrubwith cream-colored flowers.

All angiosperms are assigned to the phylumAnthophyta. Within this phylum, considerable di-versity occurs. However, two large lineages havetraditionally been recognized:class Monocotyledones (monocots)and class Dicotyledones (dicots).

The monocots include grasses,cattails, irises, lilies, orchids, andpalm trees. The dicots include thevast majority of seed plants:herbs, vines, shrubs, and mosttrees (cone-bearing trees are notangiosperms).

The terms “monocot” and“dicot” reflect the number of cot-yledons, one or two, respectively,possessed by seeds of the plants.Acotyledon is the central portionof a seed embryo to which theepicotyl (immature shoot) andradicle (immature root) are at-tached. However, one need notexamine the seed of a particularflowering plant in order to assignit to the correct class. Fortunately,each angiosperm possesses a

“syndrome” of features, any one of which may beused for the purpose of classification.

Vegetative PartsMonocots have leaves with parallel veins: The

large, easily visible veins are parallel and extendthe length of the (usually) linear leaves. In contrast,leaves of dicots are net-veined: The veins branch re-peatedly as they extend into the various portions ofthe leaf. Vascular bundles (clusters of conductingcells) within stems of monocots are scattered, incontrast to those of dicots, which are arranged intoa cylinder. These are best seen in a cross-section ofthe root. Furthermore, stems of monocots rarelyproduce wood, whereas those of many dicots (trees

688 • Monocots vs. dicots

Characteristics of Monocots vs. DicotsFeature Monocots Dicots

Embryos One cotyledon Two cotyledons

Pollen One pore Three pores

Flowers Parts in threes Parts in fours or fives

Leaf veins Parallel Netted or reticulated

Vascular bundles Scattered, complex Ordered in rings

Woody growth In 10% of species In 50% of species

Root system Adventitious Primary+adventitious

Species ~65,000 ~165,000

Examples BromeliadsGrassesIrisesLiliesOrchidsPalms

AstersCactiHerbsShrubsRosesTrees

and shrubs) increase their diame-ter yearly, as wood accumulateswithin them.

Reproductive PartsFlowers of monocots typically

have their parts arranged in threesand are said to be “3-merous.” Anexample would be a lily flower,which has these parts (designatedfrom the outside toward the centerof the flower): three sepals, threepetals, six stamens, and one pistil.Flowers of dicots have parts ar-ranged in fours or fives. Also, pol-len grains produced by flowers ofmonocots each have one pore inthe outer covering, as comparedto three pores in the case of dicotpollen grains. Thus, it is apparentthat nearly any part of a floweringplant can be used to place it intothe correct subgroup of angio-sperms. However, it is the flowers and leaves thatare most easily considered (not requiring micro-scopic examination).

New InterpretationsLike all areas of science, botany is subject to

modification and reinterpretations as a result of theaccumulation of new information. One group ofplants traditionally considered to be dicots, themagnoliids, have long been problematic. Theyhave some features of dicots, but their floral partsare free (unattached to one another) and arrangedspirally. Also, they produce pollen grains, each ofwhich has only a single pore. These features arecharacteristic of monocots. These traits have thus

caused those who study angiosperms to place theminto a new, third category, the class magnoliids. Thismost primitive group of angiosperms is thought tobe ancestral to both of the other classes. The south-ern magnolia, Magnolia grandiflora, with its huge,white flowers, is a good example of this class. Thename eudicots is applied to the still-large group thatincludes the remainder of the dicots.

Thomas E. Hemmerly

See also: Angiosperm evolution; Eudicots; Evolu-tion of plants; Flower structure; Flower types; Leafanatomy; Monocotyledones; Paleobotany; Repro-duction in plants; Seeds; Stems; Wood.

Sources for Further StudyHemmerly, Thomas E. Appalachian Wildflowers. Athens: University of Georgia Press, 2000.

Representative angiosperms of this mountain range and adjacent areas are presented infull color. Uses the traditional two-class system.

Heywood, V. H. Flowering Plants of the World. New York: Mayflower Books, 1978. Well-illustrated overview of world angiosperms. Uses the traditional two-class system.

Raven, Peter H, Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. The rationale for the three-class system is presented in thisclassic college-level botany textbook.

Strickberger, Monroe W. Evolution. 3d ed. Boston: Jones and Bartlett, 2000. Presents an over-view of plant evolution along with that of animals.

Monocots vs. dicots • 689

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MONOCOTYLEDONES

Categories: Angiosperms; Plantae; taxonomic groups

The Monocotyledones, or monocots, are a large and very distinctive class of angiosperms or flowering plants, phylumAnthophyta, consisting of some 133 families, 3,000 genera, and 65,000 species. Monocotyledones form one of the twomajor subdivisions of angiosperms, the other being the Eudicotyledones (eudicots), with about 165,000 species.

Typical monocots have a single cotyledon (seed-ling leaf), stems with scattered vascular bun-

dles, root systems composed entirely of adventi-tious roots (arising directly from stem tissues),leaves with parallel venation and sheathing bases,and flower parts in threes. Monocots lack the abilityto produce secondary growth (wood). In mostmonocots, stems remain at or below ground leveland take the form of rhizomes (horizontal stems),bulbs (short, vertical stems covered with modified,fleshy leaves), corms (short, wide stems), or tubers(wood), which produce new adventitious rootscontinually or seasonally.

The growing tips (apical meristems) or buds ofthese plants remain below ground, except whenthey rise to produce flowers, and thus are protectedagainst seasonal cold, drought, fire, and grazing an-imals. The growing tips are also surrounded and

protected by the sheathing bases of the leaves.Monocot leaves are typically long and strap-shaped with numerous parallel veins that connectindividually to the stem. Leaves grow primarilyfrom their bases, increasing in length and pushingthe earlier formed tips upward but not increasingmuch in width.

Most monocot flower parts occur in threes, suchas three sepals, three petals, six stamens, and threecarpels. Some members of the largely aquatic sub-class Alismatidae have other patterns and showthe primitive condition of apocarpy (carpels remainfree from one another). In most other monocots,carpels are completely fused together into a three-chambered pistil. (Exceptions occur also in somepalms). Monocots thus appear to have evolvedfrom primitive anthophytes and have had a long,separate history.

Ecology and VariationBecause of their predominantly underground

stem structures and basally regenerating leaves,monocots predominate in open habitats with strongseasonal contrasts or unpredictable droughts, suchas grasslands. Grasses and similar monocots areparticularly well adapted to survive drought, fire,and overgrazing, the three primary threats in grass-lands. Bulbs, corms, and tubers are other forms ofmonocot stems that allow for underground sur-vival, both in grasslands and in other regions thatexperience long winters or dry seasons.

The numerous monocots that inhabit marshesand aquatic habitats often have elongate petiolesthat grow from the base to lift their expanded leafblades above the water level (such as Sagittaria andAponogeton). The spectacular Egyptian papyrusplants have highly specialized upright stems thatconsist of a single internode (stem segment between

690 • Monocotyledones

Monocot Families Common inNorth America

Common Name Scientific Name

Arum family AraceaeCattail family TyphaceaeDaffodil family AmaryllidaceaeGrass family PoaceaeIris family IridaceaeLily family LiliaceaeOrchid family OrchidaceaeSedge family Cyperaceae

Note: For a full list of angiosperm families, see the tables thataccompany the essay “Angiosperms,” in volume 1.

Source: U.S. Department of Agriculture, National Plant DataCenter, The PLANTS Database, Version 3.1, http://plants.usda.gov. National Plant Data Center, Baton Rouge,LA 70874-4490 USA.

nodes that elongates from the base), lifting a tuft ofleaves and reproductive branches above fluctuat-ing water levels. Other submerged aquatic mono-cots may remain rooted, producing conventionalstrap-shaped leaves (such as sea grasses) or maydrift rootless, with short, whorled leaves producedalong the stem.

Many monocots have adapted to live as epiphytes(plants that live on top of other plants), particularlyon the upper branches of tropical rain-forest trees, ahabitat that is often quite dry. The orchid, bromeliad,and aroid (Philodendron, Anthurium) families in par-ticular have evolved largely in this habitat. Epi-phytic orchids have succulent leaves or roots thatstore water for use during the dry periods betweenrains. A bromeliad grows as a rosette of tightlyoverlapping leaves radiating from the central grow-ing point, forming a water-holding cup, or tank, inthe center. Aroids can be found climbing up treetrunks or perched on upper branches and are un-usual both for their flower structure and theirleaves. Flowers are tiny and crowded on the club-like spadix, which in turn is enveloped by a large,often colored, leaflike structure called a spathe,while the leaves are broad, often dissected or di-vided into separate leaflets, and net-veined—quiteunlike the typical monocot leaf.

Lack of the ability to form conventional treetrunks has led to some other novel growth forms inmonocots. One of these is the pseudostem (falsestem). In banana plants, and to a less obvious extentin gingers, heliconias, and other giant tropicalherbs, an apparent stem forms from the elongate,tubular, concentric leaf sheaths, and gets taller aseach new leaf grows up through the center.

Monocot TreesSome monocots have become arborescent (tree-

like) by growing upward and developing thick,fibrous trunks without conventional secondarygrowth. In most arborescent monocots (palms,screw pines), the thickening growth of the stemsoccurs at the growing tip, in what is known as theprimary thickening meristem. In these specializedmeristems, the dividing cells expand laterally morethan vertically. Stems reach their full thickness inthis initial growth at the tip of the plant and expandvery little after that.

In members of the Dracena family, however, anew way of thickening stems has evolved. Exten-sions from the apical meristem remain active alongthe sides of the stem and produce new parenchymatissue and whole vascular bundles (not layers ofwood). This allows the plants continually to in-crease the thickness of the stem and the amount ofvascular tissue.

Branching is limited or absent in arborescentmonocots, and leaves therefore tend to be large.Screw pines and dracenas have very long, narrowleaves, while palm leaves are broad and dissectedinto folded linear leaflets in either a pinnate(featherlike) or a palmate (fanlike) arrangement.Palm leaves are the largest in the world, reachingmore than 60 feet (18 meters) in length in some spe-cies, and are highly fibrous. Development of thesecompound leaves is complex, involving multiplemeristematic areas.

Frederick B. Essig

See also: Angiosperms; Eudicots; Monocots vs.dicots.

Sources for Further StudyDahlgren, R. M. T. The Families of the Monocotyledons: Structure, Evolution, and Taxonomy. New

York: Springer-Verlag, 1985. This comprehensive, technical reference includes an excel-lent introductory background to the biology and evolution of the monocots.

Jones, David L. Palms Throughout the World. Washington, D.C.: Smithsonian InstitutionPress, 1995. This comprehensive and beautifully illustrated reference on a large and dis-tinctive family of monocots is an example of the many books that can be found on specificgroups of plants.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. The various features of monocots are discussed and illus-trated, in comparison with other groups of plants, throughout this widely respected text-book.

Takhtajan, A. Diversity and Classification of Flowering Plants. New York: Columbia UniversityPress, 1997. This is one of the most respected references on the flowering plants. Containsa good discussion of the features of monocots.

Monocotyledones • 691

MONOCULTURE

Categories: Agriculture; economic botany and plant uses; environmental issues

Monoculture is the agricultural practice of repetitively planting a single plant species rather than growing a variety oftypes of plants.

There has been considerable debate regardingthe advantages and disadvantages of monocul-

ture agriculture. This type of plant production is asystem in which a single plant species, typically oneproducing grain (such as corn, wheat, or rice), for-age (such as alfalfa or clover), or fiber (such as cot-

ton), is grown in the same field on a repetitive basis,to the exclusion of all other species. In its most ex-treme version, a single variety of a plant species isgrown, and all plants are virtually identical to oneanother. Monoculture can be contrasted with otheragricultural production practices, such as multiple

692 • MonocultureP

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In the most extreme version of monoculture agriculture, a single variety of a plant species is grown, and all plants arevirtually identical to one another. Monoculture can also apply to perennial produce systems, such as coffee plantationslike this one.

cropping (in which sequential monoculture cropsare grown in the same year) or intercropping (inwhich two or more different crops are grown at thesame time and place). Monoculture can also applyto perennial produce systems, such as fruitingtrees, citrus crops, tea, coffee, and rubber trees.

AdvantagesMonocultures are unnatural ecological occur-

rences. They are maintained through the use ofresources such as labor, energy, and capital (fertiliz-ers, chemicals, and so on). Left to itself, a monocul-ture crop will quickly revert to being a mixed plantcommunity. However, monoculture agriculturehas several advantages that caused its widespreadadoption from the moment agriculture began.Monocultures allow agriculturalists to focus theirenergy on producing a single crop best adapted to aparticular environment or to a particular market.For example, a premium is paid for white corn,used in making snack foods.

Monoculture is an appropriate agriculturalstrategy to optimize crop yield per unit of landwhen either temperature or water conditions limitthe growing season. Monoculture agriculture lendsitself to mechanization, an important considerationwhen labor is expensive relative to energy costs.Consequently, monoculture agriculture in theUnited States has developed in concert with the re-sources required to support it—markets, credit,chemicals, seed, and machinery—and with the so-cial conditions that have caused the United States tochange from a rural to a largely urban and subur-ban population.

DisadvantagesThe disadvantages of monoculture are numer-

ous. There are apparent limits to the increase incrop yields brought about by new hybrid seeds, fer-tilizers, and pesticides. Yield increases in monocul-ture agriculture have diminished since the 1980’s.There is an economy of scale at which farm size be-comes too small to permit effective mechanizationor for which insufficient markets exist for relianceon a single crop. Focus on production of a singlecrop may lead to unbalanced diets and nutritionaldeficiencies in agricultural communities where noexternal supplies of produce are available.

More important, monoculture crops are biologi-cally unstable, and considerable effort must bemade to keep other plants and pests out. When ev-ery plant under cultivation is the same, these sys-tems are inherently susceptible to natural events(storms, drought, and wind damage) and to biologi-cal invasions by insects and plant pathogens. Theclassic example of overreliance on monoculturewas the Irish Potato Famine (1845-1850). The fam-ine was instigated by natural climatic conditionsthat allowed the plant pathogen Phytophthorainfestans (potato late blight) to destroy successivepotato crops in a population too impoverished toafford other food staples that were available.

Mark S. Coyne

See also: Agriculture: modern problems; GreenRevolution; High-yield crops; Hybridization; Mul-tiple-use approach; Nutrition in agriculture; Pesti-cides; Slash-and-burn agriculture; Sustainable agri-culture.

Sources for Further StudyCoughenour, C. Milton, and Shankariah Chamala. Conservation Tillage and Cropping Innova-

tion: Constructing the New Culture of Agriculture. Ames: Iowa State University Press, 2000.Explores causes, conditions, and significance of the changes in tillage and cropping prac-tices in the United States and Australia.

Odemerho, F. O., and A. Avwunudiogba. “The Effects of Changing Cassava ManagementPractices on Soil Loss: ANigerian Example.” Geographical Journal 159, no. 1 (March, 1993):63-69. Study compares erosion between monoculture and polyculture systems. Assessesplanting practices in reducing erosion under monoculture.

Prihar, S. S., P. R. Gajri, D. K. Benbi, and V. K. Arora. Intensive Cropping: Efficient Use of Water,Nutrients, and Tillage. New York: Food Products Press, 2000. Chapters address intensivecropping and related concerns, including efficient input use and sustainability of high-production systems.

Monoculture • 693

MOSSES

Categories: Nonvascular plants; paleobotany; Plantae; taxonomic groups

Members of the ten thousand species of the class Bryopsida or Musci in the order Bryophyta, mosses are usually tiny,fragile, nonflowering, spore-bearing plants. They are related to hornworts and liverworts.

Mosses evolved from green algae before the ex-istence of reptiles and flying insects. Fossils

from the Devonian period, about 410 million to 353million years ago, contain mosses. The first knownmosses lived 286 million to 245 million years ago,during the Permian period. By the Tertiary period,approximately 66.4 million to 1.6 million years ago,

at least one hundred moss species existed. Fos-silized mosses, including Muscites, Palaeohypnum,and Protosphagnum, resemble modern moss spe-cies structurally, indicating that mosses have notevolved quickly.

Found globally, mosses prefer damp, shady,sparsely vegetated environments and often grow

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on land adjacent to bodies of fresh water and inwoods, sometimes appearing carpetlike. Somemosses prefer rotting wood, while others thrive insoil saturated with water or bare soil. Althoughhardy, mosses are sensitive to pollutants. Mossesneed external sources of moisture to transport nu-trients. They can adapt to environments, exhibitingvariations. Although many mosses live in temper-ate zones, they also have been found living in tun-dra. Mosses can grow on rocks, mountains, andbrick and cement structures. They are frequentlyfound on tombstones and other memorials.

Mosses are categorized into three groups, thetrue mosses (Bryopsida or Musci), the peat mosses(Sphagnopsida), and the granite mosses (Andreaop-sida). The greatest number of moss species are in thesubclass Bryidae. Some subclasses contain only afew species.

Most mosses are short. Cape pygmy moss

(Ephemerum capensi), the size of a pencil dot, is thetiniest moss that can be seen without a microscope.The largest mosses are 1 meter (40 inches) long.Water mosses in the order Bryales live in streamsand ponds, sometimes adhering to tree roots orrocks. Brook moss grows shoots ranging from 30 to100 centimeters (12 to 40 inches) long. Commonmosses include hair-cap, sheet, top, slit, granite,broom, and wind-blown mosses. Luminous mosses,whose curved cells concentrate light on chloro-plasts for photosynthesis, grow in dark spaces.

Mosses consist of green stems and leaves that areusually one cell thick and sometimes have midribs.Leaf cell shape and pattern are used to identifymosses, and differences can be detected microscop-ically. Instead of roots, each moss stem has rhizoidswhich secure the plant to the ground. Lacking avascular system to conduct fluid, mosses need awatery environment to distribute male gametes.

Mosses • 695

Classification of MossesClass Andreaeopsida (granite mosses)

Families:AndreaeaceaeAndreaeobryaceae

Class Sphagnopsida (peat mosses)Families:Sphagnaceae

Class Bryopsida (true mosses)Families:ArchidiaceaeAulacomniaceaeBartramiaceaeBryaceaeCatoscopiaceaeHypopterygiaceaeMeesiaceaeMniaceaePseudoditrichaceaeRacopilaceaeRhizogoniaceaeTimmiaceaeBuxbaumiaceaeBruchiaceaeBryoxiphiaceaeDicranaceaeDitrichaceaeLeucobryaceae

Class Bryopsida (continued)FissidentaceaeDisceliaceaeEphemeraceaeFunariaceaeGigaspermaceaeOedipodiaceaeSplachnaceaeSplachnobryaceaeGrimmiaceaePtychomitriaceaeScouleriaceaeCallicostaceaeDaltoniaceaeHookeriaceaeAmblystegiaceaeBrachytheciaceaeEntodontaceaeFabroniaceaeHelodiaceaeHylocomiaceaeHypnaceaeLeskeaceaeMyriniaceaePlagiotheciaceaePleuroziopsidaceaePterigynandraceae

Class Bryopsida (continued)RhytidiaceaeSematophyllaceaeStereophyllaceaeThamnobryaceaeTheliaceaeThuidiaceaeFontinalaceaeAnomodontaceaeClimaciaceaeCryphaeaceaeHedwigiaceaeLeptodontaceaeLeucodontaceaeMeteoriaceaeNeckeraceaePterobryaceaeErpodiaceaeOrthotrichaceaeRhachitheciaceaePolytrichaceaeCalymperaceaeEncalyptaceaePottiaceaeSchistostegaceaeSeligeriaceaeTetraphidaceae

Source: U.S. Department of Agriculture, National Plant Data Center, The PLANTS Database, Version 3.1, http://plants.usda.gov.National Plant Data Center, Baton Rouge, LA 70874-4490 USA.

Life CycleMosses have two reproductive forms, referred to

as the alternation of generations. During the game-tophytic, or sexual, generation, a gametophyte cre-ates a plant body known as the thallus. Two special-ized organs are involved in sexual reproduction.The antheridium is a sac that produces male ga-metes, each with two flagella, and the archegoniumis the bottle-shaped female sexual organ. Short cellfilaments, called the paraphyses, are associatedwith the antheridium. Usually both sexual organsare located on moss leaves.

Sperm fertilizes the egg to create a maroon-colored stalk called a seta, which has a sporangium,or spore capsule, in the sporophytic generation.The sporangium consists of an urn, peristome, an-nulus, operculum, and calyptra. Capsule compo-nents vary by species. This sporangium relies onthe gametophyte to deliver necessary nourishmentand moisture. When the sporophyte dries, sporesare scattered like dust and germinate to becomethreadlike protonema, an immature moss form,which form new gametophytes.

During wet conditions, the capsule can withholdspores by closing from one to two rows of peri-stome teeth. Because spores are sensitive to mois-ture, temperature, and soil chemistry and have lim-ited supplies of energy for protonema to grow, oddsare that only one spore per million forms a maturemoss plant. Mosses also reproduce asexually bygrowing from fragments of stems and leaves thatgerminate into new plants.

UsesEcologically, mosses serve several purposes.

They help secure soil by forming patches of densemats of ground cover and absorb water to prevent

erosion. Mosses also provide nutrients that enrichsoils for future plant growth. Mosses are popularterrarium plants. They are often illegally harvestedfrom public lands, including national forests.

The word “moss” in plant names does not al-ways indicate a moss species. The club moss is, infact, an evergreen herb from the family Lycopo-diaceae, and its relatives, the spike mosses, are prim-itive vascular plants in the family Selaginellaceae.Often, people in the Northern Hemisphere usemoss to tell direction because it grows on the north-facing sides of trees and structures, where sunlightdoes not shine directly. Green algae that grow ontrees’ northern sides are referred to as moss butusually are not. Other types of algae, such as pondmoss, are incorrectly described as mosses. Spanishmoss is one of the best-known plants called a moss,but it actually is a lichen and air plant belonging tothe pineapple family, Bromeliaceae.

Some moss species in the genus Sphagnum, of theorder Sphagnales, partially decompose into carbon-ized tissues in acidic, watery areas such as bogs tobecome peat, which is significant economically as afuel and as a commercial gardening product.Empty moss cell spaces absorb water, and bogs be-come drier. Peat mosses float in mats, prevent fungiand bacteria from multiplying, and create an envi-ronment that can preserve carcasses. Prior to thedevelopment of modern medical techniques,sphagnum moss was used to dress surgical sites be-cause of its antiseptic qualities.

Elizabeth D. Schafer

See also: Algae; Bioluminescence; Bromeliaceae;Bryophytes; Hornworts; Liverworts; Lycophytes;Peat.

Sources for Further StudyConard, Henry S. How to Know the Mosses and Liverworts. 2d ed., edited by Paul L. Redfearn,

Jr. Dubuque, Iowa: Wm. C. Brown, 1979. A classic guide to identifying North Americanmoss species. Includes illustrations, index.

Crosby, Marshall R., and Robert E. Magill. A Dictionary of Mosses. St. Louis: Missouri Botani-cal Garden, 1981. Brief list of moss genera, with taxonomic information for classificationof specimens. Includes index and corrections and additions to supplement two previousprintings.

_______. Index of Mosses, 1990-1992. St. Louis: Missouri Botanical Garden, 1994. Comple-ments authors’ previous works by updating moss nomenclature and providing citationsto significant moss research publications. Includes bibliography.

Crum, Howard A., and Lewis E. Anderson. Mosses of Eastern North America. 2 vols. NewYork: Columbia University Press, 1981. Comprehensive guide to mosses indigenous to

696 • Mosses

Canada and the eastern United States. Includes illustrations, bibliographies, and index.Schenk, George. Moss Gardening: Including Lichens, Liverworts, and Other Miniatures. Port-

land, Oreg.: Timber Press, 1997. Explains how to use mosses and similar plants to land-scape decoratively. Includes color illustrations, bibliographical references, and index.

MULTIPLE-USE APPROACH

Categories: Agriculture; economic botany and plant uses; environmental issues; forests and forestry

The multiple-use approach is a concept of resource use in which land supports several concurrent managed uses ratherthan single uses over time and space.

T he multiple-use approach is a managementpractice that is teamed with the concept of sus-

tained yield. Multiple use began as a working pol-icy, generally associated with forestry, and was en-acted as law in 1960. As a concept of land-usemanagement, it has most often been applied to theuse of forestlands. Historically, multiple use hasbeen linked with another concept, that of sustainedyield.

Historical BackgroundThe history of the intertwined multiple-use and

sustained-yield approaches to land management inthe United States dates from the late 1800’s. Priorto that time, forestlands were used for timber pro-duction, rangeland for grazing, and parklands forrecreation. Little attention was given to the interre-lated aspects of land use. By the late 1800’s, how-ever, some resource managers began to see land as aresource to be managed in a more complex, inte-grated fashion which would lead to multiple use.This awakening grew out of the need for conserva-tion and sustained yield, especially in the forest sec-tor of the resource economy.

Sustained YieldSince the earliest European settlement of North

America, forest resources had been seen both as anearly inexhaustible source of timber and as an im-pediment to be cleared to make way for agriculture.This policy of removal led to serious concern by thelate 1800’s about the future of American forests. By1891 power had been granted to U.S. presidentBenjamin Harrison to set aside protected forest ar-eas. Both he and President Grover Cleveland took

actions to establish forest reserves. To direct themanagement of these reserves, Gifford Pinchot wasappointed chief forester. Pinchot was trained in Eu-ropean methods of forestry and managed resources,as noted by Stewart Udall in The Quiet Crisis (1963),“on a sustained yield basis.” The sustained-yieldbasis for forest management was thus established.Essentially, the sustained-yield philosophy restrictsthe harvesting of trees to no more than the ultimatetimber growth during the same period.

Multiple UseProperly managed, forestlands can meet needs

for timber on an ongoing, renewable basis. How-ever, land in forest cover is more than a source oftimber. Watersheds in such areas can be protectedfrom excessive runoff and sedimentation throughappropriate management. Forest areas are alsowildlife habitat and potential areas of outdoor recre-ation. The combination of forest management forrenewable resource production and complex, inter-related land uses provided the basis for the devel-opment of multiple use-sustained yield as a long-term forest management strategy.

Multiple Use Joins Sustained YieldThe merging of these two concepts took shape

over a period of many years, beginning in the earlytwentieth century. The establishment of nationalforests by Presidents Harrison and Cleveland pro-vided a base for their expansion under PresidentTheodore Roosevelt in the early 1900’s. With theactive management of Pinchot and the enthusiasticsupport of Roosevelt, the national forests began tobe managed on a long-term, multiple-use, sustained-

Multiple-use approach • 697

yield basis. The desirability of this approach even-tually led to its formalization by law: On June 12,1960, Congress passed the Multiple Use-SustainedYield Act. To some, this act was the legal embodi-ment of practices already in force. However, the actprovides a clear statement of congressional policyand relates it to the original act of 1897 that had es-tablished the national forests.

The 1960 act specifies that “the national forestsare established and shall be administered for out-door recreation, range, timber, watershed, and wild-life and fish purposes.” Section 2 of the act statesthat the “Secretary of Agriculture is authorized anddirected to develop and administer the renewableresources of the national forests for multiple useand sustained yield of the several products and ser-vices obtained therefrom.” The act gives no specif-

ics, providing a great deal of freedom in choosingways to meet its provisions. It also refrains fromproviding guidelines for management. In practice,the achievement of a high level of land manage-ment under the act has called for advocating aconservation ethic, soliciting citizen participation,providing technical and financial assistance to pub-lic and private forest owners, developing interna-tional exchanges on these management principles,and extending management knowledge.

Jerry E. Green

See also: Agriculture: modern problems; Forestmanagement; Forests; Grazing and overgrazing;High-yield crops; Monoculture; Rangeland; Sus-tainable agriculture; Sustainable forestry; Timberindustry.

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Sources for Further StudyCutter, Susan, Hilary Renwick, and William Renwick. Exploitation, Conservation, Preserva-

tion: A Geographical Perspective on Natural Resource Use. 3d ed. New York: J. Wiley & Sons,1999. Integrates physical, economic, social, and political considerations of the major natu-ral resource issues facing the world.

Hewett, Charles E., and Thomas E. Hamilton, eds. Forests in Demand: Conflicts and Solutions.Boston: Auburn House, 1982. Alook at the United States’ evolving philosophy governingrenewable resource use.

Lovett, Francis. National Parks: Rights and the Common Good. Lanham, Md.: Rowman &Littlefield, 1998. Explores issues regarding the proper balance between individual andsubgroup rights and the needs and value of the community.

Sedjo, Roger A., ed. A Vision for the U.S. Forest Service: Goals for Its Next Century. Washington,D.C.: Resources for the Future, 2000. At the centenary of the Forest Service, ten paperscover such topics as rethinking scientific management, state trust lands, and conservation.

Udall, Stewart. The Quiet Crisis. 1963. Reprint. Salt Lake City, Utah: Gibbs Smith, 1998. Goodhistorical material.

MUSHROOMS

Categories: Economic botany and plant uses; food; fungi; poisonous, toxic, and invasive plants

Members of the kingdom Fungi and division Mycota, mushrooms are members of the class Basidiomycetes. This classincludes both the hymenomycetes, which reproduce spores using a layer structure called the hymenium, and thegasteromycetes, which include the stinkhorns and puffballs.

Fossil evidence suggests that mushrooms existedninety million years ago. The order Agaricales

represents approximately four thousand species insixteen families of commonly found mushrooms.The order Polyporus consists of fungi that grow ontree limbs or stumps. Although they prefer moistenvironments, mushrooms are sometimes found indeserts, beaches, and occasionally snowdrifts.

CharacteristicsThe mushrooms familiar to most people are

hymenomycetes. They are either edible or poisonousstructures lacking chlorophyll. That is, like otherfungi, they do not make their own food throughphotosynthesis but instead are heterotrophs, whichfeed off other organisms. They are classified asthousands of species, distributed globally. Mostmushrooms consist of a fleshy fruiting body calledthe sporophore and a cylindrical stalk.

Most mushrooms with the familiar umbrella-shaped sporophore, called the cap or pileus, belongto the family Agaricaceae. The pileus has narrow

sheets called gills that contain the spores. The pileussits atop the stalk, known as the stipe. Pilei andstipes vary in size.

Sporophores grow from a mat of thin strandscalled the mycelium, or spawn, which is located be-low the soil surface. Each mycelium grows newsporophores during the annual fruiting season.Mycelial life spans range from several months tocenturies, depending on nutrient and moisturesources and suitable temperature. A honey mush-room mycelium in Michigan once spread across 40acres over fifteen hundred years.

Mushrooms grow quickly, often maturingwithin several days. A membrane joins the cap’sedges to the stem. As mushrooms mature, themembrane breaks, revealing the cap’s gills. Whenthe agaric mushroom ripens, its color changes fromwhite to pink, then brown. Some mushrooms’ colorschange when they are exposed to air or water.Mushrooms are geotropic, keeping their caps up-right and gills vertical and turning the pileus ifplaced on their sides after being picked.

Mushrooms • 699

SpeciesWild mushrooms grow in fields, forests, lawns,

and gardens. The waxy caps are among the mostcolorful mushrooms. In spring, morels emerge, of-ten thriving on burned land. Their gills resemblehoneycombs. The genus Gyromitra includes thefalse morels, which can be toxic.

The family Boletaceae includes mushrooms thatcontain layers of spores in tubes on the pileus’sundersurface. The Hydnum mushrooms appear tohave teeth, and the Clavarias look like coral. Oc-casionally, sporophores grow in circular patternspopularly called fairy rings.

The Coprinus comatus, or shaggy mane, is an-other spring mushroom which appears through thefall in open areas, with individuals or groups of thisspecies often reemerging in the same place an-nually. This mushroom has tall, brown caps thatappear shaggy because of soft scales on its surface.Unlike most mushrooms, members of Coprinushave caps that do not expand at maturity, whichcauses the compacted gills to liquefy into a black

fluid, resulting in the collective name for the onehundred species of this genus, “inky caps.”

The species Agaricaceae campestris profuselygrows in rural areas, especially during the summer.The orange or yellow funnel-capped Cantherelluscibarius thrives in European forests, tastes nutty,and has been a preferred edible mushroom sincethe days of the Roman Empire. By autumn, largemushrooms representing the genus Boletus ripen inwooded areas. Veins on caps of Boletus specieschange color from white to yellow, indicating theirreadiness for picking.

Several mushroom species live on decayingwood hosts. Rotten timber is the habitat for Poly-porus sulfurreus, which form as orange-yellowshelves that can be meters wide and weigh manykilograms. Pores beneath the layers create spores.

Poisonous MushroomsBetween sixty and seventy mushroom species

are poisonous. If consumed, poisonous mushroomspecies can cause intense physical reactions, rang-

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ing from sickness to death. The genus Amanita in-cludes the most dangerous mushrooms, includingAmanita muscarine. Only the Amanita caesarea is ed-ible. The fly mushroom, Amanita muscaria, is alarge, colorful mushroom that is deadly to insects.The white Amanita phalloides, known as the deathcap or death angel, are the most toxic mushroomsand are widely distributed. Other hazardous mush-rooms are Boletus satana (Satan’s mushroom) andClitocybe illudens (jack-o’-lantern), which glows inthe dark and is physically similar to the benign Cli-tocybe gigantea.

Alkaloid mycotoxins in mushrooms attack nerve,muscle, blood, and organ cells. Victims suffer di-gestive symptoms within eight to twelve hours af-ter ingesting mushrooms, then slip into a coma anddie several days later. Patients undergo gastrointes-tinal tract purging and antidote treatment in an ef-fort to counter poisoning.

UsesHumans have cultivated mushrooms for thou-

sands of years. Some mushrooms lack flavor or aretoo bitter or woody to consume. Others, such asIthyphallus impudicus, have a foul odor. The Pleu-rotus ostreatus tastes like oysters. Although mush-rooms are not especially nutritious because they are90 percent water, many have pleasing tastes andtextures and are low in fat.

Annually, about ten million North Americanshunt common mushrooms, especially morels. Themushroom industry relies on mushrooms picked

and sold at wholesale prices totaling millions ofdollars. Brokers ship the mushrooms to distribu-tors or retail markets.

Commercial growers produce a safe source of ed-ible mushrooms. These mushrooms are cultivatedin specially designed structures, caves, or cellars inwhich the darkness, humidity, and temperature areregulated. Beds of soil-covered straw and manureare planted with mycelia. The Agaricus bisporus, ahybrid developed by researchers, is the most com-mon commercial species. Exotic wild mushrooms,including portobello and shiitake, are cultivated ar-tificially. Liquid nitrogen is used to store spawns andenhances their endurance and quality. Scientists aregenetically designing improved mushroom strains.

Some mushrooms are hallucinogenic, and mostgovernments restrict their use as a narcotic, al-though mushrooms are smuggled into countriesthat have bans. Various cultures incorporate mush-rooms into spiritual rituals, believing the mush-rooms have magical powers. Researchers have de-termined that some mushrooms, specifically theGleophyllum odoratum and Clitocybe gibba species,can be therapeutic if thrombin inhibitors are ex-tracted to manufacture anticoagulant pharmaceu-ticals.


See also: Ascomycetes; Basidiosporic fungi; Basid-iomycetes; Bioluminescence; Culturally significantplants; Fungi; Medicinal plants; Plant biotechnol-ogy; Poisonous and noxious plants; Tropisms.

Sources for Further StudyBenjamin, Denis R. Mushrooms: Poisons and Panaceas. New York: W. H. Freeman, 1995. Com-

ments on mushrooms’ medicinal and toxic properties. Includes illustrations, biblio-graphical references, and index.

Bessette, Alan E., William C. Roody, and Arleen R. Bessette. North American Boletes: A ColorGuide to the Fleshy Pored Mushrooms. Syracuse, N.Y.: Syracuse University Press, 2000. Fo-cuses on identification and uses of a specific mushroom family. Includes illustrations, bib-liographical references, and indexes.

Fisher, David W., and Alan E. Bessette. Edible Wild Mushrooms of North America. Austin: Uni-versity of Texas Press, 1992. Guide with recipes for consuming mushrooms. Includes il-lustrations, bibliographical references, and index.

Phillips, Roger. Mushrooms of North America. Boston: Little, Brown, 1991. Provides details foridentification of common and rare edible and poisonous mushrooms. Includes illustra-tions, bibliographical references, and index.

Schaechter, Elio. In the Company of Mushrooms: A Biologist’s Tale. Cambridge, Mass.: HarvardUniversity Press, 1997. Narrates author’s experiences, supplemented with informationabout mushroom science, history, and myths. Includes illustrations, bibliographical ref-erences, tables, and index.

Mushrooms • 701

MYCORRHIZAE

Categories: Ecosystems; fungi

“Mycorrhiza” (from the Greek mukes, meaning “fungus,” and rhiza, meaning “root”) is a term describing amutualistic, symbiotic relationship between plant roots or other underground organs and fungi. Mycorrhizae are amongthe most abundant symbioses in the world.

Mycorrhizal associations have been describedin virtually all economically important plant

groups. Investigators in Europe detected fungal as-sociations in most European species of floweringplants, all gymnosperms, ferns, and some bryo-phytes, especially the liverworts. Similar patternsare predicted in other ecosystems. Continuingstudies of ecosystems, from boreal forests to tem-perate grasslands to tropical rain forests andagroecosystems, also suggest that most plantgroups are intimately linked to one or more speciesof fungus.

It is theorized that most of the plants in stablehabitats where competition for resources is com-mon probably have some form of mycorrhizal asso-ciation. Species from all of the major taxonomicgroups of fungi, including the Ascomycotina,Basidiomycotina, Deuteromycotina, and Zygomy-cotina, have been found as partners with plants inmycorrhizae.

Considering the prevalence of mycorrhizae inthe world today, botanists theorize that mycor-rhizae probably arose early in the development ofland plants. Some suggest that mycorrhizae mayhave been an important factor in the colonization ofland.

The fungal partner (or mycobiont) in a mycor-rhizal relationship benefits by gaining a source ofcarbon. Often these mycobionts are poor compet-itors in the soil environment. Some mycobiontshave apparently coevolved to the point that theycan no longer live independently of a plant host.

The plant partner in the mycorrhizal relation-ship benefits from improved nutrient absorption.This may occur in different ways; for example, themycobiont may directly transfer nutrients to theroot. Infected roots experience more branching,thus increasing the volume of soil that the plant can

penetrate and exploit. Evidence also suggests thatmycorrhizal roots may live longer than roots with-out these associations. Comparison of the growthof plants without mycorrhizae to those with fungalpartners suggests that mycorrhizae enhance over-all plant growth.

Types of MycorrhizaeMycorrhizae may be classified into two broad

groups: endomycorrhizae and ectomycorrhizae. Endo-mycorrhizae enter the cells of the root cortex.Ectomycorrhizae colonize plant roots but do not in-vade root cortex cells.

The most common form of endomycorrhizae arethe vesicular-arbuscular mycorrhizae. The fungi in-volved are zygomycetes. These mycorrhizae haveinternal structures called arbuscules, which arehighly branched, thin-walled tubules inside theroot cortex cells near the vascular cylinder. It is esti-mated that 80 percent of all plant species may havevesicular-arbuscular mycorrhizae. This type of my-corrhiza is especially important in tropical trees.

There are several other subtypes of endomy-corrhizae. Ericoid mycorrhizae, found in the familyEricaceae and closely related families, supply thehost plants with nitrogen. These are usually re-stricted to nutrient-poor, highly acidic conditions,such as heath lands. Arbutoid mycorrhizae, found inmembers of the Arbutoideae and related families,share some similarities with ectomycorrhizae inthat they form more developed structures calledthe sheath and Hartig net (described below).

Monotropoid mycorrhizae, found in the plant fam-ily Monotropaceae, are associated with plants thatlack chlorophyll. The host plant is completely de-pendent on the mycobiont, which also has connec-tions to the roots of a nearby tree. Thus the host,such as Monotropa, indirectly parasitizes another

702 • Mycorrhizae

plant by using the mycobiont as an intermediate.Orchidaceous mycorrhizae are essential for orchidseed germination.

Ectomycorrhizae are common in forest trees andshrubs in the temperate and subarctic zones. Well-developed fungal sheaths characterize these mycor-rhizae, along with special structures called Hartignets. Basidiomycetes are the usual mycobionts andoften form mushrooms or truffles. Ectomycorrhizaehelp protect the host plant from diseases by form-ing a physical fungal barrier to infection.

Anatomy and DevelopmentIndividual filaments of a fungal body are called

hyphae. The entire fungal body is called a mycelium.Root infection may occur from fungal spores thatgerminate in the soil or from fungal hyphae grow-ing from the body of a nearby mycorrhiza. When in-fection occurs, hyphae are drawn toward certainchemical secretions from a plant root.

In ectomycorrhizae, root hairs do not develop inroots after infection occurs. Infected roots have afungal sheath, or mantle, that ranges from 20 to 40 mi-crometers thick. Fungal hyphae penetrate the rootby entering between epidermal cells. These hyphaepush cells of the outer root cortex apart and con-tinue to grow outside of the cells. This associationof hyphal cells and root cortex cells is called a Hartignet. In ectomycorrhizae, the mycobionts never in-vade plant cells, nor do they penetrate the endo-dermis or enter the vascular cylinder. The root tipmay be ensheathed by fungi, but the apical meri-stem is never invaded. Main roots experience feweranatomical changes than lateral roots after infec-

tion. Lateral roots become thickened, may showthe development of characteristic pigments, andgrow very slowly. Infected roots also show differ-ent branching patterns than those of uninfectedroots.

Endomycorrhizae are highly variable in struc-ture. Many endomycorrhizae do not have sheathsor Hartig nets. In all endomycorrhizae, hyphaepenetrate into root cortex cells, while portions ofthe mycelium remain in contact with the soil. Thehyphae that remain in the soil are important in fun-gal reproduction and produce large numbers of hap-loid spores. Fungi do not invade root meristems,vascular cylinders, or chloroplast-containing cellsin the plant.

Some of the host cells contain fungal extensionscalled vesicles that are filled with lipids. Vesicles arespecialized structures that are often thick-walledand may serve as storage sites or possibly in repro-duction. Vesicles are also produced on the hyphaethat grow in the soil. Near the vascular cylinder, thehyphae branch dichotomously and form largenumbers of thin-walled tubules called arbusculesthat invade host cells. The arbuscules cause the hostmembranes to fold inward, creating a plant-fungusinterface that has a very large surface area. Thearbuscules last for about fourteen days before theybreak down on their own or are digested by the hostcell. Host cells whose fungal arbuscules have bro-ken down may be reinvaded by other hyphae.

Darrell L. Ray

See also: Ascomycetes; Basidiomycetes; Coevolu-tion; Deuteromycetes; Fungi; Legumes; Nitrogenfixation; Roots; Zygomycetes.

Sources for Further StudyDeacon, J. W. Modern Mycology. 3d ed. Malden, Mass.: Blackwell Science, 1997. An introduc-

tory text in mycology. Includes photos, diagrams.Harley, J. L., and S. E. Smith. Mycorrhizal Symbiosis. New York: Academic Press, 1983. An au-

thoritative text on the formation and biology of mycorrhizae.Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:

W. H. Freeman/Worth, 1999. A standard text for introductory botany courses at the uni-versity level. Discusses the significance of major types of mycorrhizae. Includes photos.

Mycorrhizae • 703

NASTIC MOVEMENTS

Categories: Animal-plant interactions; movement; physiology

Plants, unlike animals, are sedentary organisms, but they are capable of some limited movements. These include nasticmovements, which enable plants to adapt rapidly to changes in their environment by changing orientation.

Nastic movements and tropisms, or growthmovements, are two important, but different,

kinds of movements in plants. In nastic move-ments, the direction of movement is determined bythe anatomy of the plant rather than by the positionof the origin of the stimulus. In tropisms, the move-

ment is in a direction either toward or away fromthe origin of the stimulus. In addition, the ori-entational changes that occur in nastic movementsare temporary; they are reversible and repeatable.The tropisms, in contrast, are generally irreversible.

Nastic movements are common in certain plantfamilies, especially among the legumes (familyFabaceae, formerly Leguminosae). In legumes havingleaves that are composed of many leaflets—that is,compound—both the leaflets and the leaves mayexhibit the movements. The most widely occurringnastic movements are probably day-and-nightmovements, known as nyctinastic movements. An-other important kind is thigmonastic movements,triggered by touch or other mechanical stimuli.

Nyctinastic MovementsThe leaves of many plants respond to the daily

alternation between light and darkness by movingup and down. In these nyctinastic, or sleep, move-ments, the leaves extend horizontally (open) to in-tercept sunlight during the day and fold togethervertically (close) at night. Leguminous plants ex-hibiting nyctinastic movements include the sensi-tive plant (Mimosa pudica) and the silk tree (Albizziajulibrissin). The movements also occur in some spe-cies of oxalis (family Oxalidaceae). In the prayerplant, maranta species (family Marantaceae), theleaves, which are simple, fold at night into a verticalconfiguration that suggests praying hands.

Thigmonastic MovementsMechanical disturbances that may trigger thig-

monastic movements include touch, shaking, orelectrical or thermal stimulation. The stimulus istransmitted from touch-sensitive cells to respond-ing cells located elsewhere in the plant. Many ofthe plants that display thigmonastic movementsare ones that also exhibit nyctinastic ones, such as

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Nastic movements are controlled by hormones that re-sult in cell growth across stem tips at changing positions.When a twining plant grows, such alterations in growthpush the stem tip in different directions. When the stemtouches an obstacle such as a stake or other support struc-ture, it will spiral around that structure. Such move-ment, driven by hormones, is not the same as a tropism,which is a response to external stimuli.

some members of the Fabaceae and Oxalidaceae.The sensitive plant exhibits pronounced thig-

monastic movements. If even a single leaflet istouched, it and the other leaflets of the leaf foldupward in pairs until their surfaces touch. The sig-nal moves down the leaf stalk, or petiole, whichdroops, and then proceeds to the rest of the shoot. Ifthe plant is allowed to rest, the leaves return to theiroriginal orientation in fifteen to twenty minutes.The adaptive significance of thigmonastic move-ments to most plants is not well understood. Someevidence suggests that the movements may scareoff leaf-eating insects.

Many plants require a stronger stimulus thandoes the sensitive plant in order for a response to begenerated. A notable exception is Venus’s flytrap(Dionaea muscipula, in the family Droseraceae). Theleaves of this carnivorous plant respond in a rapid,highly specialized way, and the purpose is preda-tory, not defensive. An insect alighting on a leaf,which has two lobes, stimulates sensitive “trigger”hairs on the leaf epidermis, or surface layer of cells.Within about a half-second, the two lobes of the leafsnap shut. Enzymes digest the insect in one to sev-eral days, and the empty trap then reopens.

Physiological MechanismsThe “motor” that drives most nastic movements

is a controlled change in turgor pressure, which isthe pressure exerted on a cell wall due to movementof water into the cell. In the sensitive plant andmany other thigmonastically responsive plants,these turgor changes occur in certain large, thin-walled cells that function as motor cells, located atthe bases of the leaf blades or petioles, and leaflets,if present. The motor cells surround a central strandof vascular, or conducting, tissue, forming ajointlike thickening called a pulvinus.

Movement occurs when there are differentialturgor changes in the thin-walled cells on opposite

sides of a pulvinus, resulting in differential contrac-tion and expansion of these cells. The turgorchanges are triggered when potassium ions (K+)and other ions move into or out of the cells, andwater follows by osmosis (as in the opening andclosing of stomata in the leaf epidermis). Cells onone side of the pulvinus function as extensors, andcells on the opposite side function as flexors. Leavesor leaflets open when extensor cells accumulate K+and then swell with water and flexor cells lose K+and then shrink from loss of water. Conversely, theleaf or leaflet closes when the extensor cells shrinkand the flexor cells swell.

The mechanisms causing these ion fluxes vary. Innyctinastic movements, the fluxes and the pulvinalturgor changes that they trigger are rhythmic andare regulated by interactions between light and theplant’s innate biological clock. Phytochrome, a plantpigment involved in many timing processes, in-cluding flowering, plays a role in this regulation. Inthigmonastic movements, the touching of a leaf orleaflet generates an electrical signal called an actionpotential. This signal typically moves along thestalk of the leaflet and leaf. It is then translated intoa chemical signal that causes the ion fluxes andpulvinal turgor changes.

In Venus’s flytrap, touching of the hairs on a leafsurface generates an action potential. There are nopulvini to respond, however. The underlying bio-chemical mechanisms of trap closure are not wellunderstood. They may involve turgor changes in alayer of photosynthetic cells immediately beneaththe leaf’s upper epidermis.

Jane F. Hill

See also: Carnivorous plants; Circadian rhythms;Leaf anatomy; Osmosis, simple diffusion, and facil-itated diffusion; Photoperiodism; Thigmomorpho-genesis; Tropisms; Water and solute movement inplants.

Sources for Further StudyAttenborough, David. The Private Life of Plants. Princeton, N.J.: Princeton University Press,

1995. Written for the layperson. Discusses, with photos, thigmonasty and prey capture byVenus’s flytrap. Includes index.

Moore, Randy, et al. Botany. Dubuque, Iowa: Wm. C. Brown, 1995. A comprehensive, intro-ductory college textbook. Includes a chapter on plant responses to environmental stimuli,with a section on nastic movements. Includes references, color photos, diagrams, glos-sary, tables, and index.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. A comprehensive, introductory college-level textbook.

Nastic movements • 705

Contains a chapter on external factors and plant growth, with a section on nastic move-ments. Includes color photos, diagrams, index, and bibliography.

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology. 4th ed. Belmont, Calif.: Wadsworth,1992. College-level textbook, with a chapter on plant movements. Includes black-and-white illustrations, references, and indexes.

NITROGEN CYCLE

Categories: Biogeochemical cycles; cellular biology; ecology; nutrients and nutrition

The nitrogen cycle outlines the movement of the element nitrogen from one chemical state to another as it makes its waythrough a series of complex physical and biological interactions.

Nitrogen (N) is one of the most dynamic ele-ments in the earth’s biosphere; it undergoes

transformations that constantly convert it betweenorganic, inorganic, gaseous, and mineral forms. Ni-trogen is an essential element in all living things,where it is a crucial component of organic mole-cules such as proteins and nucleic acids. Conse-quently, nitrogen is in high demand in biologicalsystems.

Unfortunately, most nitrogen is not readilyavailable to plants and animals. Although the bio-sphere contains 300,000 terrograms (a terrogram isa billion kilograms) of nitrogen, that amount is onehundred times less nitrogen than is in the hydro-sphere (23 million terrograms) and ten thousandtimes less nitrogen than is in the atmosphere (about4 billion terrograms). Atmospheric nitrogen is al-most all in the form of nitrogen gas (N2), whichcomposes 78 percent of the atmosphere by volume.The greatest reservoir of nitrogen on the earth is thelithosphere (164 billion terrograms). Here the nitro-gen is bound up in rocks, minerals, and deep oceansediments.

Even though living things exist in a “sea” of ni-trogen gas, it does them little good. The bond be-tween the nitrogen atoms is so strong that nitrogengas is relatively inert. For living things to use nitro-gen gas, it must first be converted to an organic orinorganic form. The nitrogen cycle is the collectionof processes, most of them driven by microbial ac-tivity, that converts nitrogen gas into these usableforms and later returns nitrogen gas back to the at-mosphere. It is considered a cycle because every ni-trogen atom can ultimately be converted by each

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process, though that conversion may take a longtime. It is estimated, for example, that the averagenitrogen molecule spends 625 years in the bio-sphere before returning to the atmosphere to com-plete the cycle.

Nitrogen FixationThe first step in the nitrogen cycle is nitrogen fixa-

tion. Nitrogen fixation is the conversion, by bacte-ria, of nitrogen gas into ammonium (NH4

+) andthen organic nitrogen (proteins, nucleic acids, andother nitrogen-containing compounds). It is esti-mated that biological nitrogen fixation adds about160 billion kilograms of nitrogen to the biosphereeach year. This represents about half of the nitrogentaken up by plants and animals. The microorgan-isms that carry out nitrogen fixation are highly spe-cialized. Each one carries a special enzyme com-plex, called nitrogenase, that allows it to carry outfixation at temperatures and pressures capable ofpermitting life—something industrial nitrogen fix-ation does not allow.

Nitrogen-fixing microbes, may be either free-living or growing in association with higher organ-isms such as legumes (in which case the process iscalled symbiotic nitrogen fixation). Symbiotic nitro-gen fixation is a very important process and is onereason legumes are so highly valued as a naturalresource. Because they are able to form these sym-biotic associations with nitrogen-fixing bacteria, le-gumes can produce seeds and leaves with morenitrogen than other plants. When they die, theyreturn much of that nitrogen to soil, enriching it forfuture growth.

Mineralization and NitrificationWhen plants and animals die they undergo a

process called mineralization (also called ammoni-fication). In this stage of the nitrogen cycle, the or-ganic nitrogen in decomposing tissue is convertedback into ammonium. Some of the ammonium istaken up by plants as they grow. This process iscalled assimilation or uptake. Some of the ammo-nium is taken up by microbes in the soil. In this casethe nitrogen is not available for plant growth. If thishappens, it is said that the nitrogen is immobilized.Some nitrogen is also incorporated into the clayminerals of soil. In this case it is said that the nitro-gen is fixed—it is not immediately available forplant and microbial growth, but it may becomeavailable at a later date.

Ammonium has another potential fate, and thisstep in the nitrogen cycle is nitrification. In nitrifica-tion the ammonium in soil is oxidized by bacteria(and some fungi) to nitrate (NO3

–) in a two-step pro-cess. First, ammonium is oxidized to nitrite. Next,nitrite is rapidly oxidized to nitrate. Nitrificationrequires oxygen, so it occurs only in well-aeratedenvironments. The nitrate that forms during nitrifi-cation can also be taken up by plants and microbes.However, unlike ammonium, which is a cation(positively charged ion) and readily adsorbed bysoil, nitrate is an anion (negatively charged ion) andreadily leaches or runs off of soil. Hence, nitrate is aserious water contaminant in areas where excessivefertilization or manure application occurs.

DenitrificationObviously some process is responsible for re-

turning nitrogen to the atmosphere; otherwise or-ganic and inorganic nitrogen forms would accumu-

Nitrogen cycle • 707

Nitrogen inatmosphere

Nitrates inthe soil

Proteinin plants

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Nitrogenfixation

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Feeding

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late in the environment. The process that completesthe nitrogen cycle and replenishes the nitrogen gasis denitrification. Denitrification is a bacterial pro-cess that occurs in anaerobic (oxygen-limited) envi-ronments such as waterlogged soil or sediment.Nitrate and nitrite are reduced by denitrifying bac-teria, which can use these nitrogen oxides in placeof oxygen for their metabolism. Wetlands are par-ticularly important in this process because at leasthalf of the denitrification that occurs in the bio-sphere occurs in wetlands.

The major product of denitrification is nitrogengas, which returns to the atmosphere and approxi-mately balances the amount of nitrogen gas that isbiologically fixed each year. In some cases, however,an intermediate gas, nitrous oxide (N2O), accumu-lates. Nitrous oxide has serious environmental con-sequences. Like carbon dioxide, it absorbs infrared

radiation, so it contributes to global warming. Moreimportant, when nitrous oxide rises to the strato-sphere, it contributes to the catalytic destruction ofthe ozone layer. Besides the potential for fertilizernitrogen to contribute to nitrate contamination ofgroundwater, there is the concern that some of it canbe denitrified and contribute to ozone destruction.

The nitrogen cycle is a global cycle involvingland, sea, and air. It circulates nitrogen through var-ious forms that contribute to life on earth. When thecycle is disturbed—as when an area is deforestedand nitrogen uptake into trees is stopped, or whenexcessive fertilization is used—nitrogen can be-come an environmental problem.

Mark S. Coyne

See also: Nitrogen fixation; Nutrient cycling; Nu-trients.

Sources for Further StudyAnderson, Stanley H., Ronald Beiswenger, and P. Walton Purdom. Environmental Science. 4th

ed. New York: Maxwell Macmillan International, 1993. Probably the most readable intro-ductory-level text, intended for a college course in environmental science. Discusses eco-systems and includes sections on the nitrogen cycle, ammonification, and nitrogen fixa-tion. Suitable for the average reader; black-and-white illustrations, a glossary of terms,and an index. A good introduction to the subject.

Burt, T. P., A. L. Heathwaite, and S. T. Trudgill, eds. Nitrate: Processes, Patterns, and Manage-ment. Chichester, N.Y.: John Wiley, 1993. Explores nitrate transport patterns and considersvarious strategies for nitrate pollution, including legislative, land use, and water treat-ment policies.

Clark, F. E., and T. Rosswall, eds. Terrestrial Nitrogen Cycles: Processes, Ecosystem Strategies,and Management Impacts. Stockholm: Swedish Natural Science Research Council, 1981. Acollection of technical papers that is very readable with clear and concise abstracts to headeach article. Extensive coverage of the terrestrial nitrogen cycle with excellent referencesand index for further investigation into this complex topic. Suitable for the college-levelreader or serious layperson who wishes to read about how the nitrogen cycle is beingstudied by modern environmental scientists.

Goodwin T. W., and R. M. S. Smeille, eds. Nitrogen Metabolism in Plants. London: BiochemicalSociety, 1973. A collection of papers discussing the role of nitrogen in plant metabolism,with extensive coverage of biochemical reactions within the cell. The text is technical andsuited to a college-level audience. Well indexed to locate specific areas of interest. A goodvolume for those interested in the biological processes involved in the nitrogen cycle.

Krebs, Charles J. Ecology: The Experimental Analysis of Distribution and Abundance. 5th ed. SanFrancisco: Benjamin Cummings, 2001. Extensive coverage of the biogeochemical cyclesin chapter dealing with nutrient cycling. Referenced, with an index including many indexentries on nitrogen; the nutrient cycle is discussed throughout the book. This is an excel-lent college-level text with some mathematical models of nutrient cycles and algaegrowth. Numerous illustrations, figures, graphs, and diagrams.

Lasserre, P., and J. M. Martin, eds. Biogeochemical Processes at the Land-Sea Boundary. NewYork: Elsevier, 1986. Acollection of very accessible technical papers dealing with nutrientcycling interactions at the continental-ocean boundary. Contains excellent discussions of

708 • Nitrogen cycle

ecosystem and nutrient-cycle modeling. Suitable for the advanced student or college-level reader with an interest in land-sea interactions. Unfortunately, the book contains noindex, but it is organized to facilitate finding topics of interest.

Mlot, Christine. “Tallying Nitrogen’s Increasing Impact.” Science News 151, no. 7 (Febru-ary 15, 1997): 100. Discusses research findings that the rate at which nitrogen is madeavailable to the land has risen to the point that humans, through the use of commercialfertilizer, global cultivation of legumes, and clearing of wildlands, have become the majorforce in the nitrogen cycle.

Tamm, Carl Olof. Nitrogen in Terrestrial Ecosystems: Questions of Productivity, VegetationalChanges, and Ecosystem Stability. New York: Springer-Verlag, 1991. Local and globalchanges in nitrogen are examined, with emphasis on changes that are a result of shifts inland use and automobile and industrial pollution, found to be damaging to plant life.

NITROGEN FIXATION

Categories: Cellular biology; nutrients and nutrition; physiology

Nitrogen fixation is the process whereby elemental nitrogen from the atmosphere is converted to ammonium, an ionicform of nitrogen available to higher plants.

Nitrogen, the fourth most abundant element inmost organisms, can account for as much as 4

percent of a plant’s dry weight. The majority of thisnitrogen is present as a constituent of protein struc-ture, but it is also a component of numerous otherbiological compounds, such as the chlorophyllsand nucleic acids. Thus, for normal plant growthand development, nitrogen must be maintained atfairly high levels in the soil.

The earth’s atmosphere is about 79 percent nitro-gen, and the vast majority of atmospheric nitrogenexists in the elemental state. Unfortunately, the ele-mental form of nitrogen is of no direct value tohigher plants; they must acquire their nitrogen inthe form of either ammonium or nitrate. These twoforms of nitrogen can be supplied to the soil as fer-tilizer by humans or by nature, as the product of mi-crobial action.

There are three microbial processes that rendernitrogen into forms usable by higher plants. Theseare ammonification, nitrification, and nitrogen fixa-tion. Ammonification is the process whereby vari-ous forms of organic nitrogen, such as is present inthe proteins in plant and animal residues and ani-mal wastes (manures), are converted to ammo-nium. Nitrification is the process by which ammo-nium is converted to nitrate. Both these processes

are carried out by populations of free-living soil mi-croorganisms.

In the nitrogen fixation process, atmospheric ni-trogen is converted to ammonium. While some ofthe nitrogen fixers are free-living microbes, bacteriathat live symbiotically within the roots of a numberof plant species are also responsible for much of thisconversion in terrestrial ecosystems.

Nitrogen-Fixing BacteriaNitrogen-fixing bacteria have been shown to co-

exist with a variety of lower plants, including li-chens, liverworts, mosses, and ferns. Among themore advanced seed-bearing plants, nitrogen fixershave been found to be associated with some tropi-cal grasses and a number of shrubs and trees, suchas the alders. Agriculturally, the legumes are themost important group of plants coexisting withnitrogen-fixing bacteria. Some fifteen hundred spe-cies of legumes, including peas, beans, clover, andalfalfa, have been shown to live symbiotically withnitrogen-fixing bacteria called Rhizobium. A differ-ent species of Rhizobium infects each different spe-cies of legume.

The bacteria penetrate the root by entering thefilamentous projections of epidermal cells calledroot hairs. The epidermal cells respond to the inva-

Nitrogen fixation • 709

sion by enclosing the bacterium in a threadlikestructure referred to as an infection thread. The infec-tion thread begins to grow and branch, and, as itdoes so, the Rhizobia reproduce numerous timesinside the thread. After penetrating several layersof cells, the infection thread eventually reaches theroot cortex, where it ruptures and releases the en-cased bacteria.

The release of the bacteria induces the secretionof plant hormones that stimulate specialized rootcortical cells to divide several times. As these cellsdivide, the Rhizobia are encapsulated, and a nod-ule is formed. Within the cytosol of the nodule cells,the bacteria become nonmotile, increase in size, andaccumulate in groups of four to eight bacteroids.These bacteroids are responsible for the biochemicalconversion of elemental nitrogen to ammonium.

Chemistry of Nitrogen FixationChemically, the fixation of nitrogen requires that

six electrons and eight hydrogen ions be trans-

ferred to the atmospheric nitrogen molecule. Thisreaction is an energy-requiring process; therefore,adenosine triphosphate (ATP), the cell’s form ofstored energy, must be available for the reaction totake place. The electrons, hydrogen ions, and ATPare supplied by the cellular respiration process thattakes place in the root cells.

The electrons and hydrogen ions are transferredto the atmospheric nitrogen atom by an enzymecalled nitrogenase. This enzyme consists of two sub-units. One subunit takes the electrons and hydro-gen ions from the respiratory products and trans-fers them to the other subunit. The ATP binds withpart II of the nitrogenase, and the hydrolysis of theATP releases the energy stored in the molecule.

This energy drives the reaction and makes iteven easier to pass the electrons and hydrogenions on to the second subunit. In the last step, thenitrogenase transfers the electrons and hydrogenions to the nitrogen atom. This final transfer resultsin the production of ammonium. The ammonium

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moves out of the bacteroids into the cytosol, whereit is converted to an organic form of nitrogen thatcan be transported throughout the plant.

Rates of FixationNot all species fix nitrogen at the same rate. A

number of factors can account for these differences.Some plants form nodules much more abundantlythan others. Because of their more extensive noduleformation, these plants will fix more nitrogen thanthose that produce fewer nodules.

The nitrogenase of all rhizobial species has a ten-dency to transfer electrons to hydrogen ions ratherthan to nitrogen. As a result, hydrogen gas, whichescapes into the atmosphere, is produced. This rep-resents a loss of electrons that could have been usedto produce ammonium. Some Rhizobia species,however, have a second enzyme, called hydro-genase, which uses the hydrogen gas to producewater. ATP is produced as a byproduct of thisprocess. Consequently, these rhizobial species aremore efficient because less energy is wasted.

In addition, the fixation rate and the amount ofnitrogen fixed will vary with age or the stage ofplant development. In most cases, fixation rates arehighest when the fruits and seeds are being pro-duced. The seeds of many plants, and especiallylegumes, are high in protein. Hence, nitrogen fix-ation and transport out of the nodules must behigher at the time the seeds are developing. In fact,more than 85 percent of the total nitrogen fixation inlegumes occurs at such times.

Plant-Bacteria MutualismThe nitrogen-fixing bacteria exist in a symbiotic

relationship with their plant hosts. The bacteriasupply the plant with much-needed nitrogen,while the plant supplies the bacteria with carbohy-drates and other nutrients. Some of the energy, de-rived from the plant-supplied carbohydrates, isused in nitrogen fixation, but there is ample leftover to supply the bacteria with all the energy nec-essary for their survival.

Providing NitrogenPlant production throughout the world is lim-

ited more by the shortage of nitrogen than by anyother nutrient. The root zone that encompasses theupper 15-centimeter layer of soil contains from 100to 6,000 kilograms of total nitrogen per hectare. Thisincludes all forms of nitrogen, many of which are

not available to plants. The total nitrogen content isdetermined by a number of factors, such as the min-erals making up the soil, the kinds of vegetation,and the extent to which these factors are affectedover time by climate, topography, and the presenceof people.

Ammonium and nitrate are the only formsreadily available to plants. These two molecules, inaddition to those such as organic nitrogen com-pounds that can easily be converted to the availableforms, are the only ones of ecological or agriculturalimportance. Unfortunately, these forms of nitrogenare continually being removed from the soil. Cropremoval, leaching (the removal of minerals as waterpercolates through the soil), denitrification (the pro-cess by which anaerobic microbes convert nitratesto gaseous nitrogen-containing compounds that es-cape the soil), and erosion account for a total loss ofapproximately 125 kilograms per hectare annually.While some of the lost nitrogen can be replaced byavailable forms falling to the earth in rain, thatamount is much too low to be of value in plantgrowth. Microbial fixation and the application offertilizers are the only sources that supply sufficientnitrogen for plant growth.

Research ApplicationsThe nonsymbiotic nitrogen fixers are of extreme

ecological importance, especially in nonagricul-tural soils. Forest, desert, and prairie ecosystemsare dependent on nitrogen fixation by free-livingspecies to replace the annual nitrogen loss. Withoutit, growth of a number of plant species would suf-fer, and food chains in these ecosystems wouldsoon be disrupted. Anumber of studies have inves-tigated the advantages of incorporating free-livingnitrogen fixers into nonlegume crop production,but clear benefits are uncertain.

On the other hand, knowledge of symbiotic ni-trogen fixation have resulted in definite improve-ments in the production of legume crops. WhenRhizobium is included with seeds as they areplanted, increased yields have been observed innearly every case. There is considerable interest inenhancing the efficiency of the nitrogen-fixing pro-cess by increasing nodulation in the roots of somespecies or by incorporating the hydrogenase sys-tem into species that do not have it. An increase inbiological nitrogen fixation could enhance the ni-trogen content of the soil and decrease the depen-dency on commercial nitrogen fertilizers.

Nitrogen fixation • 711

The application of nitrogen fertilizers to non-leguminous crops was once one of the best invest-ments a farmer could make. Commercial fertilizers,however, have become very expensive because ofincreased energy costs. In addition, a number ofenvironmental problems have developed from theaccumulation of nitrates from fertilizers in rivers,ponds, and lakes. Consequently, there is a renewedinterest in symbiotic nitrogen fixation. For years,before the extensive use of nitrogen fertilizers,farmers planted legume crops alternately withother crops. The legume crops were plowed underto supply nitrogen to the soil. There has been a re-

surgence in this technique. In fact, there is consider-able interest in growing plants containing symbi-otic nitrogen fixers in the same fields with plantslacking symbiotic nitrogen fixers, to improve thenatural nitrogen balance in certain environments.There are a number of studies directed at incorpo-rating nitrogen fixation into nonlegume cropplants; this research is difficult, however, becausethe genetics of the process is so complex.

D. R. Gossett

See also: ATP and other energetic molecules; Eutro-phication; Fertilizers; Lichens; Mycorrhizae; Nitro-gen cycle; Nutrients.

Sources for Further StudyLegocki, Andrezej, Hermann Bothe, and Alfred Pühler, eds. Biological Fixation of Nitrogen for

Ecology and Sustainable Agriculture. New York: Springer, 1997. From the proceedings of theNATO Advanced Research Workshop held in Pozna5, Poland, in 1996. Includes biblio-graphical references.

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology. 4th ed. Belmont, Calif.: Wadsworth,1992. An intermediate college-level textbook for science students. Chapter 13, “Assimila-tion of Nitrogen and Sulfur,” gives an in-depth view of the role of nitrogen fixation inplant physiology. An excellent discussion of the biochemistry of the fixation reaction isprovided. Includes detailed bibliography.

Stacey, Gary, Robert H. Burris, and Harold J. Evans, eds. Biological Nitrogen Fixation. NewYork: Chapman and Hall, 1992. Leading researchers contribute reviews on all aspects ofnitrogen fixation, including molecular biology and genetics, physiology, and ecology.

Tortora, Gerard J., Berdell R. Funke, and Christine L. Case. Microbiology: An Introduction. 7thed. San Francisco: Benjamin/Cummings, 2001. An introductory college textbook forscience students. The chapter “Soil and Water Microbiology” provides a good generaldiscussion of the process from the microbiologist’s point of view. A very readable, easy-to-understand text with sufficient illustrations to explain the process. Includes list of sug-gested readings; glossary.

NONRANDOM MATING

Categories: Genetics; reproduction and life cycles

Nonrandom mating occurs in plants when there is inbreeding or assortative mating. The consequence of nonrandommating is that population genotype frequencies do not exist in Hardy-Weinberg equilibrium. Although nonrandommating does not directly drive evolutionary change—meaning allele frequencies are not altered—it can have a profoundeffect on genotype frequencies and hence can indirectly affect evolution.

There are three main ways for nonrandom mat-ing to occur. First, positive assortative mating re-

sults when individuals and their mates share one or

more phenotypic characteristics with themselves.Negative assortative mating occurs when individu-als and their mates are dissimilar phenotypically.

712 • Nonrandom mating

Inbreeding is a third form of nonrandom mating thatoccurs when individuals mate with relatives moreoften than would be expected by chance. For bothinbreeding and assortative mating, genes combinein such a way that offspring genotypes differ fromthose that are predicted by the most basic pop-ulation genetic model, described by the Hardy-Weinberg theorem. One assumption of the Hardy-Weinberg theorem, which predicts unchangingequilibrium values for genotype and allele frequen-cies, is that individuals mate at random. Unlikeother violations of this model (such as natural selec-tion, genetic drift, mutation), nonrandom matingaffects genotype but not allele frequencies.

InbreedingInbreeding is very common in many plant spe-

cies for two main reasons. First, seed dispersaltends to follow a leptokurtic distribution, such thatmost seed falls near the parent plant. This resultsin near neighbors that are closely related and in-creases the probability that short-distance pollenmovement will result in mating among relatives.In small populations with a limited number of po-tential mates, such matings between relatives arealso common. Second, most flowering plants arehermaphroditic or monoecious. Thus, individualplants produce both male and female gametes andare capable of self-fertilization, the most extremeform of inbreeding.

The degree to which inbreeding occurs in a pop-ulation depends upon the probability that an indi-vidual will mate with a relative or with itself. Aplant’s mating system is characterized by the degreeto which self-fertilization occurs and can range fromcomplete outcrossing to complete self-fertilization,or selfing. While certain plant families tend to becharacterized by a particular mating system (suchas the inability to self-fertilize in the passionflowerfamily, Passifloraceae), others exhibit great diver-sity in the levels of inbreeding among species (thegrasses, Poaceae, and the legumes, Fabaceae).

For species with a mixed mating system, andwhich therefore engage in both selfing and out-crossing, the degree to which individual offspringare inbred is highly variable. Flowers with multipleovules within an ovary can produce fruits withboth selfed and outcrossed seeds. Some plants, suchas violets (Viola) and jewelweed (Impatiens), pro-duce morphologically distinct flowers for selfingand outcrossing.

Consequences of InbreedingInbreeding has a larger evolutionary impact

than assortative mating because it can affect allgenes in the population. It can have negative conse-quences for plant survival and reproduction (fit-ness) because it tends to increase homozygosityand decrease heterozygosity. In response to thesenegative effects, collectively known as inbreedingdepression, plants have evolved numerous adapta-tions to reduce levels of inbreeding. Although evi-dence for inbreeding depression in plants has beenfound, many species of plants are almost com-pletely self-fertilizing and do not appear to sufferfitness consequences.

Under certain conditions, inbreeding may beadvantageous. For example, rare plants or plantswith rare pollinators may have few opportunitiesfor outcrossing, and thus, self-fertilization providesa level of reproductive assurance. Many weedyplant species that tend to colonize disturbed sitesare, in fact, capable of self-fertilization. Commoncrop weeds such as velvetleaf (Abutilon theophrasti)and shepherd’s purse (Capsella bursa-pastoris) pre-dominantly self-fertilize. In these species, inbreed-ing may provide benefits that outweigh any as-sociated costs. It has also been suggested thatinbreeding species are better able to adapt to localenvironmental conditions because fewer malad-apted genes from other populations would enterthrough outcrossing.

Reducing Inbreeding DepressionInbreeding depression (which occurs when al-

leles that decrease fitness drift to fixation, causinga decrease in average fitness within a population)is reduced when plants are genetically or morpho-logically unable to self-fertilize. Genetic self-incompatibility, which is thought to occur in morethan forty different plant families (for example, Bras-sicaceae, Solanaceae, and Asclepiadaceae) prevents mat-ing between individuals that share certain genesthat are involved in the interaction between pollengrains and the stigma or style. Morphological adap-tations that reduce self-fertilization include thosethat separate anther and stigma maturation in time(protandry and protogyny) or space (heterostyly).

The individual hermaphroditic flowers of prot-androus plants, such as phlox, shed their pollenprior to the time when the stigma on the sameflower is receptive. Protogyny, which is less com-mon than protandry, occurs when stigma receptiv-

Nonrandom mating • 713

ity occurs first (as in Plantago lanceolata). Dioeciousplant species, such as date palms (Phoenix) andmarijuana (Cannabis sativa), avoid selfing by havingunisexual male and female flowers on separate in-dividuals. In some hermaphroditic species, selfingis avoided when flower morphology favors crossesbetween certain flower phenotypes, as in the case ofheterostyly.

Assortative MatingAssortative mating generally affects only those

traits important for reproduction. Many primrose(Primula) species are distylous, having two types offlowers. Flowers with the pin morphology have atall style and relatively short stamens, while flow-ers with thrum morphology have a short style andlong stamens. Insect-mediated pollen transfer re-sults in matings between pin and thrum but not be-tween two pin or two thrum plants. The result is

nonrandom negative assortative mating. Unlike in-breeding, negative assortative mating tends to in-crease the level of heterozygosity in a population, atleast for those traits that are involved in mate choice(such as relative style length).

Positive assortative mating, like inbreeding, re-sults in increased homozygosity and decreasedheterozygosity. Positive assortative mating forflowering time, for example, is common in manyplant populations because individuals that flowerearly in the season will tend to mate with otherearly-flowering individuals.

Cindy Bennington

See also: Angiosperm life cycle; Flower types; Ge-netics: mutations; Genetics: post-Mendelian; Hardy-Weinberg theorem; Hybridization; Pollination; Pop-ulation genetics; Reproduction in plants; Speciesand speciation.

Sources for Further StudyAttenborough, D. The Private Life of Plants. Princeton, N.J.: Princeton University Press, 1995.

Describes self-fertilization and plant adaptations that promote outcrossing. Includescolor photographs.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. This textbook describes effects of inbreeding and mecha-nisms plants employ to promote outbreeding.

Willson, M. F. Plant Reproductive Ecology. New York: John Wiley and Sons, 1983. Coverage ofthe evolution of sex and mating systems in plants. Topics include inbreeding, heterostyly,dioecy, and self-incompatibility.

NORTH AMERICAN AGRICULTURE

Categories: Agriculture; economic botany and plant uses; food; world regions

North American crops include grains, legumes, fruits, vegetables, and plants for clothing and other nonfood uses. A dis-cussion of modern agriculture in North America cannot be complete without some attention to agribusiness, the systemof businesses associated with agricultural production in an industrialized society.

In North America, agriculture generally has be-come mechanized and heavily dependent upon

an integrated system of supporting agribusinesses,although traditional practices continue in Mexico.In the United States and Canada, most farmersand ranchers depend heavily upon technology, al-though groups such as the Amish have rejected au-tomation and continue to use animal power for

traction. Most farmers practice monoculture, relyingupon a single crop for their primary income, andhave expanded to very large acreages in order totake advantage of economies of scale. Such farmsare referred to in terms of the primary crop, for ex-ample, a dairy farm, a cattle ranch, or a wheat farm.Some small farms are run by part-time farmers whoalso have other occupations.

714 • North American agriculture

Denver

Oklahoma City

Los Angeles

San Diego

San Francisco

Miami

New York

Boston

Philadelphia

Quebec

Montreal

Seattle

New OrleansWaco

Monterrey

Minneapolis

Detroit

Toronto

Chicago

Vancouver

Oaxaca

Guadalajara

Washington, D.C.

MexicoCity

Ottawa

CANADA

GREENLAND

UNITED STATES

MEXICO

A t l a n t i c

O c e a n

C a r i b b e a n S e a

P a c i f i c

O c e a n

G u l f

o f

M e x i c o

Fish Fish

TimberTimber

Potatoes

Timber

SpringWheat

Apples

Apples

Pears

Pears

Cotton

Citrus

Citrus

Sheep

Fruitsand

Vege-tables

Beef Cattle

OrangesCatfish

Sugar CaneRice

Tobacco

DairyCattle

WinterWheat Corn

Corn

Coffee

Oil Seeds

FlaxSoybeans

Hogs

Selected Agricultural Products of North America

Within the United States, there were 2,192,000farms, cultivating a total of 954,000,000 acres, in the1990’s. These farms produced net returns of $44.1billion. Although farmers represented less than 2percent of the U.S. population, they successfullyfed the country at a high standard of living, pro-duced grain and other products for export, and stillmaintained a surplus carryover of as much as 2 per-cent of the total grown.

Regional Crops and CultivationModern farming techniques in the United States

and Canada require specialization in a single cashcrop. Such specialized farms tend to cluster by re-gion, where the climate and soil quality are appro-priate to a given crop. The supporting agribusi-nesses—such as suppliers of implements, chemicalfertilizers and pesticides, and grain elevators—tendto specialize in products and activities that supportthe primary crops of their given area.

Wheat, the most important cereal grain in West-ern diets, grows in the broad, open lands of theGreat Plains, in Kansas, Nebraska, North and SouthDakota, and the Canadian provinces of Alberta andSaskatchewan.

In the southern part of this region, the primarycrop is winter wheat, which is planted in the fall, isdormant during the winter, completes its growth inspring, and is harvested in midsummer. In many ofthese areas, a farmer then can plant a crop of soy-beans, a practice known as double-cropping. The soy-beans often can be harvested in time to plant the fol-lowing year’s wheat crop in the fall. Farther to thenorth, where the weather is too harsh for wheat tosurvive the winter, farmers plant spring wheat,which completes its entire growth during thespring and summer and is harvested in the fall.Wheat is used to make bread, pasta, and manybreakfast cereals and is an ingredient in numerousother products.

Corn, which originally was domesticated byAmerican Indians, is the best producer per acre. Itrequires a longer growing season than wheat, so ar-eas where it can be grown economically are limited.The Midwestern states—Iowa, Illinois, Indiana,Ohio—are the principal areas for cultivation of cornand frequently are referred to as the Corn Beltstates. Much corn is used as livestock feed, al-though a considerable amount is processed into hu-man foods as well, often in the form of cornstarchand corn-syrup sweeteners.

Rice requires flooded fields for successful culti-vation, so it can be grown only in areas such as Lou-isiana, where large amounts of water are readilyavailable. Because labor costs are the primary lim-iter in U.S. agriculture, American rice growers usehighly mechanized, single-field growing tech-niques rather than the labor-intensive transplanta-tion technique used in Asian countries. Laser levelsand computerized controls tied into the Global Po-sitioning System enable farmers to prepare smoothfields with a slight slope for efficient flooding anddrainage. Because the ground is usually wet duringtilling and harvesting, the machinery typicallyused in growing rice is fitted with tracks instead ofwheels to reduce soil compaction.

Rye, oats, and barley are other major grain crops,although none form the backbone of an area’s econ-omy to the extent that wheat, corn, and rice do.Oats, once a staple feed grain for horses, now isused mainly for breakfast cereals, while most bar-ley is malted for brewing beer. Rye typically is usedin the production of specialty breads.

Legumes, such as soybeans and alfalfa, form thenext major group of crops produced in North Amer-ica. In addition to being an important source of pro-tein in human and livestock diets, legumes are im-portant in maintaining soil fertility. Nodules on theirroots contain bacteria that help to transform nitro-gen in the soil into compounds that plants can use.Because of this, soybeans have also become a regularrotation crop with corn in much of the U.S. Midwest.

Both corn and soybeans can be grown with thesame machinery and sold to the same markets, al-though harvesting corn requires a specialized corn-header that pulls down the stalks and breaks loosethe cob on which the corn kernels grow, rather thanthe generalized grain platform used with soybeansand small grains. Soybeans for human consump-tion generally are heavily processed and becomefiller in other foods, although there is a market fortofu (bean curd) and other soybean products.

Other crops include edible oil seeds, such as sun-flower seeds and safflower seeds, which are gener-ally grown as rotation crops with corn or wheat.Sugarcane is grown in Louisiana and other areas onthe coast of the Gulf of Mexico that have the neces-sary subtropical climate. Many varieties of fruitsand vegetables are grown in California’s irrigatedvalleys; Florida grows much of the United States’juice oranges. Other citrus crops are grown in Ala-bama, Mississippi, and Texas, where these warmth-

716 • North American agriculture

loving trees will not be damagedby frost. Fruits such as apples andpears, which require a cold pe-riod to break dormancy and setfruit, are grown in northern statessuch as Michigan and Washing-ton and the eastern provinces ofCanada.

Fibrous PlantsIn addition to food plants, the

production of fibers for textiles isan important part of Americanagriculture, although such artifi-cial fibers as nylon and polyesterhave taken a share of the market.Cotton and flax also are impor-tant sources of natural fibers. Cot-ton requires a long growing sea-son and relatively high rainfalllevels; therefore, it generally isgrown in an area in the southernUnited States often referred to asthe Cotton Belt. Flax has a shortergrowing season and is oftenplanted in rotation with such small grains as wheatand oats. Flax stems are used to produce linen, andedible oils and meal are obtained from its seeds.

Trees have become a cultivated species, al-though their long growth cycle has limited the abil-ity of humans to create particular varieties. Duringthe twentieth century, concerns about the environ-mental damage done by the clear-cutting of virginforests for lumber and paper encouraged manycompanies to reseed the cut areas with tree speciesthat could be harvested thirty or forty years later.Another form of tree farming, although on a muchsmaller scale, is the production of small evergreensfor Christmas trees.

The Business of FarmingBecause of the intense specialization of modern

mechanized agriculture, farming has become abusiness interlocked with a number of supportingbusinesses. Farm management—the control of cap-ital outlay, production costs, and income—has be-come as vital to a farmer’s economic survival asskill in growing the crops themselves. Such organi-zations as Farm Business/Farm Management helpfarmers develop the skills needed to farm moreproductively and economically.

Farmers also have had to become actively in-volved in the marketing of their crops to ensurean adequate income. In many areas of the UnitedStates and Canada, farmers have banded togetherin cooperatives to gain economic leverage in buy-ing supplies and selling their products. Some ofthese cooperatives have taken on some of the pre-liminary and intermediate steps in transformingthe raw farm products into consumer goods, thusincreasing the prices farmers receive from buyers.

Modern farmers also are concerned with themanagement of the resources that support agricul-ture. In earlier generations, it often was assumedthat natural resources were unlimited and couldbe used and abused without consequence. The re-sult of this ignorance was ecological destructionsuch as the Dust Bowl of the 1930’s, in which thetopsoil over large areas of U.S. Plains States dried upand blew away in the wind, rendering the landunfarmable. To prevent more disasters and theeconomic dislocation they produced, various soilconservation measures were introduced throughgovernment programs that gave farmers financialincentives to change their practices. The use ofcontour plowing and terracing on steeply slopinghillsides helped to slow the movement of water that

North American agriculture • 717

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could carry away soil, thus preventing the forma-tion of gulleys. Reduced tillage techniques allowedmore plant residue to remain on the surface of thesoil, protecting it from the ravages of both wind andwater.

Leigh Husband Kimmel

See also: African agriculture; Agriculture: modernproblems; Asian agriculture; Australian agricul-ture; Central American agriculture; Corn; Erosionand erosion control; European agriculture; Grazingand overgrazing; Green Revolution; High-yieldcrops; Legumes; Monoculture; North Americanflora; Rice; South American agriculture; Wheat.

Sources for Further StudyBonnano, Alessandro. From Columbus to ConAgra: The Globalization of Agriculture and Food.

Lawrence: University Press of Kansas, 1994. Discusses the history of the food industry,agricultural businesses, and international trade.

Danborn, David B. Born in the Country: A History of Rural America. Baltimore: Johns HopkinsUniversity Press, 1995. Integrates social, political, and economic history, focusing on farmpeople rather than residents of small towns. Treats agriculture, rural culture and society,and agrarian policy as an inseparable web.

Fussell, Betty Harper. Story of Corn. New York: Alfred A. Knopf, 1992. An account of the his-tory of corn in the New World. Blends history and myth, science and art, anecdote and im-age, personal narrative and epic to tell the story of this amazing crop and the people whofor centuries have planted, eaten, worshiped, processed, and profited from it.

Goreham, Gary, et al., eds. Encyclopedia of Rural America. Santa Barbara, Calif.: ABC-Clio,1997. Covers a broad range of topics, such as agriculture, economics, the environment,health, and political and social science.

Hassanien, Neva. Changing the Way America Farms: Knowledge and Community in the Sustain-able Agriculture Movement. Omaha: University of Nebraska Press, 1999. Describes two or-ganizations, the Ocooch Grazers Network and the Wisconsin Women’s SustainableFarming Network, to trace how some farmers are developing their own collective localknowledge as a basis for moving toward an ecologically, economically, and sociallysound agricultural system.

NORTH AMERICAN FLORA

Category: World regions

The world’s major biomes are all represented in the diverse vegetation of North America, from Arctic tundra in the northto deserts in the Southeast and the grasslands, wetlands, and various forest biomes between.

Forest is the native vegetation of almost half ofmainland Canada and the United States. Before

European settlement, forestlands dominated theeastern, and much of the northern, part of the conti-nent. Grasses covered a large part of the continentalinterior. Desert vegetation is native in the South-west, tundra in the far north. Over much of the con-tinent, however, human activity has virtually elimi-nated native vegetation.

Coniferous Forests of the WestAlong the Pacific coast from Alaska to Northern

California, evergreen coniferous forest grows luxuri-antly, watered by moisture-laden winds blowingfrom the ocean. This lowland forest includes someof the largest and longest-lived trees in the world.North of California, characteristic trees include Sitkaspruce, western hemlock, and western red cedar.Douglas fir, one of the major timber species in

718 • North American flora

North America, is also common. The northwestcoastal coniferous forest is sometimes called tem-perate rain forest because, in its lushness, it resem-bles the tropical rain forests. Many of the trees of thecoastal forest have been cut for timber.

In California, the dominant coastal conifer spe-cies is the coast redwood. The tallest tree in theworld, coast redwood reaches 330 feet (100 meters)and can live two thousand years. Coniferous forestalso grows along the Cascade Mountains and theSierra Nevada. Trees of the Cascades include moun-tain hemlock and subalpine fir at high elevationsand western hemlock, western red cedar, and firssomewhat lower. Sierra Nevada forests includepines, mountain hemlock, and red fir at high eleva-tions; and red and white firs, pines, and Douglas firsomewhat lower. Ponderosa pine is dominant atlow mountain elevations in both of these Pacificranges.

The giant sequoia, long thought to be the largestliving organism on earth, grows in scattered grovesin the Sierra Nevada. (The largest organism actu-ally may be a very old tree root-rot fungus thatcovers 1,500 acres in Washington State.) Althoughshorter than the coast redwood, the giant sequoia islarger in trunk diameter and bulk. It can reach 260feet (80 meters) tall and 30 feet (10 meters) in cir-cumference.

Coniferous forest also dominates the RockyMountains and some mountainous areas of Mex-ico. In the Rockies, Englemann spruce and subal-pine fir grow at high elevations, and Douglas fir,lodgepole pine, and white fir somewhat lower.Ponderosa pine grows throughout the Rockies atlow elevations and is a dominant tree in westernNorth America.

Boreal Coniferous ForestJust south of the Arctic tundra in North America

is a broad belt of boreal, coniferous, evergreen forest. Itis often called taiga, the Russian name for similar co-niferous forests growing in northern Eurasia. How-ever, in large areas of northeastern Siberia, the dom-inant tree is larch, which is deciduous, whereas theNorth American taiga is mostly evergreen. Taigais the most extensive coniferous forest in NorthAmerica, covering nearly 30 percent of the landarea north of Mexico. It grows across Alaska andCanada and southward into the northern GreatLakes states and New England. White spruce andbalsam fir dominate much of the Canadian taiga.

Eastern Deciduous ForestAforest of mainly broad-leaved, deciduous trees

is the native vegetation of much of eastern NorthAmerica. Narrow fingers of this forest, growingalong rivers, penetrate westward into the interiorgrasslands. Early settlers from Europe cut most ofthe eastern forest, but second-growth forest nowcovers considerable areas. The plants are closely re-lated to plant species of the temperate deciduousforests in Europe and Asia. In contrast, the plants ofother biomes in North America are generally notclosely related to the plants that occur in the samebiomes elsewhere in the world, although they looksimilar.

In the eastern deciduous forest, maple and oakare widespread—maples especially in the north,oaks in the south. There are major subdivisionswithin the forest. These include oak and hickoryforests in Illinois, Missouri, Arkansas, easternTexas, and also in the east—Pennsylvania, Virginia,and West Virginia—where oak and chestnut forestformerly predominated; beech and maple forest inMichigan, Indiana, and Ohio; and maple and bass-wood forest in Wisconsin and Minnesota. The for-est in parts of Michigan, Wisconsin, Minnesota, andNew England contains not only deciduous treesbut also evergreen conifers, including pines andhemlock. Vast native pine stands in the Great Lakesstates have been cut for lumber.

Plant diseases have changed the composition ofthe eastern forest. American chestnut was once animportant tree but has now nearly disappeared as aresult of an introduced fungal disease. Dutch elmdisease is similarly devastating American elms.

Other ForestsThe southeastern United States, excluding the

peninsula of Florida, once supported open standsof pine and also mixed evergreen and decidu-ous forest. The mixed forest included a variety ofpines, evergreen oaks, and deciduous trees. Inmuch of Florida, the native vegetation is a mix-ture of deciduous and evergreen trees that are sub-tropical rather than temperate. In many parts ofthe Southeast, people have replaced the nativevegetation with fast-growing species of pines fortimber production. In Mexico, tropical rain for-ests are prominent on the west coast, in the southand east, and in Yucatán. On the south coasts ofMexico and Florida, swamps of mangrove trees arecommon.

North American flora • 719

Central GrasslandsThe central plains of North America, a wide

swath from the Texas coast north to Saskatchewan,Canada, were once a vast grassland, the prairie. Theclimate there is too dry to support trees, exceptalong rivers. From west to east, there is a transitionfrom the more desertlike short-grass prairie (theGreat Plains), through the mixed-grass prairie, tothe moister, richer, tall-grass prairie. This change isrelated to an increase in rainfall from west to east.Grasses shorter than 1.5 feet (0.5 meter) dominatethe short-grass prairie. In the tall-grass prairie, somegrasses grow to more than 10 feet (3 meters). Color-ful wildflowers brighten the prairie landscape.

Grassland soil is the most fertile in North Amer-ica. Instead of wild prairie grasses, this land nowsupports agriculture and the domesticated grassescorn and wheat. The tall-grass prairie, which hadthe best soil in all the grasslands, has been almostentirely converted to growing corn. Much of thegrassland that escaped the plow is now grazed bycattle, which has disturbed the land and aided thespread of invasive, nonnative plants.

Other outlying grasslands occur in western

North America. Between the eastern deciduous for-est and the prairie is savanna, a grassland with scat-tered deciduous trees, mainly oaks. Savanna alsooccurs over much of eastern Mexico and southernFlorida.

Scrub and DesertIn the semiarid and arid West, the natural vege-

tation is grass and shrubs. Over a large part of Cali-fornia, this takes the form of a fire-adapted scrubcommunity called chaparral or Mediterranean scrub,in which evergreen, often spiny shrubs form densethickets. The climate, with rainy, mild winters andhot, dry summers, is like that around the Mediterra-nean Sea, where a similar kind of vegetation, calledmaquis, has evolved. However, chaparral and ma-quis vegetation are not closely related genetically.Humans have greatly altered the chaparral throughovergrazing of livestock and other disturbances.

The North American deserts, which are locatedbetween the Rocky Mountains and the Sierra Ne-vada, cover less than 5 percent of the continent.Shrubs are the predominant vegetation, althoughthere are many species of annuals. Desert plants,

720 • North American floraP

ho

toD

isc

Wildflowers grow along a trail at the Grand Canyon in Arizona.

commonly cacti and other succulents, are sparselydistributed.

In Southern California, Arizona, New Mexico,West Texas, and northwestern Mexico, there arethree distinct deserts. The Sonoran Desert stretchesfrom Southern California to western Arizona andsouth into Mexico. A characteristic plant of theSonoran is the giant saguaro cactus. To the east ofthe Sonoran, in West Texas and New Mexico, is theChihuahuan Desert, where a common plant is theagave, or century plant. North of the SonoranDesert, in southeastern California, southern Ne-vada, and northwestern Arizona, is the MojaveDesert, where the Joshua tree, a tree-like lily, is awell-known plant. It can reach 50 feet (15 meters) inheight. Creosote bush is common in all threedeserts. To the north, the Mojave Desert grades intothe Great Basin Desert, which is a cold desert, largeand bleak. The dominant plant of the Great BasinDesert is big sagebrush. Plant diversity there islower than in the hot American deserts.

Deserts dominated by grasses rather than shrubsonce occurred at high elevations near the Sonoranand Chihuahuan Deserts. Much of this area hasbeen overtaken by desert scrub, including creosotebush, mesquite, and tarbush. Cattle grazing mayhave been a factor in this change. Desert is veryfragile; even one pass with a heavy vehicle causeslasting damage.

TundraTundra vegetation grows to the northern limits

of plant growth, above the Arctic Circle, in Canada.

The flora consists of only about six hundred plantspecies. In contrast, tropical regions that are smallerin area support tens of thousands of plant species.Arctic tundra is dominated by grasses, sedges,mosses, and lichens. Some shrubby plants also growthere. Most tundra plants are perennials. During theshort Arctic growing season, many of these plantsproduce brightly colored flowers. Like desert, tun-dra is exceptionally fragile, and it takes many yearsfor disturbed tundra to recover. Tundra also occurssouthward, on mountaintops, from southern Alaskainto the Rocky Mountains, the Cascades, and the Si-erra Nevada. This alpine tundra grows at elevationstoo high for mountain coniferous forest.

Coastal VegetationAlong the coasts of the Atlantic Ocean and the

Gulf of Mexico, the soil is saturated with water andis very salty. Tides regularly inundate low-lyingvegetation. The plants in these salt marsh areas con-sist mainly of grasses and rushes. Marshes are a vi-tal breeding ground and nursery for fish and shell-fish. They play an important role in absorbing andpurifying water from the land. Coastal marshes arebeing lost to development at a rapid rate.

Jane F. Hill

See also: Arctic tundra; Biological invasions; Bi-omes: types; Forests; Deserts; Grasslands; Graz-ing and overgrazing; Mediterranean scrub; NorthAmerican agriculture; Savannas and deciduoustropical forests; Taiga; Tundra and high-altitudebiomes.

Sources for Further StudyBraun, E. Lucy. Deciduous Forests of Eastern North America. New York: Macmillan, 1950. Re-

print. New York: Free Press, 1985. Botanist Braun’s devotion to land preservation was oneof the pivotal influences in the then-developing field of ecology. This book describes indetail the trees and shrubs in the deciduous forests of Kentucky, Tennessee, Ohio, Vir-ginia, West Virginia, and Pennsylvania.

Foster, David R. New England Forests Through Time: Insights from the Harvard Dioramas. Cam-bridge, Mass.: Harvard University Press, 2000. Today, in many ways, the New Englandregion is more natural than at any time since the American Revolution. Historical and en-vironmental lessons in this book are told through world-renowned dioramas in HarvardUniversity’s Fisher Museum.

MacMahon, James A. Deserts. The Audubon Society Nature Guides. New York: RandomHouse, 1998. Desert ecology, with life histories of characteristic flora and fauna, rangemaps, and illustrations.

Randall, John M., and Janet Marinelli, eds. Invasive Plants: Weeds of the Global Garden. Brooklyn,N.Y.: Brooklyn Botanic Garden, 1996. Refreshingly simple explanation of environmentalimpacts and how to control the problem of invasive species. With excellent illustrations.

North American flora • 721

NUCLEAR ENVELOPE

Categories: Anatomy; cellular biology; physiology; transport mechanisms

The nuclear envelope is the outer covering of the nucleus in plant and other eukaryotic cells that acts as a barrier separat-ing the nuclear contents from the surrounding cytoplasm.

The nuclear envelope is a double membrane sys-tem, consisting of two concentric membranes.

The membranes are separated by a fluid-filledspace called the perinuclear cisterna that measuresabout 20 to 40 nanometers. Like other plant cellmembranes, the nuclear envelope consists of twobilayers, both made of phospholipids, in which nu-merous proteins are embedded. Attachment sitesfor protein filaments are stitched on the innermostsurface of the nuclear envelope. These protein fila-ments anchor the molecules of deoxyribonucleicacid (DNA) to the envelope and help to keep themorganized. The network of filaments that enmeshthe nuclear envelope provides stability.

Large numbers of ribosomes are located on theouter surface of the envelope. The outermost mem-brane is continuous with the organelle called roughendoplasmic reticulum (RER) in the cytoplasm of theplant cell, which also has ribosomes attached to it.The space between the outer and inner membranesis also continuous with the rough endoplasmic retic-ulum space and can fill with newly synthesized pro-teins, just as the RER does. When the nuclear enve-lope breaks down during cell division, its fragmentsare similar to portions of the endoplasmic reticulum.This can be commonly observed at the root andshoot apices of a plant, where active mitosis (regu-lar cell division for growth) occurs. Both bilayers ofthe envelope are fused at intervals to form manynuclear pores, which consist of protein complexesin clusters. The two membranes enclose a flattenedsac and are connected at the nuclear pore sites.

FunctionsThe nuclear envelope surrounds the fluid por-

tion of the nucleus, called the nucleoplasm, in allplant cells. The ribosomes on the outer surface ofthe envelope serve as sites for protein synthesis inaddition to ribosomes located in the cytoplasm. The

envelope is selectively permeable and thereforeregulates the passage of materials and energy be-tween the nucleoplasm and the cytoplasm. The en-velope allows certain cell activities to be localizedwithin the nucleus or outside in the cytoplasm. Italso permits many different activities to go on si-multaneously within and outside of the nucleus.

Like other membranes of the plant cell, the nu-clear envelope membranes serve as importantwork surfaces for many chemical reactions inplants that are carried out by enzymes bound to themembranes. These functions are essential in theplant for the transport and use of minerals andwater from the roots to cells of the stem and leaves.They are also important for the movement and useof carbohydrates, proteins, lipids, nucleic acids,and other chemical compounds in plant cells afterphotosynthesis, protein synthesis, and other bio-chemical activities have occurred.

Nuclear PoresNuclear pores, of about 100 nanometers in diame-

ter, perforate the nuclear envelope. These poreslook like wheels with eight spokes when observedfrom the top. Each contains eight subunits over theregion where the inner and outer membranes join.They form a ring of subunits that are 15 to 20 nano-meters in diameter. At the lip of each pore, the innerand outer membranes of the nuclear envelope arefused.

Each nuclear pore serves as a water-filled chan-nel, and the arrangement allows transport in andout of the nucleus to occur in several ways. The nu-clear pores allow the passage of materials to the cy-toplasm from the interior of the nucleus, and viceversa, but the process is highly selective, permittingonly specific molecules to pass through these open-ings. An intricate protein structure called the porecomplex lines each pore and regulates the entry and

722 • Nuclear envelope

exit of certain large macromolecules and particles.Large molecules, including the subunits of ribo-somes, cross the bilayers at the pores in highly con-trolled ways. Ions and small, water-soluble mole-cules cross the nuclear envelope at the pores.

Studies show that the pore can actually dilatemore when it gets the appropriate signal. Studieshave also shown that the signal is in the peptide se-quences of the molecules. These signals are recogni-tion sequences rich in the amino acids lysine,arginine, and proline. Nuclear pores in plantstherefore exert control over the movement of mate-rials. This is, for example, demonstrated in the factthat if a nucleus is extracted from a cell and placedinto water, it swells; this can happen only if thepores prevent material from oozing out as the nu-cleus absorbs water.

Nuclear LaminaThe nuclear lamina is a layer of specific proteins,

called lamins, attached to inside membrane of the

nuclear envelope. The layer consists of thin fila-ments (intermediate filaments) that are 30 to 40nanometers thick. Each filament is a polymer oflamin. There are two types of lamin: A-type laminsare inside, next to the nucleoplasm, and B-typelamins are near the inner part of the nuclear mem-brane. The nuclear lamina surrounds the nucleus,except at the nuclear pores. The lamina serves as askeletal framework for the nucleus. It may be in-volved in the functional organization of the nucleusand may also play an important role in the break-down and reassembly of the nuclear envelope dur-ing mitosis.

Samuel V. A. Kisseadoo

See also: Cell cycle; Cell wall; Cytoskeleton; Cyto-sol; Endoplasmic reticulum; Eukaryotic cells; Liquidtransport systems; Membrane structure; Mitosis andmeiosis; Nucleus; Plasma membranes; Water andsolute movement in plants.

Sources for Further StudyCampbell, Neil A., and Jane B. Reece. Biology. 6th ed. New York: Benjamin Cummings, 2002.

Basic textbook has clear definition and description of the nuclear envelope and its compo-nents. Includes illustrations, pictures, glossary, bibliography, and index.

Fawcett, Don W. A Textbook of Histology. 12th ed. New York: Chapman & Hall, 1994. Good ac-count of structure and function of the pores of the nuclear envelope. Includes illustrationsand pictures.

Mauseth, James D. Botany. Boston: Jones and Bartlett, 1998. Throws light on the functions ofthe nuclear envelope in plants. Includes photographs, illustrations, glossary, and index.

Solomon, Eldra P., Linda Berg, and Diana Martin. Biology. 6th ed. Pacific Grove, Calif.:Brooks/Cole, 2002. Focuses on functions of the membranes and gives general structure ofthe nuclear envelope. Includes review questions, glossary, bibliography, and index.

Starr, Cecie. Biology. 4th ed. Pacific Grove, Calif.: Brooks/Cole, 2002. Focuses on structure ofthe nuclear envelope and transport across the membranes and pores. Includes good pho-tographs, illustrations, micrographs, glossary, bibliography, and index.

NUCLEIC ACIDS

Categories: Cellular biology; genetics; reproduction and life cycles

Nucleic acids are the genetic material of cells, including DNA and the various types of RNA.

Nucleic acids were discovered in the mid-nineteenth century, but their role as genetic

material was not substantiated until the mid-

twentieth century. When chromosomes were dis-covered at the beginning of the twentieth century,they were quickly identified as the genetic material

Nucleic acids • 723

of the cell. Chromosomes were found to be com-posed of nucleic acids and proteins. Through theexperiments of Fred Griffith on transformation inpneumonia bacteria and the work of Alfred Hersheyand Martha Chase on bacteriophages, by 1952 mostbiologists recognized deoxyribonucleic acid (DNA)as containing the genes. James Watson and FrancisCrick provided the capstone to science’s initial un-derstanding of nucleic acids when they determinedthe double helix structure of DNA in 1953.

Heredity is the process by which the physicaltraits of an organism are passed on to its offspring.At the molecular level, DNA contains the informa-tion necessary for the transmission of genetic char-acteristics from one generation to the next, as wellas the information required for the new organismsto grow and to live. DNAis the chemical basis of he-redity and provides the synthesis of new proteins,such as enzymes.

DNA and RNAThere are two types of nucleic acids within cells,

the single-stranded ribonucleic acid (RNA) and thedouble-stranded DNA. Each kind has specificroles. DNAwas isolated in 1869 by German chemistFriedrich Miescher. The substance that Miescherfound was white, sugary, and slightly acidic, and itcontained phosphorus. Because it occurred onlywithin the nuclei of cells, he called it “nuclein.” Thename was later changed to deoxyribonucleic acid,to distinguish it from ribonucleic acid, which is alsofound in cells.

In eukaryotic cells, DNAis present in the chromo-somes of the nucleus and within the mitochondriaand chloroplasts. Bacteria, yeasts, and molds, in ad-dition to the chromosomes, contain circular strandsof DNA, called plasmids, within the cytoplasm oftheir cells. Plasmids are relatively small, circularstrands of DNA that exist independently of thechromosome. Plasmids typically have only twenty-five or thirty genes, which are not essential to thehost cell but often confer antibiotic resistance, theability to pass DNA to other bacterial cells, andother useful functions. Some plasmids are onlyfound as single copies, whereas others occur asmany copies. The mitochondria and chloroplasts ofeukaryotic cells are self-replicating and contain atiny circular chromosome (DNA) resembling aplasmid of a bacterium.

Viruses (minute parasites that infect specifichosts) contain only one type of nucleic acid—either

DNA or RNA, never both, and the DNA or RNAcan be single- or double-stranded. The DNA ofsome viruses can integrate into the DNA of the hostcell. In this state, the viral DNA replicates as thehost DNA replicates. The genetic apparatus of avirus, whether RNA or DNA, is much the same asthat of bacteria but is far less complex. Even largeviruses (such as the pox virus) have only a few hun-dred genes. Smaller viruses (such as the polio virus)have considerably fewer.

Chemical StructureBoth DNA and RNA are long-chained polymers

made up of nucleotides. The nucleotides, in turn,are made up of a nitrogenous base, a ribose ordeoxyribose sugar, and a phosphate. All the basesof DNA and RNA are heterocyclic amines. Two, ad-enine and guanine, are called purines; the otherthree, cytosine, thymine, and uracil, are called py-rimidines. The two purines and one of the pyrimi-dines, cytosine, occur in both RNA and DNA. Ura-cil is found only in RNA, while thymine occurs onlyin DNA.

The purines are nine-membered heterocyclicrings with nitrogen occurring in place of carbon atseveral positions. Adenine and guanine differ inthe functional groups attached to them. The pyrimi-dines are six-membered heterocyclic rings, with ni-trogen in place of two of the carbon atoms. Like thepurines, the three pyrimidines also differ in the spe-cific functional groups attached to them.

The ribose sugars are made up of a five-mem-bered heterocyclic ring containing one oxygenatom between carbons one and four. A fifth carbon(number five) is not part of the ring and is bondedto carbon number four. Along with hydrogen at-oms attached to each carbon, there is one hydroxylgroup (OH) attached to each of the four hetero-cyclic carbon atoms in the ribose sugar of RNA.(The fifth carbon has a phosphate group attached toit.) The sugar of DNA is called D-deoxyribose be-cause a hydroxyl group is missing from the sec-ond carbon, having been replaced by a hydrogenatom—thus the name deoxyribonucleic acid.

The sugar-base combination is called a nucle-oside. The purines are linked to carbon one of thesugars with the nitrogen at position one. The nu-cleoside of guanine and ribose is guanosine; it isadenosine for adenine and D-ribose. The pyrimi-dines of RNA, when attached to ribose, are uridineand cytosine. In DNA the nucleoside names are

724 • Nucleic acids

deoxyadenine, deoxyguanosine, deoxythymidine,and deoxycytidine.

Nucleotides are phosphate esters of nucleosides.In these molecules, a phosphate group (phosphoricacid) is attached to carbon five (called the 5′ carbon)of the sugar (ribose or deoxyribose) in the nu-cleoside. Nucleotides are named by combining thename of their nucleoside with a word describingthe numbers of phosphates attached to it. Gua-nosine monophosphate, for example, is the name ofthe phosphate ester of guanosine, which is oftenabbreviated as GMP.

Individual nucleotides also occur in cells. Thesefree nucleotides usually exist as diphosphates ortriphosphates. Examples of these are adenosinediphosphate (ADP) and adenosine triphosphate(ATP). ATP is the universal energy source for theanabolic processes of all cells, including the forma-tion of the DNA and RNA.

DNA Structure and FunctionDNA can be an extremely long molecule that is

tightly wound within the nuclei of eukaryotic cellsand within the cytoplasm of prokaryotic cells. Nu-clear DNA is linear, whereas prokaryotic DNA iscircular. (If the DNA in a human cell could bestretched out, it would measure roughly 2 meters,or 6 feet long; bacterial DNA would be about 1.5millimeters long, or just over 0.5 inch.) Using a typi-cal lily as a point of reference, DNAis packaged intotwenty-four chromosomes, twelve of which arecontributed by the pollen and twelve by the egg.Every cell derived from the fertilized egg (zygote)will have exactly the same amount of DNAcontain-ing exactly the same genetic information. Withinthe cytoplasm, several mitochondria (the sites ofrespiration) and chloroplasts (the sites of photosyn-thesis) are found, both of which contain their ownDNA, which is circular and resembles prokaryoticDNA in many respects.

DNA is a double-stranded spiral; its shape iscalled the double helix. Structurally, it may be com-pared to a ladder, with the rails or sides of the lad-der consisting of alternating deoxyribose sugar andphosphate molecules connected by phosphodiesterbonds between the 5′ carbon of one sugar and the 3′carbon of the other. The rungs of the ladder consistof purine (adenine and guanine, often abbreviatedas A and G, respectively) and pyrimidine (cytosineand thymine, often abbreviated as C and T, respec-tively) building blocks from the opposite strands,

held together by hydrogen bonds. The buildingblocks pair with each other consistently in what arecalled complementary pairs: Adenine always pairswith thymine with two hydrogen bonds, and cyto-sine always pairs with guanine with three hydro-gen bonds. Consequently, the attraction betweencytosine and guanine is stronger than that betweenadenine and thymine.

Because of this arrangement, the sequence of thepurine and pyrimidine building blocks on onestrand is complemented by the sequence of build-ing blocks on the other strand. The specificity of thebase pairing between the two strands allowsstrands to fit neatly together only when such pair-ing exists. Each DNA strand has a 5′ end with ahydroxyl group attached to the 3′ carbon of adeoxyribose sugar. When connected, the twostrands are actually in an opposite orientation andare referred to as being antiparallel. This is best ob-served by looking at one end of the double-stranded molecule. One strand terminates with a 5′phosphate group and the other with a 3′ hydroxylgroup.

The specific nucleotide composition in a speciesis essentially constant but can vary considerablyamong organisms. Regardless, the amounts of ade-nine and thymine are always the same, as are theamounts of guanine and cytosine because of the re-quired complementary pairing. Due to the greaterstrength of G-C bonds, organisms with a high GCcontent have DNA that must be heated to a highertemperature to denature, or separate, the strands.Some bacteria that live in hot springs have an espe-cially high GC content.

The instructions contained within the DNAmol-ecules occur in segments called genes. Most genesinstruct the cell about what kind of polypeptide (mol-ecule composed of amino acids used to make func-tional proteins) to manufacture. These polypep-tides lead to the formation of enzymes and otherproteins necessary for survival of the cell. Othergenes are important in coding for the production ofantibodies, RNA, and hormones.

RNA TypesThe DNA acts as a template to make three kinds

of RNA: messenger RNA (mRNA), transfer RNA(tRNA), and ribosomal RNA (rRNA). Each kind ofRNA has a specific function. RNA is not found inchromosomes and is located elsewhere in the nu-cleus and in the cytoplasm.

Nucleic acids • 725

The largest and most abundant RNA is rRNA.Between 60 and 80 percent of the total RNA in cellsis rRNA, and it has a molecular weight of severalmillion atomic mass units. The rRNAcombines withproteins to form ribosomes, which are the sites for thesynthesis of new protein molecules. About 60 per-cent of the ribosome is rRNA, and the rest is pro-tein. Although single-stranded, rRNA moleculesfold into specific functional shapes that involve thepairing of portions of the molecule to form double-stranded regions. The precise shape of rRNAs is im-portant for their function, and some of them actu-ally have catalytic properties, just as enzymes do.RNAs of this type are sometimes called ribozymes.

Molecules of mRNA carry the genetic informa-tion from DNA to the ribosomes. The process ofconverting the DNAcode of a gene into an mRNAiscalled transcription. When attached to the ribo-somes, mRNAs direct protein synthesis in a processcalled translation. The size of the mRNA moleculedepends upon the size of the protein molecule to bemade. In prokaryotes (such as bacterial cells), aswell as in mitochondria and chloroplasts, mRNAsare ready to take part in translation even while tran-scription of the remainder of the mRNA is takingplace. In eukaryotes (cells of most other forms oflife), the mRNAs transcribed from nuclear DNAareinitially much larger than they are later, when theyparticipate in translation. These mRNAs must beprocessed to removed large pieces of noncodingRNA, called introns, and to modify both ends of themRNA in specific ways. After introns are removed,the remaining codon regions, called exons, arespliced together by splicesomes (a complex systemcomposed of proteins and small RNAs). Once allthe modifications are complete, the mRNA is readyto be translated. A small number of mRNAs aretranslated in the nucleus, but most are transportedto the cytoplasm first.

The smallest of the three main kinds of RNA istRNA. Each of the tRNA molecules consists ofabout one hundred nucleotides in a single chainthat loops back upon itself in three places, formingdouble-stranded regions that result in a structurethat, when viewed in two dimensions, could becompared to a cross or clover leaf. The function oftRNA is to bring amino acids to the ribosomes to beused in the formation of new proteins. Each of thetwenty amino acids found in proteins has at leastone particular tRNA molecule to carry it to the siteof protein synthesis.

The cloverleaf shape of the tRNA molecule ismaintained by hydrogen bonds between basepairs. The other parts of the molecule that do nothave hydrogen-bonded base pairs exist as loops.Two parts of every tRNA molecule have significantbiological functions. The first is the place where thespecific amino acid to be transferred is attached.This is located at the longest free end of the three-looped structure, often called the stem, where it isspecifically attached to an adenine of an adeninemonophosphate nucleotide. The second importantsite is the loop at the opposite end of the moleculefrom the stem. This loop contains a specific three-base sequence that represents a code for the aminoacid that is being transferred by the tRNA. Thisthree-base sequence is called an anticodon andplays an important role in helping place the aminoacid in the correct position in the protein moleculeunder construction.

Jon P. Shoemaker, updated by Bryan Ness

See also: Chloroplast DNA; Chromosomes; DNAin plants; DNA replication; Extranuclear inheri-tance; Gene regulation; Genetic code; Genetic equi-librium: linkage; Mitochondrial DNA; Proteins andamino acids; RNA.

Sources for Further StudyBettelheim, Frederick A., William H. Brown, and Jerry March. Introduction to General, Or-

ganic, and Biochemistry. 6th ed. New York: Saunders College Publishing, 2001. Excellent il-lustrations of the nucleic acids, along with a good historical background.

Blackburn, G. Michael, and Michael J. Gait, eds. Nucleic Acids in Chemistry and Biology. 2d ed.New York: Oxford University Press, 1996. Discusses the impact of sophistication of nu-cleic acids synthesis on the biotechnology industry.

Diamond, R., et al., eds. Molecular Structures in Biology. New York: Oxford University Press,1993. Authoritative surveys of techniques used in structure determination, modelingstudies, and specific groups of biomacromolecules.

Hecht, Sidney M., ed. Bioorganic Chemistry: Nucleic Acids. New York: Oxford University

726 • Nucleic acids

Press, 1996. Part of the Topics in Bioorganic and Biological Chemistry series; includesmethods of preparation, mapping, analysis, binding, and catalysis of nucleic acids.

Prescott, Lansing M., John P. Harley, and Donald A. Klein. Microbiology. 5th ed. Boston:McGraw-Hill, 2002. Atextbook that contains excellent illustrations and good discussionsof nucleic acids.

Seager, Spencer L., and Michael R. Slabaugh. Chemistry for Today: General, Organic, and Biochem-istry. 4th ed. Pacific Grove, Calif.: Brooks/Cole, 2000. Agood presentation of the historicalbackground leading to the discovery of DNA structure; good coverage of nucleic acids.

Watson, James D. Molecular Biology of the Gene. 4th ed. Menlo Park, Calif.: Benjamin/Cummings, 1987. This is a concise, well-written, and well-illustrated popular text.

NUCLEOLUS

Categories: Anatomy; cellular biology; physiology

The most prominent organelle of the eukaryotic cell is the nucleus. Within the nucleus, substructures called nucleoli arevisible by light microscopy, each nucleolus as a small spot.

Generally, nucleoli are most prominent in non-dividing cells, and they are not visible in di-

viding cells in which the nuclear membrane hasdisassembled. The nucleolus is composed of genesthat encode the ribonucleic acid (RNA) moleculesfound in ribosomes (ribosomal RNA, or rRNA), the“working” copies or transcripts of these genes (therRNA itself), and proteins. Nucleoli are the sites ofribosome synthesis in eukaryotic cells.

The ribosome is the site of protein synthesis in thecell. All proteins made by cells are made on ribo-somes, so cells need an abundant supply of ribo-somes to support the synthesis of all the cellularproteins. Mature ribosomes are found in the cyto-plasm of eukaryotic cells, either attached to themembrane of the endoplasmic reticulum or “free”within the cytoplasm. Although ribosomes func-tion outside the nucleus in the cytoplasm, they aresynthesized in the nucleus at the nucleolus.

Nucleolar StructureNucleoli are clearly visible with light micros-

copy. They disassemble and reassemble with thecell cycle and exhibit different staining propertiesthan the rest of the nucleus. They range in diameterfrom 1 micrometer in small yeast cells to 10 microm-eters in larger cells, such as the root cells of peaplants and wheat.

When observed using electron microscopy, nu-

cleoli appear to have roughly three distinct regions:the fibrillar center (FC), the dense fibrillar center(DFC), and the granular component (GC). In someplant cells, the nucleolus also has a centrally locatedclear region, sometimes called the nuclear vacuole.Although the precise molecular structures withinnucleoli have not yet been thoroughly described,the regions of the nucleolus appear to correspondroughly to content and function, rRNA transcrip-tion, processing, and assembly.

rRNA GenesA typical cell contains around one million ribo-

somes. To produce this number of ribosomes, a cellneeds to be able to mass-produce the ribosomalcomponents. To provide enough ribosomes fordaughter cells produced from cell divisions, a cellmust synthesize new ribosomes at the rate of sev-eral hundred per second. Additionally, these ribo-somes must be exported from the nucleus into thecytoplasm, where proteins are synthesized.

In order to mass-produce the RNA componentsof ribosomes, the three rRNA genes that are tran-scribed (copied) in the nucleolus are present in mul-tiple copies. The genes for the 5.8S, 18S, and 28SrRNAs (rRNAs are named for their sedimentationrate, “S,” a measure of their size) are arranged intandem repeats on the chromosomes of the cells.Typically, each repeat contains the 18S, the 5.8S, and

Nucleolus • 727

the 28S genes, in that order, preceded by a regioncalled a nontranscribed spacer (NTS) and separatedby internal nontranscribed spacers. The NTS’s aretypically as small as two thousand to three thousandbase pairs of deoxyribonucleic acid (DNA) in yeastand plants. These spacer regions may be involvedin regulating gene expression in these regions ofDNA or in attaching these areas of the chromo-somes to protein structures inside the nucleolus.

Among different organisms, the number of cop-ies of rRNAgenes varies greatly. In fact, the numberof copies of rRNA genes can even vary within dif-ferent cells of a single organism. The human ge-nome (haploid set) contains approximately onehundred copies of these genes, but many organ-isms, especially plants, have several thousand cop-ies of the rRNAgenes. By having hundreds or thou-sands of copies of the genes, the cells are able tomass-produce the rRNAs that these genes encode.

Nucleolar Organizing RegionsThe genes that encode the rRNAs are found

along metaphase chromosomes at constrictionscalled nucleolar organizing regions, or NORs. Whenthe cell completes division and reenters interphase,NORs are the sites where the nucleoli will form. Inplants, experiments to stain the specific rRNAgenes reveal a complex organization of the genomicDNA. Large areas of inactive or untranscribedrRNA gene repeats appear at the periphery of theplant nucleoli, and several spots containing inac-tive rRNA genes also appear at the center of the nu-cleoli. However, many areas of active transcriptionof a single copy of the rRNA genes appear dis-persed throughout the nucleolus. These areas of ac-tive rRNA gene transcription may correspond tothe fibrillar center regions.

Ribosome Assembly and rRNA ProcessingThe three rRNA genes are initially copied as one

large molecule of RNA, called the pre-rRNA. Thislarge piece of RNA is processed to make the threesmaller 18S, 5.8S, and 28s rRNAs that will become

part of the ribosome. During processing the non-transcribed spacer sequences are removed. Eachpre-rRNA molecule moves away from the genomicmaterial as it is processed in several steps. Pro-cessing of the pre-rRNA molecule appears to re-quire many accessory proteins and other RNA mol-ecules, called small, nucleolar RNAs (snoRNAs).

The nucleolus contains many proteins that func-tion in rRNA transcription and processing as wellas transport of ribosome components into and outof the nucleus. One of these proteins, nucleolin,which is specific to the nucleolus, is found in thedense fibrillar center and appears to be involved indifferent stages of ribosome synthesis. Nucleolintravels between the nucleus and cytoplasm andtherefore may also be involved in transport of ribo-some components between the two cellular com-partments.

For ribosome synthesis, ribosome proteins mustbe assembled with the three nucleolar rRNAs. Theribosomal proteins are synthesized, like other cel-lular proteins, on existing ribosomes in the cyto-plasm. These proteins must be transported into thenucleus, where they associate with the pre-rRNAinthe nucleolus even while it is being processed. Afourth rRNA, the 5S RNA, is transcribed in a sepa-rate part of the nucleus and transported to the nu-cleolus to be assembled with the other ribosomalcomponents.

The ribosome consists of a small subunit and alarge subunit. The small subunit, which containsthe 18s rRNA and many proteins, is produced andexported from the nucleus separately from thelarge subunit, which contains the 28S, 5.8S, and 5SrRNAs. The two subunits continue to mature in thecytoplasm, and matured subunits recognize mes-senger RNA within the cytoplasm to assemble afunctional protein-synthesizing ribosome.

Michele Arduengo

See also: DNA in plants; DNA replication; Nucleicacids; Nucleus; Plant cells: molecular level; Pro-teins and amino acids; RNA.

Sources for Further StudyCooper, Geoffrey M. The Cell: A Molecular Approach. Sunderland, Mass.: Sinauer Associates,

1997. Standard college-level text contains an entire chapter on the nucleus, with consider-able information on the nucleolus and ribosome synthesis.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Introductory botany text provides a chapter introducingeukaryotic cells in general and plant cells specifically.

728 • Nucleolus

NUCLEOPLASM


The nucleoplasm is the protoplasm contained within the plant cell nucleus and can best be described as the fluid-filledmatrix that is contained within the nuclear membrane.

The nucleoplasm is a distinct entity of the plantcell nucleus, bounded, protected, and sepa-

rated from the cytoplasm by the double-membranenuclear envelope. Connections between the cytoplasmand nucleoplasm are closely regulated by nuclearpores that penetrate the nuclear envelope at in-tervals. Interiorly, these nuclear pores connect to acomplex of nucleoplasmic channels that lead intothe pores and serve simultaneously to direct andregulate exchange of materials between the nucleo-plasm and the cytoplasm.

The nucleoplasm consists of a viscous mix ofwater, in which various substances and structuresare dissolved or carried, and an underlying intra-nuclear ultrastructure. Comparisons of the aque-ous phase of nucleoplasm with that of cytoplasmusing a technique called the Stokes-Einstein equa-tion reveal that diffusion rates are 1.2 times slowerthrough cytoplasm than nucleoplasm, indicatingthat nucleoplasm is a less viscous fluid than cyto-plasm. Substances in the nucleoplasm include ions,enzymes, minerals, and some organic moleculesand macromolecules. The nucleoplasm is especiallyrich in protein enzymes and protein constituents in-volved in the synthesis of deoxyribonucleic acid(DNA) and the various types of ribonucleic acid(RNA), the precursor molecules of RNA, and thenucleotides from which they are assembled. Someof these proteins direct initial transcription, whileothers function in the further modification of theRNA molecules for packaging and transport to thecytoplasm.

Prominent structures located within the inter-phase nucleoplasm (the resting cell or the non-replicating cell) include organelles called nucleoliand the unwound DNA, called chromatin. The nu-cleoli resemble miniature nuclei and are the sites ofsynthesis of precursor RNA molecules and theirassembly.

The other major components in nucleoplasm in-clude the DNA chromosomes seen during mitosis.During cell interphase most of the DNA chromo-somes exist as unwound chromatin that extendthrough the nucleoplasm. Two distinct types ofchromatin are recognized. Diffuse, or uncondensed,chromatin is called euchromatin and exists as thinthreads that extend throughout much of the nu-cleoplasm. Small sections of DNA that remain un-condensed during cell interphase are called hetero-chromatin. The chromatin can be further subdividedinto gene-rich R bands concentrated within the nu-cleus and gene-poor G bands found in the periph-eral regions of the nucleus.

The space between these chromatin elements iscalled the interchromatin space. Ultrastructure stud-ies have revealed a surprisingly complex structurewithin this interchromatin space. It consists of sev-eral components, including a nuclear lamina, nu-clear matrix, thousands of highly organized sitescalled foci, and nuclear speckles.

The nuclear lamina and nuclear matrix consist ofprotein microfilaments—fine tubular structures—which apparently provide structural support forthe interior of the nucleus as well as a frameworkfor further nuclear structure and function. Thelamina consists of lamin proteins that help shapeand maintain the inner membrane of the nuclearenvelope. The nuclear matrix functions in pore for-mation and may also be continuous with the nu-clear lamina in conferring an interior structure orshape to the nuclear envelope. The nuclear matrixalso binds chromosomes, or parts of chromosomes,to specific sites on the inner nuclear membrane.

Foci and nuclear speckles are both involved in theproduction of the several types of RNA moleculesmanufactured by the nucleus. Foci are apparentlysites that are functionally associated with gene-richchromatin. Some scientists argue, in fact, that foci

Nucleoplasm • 729

are actually functional domains on active chroma-tin. Whether active chromatin sites or separate enti-ties, the foci contain protein enzymes and assemblycomponents involved in the transcription andsplicing of ribonucleoprotein (RNP) molecules as-sociated with the production of RNA precursor orsubassembly molecules. The production of a spe-cific RNP molecule may involve the activity of hun-dreds or thousands of discrete foci. Once com-pleted, the molecular products of the foci aretransported to nuclear speckles for further modifi-cation.

Nuclear speckles appear to be clusters of RNP.They are highly organized sites that are separatedfrom other nuclear compartments by nucleoplasm.

Their role and importance is still uncertain, butthey are probably nuclear structures directly in-volved in the final assembly and modification ofthe various types of RNA molecules from RNPmolecules supplied by the foci.

Although much of the structure and role of thenucleoplasm remains to be investigated, it is in-creasingly clear that the nucleoplasm has a complexultrastructure underlying and facilitating its myr-iad functions.

Dwight G. Smith

See also: Chromatin; Cytoplasm; Cytosol; Mitosisand meiosis; Nuclear envelope; Nucleic acids; Nu-cleolus; Nucleus; Plant cells: molecular level.

Sources for Further StudyBusch, Harris. The Cell Nucleus. Vol. 1. New York: Academic Press, 1974. This somewhat

older volume provides a very detailed summary of the nuclear interior, including the roleand structure of the nucleoplasm.

Lodish, Harvey, et al. Molecular Cell Biology. 4th ed. New York: W. H. Freeman, 2000. One ofthe definitive books on cell biology. All topics about cells are well and thoroughly cov-ered. This provides a good general introduction to the structure and functioning of allnucleus components in a highly readable style.

Smith, C. A., and E. J. Wood. Cell Biology. 2d ed. New York: Chapman and Hall, 1999. Ad-vanced textbook of cell biology provides a basic but thorough coverage of the latest infor-mation about all aspects of cell biology. Well illustrated and well written for both under-graduate and graduate students.

NUCLEUS


A eukaryotic cell’s nucleus directly or indirectly controls virtually all cellular physiological activities, including initia-tion, regulation, and termination of enzymatic events. It is also the repository of genetic information (the genome), hous-ing and protecting the chromosomes and the genes that they carry. In all eukaryotic cells the nucleus is generally thelargest and most centrally located cell structure, although in plant storage cells certain kinds of vacuoles may be largerand more conspicuous.

The nucleus, from the Greek word nucle (mean-ing “pit” or “kernel”) is the command and con-

trol center of the cell. The six basic functions of thenucleus are, first, to protect and store genes, ul-timately protecting the deoxyribonucleic acid(DNA) on which the genes are organized from therest of the cell; second, to organize genes into chro-mosomes to facilitate their movement and distribu-

tion during cell division; third, to organize the un-coiling of DNA during the copying of genes for theproduction of thousands of proteins; fourth, tomanufacture and transport regulatory molecules,mostly enzymes and other gene products, into thecytoplasm; fifth, to manufacture subunits of ribo-somes; and sixth, to respond to hormones and otherchemical signals received via the nuclear pores.

730 • Nucleus

ComponentsStructurally, the nucleus consists of several dis-

tinct parts: a nuclear envelope, nucleoplasm, chroma-tin, and one or more suborganelles called nucleoli.

The nuclear envelope forms a protective barrierthat isolates the nucleus from the cytoplasm of thecell. The envelope consists of two unit membranes(a double-unit membrane) which are structurallysimilar to other membranes of the cell. The outermembrane is closely associated with the cell’s endo-plasmic recticulum (ER) and may be continuouswith it. Like the rough ER of the cytoplasm, theouter nuclear membrane has ribosomes embeddedin it. Some scientists, in fact, suggest that the nu-clear envelope is just a localized and specializedversion of the ER. The inner nuclear membrane islined with a fibrous layer, called the nuclear lamina,which provides strength and structure to the nu-cleus shape and may also serve as a binding site forsome chromatin.

At intervals, the nuclear envelope is perforatedby small pores which function as communicationchannels for the controlled exchange of materialsbetween the nucleus and the cytoplasm. Collec-tively, the nuclear pores cover about 10 percent of thesurface of the nucleus. Each nuclear pore is a com-plex consisting of a central pore that has been esti-mated at 30-100 nanometers in diameter. The selec-tively permeable nuclear pores function as entryand exit ways for a variety of water-soluble mole-cules, mostly nuclear products, such as ribosomesubunits, messenger RNA (ribonucleic acid) mole-cules, and chromosomal proteins.

The protoplasm within the nucleus is called nu-cleoplasm. Like cytoplasm, it consists of a jellylikemix of substances and organelles but differs in hav-ing a higher concentration of nucleotides and otherorganic molecules that are used in the synthesis ofDNA and RNA.

Major structures within the nucleoplasm in-clude the DNA and usually one organelle—butsometimes several—called the nucleolus. Exceptduring cell division, the molecules of DNAoccur as

a network of unwound fibers called chromatin.During cell division molecular strands of DNA coiland supercoil around histone proteins to condenseand form the chromosomes. The number of chro-mosomes found within the nucleus are specific foreach species of plant and animal. Humans, for ex-ample, have forty-six chromosomes, tobacco hasforty-eight, corn has twenty, carrots have eighteen,and peas have fourteen chromosomes.

The nucleolus is the largest visible organellewithin the nucleus. It is typically associated withspecific regions of chromosomes, called nuclear or-ganizer regions, which contain genes that direct syn-thesis of ribosomal subunits. The main products ofnucleolus activity are the units of ribosomal RNA(rRNA). These subunits are eventually complexedwith ribosomal proteins and transported from thenucleus into the cytoplasm by special carrier pro-teins. Other sites within the nucleus called functionaldomains control the synthesis of messenger (pre-mRNA), transfer (tRNA) molecules. Once formed,these molecules are then complexed with proteinsand transported as nucleoproteins to the cytoplasm.

Cell DivisionAlthough seemingly both stable and durable,

the nucleus disappears from normal view and isreformed during cell division in almost all plantsexcept yeasts, which retain a clearly defined nu-cleus throughout the division process. In other eu-karyotic plant cells the nucleus disappears earlyduring the prophase of mitosis, when the nuclearenvelope is enzymatically fragmented into small,nearly invisible vesicles. These are not reassembleduntil the final events of telophase, when they re-form around the chromosomes and are controlledby the lamina of the daughter cells.

Dwight G. Smith

See also: Chromosomes; Chromatin; Cytoplasm;Cytosol; DNA in plants; DNA replication; Eukary-otic cells; Mitosis and meiosis; Nuclear envelope;Nucleolus; Nucleoplasm.

Sources for Further StudyBecker, W. M., L. J. Kleinsmith, and J. Hardin. The World of the Cell. 4th ed. San Francisco:

Benjamin/Cummings, 2000. This very well-written college-level textbook describes thenucleus within the larger context of the structure and function of the cell.

Ross, M. H., L. J. Romrell, and G. I. Kaye. Histology: A Text and Atlas. 3d ed. Philadelphia: Wil-liams and Wilkins, 1995. A detailed and well organized discussion of the structure of thecell nucleus and its role within the eukaryotic cell.

Nucleus • 731

Strauss, Phyllis R., and Samuel H. Wilson. The Eukaryotic Nucleus: Molecular Biochemistry andMacromolecular Assemblies. Vol. 1. Caldwell, N.J.: Telford Press, 1990. This volume in-cludes seventeen papers which provide an exhaustive and very technical description ofthe structure and function of the nucleus.

Strouboulis, J., and A. P. Wolffe. 1996. “Functional Compartmentalization of the Nucleus.”Journal of Cell Science 109 (1991). This somewhat technical article presents the latest find-ings on the structure and function of the nucleus.

NUTRIENT CYCLING

Categories: Biogeochemical cycles; ecosystems; nutrients and nutrition

Within an ecosystem, nutrients move through biogeochemical cycles. Those cycles involve chemical exchanges of ele-ments among the earth’s atmosphere, water, living organisms, soil, and rocks.

All biogeochemical cycles have a common struc-ture, sharing three basic components: inputs,

internal cycling, and outputs.

Input of NutrientsThe input of nutrients to an ecosystem depends

on the type of biogeochemical cycle. Nutrients witha gaseous cycle, such as carbon and nitrogen, enteran ecosystem from the atmosphere. For example,carbon enters ecosystems almost solely throughphotosynthesis, which converts carbon dioxide toorganic carbon compounds. Nitrogen enters ecosys-tems through a few pathways, including lightning,nitrogen-fixing bacteria, and atmospheric deposi-tion. In agricultural ecosystems, nitrogen fertiliza-tion provides a great amount of nitrogen influx,much larger than by any other influx pathways.

In contrast to carbon and nitrogen with inputfrom the atmosphere, the input of nutrients such ascalcium and phosphorus depends on the weather-ing of rocks and minerals. Soil characteristics andthe process of soil formation have a major influenceon processes involved in nutrient release to recy-cling pools. Supplementary soil nutrients comefrom airborne particles and aerosols, as wet or drydepositions. Such atmospheric deposition can sup-ply more than half of the input of nutrients to someecosystems.

The major sources of nutrients for aquatic eco-systems are inputs from the surrounding land.These inputs can take the forms of drainage water,detritus and sediment, and precipitation. Flowing

aquatic systems are highly dependent on a steadyinput of detrital material from the watershedthrough which they flow.

Internal CyclingInternal cycling of nutrients occurs when nutri-

ents are transformed in ecosystems. Plants take upmineral (mostly inorganic) nutrients from soilthrough their roots and incorporate them into liv-ing tissues. Nutrients in the living tissues occur invarious forms of organic compounds and performdifferent functions in terms of physiology and mor-phology. When these living tissues reach senes-cence, the nutrients are usually returned to the soilin the form of dead organic matter. However, nitro-gen can be reabsorbed from senescent leaves andtransferred to other living tissues. Various micro-bial decomposers transform the organic nutrientsinto mineral forms through a process called miner-alization. The mineralized nutrients are once againavailable to the plants for uptake and incorporationinto new tissues. This process is repeated, formingthe internal cycle of nutrients. Within the internalcycles, the majority of nutrients are stored in or-ganic forms, either in living tissues or dead organicmatter, whereas mineral nutrients represent a smallproportion of the total nutrient pools.

Output of NutrientsThe output of nutrients from an ecosystem rep-

resents a loss. Output can occur in various ways,depending on the nature of a specific biogeochem-

732 • Nutrient cycling

ical cycle. Carbon is released from ecosystems tothe atmosphere in the form of carbon dioxide viathe process of respiration by plants, animals, andmicroorganisms. Nitrogen is lost to the atmospherein gaseous forms of nitrogen, nitrous oxide, andammonia, mostly as by-products of microbial ac-tivities in soil. Nitrogen is also lost through leach-ing from the soil and carried out of ecosystemsby groundwater flow to streams.Leaching also results in export ofcarbon, phosphorus, and other nu-trients out of ecosystems.

Output of nutrients from ecosys-tems can also occur through surfaceflow of water and soil erosion.However, loss of nutrients from oneecosystem may represent input toother ecosystems. Output of organicmatter from terrestrial ecosystemsconstitutes the majority of nutrientinput into stream ecosystems. Or-ganic matter can also be transferredbetween ecosystems by herbivores.For example, moose feeding onaquatic plants can transport nutri-ents to adjacent terrestrial ecosys-tems and deposit them in the formof feces.

Considerable quantities of nutri-ents are lost permanently from eco-systems by harvesting, especially infarming and logging lands, whenbiomass is directly removed fromecosystems. Fire usually results inthe loss of large amounts of nutri-ents. Fire kills vegetation and con-verts portions of biomass and or-ganic soil matter to ash. Fire causesloss of nutrients through volatiliza-tion and airborne particulate. Afterfire, many nutrients become readilyavailable, and nutrients in ash aresubject to rapid mineralization. Ifnot taken up by plants during vege-tation recovery, nutrients are likelyto be lost from ecosystems throughleaching and erosion.

The Hubbard Brook ExampleNutrient cycling has been studied

in several intact ecosystems. One of

the most notable experiments was conducted in theHubbard Brook experimental forest in New Hamp-shire. The experimental forest was established ini-tially for forest hydrology research. Begun in theearly 1960’s, one of the longest-running studies ofwater and nutrient dynamics of forest ecosystemshas been on the Hubbard Brook site. Both waterand nutrient concentrations in precipitation inputs

Nutrient cycling • 733P

ho

toD

isc

Supplementary soil nutrients come from airborne particles and aerosols,as dry or wet depositions, such as this fog. Such atmospheric depositioncan supply more than half of the input of nutrients to some ecosystems.

and stream outputs were regularly monitored, al-lowing estimations of nutrient balances over thewatershed ecosystems.

One of the major findings from the HubbardBrook study was that undisturbed forests exhibitregularity and predictability in their input-outputbalances for water and certain chemical elements.Nitrogen, however, shows a more complex, but stillexplicable, pattern of stream concentrations. Lossesof nitrates from the control watershed are higher inthe dormant season, when biological activity is low.Losses are near zero during the growing season,when biological demand for nitrogen by plants andmicrobes are high. Removal of vegetation in theHubbard Brook forest had a marked effect on waterand nutrient balances. Summer stream flow duringthe devegetation experiment was nearly four timeshigher than in the control watershed. The increasein stream flow, combined with increases in the con-centration of nutrients within the stream, resultedin increases in loss rates of nitrate much higher thanthose of undisturbed areas. Similarly, loss of potas-sium used in large quantities by plants showed agreat increase.

Nutrient Uptake and CompetitionEcosystem nutrient cycling is critical for plant

growth and ecosystem productivity. Plant uptakeof essential nutrient elements is related to nutrientavailability, root absorption surface, rooting depth,and uptake kinetics of roots. A nutrient-rich siteusually supports more plants of different speciesthan a site with fewer available nutrients. Nutrientcompetition among plants is usually manifestedthrough physiological, morphological, and ecolog-ical traits. Usually grasses and forbs can coexist inone grassland ecosystem, for example, through dif-ferent rooting depth. To compete for less solublenutrients such as phosphorus, plants usually ex-tend their root surfaces using symbiotic relation-ships with mycorrhizae. Differential seasonality innutrient uptake and rooting depth become morecritical to compete limited nutrients.

Yiqi Luo

See also: Carbon cycle; Ecosystems: studies; Ero-sion and erosion control; Hydrologic cycle; Nitro-gen cycle; Nutrients; Nutrition in agriculture; Phos-phorus cycle; Root uptake systems; Soil.

Sources for Further StudyAber, J. D., and J. M. Melillo. Terrestrial Ecosystems. 2d ed. San Diego: Academic Press, 2001.

Advanced ecosystem ecology book contains a comprehensive discussion of nutrient cy-cling.

Likens, G. E., et al. Biogeochemistry of a Forested Ecosystem. New York: Springer-Verlag, 1977.The book describes studies of nutrient cycling in the Hubbard Brook watershed.

Schlesinger, W. H. Biogeochemistry: An Analysis of Global Change. 2d ed. San Diego: AcademicPress, 1997. A comprehensive book on nutrient cycling.

NUTRIENTS

Categories: Cellular biology; nutrients and nutrition

Plant nutrients are the molecular compounds necessary to maintain plant life.

Plants, like animals and other forms of life, re-quire a great diversity of compounds. Mole-

cules of these compounds are used to build andmaintain cells and to perform other necessary lifeprocesses, such as growth and reproduction, aswell as respiration and photosynthesis.

Plants manufacture, by the process of photosyn-thesis, simple sugars. From these are formed poly-saccharides (starches, cellulose) and, eventually, avariety of lipids (fats, oils, and waxes), hundreds ofdifferent protein molecules, and many other or-ganic compounds. Normal functioning of plants re-

734 • Nutrients

quires that they absorb, usually by roots from soil, avariety of chemical elements. From these elements,which are combined with water, carbon dioxide,and other compounds, the more complex com-pounds are formed.

Plant NutritionPlant nutrition involves the uptake from the en-

vironment of the raw materials needed for per-forming a variety of biochemical processes, the con-duction of these substances to all parts of the plant,and their use in growth and metabolic processes.Most of these materials are in the form of ions dis-solved in soil water. The water occupies the spacesbetween solid soil particles and is absorbedthrough the roots and then conducted upward bymeans of xylem tissue located in roots, stems, andleaves.

Present in soil water, and therefore in a plantgrowing in that soil, are more than sixty chemicalelements. Some of these, however, are present onlyincidentally and perform no known function. Oth-ers, known as essential elements, are used in someparticular way by the plant. These elements havebeen the object of study by many plant physiolo-gists.

Essential ElementsBy the mid-1800’s, chemistry had become suffi-

ciently advanced that botanists could analyze thechemical content of plants. They discovered that inthe ash remaining after a plant had been burnedwere a variety of minerals. In order to be consideredan essential element, that element must be neces-sary for normal metabolic processes, growth, andreproduction; another element cannot replace it.Essential elements are classified as either macro-nutrients or micronutrients. Although both types ofnutrients are required, macronutrients are neededin much larger amounts.

To recognize the difference between the two cat-egories of minerals, one may consider the follow-ing. When freshly harvested plants are heated in anoven to remove the water, the dry matter remainingcan be analyzed. Macronutrients are those requiredin concentrations of at least 1,000 milligrams per ki-logram of dry plant matter. In contrast, micronutri-ents are required in much smaller or even traceamounts, generally less than 100 milligrams per ki-logram of dry matter.

Among the macronutrients, nitrogen is a key ele-ment needed in large amounts to form proteins,nucleotides, chlorophyll, and coenzymes. Phos-

Nutrients • 735

Plant NutrientsNutrient Functions

MacronutrientsCalcium Formation of cell walls, regulator of membrane permeability, enzyme regulatorCarbon Basic to all structures and functionsHydrogen Basic to all structures and functionsMagnesium Chlorophyll component, enzyme activatorNitrogen Formation of amino acids, proteins, nucleotides, nucleic acids, chlorophyll, coenzymesOxygen Basic to all structures and functionsPhosphorus Formation of ADP and ATP, nucleic acids, coenzymes, phospholipidsSulfur Component of proteins, coenzyme A, thiamine, biotin

MicronutrientsBoron Calcium metabolism, nucleic acid synthesis, cell membranes, carbohydrate synthesisChlorine Osmosis, photosynthesisCopper Redox enzymesIron Chlorophyll synthesis, electron transport system, cellular respirationManganese Enzyme activator, chloroplast membrane activity, photosynthesisMolybdenum Nitrogen fixationZinc Enzyme activator, auxin synthesis

Some species need: Aluminum, cobalt, silicon, sodium

phorus is also required to build nucleic acids, but itis also necessary to form adenosine triphosphate(ATP) and adenosine diphosphate (ADP), energy-carrying compounds vital to all cells. Among theseveral functions of calcium are its roles as a com-ponent of cell walls and in changing the permeabil-ity of cell membranes. Magnesium is necessary toactivate certain enzymes; also a single atom of mag-nesium occupies a central position in each moleculeof chlorophyll. Sulfur is essential to synthesizingproteins, coenzyme A, thiamine, and biotin.

Among the micronutrients, iron functions in theelectron transport system, playing a role in cellularrespiration; also it is vital for chlorophyll synthesis.Chlorine helps to maintain an ionic balance bycontrolling osmosis. Manganese is involved in acti-vating many enzymes. Boron is believed to be in-volved in carbohydrate synthesis; also it is requiredfor nucleic acid synthesis. Zinc is required for thesynthesis of the plant hormone auxin; it also is in-volved in enzyme activation. Copper is present inthe active site of redox enzymes and electron carri-ers. Nickel plays a role in enzyme functioning in themetabolism of nitrogen. Molybdenum is requiredfor nitrogen fixation, the process by which freenitrogen (N2) in the air is converted into nitrates.

Identifying Essential ElementsTo identify a particular mineral as essential, a

classic protocol (series of steps) is followed. Themethod used today was developed by Julius vonSachs, an early German plant physiologist, in 1860:Two seedlings of the same kind of plant are grownin separate containers. One container containswhat is considered to be a complete growth me-dium. The other container contains all but a singlemineral. If the growth of the seedling in the lattercontainer is abnormal, or if its normal floweringand seed production are not successful, the missingelement is determined to be essential.

Just as animals suffer symptoms when certainvitamins or minerals are absent in their diets, soplants are affected when they suffer a deficit in aparticular essential element. Such symptoms areused by farmers and gardeners to determine whichtypes of fertilizer need to be added to the soil in or-der to correct the problem. Some of the more com-mon mineral deficiencies include nitrogen, iron,magnesium, calcium, and phosphorus deficiencies.For example, because plants require large amountsof nitrogen for growth, this mineral is often not

present in sufficient amounts. A typical symptom isa uniform yellowing of the older leaves, a conditioncalled chlorosis. An iron deficiency is diagnosedwhen the youngest leaves turn yellow. In the caseof a magnesium deficiency, the older leaves are yel-low between the veins. A calcium deficiency causesthe growing points to die back; young leaves areyellow and crinkly. Plants with a phosphorus defi-ciency turn dark green and have leaves with purpleveins.

Alternative Nutrient SourcesWhereas most plants, both wild or cultivated,

absorb required nutrients from soil water, someplants obtain nutrients by other means. Plants suchas Venus’s fly trap, the pitcher plant, and variousother carnivorous plants supplement soil nutrientsby trapping insects or other small animals. Once ananimal is caught, its protein is digested, yielding ni-trogen that is made available to the plant. Intricatetraps and other means of attracting and capturinginsects have evolved over long periods of time. Thetraps are commonly modified leaves. Carnivorousplants generally inhabit sunny habitats with soilsthat have low levels of nitrogen. Thus nitrogenfrom animal protein is necessary to supplementthat obtained in the more usual way from the soil.

Because of the special role of the proteins in plant(and animal) cells, nitrogen, a key element requiredto form proteins, is of special concern. Many plantspecies, especially those of the pea or legume fam-ily (Fabaceae) have root nodules, or mycorrhizae,filled with bacteria that are able to convert free ni-trogen into compounds of nitrogen called nitrates.This process, called nitrogen fixation, makes nitratesavailable to the plant (as well as to the bacteria) as ausable source of nitrogen. Also, the surroundingsoil is enriched, making nitrogen available to otherplants growing in the soil. Thus, nitrogen takenfrom the soil in large quantities, especially by culti-vated crops, is replaced. The common agriculturalpractice of rotating crops, in which a leguminouscrop such as alfalfa or clover is alternated with cornor wheat, which removes large amounts of nitrogenfrom the soil, takes advantage of the nitrogen thathas been fixed by legumes.

Hydroponic cultures allow for the growing ofplants without soil. The plants survive on solutionsthat contain water and essential minerals. Plantsare typically grown in greenhouses with their rootsbathed in circulating water containing the miner-

736 • Nutrients

als. Provision must be made to keep the water aer-ated. Hydroponic plants can be grown year-round,but the added expense of hydroponic culture limitsits use to fruits and vegetables made available tospecialty markets.

Thomas E. Hemmerly

See also: Biofertilizers; Carbohydrates; Carnivo-rous plants; Fertilizers; Hydroponics; Lipids; Nitro-gen fixation; Nutrition in agriculture; Photosynthe-sis; Proteins and amino acids; Root uptake systems;Soil; Water and solute movement in plants.

Sources for Further StudyPurves, W. K., et al. Life: The Science of Biology. 6th ed. New York: W. H. Freeman, 2001. Chap-

ter 36 of this excellent college text deals with plant nutrients.Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:

W. H. Freeman, 1999. This comprehensive and lavishly illustrated textbook of botany ex-plores various aspects of the relationships of plants to soil and minerals.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology. 3d ed. Sunderland, Mass.: Sinauer Asso-ciates, 2002. This standard textbook covers all aspects of plant physiology, includingplant nutrients and their use.

NUTRITION IN AGRICULTURE

Categories: Agriculture; economic botany and plant uses; nutrients and nutrition

Extensive research has been conducted to investigate the nutritional requirements of different crops and ways to enhancesoil fertility, which has greatly benefited agricultural production.

Even though people have known for more thantwo thousand years that adding mineral ele-

ments, such as plant ash or lime, to the soils can im-prove plant growth, the systematic study of plantnutrition is a relatively young science, consideringhumanity’s long history of cultivating crops. About250 years ago, farmers and gardeners started to askthe question, “What makes plants grow?” It waswidely believed that soil humus, a brown or blackorganic substance resulting from the partial decayof plant and animal matter, provided plants withcarbon for making sugar and starch, and sub-stances such as saltpeter, lime, and phosphates sim-ply helped the humus to be more useful. It was notuntil around 1840 that the German chemist Justusvon Liebig (1803-1873) helped to compile and sum-marize the scattered information on the importanceof mineral elements for plant growth, and plant nu-trition began to be established as a scientific disci-pline. Since then, great progress has been made inthe study of plant nutrition.

It is now known that, aside from carbon, hydro-gen, and oxygen, which plants get from air and

water, about a dozen other nutrients are neededfor plant growth. They can be divided into threeclasses. The primary (or major) nutrients, includingnitrogen, phosphorus, and potassium, are needed inlarger quantities than are the secondary nutrients,calcium, magnesium, and sulfur. These in turn are re-quired in greater quantity than the trace (or minor)nutrients iron, boron, manganese, copper, zinc,molybdenum, chloride, and nickel. These nutrientsare contained in the minerals and organic matter inthe soil. Many more elements are found in bothsoils and plants—for instance, aluminum, cobalt,fluorine, iodine, and sodium. They may not beneeded by all plants and may be either beneficial ortoxic to plant growth. Silicon, sodium, and cobaltare beneficial to some plants.

Soil and Plant NutrientsSoil is the natural medium in which crops grow.

The nutrient content of a soil and the availability ofthe nutrients to crops are important factors that de-termine a soil’s productivity. Soil nutrients aremostly present in forms not immediately available

Nutrition in agriculture • 737

to plants, such as being adsorbed onto or as constit-uents of soil mineral particles, or in organic matter.They become available through slow processes,such as biological decomposition of organic mattercalled mineralization, chemical reactions on soilminerals called weathering, and release from soilparticles.

Soil nutrients in agricultural land may be gradu-ally lost with the removal of harvested products,such as grain and straw, which take with them con-siderable amounts of all nutrients. In addition, nu-trients may be lost from the soil through leachingand erosion. Therefore, it is common for field cropsto suffer from nutrient deficiencies. On the otherhand, certain soils may present the crops with prob-lems of mineral toxicity, that is, excess amounts ofparticular elements. To maintain healthy growth ofcrops, it is necessary to correct these problems.

Soil testing, visual diagnosis of the plant, andplant tissue analysis can all be used to evaluate thefertility status of the soil and detect deficiencies andtoxicities in soil nutrients. The results of such testsprovide the basis for recommendations on the ap-plication of fertilizer and soil amendments.

Coping with Nutrient DeficienciesBased on knowledge of plant nutrition and ex-

perience accumulated over a long period of time,nutrient management practices have been estab-lished to enhance soil fertility and overcome cropnutritional problems by balancing the use of min-eral fertilizers combined with organic and biologi-cal sources of plant nutrients. In practice, differentmethods may have both advantages and disadvan-tages, depending on the particular set of local con-ditions.

Organic sources of plant nutrients include farm-yard manure (animal waste products), green ma-nure (plant products), and compost. They containsmall amounts of nutrients and are often bulky innature. When added to the soil, their main value isto provide organic matter that promotes microbialactivity and improves soil structure, aeration, andwater-holding capacity, enabling the soil to re-spond better to fertilizers and irrigation. The or-ganic matter may also supply micronutrients andhelp to make the phosphate in the soil more avail-able to crops. The disadvantages of applying or-ganic manures are that they may be expensive, dif-ficult to handle, or likely to release excess nutrientsinto the environment.

Nitrogen availability is one of the most limitingfactors in crop productivity. Although nitrogen isabundantly available in the air, this form of it is notdirectly usable by the plants. Some crops, such aslegumes, can form a beneficial relationship calledsymbiosis with certain bacteria capable of convert-ing atmospheric nitrogen to ammonia, a processknown as biological nitrogen fixation. The bacteriasupply the plant with nitrogen (ammonia), whilethe plant provides the bacteria with organic com-pounds for use as energy. Legumes can be used as asource of nitrogen when planted with cereals. Fast-growing legumes can be grown early in the seasonand then ploughed under to provide nitrogen forthe main crop. Recent research has identified themechanisms controlling the expression of the nitro-gen-fixation genes at the molecular level, and oneof the goals of ongoing research is to explore vari-ous approaches to constructing a viable nitrogen-fixing system for use with nonlegumes.

Low soil phosphorus availability is another con-straint on plant growth. Phosphorus is progres-sively lost from soil through weathering, and reac-tions with various soil constituents substantiallyreduce phosphorus available to plants. One re-search effort has been directed toward investigat-ing various root characteristics that are useful inphosphorus uptake by plants. The findings mayhelp farmers to breed for crops that are better ableto grow in soils with low phosphorus.

Nutrient removal from cultivated land usuallyexceeds the natural rate of nutrient input. One rem-edy is to add appropriate and balanced fertilizersback to the soil. Mineral fertilizers are widely usedto supply either single nutrients or multiple nutri-ents in combination. Foliar spray is effective for cor-recting deficiencies in micronutrients. Timing anddosage of applications is important, because thenutritional requirements of a crop vary with itsstage of growth. Insufficient supply reduces cropyields, but applying too much or at a wrong time isnot only wasteful but also potentially harmful tothe environment.

Overcoming Nutrient ToxicitiesSoil can become acidified as a result of its own

physical properties, microbial activity, climate,vegetation, and the addition of acidifying fertiliz-ers. As a result, important nutrients can be lost, andtoxicities from aluminum and manganese may oc-cur. Liming, the application to land of a material

738 • Nutrition in agriculture

containing calcium, usually chalk or limestone, isoften used as a standard measure in order to reduceproblems of soil acidity.

Salts introduced in irrigation water, blown in-land from oceans, or produced by weathering mayaccumulate in topsoil and cause toxicity to crops.This is normally controlled by applying more waterthan the crop can use, so that excess salts areleached downward below the root zone. It is impor-tant, however, that the irrigation water does nothave high salt content and that drainage is not aproblem. The addition of calcium salts, such as cal-cium sulfate, together with organic manures, is alsoeffective in treating salt-affected soils.

One focus of research in the field of plant nutri-tion has been the study of the mechanisms plantsemploy in avoiding or tolerating toxic elements,such as aluminum, manganese, and salts, and ac-

cessing scarce nutrients, such as phosphorus, in thesoil. Knowing how plants cope with various nutri-ent stresses will help the effort of breeding cropsthat are better able to withstand adverse soil nutri-ent conditions and achieve high yields by makinguse of their own genetic potential.

Zhong Ma

See also: Agriculture: modern problems; Agricul-ture: traditional; Agriculture: world food supplies;Agronomy; Biofertilizers; Composting; Eutrophi-cation; Fertilizers; Green Revolution; High-yieldcrops; Hydroponics; Nitrogen cycle; Nitrogen fixa-tion; Nutrients; Organic gardening and farming;Phosphorus cycle; Roots; Root uptake systems;Slash-and-burn agriculture; Soil management; Sus-tainable agriculture.

Sources for Further StudyGlass, A. D. M. Plant Nutrition: An Introduction to Current Concepts. Boston: Jones and Bart-

lett, 1989. This is an introductory textbook to plant mineral nutrition. Includes charts, il-lustrations, photographs, and index.

Lagreid M., O. C. Bockman, and O. Kaarstad. Agriculture, Fertilizers, and the Environment.Wallingford, England: CABI, 1999. Presents environmental and sustainability issues re-lated to fertilizer use and its role in ensuring adequate food supplies. Includes illustra-tions, tables, bibliographical references, and index.

Ludwick, A. E., et al., eds. Western Fertilizer Handbook. 8th ed. Danville, Ill.: Interstate, 1995.An educational text for developing a better understanding of agronomic principles andpractices, with convenient references on soil, water, plants, and fertilizer. Includes charts,tables, photographs, and index.

Marschner, H. Mineral Nutrition of Higher Plants. 2d ed. San Diego: Academic Press, 1995.Comprehensive discussion of plant nutritional physiology and plant-soil relationships.Includes illustrations, tables, bibliographical references, and index.

Prasad, R., and J. F. Power. Soil Fertility Management for Sustainable Agriculture. Boca Raton,Fla.: CRC Lewis, 1997. Presents the principles of controlling soil fertility in the various cli-mates of the world, with examples of soil fertility management for sustainable agricul-ture. Includes illustrations, tables, bibliographical references, and index.

Nutrition in agriculture • 739

OIL BODIES


Oil bodies, also called oleosomes, are plant cell organelles that serve as storage structures for lipids, primarilytriacylglycerols.

Oil bodies are found in many plant seeds andrepresent an efficient storage form of energy

for a germinating embryo. Other, non-oilseed,plants rely primarily on starch as a storage form ofenergy in their seeds. On a weight basis, oils containmore the twice the amount of energy as starches,because they contain more carbon and hydrogen at-oms and fewer oxygen atoms than starch. Oil thusrepresents a more compact storage form of energy.Some high oil content species include castor beans,canola, safflower, peanuts, sunflower, macadamianuts, and hazelnuts. All of these contain about 50percent or more oil on a dry-weight basis. Despitethe ubiquitous presence of corn and soybean oil inthe marketplace, these oils contain only 5 percentand 17 percent oil, respectively. Although oil bodiesare found predominantly in seeds, they may also bepresent in fruits, such as olives and avocados.

StructureOil bodies are unique in structure. They are sur-

rounded by single-layer phospholipid membrane,as opposed to the bilayer membrane found aroundmost other plant organelles. The single-layer mem-brane is derived from the endoplasmic reticulum(ER), from which oil bodies are believed to origi-nate. Triacylglycerols are synthesized in the ER andaccumulate within the two layers of its phospho-lipid membrane. Eventually the two layers are sep-arated as oil accumulates and pinches off to formthe oil body. The hydrophobic (water-avoiding)tails of the resulting phospholipid monolayer areassociated with the oily interior, and the polar

heads face outward. In addition, proteins calledoleosins are present in the single-layer membraneand are thought to prevent fusion with other oilbodies. Oleosomes appear to be consistent in size,about 1 micron. Thus, a cell with a large amount ofoil will have more oil bodies present.

FunctionWhen seeds imbibe water and germination is

initiated, many metabolic activities are activated. Inorder for a germinating seed to utilize the energy inan oil body, the triacylglycerols must be convertedto a form of sugar, typically sucrose, that can beused by the growing embryo. The sequence of en-zymatic events that makes this possible involvesthe close coordination of three organelles: theoleosome, the mitochondrion, and the highly spe-cialized glyoxysome. It is in the glyoxysome wherethe many carbon atoms present in the fatty acids ofthe triacylglycerols are removed, two at a time, in aprocess called oxidation. These reactions, togetherwith those of the mitochondria, are often referred toas the glyoxylate cycle. The end products allow for re-verse glycolysis, which produces sugars that can bemetabolized by typical respiratory pathways in thegrowing seedling.

Thomas J. Montagno

See also: Cytoplasm; Endoplasmic reticulum; Gly-colysis and fermentation; Lipids; Membrane struc-ture; Mitochondria; Plasma membranes; Respira-tion.

Sources for Further StudyDennis, David T., David B. Layzell, Daniel D. Lefebre, and David H. Turpin, eds. Plant Me-

tabolism. 2d ed. Singapore: Longman, 1997. Advanced textbook with three chapters de-voted to lipid formation and breakdown. Includes illustrations, bibliographic references,and index.

740

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology. 4th ed. Belmont, Calif.: Wadsworth,1992. Textbook with a chapter devoted to lipids and other natural products. Includes il-lustrations, photographs, bibliographic references, and index.

OLD-GROWTH FORESTS

Categories: Forests and forestry; ecosystems; environmental issues

Ancient ecosystems, old-growth forests consist of trees that have never been harvested. These forests are, in some cases,the only habitat for a number of plant and animal species.

The timber industry views the large, old treesas a renewable source of fine lumber, but envi-

ronmentalists see them as part of an ancient andunique ecosystem that can never be replaced. Inthe 1970’s scientists began studying the uncut for-ests of the Pacific Northwest and the plants and ani-mals that inhabited them. In a U.S. Forest Servicepublication, Ecological Characteristics of Old-GrowthDouglas-Fir Forests (1981), Forest Service biologistJerry Franklin and his colleagues showed that theseforests were not just tangles of dead and dying treesbut rather a unique, thriving ecosystem made up ofliving and dead trees, mammals, insects, and evenfungi.

Old-Growth Forest EcosystemThe forest usually referred to as old growth oc-

curs primarily on the western slope of the CascadeMountains in southeast Alaska, southern BritishColumbia, Washington, Oregon, and Northern Cal-ifornia. The weather there is wet and mild, ideal forthe growth of trees such as Douglas fir, cedar,spruce, and hemlock. Studies have shown thatthere is more biomass, including living matter anddead trees, per acre in these forests than anywhereelse on earth. Trees can be as tall as 300 feet (90 me-ters) with diameters of 10 feet (3 meters) or moreand can live as long as one thousand years. The for-est community grows and changes over time, notreaching biological climax until the forest primarilyconsists of hemlock trees, which are able to sproutin the shade of the sun-loving Douglas fir.

One of the most important components of theold-growth forest is the large number of standingdead trees, or snags, and fallen trees, or logs, on theforest floor and in the streams. The fallen trees rot

very slowly, often taking more than two hundredyears to decompose completely. During this timethey are important for water storage, as wildlifehabitat, and as “nurse logs” where new growth canbegin. In fact, seedlings of some trees, such as west-ern hemlock and Sitka spruce, have difficulty com-peting with the mosses on the forest floor and needto sprout on the fallen logs.

Another strand in the complex web of the forestconsists of mycorrhizal fungi (mycorrhizae), whichattach themselves to the roots of the trees and en-hance their uptake of water and nutrients. Thefruiting bodies of these fungi are eaten by smallmammals such as voles, mice, and chipmunks,which then spread the spores of the fungi in theirdroppings. There are numerous species of plantsand animal wildlife that appear to be dependent onthis ecosystem to survive.

Protecting the ForestBy the 1970’s most of the trees on timber indus-

try-owned lands had been cut. Their replanted for-ests, known as second growth, would not be readyfor harvest for several decades, so the industry be-came increasingly dependent on public lands fortheir raw materials. Logging of old growth in thenational forests of western Oregon and Washingtonincreased from 900 million board feet in 1946 tomore than 5 billion board feet in 1986.

Environmentalists claimed that only 10 percentof the region’s original forest remained. Deter-mined to save what was left, they encouraged theuse of the evocative term “ancient forest” to coun-teract the somewhat negative connotations of “oldgrowth.” Then they were given an effective toolin the northern spotted owl. This small bird was

Old-growth forests • 741

found to be dependent on old growth, and its list-ing under the federal Endangered Species Act in1990 caused a decade of scientific, political, and le-gal conflict.

Under law, the U.S. Forest Service was requiredto protect enough of the owl’s habitat to ensure itssurvival. An early government report identified7.7 million acres of forest to be protected for thebird. Later, the U.S. Fish and Wildlife Service rec-ommended 11 million acres. In 1991 U.S. DistrictCourt judge William Dwyer placed an injunctionon all logging in spotted owl habitat until a compre-hensive plan could be finalized. The timber indus-try responded with a prediction of tens of thousandsof lost jobs and regional economic disaster. In 1993President Bill Clinton convened the Forest Summitconference in Portland, Oregon, to work out a so-lution. The Clinton administration’s plan, thoughapproved by Judge Dwyer, satisfied neither the in-

dustry nor the environmentalists, and protests, law-suits, and legislative battles continued.

As the twentieth century came to an end, timberharvest levels had been significantly reduced, theNorthwest’s economy had survived, and addi-tional values for old-growth forests were found:habitat for endangered salmon and other fish, asource for medicinal plants, and a repository forbenefits yet to be discovered. The decades-longcontroversy over the forests of the Northwest had adeep impact on environmental science as well asnatural resource policy and encouraged new inter-est in other native forests around the world, fromBrazil to Malaysia to Russia.

Joseph W. Hinton

See also: Conifers; Forests; Forest management;Logging and clear-cutting; Mycorrhizae; Sustain-able forestry; Timber industry.

742 • Old-growth forests

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Sources for Further StudyDietrich, William. The Final Forest: The Battle for the Last Great Trees of the Pacific Northwest.

New York: Penguin Books, 1993. Studies the old-growth controversy with a balancedlook at individuals affected by it, conservationists and loggers alike.

Durbin, Kathie. Tree Huggers: Victory, Defeat, and Renewal in the Northwest Ancient Forest Cam-paign. Seattle: Mountaineers, 1996. A detailed examination of the politics of the old-growth controversy, with special emphasis on events between 1987 and 1996.

Kelly, David, and Gary Braasch. Secrets of the Old Growth Forest. Salt Lake City, Utah: Gibbs-Smith, 1988. A coffee-table book with color photographs and extensive text that providesan excellent introduction to the topic.

Maser, Chris. Forest Primeval: The Natural History of an Ancient Forest. Corvallis: Oregon StateUniversity Press, 2001. A fascinating biological biography of a hypothetical thousand-year-old forest.

OOMYCETES

Categories: Molds; Protista; taxonomic groups; water-related life

Oomycetes are a diverse group of fungus-like eukaryotic microorganisms that form the phylum Oomycota. First classi-fied within the kingdom Fungi, oomycetes are now unambiguously recognized as distinct from fungi and more closelyrelated to heterokonts, such as brown algae, phylum Phaeophyta.

The more than five hundred species of oomy-cetes, commonly known as water molds, white

rusts, or downy mildews, are essentially sapro-phytic but include pathogens of plants, insects,crustaceans, fish, vertebrate animals, and variousmicrorganisms. Plant pathogenic oomycetes causedevastating diseases on several crop, ornamental,and native plants. Animal pathogenic oomycetescan cause severe losses in aquaculture and fisher-ies. Both have had a significant impact on humanhistory.

Evolutionary HistoryTraditionally, and due essentially to their fila-

mentous growth habit, oomycetes have been clas-sified in the kingdom Fungi. However, modernmolecular and biochemical analyses as well as mor-phological features suggest that oomycetes sharelittle taxonomic affinity with filamentous fungi butare more closely related to brown algae (phylumPhaeophyta) in the kingdom Protista. This position issupported by molecular phylogenies based on ribo-somal RNA (ribonucleic acid) sequences, compiledamino acid data for mitochondrial proteins, andprotein encoding chromosomal genes. The oomy-

cetes also display a number of biochemical andmorphological characteristics that distinguish themfrom the fungi and confirm their affinity to brownalgae and other heterokonts.

The cell walls of oomycetes are composedmainly of glucans and cellulose and, unlike fungalcell walls, contain little or no chitin. The zoosporesdisplay two flagella, with an ultrastructure similarto that of the flagella of the motile spores of hetero-kont algae. The oomycetes also contain the energystorage chemical mycolaminarin, a molecule that isalso found in kelps and diatoms.

Taxonomic ClassesThe subdivison of oomycetes into taxonomic

classes remains under debate. Typically fourclasses of oomycetes are identified. These includeSaprolegniales, Leptomitales, Lagenidales, andPeronosporales. Some authors elevated the plantpathogenic genera Phytophthora and Pythium to aseparate class, named Pythiales. Recent molecularphylogenetic studies using ribosomal and mito-chondrial sequences have started to unravel theevolutionary relationships between the differentclasses of oomycetes. The Peronosporales and

Oomycetes • 743

Pythiales, which together account for the majority ofplant pathogenic genera, form an ancient mono-phyletic group, suggesting that acquisition of plantpathogenicity probably occurred early in the evo-lution of this lineage. Most of the saprophytic andanimal pathogenic species are restricted to theother classes. Aphanomyces, a genus with strong af-finity to the Saprolegniales, includes both animaland plant pathogenic species.

General FeaturesThe oomycetes inhabit primarily aquatic and

moist soil habitats. They are often very abundantand can be easily cultured from both freshwaterand saltwater ecosystems, as well as from a varietyof agricultural or natural soils. However, severalspecies are mainly terrestrial, including obligatebiotrophic pathogens of plants that depend on aircurrents to disperse their spores.

The basic somatic structure of a majority ofoomycete species is an extending funguslikethread, the hypha, that grows into a branched net-work of filaments, the mycelium. Oomycetes areknown as coenocytic organisms; that is, their myce-lium lacks septa or crosswalls that divide thehypha, except to separate it from the reproductiveorgans.

Both asexual and sexual reproductive structuresoccur. The primary asexual reproductive organ isthe sporangium that differentiates at the tip of avegetative hypha to produce and release motilezoospores with two flagella. The zoospores can ger-minate directly or indirectly to produce a vegeta-tive mycelium or can differentiate into secondaryzoospores. Sexual reproduction involves the inter-action of a male antheridia with a female oogoniathrough a fertilization tube that allows the male nu-clei to migrate into the oogonium. Some oomycetesare self-fertile or homothallic, whereas others areself-sterile or heterothallic and require that strainswith different mating types come into contact toachieve sexual reproduction. The sexual spores arethe oospores, which can survive desiccation andstarvation over long periods of time. Under favor-able environmental conditions, the oospores ger-minate to form vegetative mycelium or to releasezoospores. Oospores are also the structures thatgave the oomycetes their name of egg fungi.Oomycetes are diploid in the dominant vegetativephase with meiosis occurring only during gameto-genesis.

Economic ImportanceSaprophytic oomycetes play an important role in

the decomposition and recycling of decaying mat-ter in aquatic and soil environments. In addition,both plant and animal pathogenic oomycetes cancause serious economic impact by destroying crop,ornamental, and native plants as well as fish andother aquatic organisms.

Typically, oomycete diseases are difficult to man-age and require the use of specific chemicals (fungi-cides or oomycides). Sources of sustainable geneticresistance in plants to oomycetes are limited. In ad-dition, most oomycetes, such as Phytophthora, ex-hibit tremendous ability to adapt to chemical andgenetic resistance through the development of newresistant strains. For example, the appearance in the1990’s of Phytophthora infestans strains resistant tothe chemical metalaxyl resulted in potato late blightepidemics in the United States that were severe anddestructive. Most modern research focuses on inno-vative approaches for the management of oomycetediseases, including the use of plant breeding, ge-netic engineering, and genomic technologies.

Plant-Pathogenic OomycetesPlant-associated oomycetes may be facultatively

or obligately pathogenic. Pathogenic oomycetesform specialized infection structures, also found infungi, such as appressoria (penetration structures)and haustoria (feeding structures). Plant-pathogenicoomycetes include about sixty species of the ge-nus Phytophthora, several genera of the biotrophicdowny mildews, and more than one hundred spe-cies of the genus Pythium. Phytophthora species causesome of the most destructive plant diseases in theworld and are arguably the most devastating patho-gens of dicotyledonous plants.

The most notable plant-pathogenic oomycete isPhytophthora infestans, the Irish Potato Faminepathogen, which causes late blight, a disease of po-tato and tomato. Introduction of this pathogen toEurope in the mid-nineteenth century resulted inthe potato blight famine and the death and dis-placement of millions of people. Today, Phytoph-thora infestans remains a prevalent pathogen caus-ing multibillion dollar losses in potato productionworldwide.

Other economically important Phytophthora dis-eases include root and stem rot caused by Phytoph-thora sojae, which hampers soybean production inseveral continents, and black pod of cocoa caused

744 • Oomycetes

by Phytophthora palmivora and Phytophthora mega-karya, a recurring threat to worldwide chocolateproduction. The introduction of exotic plant patho-genic oomycetes to natural ecosystems can alsocause devastating effects. For example, Phytoph-thora cinnamomi has decimated native plants inAustralia and South America. More recently, sud-den oak death, a disease caused by a new species,Phytophthora ramorum, has emerged as a severe dis-ease of oak trees along the Pacific Coast of theUnited States. Other notorious oomycete pathogensinclude the obligate biotrophs Plasmopara viticola,the agent of downy mildew of grapevine, as well asAlbugo and Peronospora species, which cause whiterust and downy mildews on several crops.

Animal-Pathogenic OomycetesAnimal-pathogenic oomycetes are common. At

least one oomycete species, Pythium insidiosum, isknown to infect mammals, sometimes including hu-mans, fatally. However, most economic impact on

animals is caused by oomycetes that infect fish, fisheggs, and crustaceans. Examples include Saproleg-nia parasitica, a ubiquitous pathogen of fish that iscommon in aquaria and can cause severe losses inaquaculture, particularly when fish density is toohigh or fish diet is unbalanced. Another importantanimal pathogenic oomycete is Aphanomyces astaci,the agent of crayfish plague that decimated Euro-pean crayfish populations following its introduc-tion from North America. Animal-pathogenicoomycetes can also have beneficial effects. At leastone species of oomycetes, the insect pathogenLagenidium giganteum, has been commercialized bya California company as a biocontrol agent for mos-quitoes.

Sophien Kamoun

See also: Algae; Biopesticides; Brown algae; Cla-distics; Diseases and disorders; Eukaryotic cells;Flagella and cilia; Fungi; Heterokonts; Molecularsystematics; Protista; Resistance to plant diseases.

Sources for Further StudyAlexopoulos, C. J., C. W. Mims, and M. Blackwell. Introductory Mycology. 4th ed. New York:

John Wiley & Sons, 1996. This textbook contains one chapter on oomycetes. Includes illus-trations, bibliographical references, and index.

Erwin, D. C., and O. K. Ribeiro. Phytophthora Diseases Worldwide. St. Paul, Minn.: APS Press,1996. This monumental treatise is the main reference work for Phytophthora. It includes adescription of methods of study and chapters detailing each of the more than sixty speciesof Phytophthora. Includes illustrations, bibliographical references, and index.

Margulis, L., and K. V. Schwartz. Five Kingdoms: An Illustrated Guide to the Phyla of Life onEarth. New York: W. H. Freeman, 2000. This book includes one chapter on oomycetes thatdiscusses the general features of their life cycles and habits. Includes illustrations, biblio-graphical references, and index.

ORCHIDS

Categories: Angiosperms; economic botany and plant uses; gardening; taxonomic groups

Orchids are the largest family of monocots among the angiosperms (flowering plants), with between twenty-five thou-sand and thirty thousand species, and new species are continually being described.

There are numerous natural and artificial hy-brids of orchids (family Orchidaceae). The floral

and vegetative variation in this family is enormous,and most orchid specialists consider the orchids tobe undergoing a great deal of evolutionary activity

as well. Orchids follow the general monocot pat-tern of having flower parts in three’s. They differfrom other monocots in several ways, however.

First, the stamens are fused together. Orchids aredivided into two major groups based on their an-

Orchids • 745

thers and stamens: the diandrous orchids, whichhave two functional anthers and a sterile stamen,which in the lady’s slipper orchids (such as Cypri-pedium, Selenipedium, Paphiopedilum, and Phragmi-pedium) becomes modified into a staminode, andthe monandrous orchids (all other orchids), whichhave one functional anther.

Second, the stamens are fused with the pistil toform a structure called a column, which has at its tipa rostellum, a gland that separates the pollinia fromthe stigmatic surface. The fusion of the stamens andpistils into a column also occurs in the milkweedfamily (Asclepiadaceae) in the dicots.

Third, the pollen grains are formed together intopackets called pollinia.

Fourth, flowers have bilateral symmetry orasymmetry, as in Tipularia.

Fifth, resupination is the most common conditionfor orchid flowers. In resupinate orchids, thepedicel and ovary undergo a 180 degree twisting

process so that as the flower matures the lip is low-ermost and the column uppermost. Although someorchids (such as Calopogon, Polystachya, Malaxis,and Platanthera nivea) are nonresupinate, most areresupinate.

Sixth, the third petal, or lip, is usually highlymodified in size, shape, or color when compared tothe other two petals.

Seventh, the ovary is inferior.Eighth, orchids produce large numbers of tiny

seeds that lack endosperm. Most orchids havebisexual flowers; however, some (Catasetum, Cyc-noches, Mormodes) produce unisexual (male or fe-male) flowers. Flowers may last only a day (Sobra-lia), or for more than a month (some Paphiopediliumand Dendrobium).

PollinationPollination ranges from promiscuous to pseudo-

copulation. Promiscuous pollination occurs whenalmost any appropriate insect can transfer the pol-linia successfully from one flower to another. Pseu-dopollination involves flowers which mimic thesize, shape, smell, or movements of female waspsor bees (Ophrys, Caladenia, Drakaea). These flowersattract the male wasp or bee, which attempts tomate with the flower. Pollinia are thereby attachedto the insect’s body. Eventually the frustrated maleleaves the flower and finds another flower, againtrying to mate with it but succeeding only in depos-iting the pollinia from the first flower onto the sec-ond flower’s stigmatic surface.

Mycorrhizae and Seed GerminationTo germinate, orchid seeds, which lack endo-

sperm, must form a symbiotic relationship with themycelium of an appropriate fungus (mycorrhiza)which provides the seeds with nutrients and water.

Vegetative Plant StructureVegetative plants are herbs or, rarely, vines (Va-

nilla), perennials or, rarely, annuals (Zeuxinestrateumatica). Vegetative parts are normally roots,stems, and leaves, which can undergo many differ-ent modifications. The roots of terrestrial orchidsare similar to other terrestrial monocots and arenonphotosynthetic. Some are modified into foodstorage structures (tuberoids). Roots of epiphyticorchids have a specialized outer multilayered tis-sue called the velamen, which forms a spongy,whitish sheath around the root. The velamen al-

746 • Orchids

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lows absorption of water and dissolved mineral nu-trients.

Stems are frequently modified into creeping rhi-zomes and often into water- and food-storage struc-tures, such as corms, tubers, or pseudobulbs. Thesestructures are usually aboveground and photosyn-thetic. Plants may be minute (some Bulbophyllumand Platystele) or gigantic (about 44 feet tall, such asSobralia altissima) or may form large clusters ofpseudobulbs weighing several hundred pounds(Grammatophyllum).

Growth Patterns, Distribution, and HabitatsThere are two basic growth forms in orchids. In

the sympodial form, successive new stem growthoriginates from the base of the preceding stemgrowth (as in Cattleya and Cypripedium). In themonopodial form, new growth comes from an apicalmeristem; these plants often become quite tall (suchas Vanda).

Orchids occur worldwide, from the Arctic to thetropical rain forests but do not grow wild in Antarc-tica. Most tropical orchids are epiphytic (growingon other plants) or epilithic (growing on rock sur-

faces), and a relatively small number, including thetemperate and Arctic orchids, are terrestrial. Thegenus Rhizanthella in Australia is subterranean.

Economic UsesUses include a complex starch (called salep) used

as food in Turkish and other Middle-Eastern confec-tions. Salep is made from the dried tuberoidal rootsof Dactylorhiza, Eulophia, and Orchis. In the Americas,the Mayans and the Aztecs cultivated Vanilla vinesfor the fleshy fruit, or bean, which was fermented tomake vanilla for use as food flavoring and perfume.Some orchids have been used as sources of fibersfor weaving or basket making. Currently, many dif-ferent orchids are important as cut flowers or aspotted plants, including Cattleya, Dendrobium, Cym-bidium, Paphiopedilium, and Laelia. Advances madeduring the 1990’s in mericloning have greatly re-duced the cost of producing high-quality plants.

Lawrence K. Magrath

See also: Angiosperm evolution; Animal-plant in-teractions; Garden plants: flowering; Monocots vs.dicots; Monocotyledones; Mycorrhizae.

Sources for Further StudyBrown, P. M. Wild Orchids of the Northeastern United States: A Field Guide. Ithaca, N.Y.:

Comstock, 1997. Includes keys, distribution maps, line drawings, and color plates.Flora North America Editorial Committee. Orchidaceae. Vol. 26 in Flora of North America North

of Mexico. New York: Oxford University Press, 2001. This volume contains keys to generaand species, descriptions of genera and species, line drawings, and distribution maps.

Northen, R. T. Miniature Orchids. New York: Van Nostrand Reinhold, 1980. Covers miniatureorchids, how to grow them, and a selection of recommended species. Includes black-and-white and color illustrations.

Richter, W. Orchid Care: A Guide to Cultivation and Breeding. New York: Van NostrandReinhold, 1972. Basic guide to cultivation, discussing light, humidity, temperature, wa-tering, potting, and how to pollinate orchids. Includes good line drawings.

ORGANIC GARDENING AND FARMING

Categories: Agriculture; economic botany and plant uses; environmental issues; food; gardening

Organic growing strives to produce healthy soils and plants through practices that replenish and maintain soil fertility.Organic farmers avoid the use of synthetic and often toxic fertilizers and pesticides.

At the beginning of agricultural history, farmersbelieved that plants “ate the soil” in order to

grow. During the nineteenth century, Germanchemist Justus von Liebig discovered that plants

Organic gardening and farming • 747

extracted nitrogen, phosphorus, and potash fromthe soil. His findings dramatically changed agricul-ture as farmers found they could grow crops in anytype of soil, even sand and water solutions, if theright chemicals were added.

By the late twentieth century, diversified familyfarms in industrialized nations had given way tohuge, specialized operations. Crop yields wereraised with the use of chemicals, but farms andtheir soil and water were not being used efficiently.Over time, the organic quality of the soil was lost,even though the agricultural chemicals remained inthe soil.

Chemicals were found to leach into the watersupply. In 1988 the U.S. Environmental ProtectionAgency (EPA) found that groundwater in thirty-two states was contaminated with seventy-fourdifferent agricultural chemicals. Leaching causesonce-friable and fertile soils to become nonpro-ductive. Further, the use of chemical insecticideswas having toxic effects on both the foods grownand the farmworkers encountering them. Aboutforty-five thousand accidental pesticide poisoningsoccur in the United States each year; in 1987 the EPAranked pesticides as the third leading environmen-tal cancer risk. A National Academy of Sciencestudy estimated that twenty thousand people eachyear get cancer because of pesticides alone.Growing public awareness of the effects of the tra-ditional agricultural reliance on synthetic chemi-cals is reflected in a growing demand for organi-cally produced foods.

Tenets of Organic FarmingOrganic farming represents the use of diversi-

fied farming practices that emphasize workingwith nature to create an ecologically sound and sus-tainable system of agriculture. Certified organicfarmers are bound by practices that are free of syn-thetic chemicals and genetic engineering. Landsmust be chemical-free for at least three years beforeproducts grown on them can be certified as organic.The organic certification further requires that bothplants and livestock be raised without the use ofchemicals, antibiotics, hormones, or synthetic feedadditives.

Certified organic farmers must comply withboth organic regulations in their state and the 1990Organic Foods Production Act (OFPA). In the late1990’s the U.S. Department of Agriculture (USDA),through the National Organic Standards Board

(NOSB), began working to develop standards andregulations that would ensure consistent nationalstandards for organic products. There are six areasof standards that the NOSB is examining: crops;livestock and livestock products; processing, pack-aging and labeling; accreditation; international is-sues; and materials. There is interest in developingglobal standards for the importation and exporta-tion of organic products.

Soil FertilityLike all farmers, organic farmers must ensure

soil fertility and control unwanted plants and pests.Organic farmers build organic materials in the soilby the addition of green manure, compost, or animalmanure. Green manure is the term used for cropsthat are grown to introduce organic matter and nu-trients into the soil. Such crops are raised expresslyfor the purpose of being plowed under rather thansold to the consumer. Green manure crops protectsoil against erosion, cycle nutrients from lower lev-els of the soil into the upper layers, suppress weeds,and keep much-needed nutrients in the soil.

Legumes are an excellent green manure crop be-cause they are efficient at extracting nitrogen fromthe air and transferring it into the soil, leaving asupply of nitrogen for the next crop. The legumeshave nitrogen nodules on their roots; when the le-gumes are tilled under and decompose, they addmore nitrogen to the soil. As plants decay, they alsomake insoluble plant nutrients such as carbon diox-ide and organic acids available in the soil.

Much of the organic material derived from greenmanure comes in the form of decaying roots. Al-falfa, one type of legume, sends its roots several feetdown into the soil. When alfalfa plants are turned,the entire root system decomposes into organic ma-terial. Thus they help improve water retention andsoil quality at the same time. Some examples of le-gumes used for green manure are sweet clover, la-dino clover, and trefoil. Grasses used for green ma-nure include rye, redtop, and timothy grass.

Greater fertility can be achieved if green manureis grazed by animals and animal manure is depos-ited on the soil. When grass is grazed, a proportionof the roots die and rot to form humus. This humusis the stable organic material that acts as a catalystfor allowing plants to find nutrients.

Animal manure is also used as an organic fertil-izer. Rich in nitrogen, the best animal manurescome from animals with high-protein diets. For ex-

748 • Organic gardening and farming

ample, beef cattle being fattened for market con-sume a higher level of protein than dairy cattle thatare producing milk for market. The application ofmanure to fields improves the structure of the soil,raises organic nitrogen content, and stimulates thegrowth of soil bacteria and fungi necessary forhealthy soils.

Compost is particularly useful to the small-scalefarmer or gardener who does not raise livestock orhave the acreage to grow a green manure crop toplow under. Composting is a natural soil-buildingprocess that began with the first plants that existedon earth. It continues to be employed as a naturalprocess today. As leaves and dead plants, animals,and insects decompose into the soil, they form arich, organic layer. Compost can be made by farm-ers and gardeners by using alternating layers of

carbohydrate-rich plant cuttings and leaves, animalmanure, and topsoil and allowing it to decay andform a rich, organic matter to be added to the soil.

Crop RotationPlanting the same crop year after year on the

same piece of ground results in depleted soil. Cropssuch as corn, tobacco, and cotton remove nutrients,especially nitrogen, from the soil. In order to keepthe soil fertile, a legume should be planted the yearafter a cash crop in order to add nitrogen andachieve a balanced nutrient level. Planting a winter-cover crop such as rye grass will help protect landfrom erosion and will, when plowed under in thespring, provide a nutrient-rich soil for the growthof a cash crop. Crop rotation also improves the physi-cal condition of the soil because different crops vary

Organic gardening and farming • 749P

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A farmer tends organically grown cucumber plants.

in root depth and respond to either deep or shallowsoil preparation.

Organic Insect and Weed ControlMonoculture puts a large amount of the same

crop in proximity to pests that are destructive tothat particular crop. Insect offspring can multiplyout of proportion when the same crops are grown inthe same field year after year. Because insects aredrawn to the same home area, they are less able toproliferate if the crop is changed to one they cannoteat. For this reason, organic farmers rely on crop ro-tation as one aspect of insect control. Rotating cropscan also help control weeds. Some crops and culti-vation methods inadvertently allow certain weedsto thrive. Crop rotation can incorporate a successorcrop that eradicates those weeds. Some crops, suchas potatoes and winter squash, work as “cleaningcrops” because of the different styles of cultivationthat are used on them.

Organic farmers and gardeners believe thatplants within a balanced ecosystem are resistant todisease and pest infestation. Therefore, the wholepremise of organic farming is working with natureto help grow healthy, unstressed plants. Ratherthan using chemicals, which often create resistantgenerations of pests and thus the need for newerand stronger chemicals, organic farmers havefound natural ways to diminish pest problems.

Organic farmers strive to maintain and replenishsoil fertility to produce healthy plants resistant toinsects. They also try to select plant species that arenaturally resistant to insects, weeds, and disease.Crop rotation, as already discussed, is one methodof keeping infestation down. Rows of hedges, trees,or even plants that are not desirable to insectsplanted in and around the crop field can act as bar-riers to pests. They can also provide habitat forpests’ natural enemies, including birds, beneficialinsects, and snakes.

Beneficial insects can be ordered through the mailto help alleviate insect pests. The ladybug and thepraying mantis are two insects that can help ridfarms or gardens of aphids, mites, mealy bugs, and

grasshoppers. Just one ladybug can consume fiftyor more aphids per day. Because most people orderten thousand to twenty thousand ladybugs fortheir garden, and one ladybug can lay up to onethousand fertile eggs, the overall cost of biopesti-cides is much less than that of buying insecticides.Trichogramma, also available by mail order, is asmall wasp that will destroy moth eggs, squashborers, cankerworms, cabbage loopers, and cornearworms.

Organic farmers and gardeners also rely on whatis called an insecticide crop. Garlic planted near let-tuce and peas will deter aphids. Geraniums ormarigolds grown close to grapes, cantaloupes,corn, and cucumbers will deter Japanese and cu-cumber beetles. Herbs such as rosemary, sage, andthyme planted by cabbages will deter white butter-fly pests. Potatoes will repel Mexican bean beetles ifplanted near beans, and tomatoes planted near as-paragus will ward off asparagus beetles. Naturalinsecticides such as red pepper juice can be used forant control, while a combination of garlic oil andlemon may be used against fleas, mosquito larvae,houseflies, and other insects.

Organic farming relies on the physical control ofweeds, especially through the use of cutting (culti-vation) or smothering (mulching and hilling). Cul-tivation is the shallow stirring of surface soil to cutoff small developing weeds and prevent more fromgrowing. Cultivating tools include tractors, wheelhoes, tillers, or, for small gardens, hand hoes.

Mulch is a soil cover that prevents weeds fromgetting the sunlight they need for growth. Mulchingwith biodegradable materials can help build soilfertility while controlling weeds. Plastic mulchescan be used on organic farms as long as they are re-moved from the field at the end of the growing orharvest season.

Dion Stewart and Toby R. Stewart

See also: Agriculture: modern problems; Biopesti-cides; Composting; Herbicides; Hydroponics; Le-gumes; Monoculture; Nitrogen cycle; Nitrogen fix-ation; Nutrients; Nutrition in agriculture; Pesticides.

Sources for Further StudyBoard of Agriculture of the National Research Council. Alternative Agriculture. Washing-

ton, D.C.: National Academy Press, 1989. Provides interesting information on organicfarming.

Carson, Rachel. Silent Spring. 1962. Reprint. Boston: Houghton Mifflin, 1994. Classic book onthe detriments of the agricultural use of pesticides and other chemicals.

750 • Organic gardening and farming

Coleman, Eliot. The New Organic Grower. White River Junction, Vt.: Chelsea Green, 1995. Aneasy-to-read manual on organic concepts for home or market gardeners.

Community Alliance with Family Farmers. National Organic Directory. Davis, Calif.: Author,2001. Provides information on organic farming and lists producers and products.

Engelken, Ralph, and Rita Engelken. The Art of Natural Farming and Gardening. Greeley, Iowa:Barrington Hall Press, 1985. An interesting story of two decades of organic farming inIowa.

Hamilton, Geoff. The Organic Garden Book. New York: D. Kindersley, 1994. An excellent bookfor gardeners who wish to grow vegetables, flowers, and fruits organically.

OSMOSIS, SIMPLE DIFFUSION, ANDFACILITATED DIFFUSION

Categories: Cellular biology; transport mechanisms

Osmosis, simple diffusion, and facilitated diffusion are the processes by which water and other substances—usuallysmall molecules and ions—cross cell membranes.

Transport of materials across cellular mem-branes is essential to the functioning of plants

and other living organisms. It is the movement ofmaterials across these semipermeable barriers thatprovides the conditions necessary for life, not onlyfor the plasma membrane separating a cell from itsenvironment but also for membranes surroundingorganelles within cells.

Unless cells are able to maintain a stable internalenvironment (homeostasis), growth, development,and metabolism are not possible. Thus, under-standing how substances move across membranesand how membranes select which substances to ad-mit and exclude leads to a better understanding ofhomeostasis and its maintenance.

Some substances require metabolic energy tocross membranes in a process called active transport.All other movement across membranes, however,is through either simple or facilitated diffusion. Os-mosis is a special case of simple diffusion involvingjust the movement of water.

Cell MembranesCell membranes, typically about eight nanome-

ters thick, serve as barriers between the cell’s cyto-plasm and the extracellular environment. The cellmembrane’s lipid constituents (primarily phospho-

lipids) make it fundamentally insoluble in waterand therefore impermeable to all but the smallestuncharged polar molecules and a variety of lipidsoluble molecules. It is necessary, though, for someof these substances to cross this barrier.

The currently accepted structure of the cell mem-brane was proposed by Jonathan Singer and GarthNicolson in 1972. According to their fluid mosaicmodel, a membrane consists of a double layer ofphospholipids, called a lipid bilayer. Phospho-lipids are more or less linear molecules with a hy-drophilic end (water-soluble or, literally, “water-loving”), often called the “head,” and a hydro-phobic end (water-insoluble or “water-fearing”),often called the “tail.” Within the bilayer, the lip-ids naturally orient themselves with their hydro-philic heads facing toward the aqueous fluid oneach side—the cytoplasm on one side, the extra-cellular fluid on the other—and their hydropho-bic tails touching one another within the bilayer.Phospholipids on each side of the membrane arefree to move around, a property called fluidity.Among thephospholipids float various proteins.They, too, are free to move unless constrained byanchors to cytoplasmic or extracellular structuresor by limits on the fluidity of the lipid. Lipids andproteins that are exposed to the outside of the cell

Osmosis, simple diffusion, and facilitated diffusion • 751

may also have oligosaccharides (short chains ofsugar molecules) attached to them, making themglycolipids and glycoproteins. The carbohydrateportion recognizes and is recognized by specific

binding substances on other cells or in the environ-ment.

Concentration and Electrochemical GradientsSubstances crossing the membrane barrier by os-

mosis or diffusion will cross only from the side ofthe membrane with a higher concentration of thesubstance to the side with a lower concentration ofthe substance. If the substance is an ion (all of whichare electrically charged), the concentration gradientof the ion and the difference in electrical chargeacross the membrane (called the membrane potential)together determine the direction of diffusion. Theconcentration gradient and the membrane poten-tial together are referred to as an electrochemical gra-dient. Ions will only go down their electrochemicalgradient.

If the concentration gradient and the membranepotential are lined up in the same direction, it iseasy to determine which way the ion involved willtravel. For example, if chloride ions (Cl–), whichhave a negative charge, are in higher concentrationoutside the cell than inside the cell, they will be ableto move into the cell by facilitated diffusion. This isbecause all cells have a negative membrane poten-tial, which is to say, the electrical charge in the cyto-plasm is more negative than the electrical charge onthe outside. Thus, for this Cl– example, both theconcentration gradient and the membrane poten-tial are aligned in the same direction. On the otherhand, if the Cl– concentration is higher inside thecell than outside, Cl– could potentially travel in ei-ther direction. If the concentration gradient is verylarge and the membrane potential is only slightlynegative, Cl– would likely move out of the cell,whereas if the concentration gradient is small, andthe membrane potential is very negative, Cl– willlikely move into the cell.

Routes Across the MembraneRegardless of the direction substances will po-

tentially be able to travel across the membrane, theywill be able to do so by only one of three differentprocesses: osmosis, simple diffusion, or facilitateddiffusion. Water molecules, which are very small,can penetrate the lipid bilayer by passing betweenthe phospholipids in a process called osmosis, al-though the exact mechanism is poorly understood.A few other hydrophilic molecules, such as metha-nol and ethanol, can cross a lipid bilayer. Apparentlythey, too, can penetrate the otherwise inhospitable

752 • Osmosis, simple diffusion, and facilitated diffusion

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A. In osmosis, water (small dots) outside the plant cellmoves from a region of greater concentration, outside,across the cell wall and the cell membrane to a region ofless concentration initially occupied by more solutes(large dots) inside the cell.B. The cell’s uptake of water increases the volume of thecytoplasm and presses the cell membrane against the cellwall. The resulting turgor pressure or turgidity keeps theplant from wilting. The opposite occurs in dry conditionsand causes the plant to wilt.

hydrophobic environment of the membrane be-cause of their small size and lack of charge. Hydro-phobic substances, including dissolved gases suchas oxygen, can also cross the membrane essentiallyunrestrained, and because they are lipid-solublethey simply “dissolve” into the membrane andthrough to the other side.

Hydrophilic substances, particularly chargedmolecules (ions), however, cannot cross a lipidbilayer by simple diffusion. They can cross mem-branes only with assistance from specialized trans-port proteins. These proteins penetrate both sidesof the bilayer, with one part exposed to the cyto-plasm and the opposite part to the extracellularfluid. Many transport proteins appear to work by a“shuttle” type of mechanism, binding to recog-nized substances on one side of the membrane andreleasing them on the other side. Like many otherprocesses involving proteins, the activity of trans-port proteins depends critically on the shape ofthe protein itself and on its ability to recognize alimited group of substances. Some membrane pro-teins form complexes of several proteins that formaqueous channels of the proper dimensions andcharge distribution to serve as pores for specificmolecules or ions.

OsmosisBecause living cells occupy an aqueous environ-

ment, water will always be present on both sides ofa membrane. If one side has a higher concentrationof solutes (dissolved material) in it than the otherside, it will have a correspondingly lower con-centration of water. Driven by random molecularmovement, more water molecules will bump intoone another and move out of the region of lowersolute concentration than move into it, until even-tually the water is uniformly distributed on bothsides and the solute concentrations are the same.This net movement driven by the difference in sol-ute concentration is called osmosis.

Osmosis is of fundamental importance in a vari-ety of living systems. Organisms originally evolvedin the ocean, so the cytoplasm of most cells has asolute concentration similar to seawater. Even ter-restrial organisms have evolved mechanisms thatmaintain proper solute concentrations in tissue flu-ids. If the balance deviates from relatively narrowlimits, there can be serious consequences.

For example, the freshwater green alga Spirogyrahas a higher solute concentration than the sur-

rounding water. Consequently, the net movementof water is from outside the algal cells to the cyto-plasm. If this movement of water were to continueindefinitely, it would eventually cause such abuildup of pressure inside the algal cells that theywould burst, except that algal cells (as well as allplant cells) are surrounded by a rigid cell wall. Asthe pressure builds, the cytoplasm swells in vol-ume and presses the cell membrane against the cellwall, and the cell is said to be turgid. This same pro-cess occurs in terrestrial plants when roots absorbwater from the soil, and this enables stem and leafcells to maintain the turgidity required to keep theplant from wilting. In these examples, the cyto-plasm is said to have a lower (more negative) waterpotential than the water outside. A solution with alower water potential than another solution acrossa semipermeable membrane is said to be hypertonic(or hyperosmotic), while the other solution is saidto be hypotonic (or hypo-osmotic). If the solute con-centrations are the same on both sides of a semi-permeable membrane, they are said to be isotonic(or iso-osmotic).

If the same green algae is placed in concentratedsaltwater, the cytoplasm will now be hypotonic(and the saltwater will be hypertonic), and the netflow of water will be out of the algal cells. When thisoccurs in plants, the cytoplasm shrinks in volume,and the cell membrane pulls away from the cellwall in a process called plasmolysis. As a result,turgor pressure drops, and in the case of terrestrialplants in salty or dry soil, they begin to wilt. Somefreshwater protozoa have specialized structurescalled contractile vacuoles that serve as pumps tomaintain hypertonic conditions in the cytoplasm.

DiffusionBoth simple and facilitated diffusion involve the

movement of a substance down a concentration orelectrochemical gradient between the two compart-ments separated by a membrane The main differ-ence between the two mechanisms is that sub-stances moving by simple diffusion move throughthe lipid portion of the membrane, while sub-stances using facilitated diffusion require special-ized membrane proteins to allow their transport.

In simple diffusion, dissolved gases, such as oxy-gen and carbon dioxide, can cross membranes, ascan lipid soluble substances. One type of importantsmall molecule that crosses membranes by free dif-fusion are steroid hormones. These extremely po-

Osmosis, simple diffusion, and facilitated diffusion • 753

tent regulators of cell metabolism readily diffusethrough cellular membranes because their hydro-phobic nature makes them dissolve readily inlipids. Within cells, they bind to specific receptors,which pass messages on to other proteins that con-trol various biochemical events.

Facilitated DiffusionAll other materials that enter and leave cells

down their chemical or electrochemical gradientsdo so by facilitated diffusion. Forming this path-way is the role of the particular membrane proteins,called carrier proteins, transport proteins, or per-meases. Such proteins allow larger, polar moleculessuch as sugars and amino acids to be taken up bycells. They control the response of cells to certaingrowth factors and hormones, whose binding tothe cell membrane causes channels for facilitateddiffusion to open or close selectively.

Extremely specific recognition by the transport

protein enables material to be transported. For thisreason, membranes may readily transport one typeof molecule but be completely impermeable to an-other molecule, even a closely related one. Mem-branes with selective permeability probably repre-sent one of the most important innovations in livingorganisms. Without membranes, separate com-partments could not be maintained to separate thevarious incompatible biochemical reactions of liv-ing systems. Without the semipermeability of bio-logical membranes, the environment within thevarious compartments could not be continuouslyreplenished and would be unable to respond to acontinuously changing environment.

Mary Lee S. Ledbetter, updated by Bryan Ness

See also: Active transport; Cell wall; Cells and dif-fusion; Liquid transport systems; Membrane struc-ture; Plasma membranes; Vesicle-mediated trans-port; Water and solute movement in plants.

Sources for Further StudyBeyenbach, Klaus W., ed. Cell Volume Regulation. New York: Karger, 1990. Discusses cellular

control mechanisms, biological transport, cell membranes, cell physiology, osmotic pres-sure, and water-electrolyte balance. Includes bibliographic references and index.

Campbell, Neil A. Biology. 6th ed. San Francisco: Benjamin/Cummings, 2001. This introduc-tory textbook is an excellent source of information about membrane structure and func-tion. It is distinguished by extraordinary clarity of explanation, beautiful illustrations, in-cluding both line drawings and micrographs, and an engaging, lively style that capturesreaders’ interest.

Karp, Gerald. Cell and Molecular Biology: Concepts and Experiments. 3d ed. New York: J. Wiley,2002. Oriented toward the physiological aspects of cell biology, this textbook has a rigor-ous yet accessible discussion of the thermodynamics of the electrochemical gradient. Itwill be most useful to those not deterred by mathematical expressions describing physi-cal events.

Kleinsmith, Lewis J., and Valerie Kish. Principles of Cell and Molecular Biology. New York:HarperCollins, 1995. Describes the experimental basis for the current understanding ofcell structure and function. Discusses osmosis, simple diffusion, and facilitated diffusion.Though some of the discussion is rather advanced, the illustrations of mechanisms of fa-cilitated diffusion are particularly helpful.

Petty, Howard R. Molecular Biology of Membranes: Structure and Function. New York: Plenum,1993. Introduces the molecular biology of cell membranes. Familiarity with biochemistryis assumed; knowledge of elementary physics would be helpful.

Vance, Dennis, E., and Jean E. Vance, eds. Biochemistry of Lipids, Lipoproteins, and Membranes.New York: Elsevier, 1991. Advanced textbook covers the major areas in the fields of lipid,lipoprotein, and membrane biochemistry as well as molecular biology. Provides a clearsummary of these research areas for working scientists.

754 • Osmosis, simple diffusion, and facilitated diffusion

OXIDATIVE PHOSPHORYLATION

Categories: Cellular biology; transport mechanisms

Oxidative phosphorylation is the sequence of reactions in mitochondria that convert energy from food into cellular en-ergy by synthesizing ATP, the primary energy currency of cells. To drive the second step in oxidative phosphorylation,electrons must be passed to one of the electron carrier molecules of the electron transport system.

The ability to convert the energy from food mole-cules into cellular energy efficiently is crucial to

cell survival. The central conversion system is oxi-dative phosphorylation, a sequence of reactionsthat take place in mitochondria (a type of organellefound in plant cells). These reactions take high-energy electrons and use them to make adenosinediphosphate (ADP) and inorganic phosphate (Pi).The name “oxidative phosphorylation” derivesfrom the fact that organic molecules are oxidized toprovide the electrons that are used as an energysource to facilitate the phosphorylation of ADP.

Oxidative phosphorylation may be divided intofour general steps:

• obtaining high-energy electrons

• transferring energy from the electrons into cel-lular energy, accomplished via the electrontransport system

• using the cellular energy from the electrontransport system to establish a proton (H+) gra-dient across the inner mitochondrial mem-brane

• using the stored energy of the proton gradientto drive the synthesis of adenosine triphosphate(ATP)

Finding Electrons: OxidationThe main source of the electrons for the first step

of oxidative phosphorylation is the chemical oxida-tion of organic molecules. Sources can include gly-colysis, fatty acid oxidation, and the Krebs cycle (citricacid cycle). Chemical oxidation results in a loss ofelectrons by the oxidized molecule; when the elec-trons are removed from a molecule they are pickedup by an electron acceptor, or electron carrier. Elec-tron carriers are molecules that can transport elec-

trons between molecules in the cell, much as a pack-age delivery service will carry a box between twoaddresses. The most common electron carriers arenicotinamide adenine dinucleotide (NAD+) and flavinadenine dinucleotide (FAD). Each one of these mole-cules can carry two electrons at a time. Once NAD+

or FAD molecules accept electrons, they are said tobe chemically reduced and are denoted as NADHor FADH2.

Electron TransportTo drive the second step in oxidation phosphor-

ylation, the electrons carried by NADH and FADH2

must be passed to one of the electron carrier mole-cules of the electron transport system. The carriersthat receive the electrons then pass them to othercarriers in the system. Every time one carrier givesa pair of electrons to another, a small amount ofenergy is released. This energy is used by themitochondrion to pump H+ across the inner mito-chondrial membrane. The electron pair continuesto be transferred through a series of electron carri-ers until any extra energy they carry has been usedfor pumping H+. The last carrier transfers what arenow low-energy electrons to oxygen, which is thefinal electron acceptor in the series. When the elec-trons are accepted by the oxygen, it combines withtwo H+ to form a molecule of water.

The components of the electron transport systemare embedded in the inner mitochondrial mem-brane, and they are arranged in four large com-plexes. Each complex contains a component re-sponsible for picking up the electrons and a proteinportion that delivers the electrons to the next carrierin the chain. Each complex also contains additionalproteins—in some cases as many as twenty—thatattach the complex to the inner mitochondrialmembrane.

Oxidative phosphorylation • 755

In addition to these four complexes, there aretwo smaller electron carriers that can transportelectrons between the larger complexes. One ofthese, cytochrome c, is a small protein. The otherelectron carrier is called ubiquinone, or coenzyme Q.Every time an electron pair is delivered to a newcarrier, it loses a certain amount of energy, and eachof the carriers can only accept electrons that have aparticular amount of energy. Therefore, an electronpair moves through the electron transport systemin an exact order, going only to carriers that are ableto accommodate electrons of a precise energy level.

The order in which electrons move through allthe components of the electron transport systemhas been determined by Britton Chance and severalother investigators. An electron pair entering thesystem would proceed as follows. (The four com-plexes are identified by Roman numerals I-IV.)

NADH → complex I → ubiquinone → complex III→ cytochrome c → complex IV → oxygen

Complex II accepts electrons directly from FADH2,found in the matrix of the mitochondrion, thenpasses them to ubiquinone.

Establishing the Proton GradientThe energy harvested during electron transport

is used in the third step of the process to create anH+ gradient. Three of the complexes (I, III, and IV)contain an additional protein component that isable to use the harvested energy to move protonsfrom the matrix across the inner membrane, intothe space between the inner and outer membranes,which is called the intermembrane space. The accu-mulation of protons in this intermembrane spaceresults in proton concentration gradient across theinner membrane, from a high proton concentrationin the intermembrane space to a low proton concen-tration in the matrix of the mitochondrion. The pro-ton concentration gradient represents a stored formof energy, much like the capacitor in an electronicdevice.

ATP SynthesisFinally, in the fourth and final step of oxidative

phosphorylation, the proton gradient is used todrive the synthesis of ATP. The energy from the pro-ton gradient is used by an ATP-synthesizing en-zyme, also found in the inner membrane of themitochondrion called ATP synthase. ATP synthase

is a very large molecule. At very high magnifica-tion, the ATP synthase molecule looks like a lolli-pop sticking out from the inner membrane into thematrix of the mitochrondrion.

In 1961 Peter Mitchell proposed that the step-wise transfer of electrons by the electron transportsystem and the proton gradient worked together tosynthesize ATP. His proposal, called the chemios-motic hypothesis, represented a radical departurefrom other ideas at the time. At first the chemi-osmotic hypothesis found little support among sci-entists, but the chemiosmotic hypothesis has stoodthe test of time. Although some details remain to beworked out, the experimental evidence accumu-lated by Mitchell, as well as by many other investi-gators since 1961, overwhelmingly supports thismodel. Mitchell received the Nobel Prize in Chem-istry in 1978 for his proposal of the chemiosmotichypothesis and the elegant research he performedin its support.

Work from the laboratory of scientist EfraimRacker has demonstrated that there are differentfunctions for the two parts of the lollipop. Thespherical part of the lollipop can be removed bymechanical shaking and has been found still to beable to synthesize ATP. Racker called the sphere F1,or coupling factor 1. The “stick” portion of the lolli-pop is embedded in the inner mitochondrial mem-brane. This part of the enzyme acts as a tunnel forprotons to travel back into the mitochondrion. Thestick portion of the enzyme can be inactivated bythe antibiotic oligomycin, so it is called the Fo, oroligomycin-sensitive factor. Another name for theATP synthase is thus FoF1ATPase.

The fine details of how the movement of protonsthrough the channel in the stick drives ATP synthe-sis have not been completely worked out. Thespherical portion of the molecule can make ATPwithout the proton gradient. Once synthesized,however, the ATP remains tightly attached, so noadditional ATP can be made by the enzyme. Thelarge concentration of protons in the intermem-brane space causes a net flux of protons back intothe matrix through the tunnel provided by the ATPsynthase. Paul Boyer suggested that when protonsmove through the lollipop stick into the mitochon-drion, the sphere changes its shape. This shapechange, in turn, causes the enzyme to release newlysynthesized ATP. The ATP can now be used by thecell, and the enzyme can now make more ATP.Boyer’s proposal, which has gradually gained ac-

756 • Oxidative phosphorylation

ceptance among researchers, is reasonable. Manybiological enzyme reactions work by using similarshape changes to attach and release the substrates.A molecule of ATP is released for every three hy-drogen ions that are returned to the matrix.

Alina C. Lopo

See also: ATP and other energetic molecules; Car-bohydrates; Cell-to-cell communication; Energyflow in plant cells; Exergonic and endergonic reac-tions; Glycolysis and fermentation; Krebs cycle;Lipids; Mitochondria; Proteins and amino acids.

Sources for Further StudyBerg, Jeremy Mark, John Tymoczko, and Lubert Stryer. Biochemistry. 5th ed. New York: W. H.

Freeman, 2001. Includes descriptions of oxidative phosphorylation and the electrontransport system. Full-color diagrams aid in understanding what is known about thestructure and function of the components of the electron transport system. This is acollege-level biochemistry textbook, and background in chemistry will be useful; how-ever, many of the figures provide useful illustrative material for all levels.

James, D. C., and G. S. Matthews. Understanding the Biochemistry of Respiration. New York:Cambridge University Press, 1991. In-depth coverage of respiration is presented in aclear, step-by-step format designed to be used by undergraduate students.

Levings, Charles S., III, and Indra K. Vasil, eds. The Molecular Biology of Plant Mitochondria.Boston: Kluwer Academic, 1995. From the Advances in Cellular and Molecular Biology ofPlants series. Includes bibliographical references and index.

Mitchell, Peter. “Keilin’s Respiratory Chain Concept and Its Chemiosmotic Consequences.”Science 206 (1979): 1148-1159. This is the lecture that Peter Mitchell delivered at the NobelPrize ceremonies. Mitchell summarizes the work that led him to propose the hypothesisas well as subsequent work in its support.

Wolfe, Stephen L. Biology: The Foundations. Belmont, Calif.: Wadsworth, 1983. This is an in-troductory biology text. Chapter 7 includes an excellent description of oxidative phos-phorylation and of the chemical reactions that provide energy to drive the process. Con-tains numerous simple diagrams of the process as well as several information boxes andsupplements relating oxidative phosphorylation to the whole organism.

OZONE LAYER AND OZONE HOLE DEBATE

Categories: Environmental issues; pollution

Ozone, a form of the element oxygen, forms naturally in the stratosphere and provides the earth with a filter from ultravi-olet radiation. Some human activities have been linked to a decrease in the amount of ozone present, an effect that hasbeen described as a hole in the ozone layer. The size of the hole located over Antarctica fluctuates with the seasons but overthe past several decades has steadily increased in average size.

Ozone is a highly reactive form of oxygen. It iscomposed of three oxygen atoms in a mole-

cule (O3) rather than the more usual two atoms (O2).Ozone is formed from the more common diatomicoxygen where high energy is present. Near theearth, ozone forms in high-temperature combus-tion processes, such as in automobile engines andin electrical sparks.

In the stratosphere, ozone forms because ofhigh-energy ultraviolet radiation. Once formed, itis quick to react with other molecules. Near theearth there are many molecules with which to react,and the ozone concentration remains low. In thestratosphere there are few molecules present, soozone concentration builds up and forms what isknown as the ozone layer. Ozone also disappears

Ozone layer and ozone hole debate • 757

naturally by decomposing to ordinary oxygen, sothere is a natural limit to the concentration that ac-cumulates, and equilibrium, or a “steady state,” oc-curs. The ozone layer is actually quite diffuse, andthe ozone concentration is never very high.

Changes in the Ozone LayerSince the mid-1950’s, measurements of ozone

concentrations in the atmosphere have been maderegularly. In the early 1970’s analysis of the mea-surements suggested that something new wascausing a reduction in the concentration of ozone inthe stratosphere, particularly in the region over theSouth Pole. Continued measurements established asimilar change above the North Pole and a spread-ing of the effect over a larger area. Laboratory

experiments show that molecular fragments con-taining unpaired electrons are very effective inspeeding the decomposition of ozone. This cata-lytic effect is particularly strong in the presence ofsmall ice crystals, as are present in the stratospherein the polar regions in winter.

ChlorofluorocarbonsChlorofluorocarbons (CFCs) are a class of chemi-

cals that have found wide use as propellants inaerosol cans, cleaning solvents for electronic circuitboards, and working fluids in air conditioning andrefrigeration. These molecules are very stable,which is a prime factor in their utility, but this prop-erty also allows the molecules to drift into thestratosphere after they are released. Most other es-

758 • Ozone layer and ozone hole debate

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Area of Antarctica

Ozone values < 220 Dobson unitsAverage area - 30 day maximumVertical lines = minimum & maximum area

Source: Goddard Space Flight Center, National Aeronautics and Space Administration,http:\\toms.gsfc.nasa.gov\multi\oz_hole_area.jpg; accessed April 15, 2002.

caping molecules react or are washed outby precipitation before they gain muchheight in the atmosphere.

In the stratosphere, CFCs decomposeby irradiation and form molecular frag-ments to which ozone is sensitive. CFCsare not the only artificial cause of ozonedepletion, but they have been recognizedas a major contributor. Much of what isknown about the way that the ozone layerforms and decomposes comes from thework of Paul Crutzen, Mario J. Molina,and F. Sherwood Rowland, who receivedthe 1995 Nobel Prize in Chemistry fortheir work on this subject.

The Importance of OzoneOzone is decomposed when the en-

ergy available in part of the ultraviolet re-gion of the spectrum is absorbed by themolecule. When the energy is used insuch a fashion it is no longer present in thesunlight that comes through the strato-sphere to the earth. Thus, the ozone layeracts as a filter to limit the earth’s exposureto high-energy light. This type of energy,if it does make it to the earth, is capable ofcausing the reaction of other molecules,including those of biological importance.The evidence is overwhelming that theprimary cause of nonmelanoma skin can-cers is chronic long-term exposure to ul-traviolet light. Increased ultraviolet lev-els also cause cellular modifications inplants, including food crops, which may lead totheir death. Of particular concern is the inhibitionof photosynthesis in the phytoplankton, the photo-synthetic organisms that form the base of the oceanfood chain. With a diminishing level of filtering,one would expect that there would be a global in-crease in the effects of overexposure to ultravioletradiation.

The Ozone DebateSome scientists believe that ozone depletion is

part of a natural cycle related to sunspot activity.Knowledge of what has happened in the distantpast is circumstantial and not easy to interpret, butmost scientists agree that human activities play asignificant role in the current decrease in the ozonelayer. In terms of the human contribution, CFCs

have received the major attention, and their pro-duction was severely limited by internationalagreement in the 1987 Montreal Protocol and laterrevisions.

CFCs are no longer used as propellants, andtheir role as cleaners is all but over. However, theiruse as refrigerant fluids continues while economi-cally viable, safe substitutes are being sought. Peo-ple in developed countries have become extremelydependent on air conditioning; nearly all largebuildings are designed to be air-conditioned ratherthan open to the outside. Part of the controversyconcerning banning CFCs is based on ethical con-siderations. The developed countries used CFCs tohelp build their economies, so prohibiting CFC usein developing countries is considered inequitableby some. The search for substitutes has proved dif-

Ozone layer and ozone hole debate • 759

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ficult, with economic, safety, and environmentalconcerns all placing limits on what is acceptable.

Kenneth H. Brown

See also: Air pollution; Greenhouse effect; Photo-synthesis; Phytoplankton; Rain forests and the at-mosphere.

Sources for Further StudyAsimov, Isaac. What’s Happening to the Ozone Layer? Milwaukee, Wis.: Gareth Stevens, 1992.

Good source for general readers; explains related concepts very clearly.Brune, William. “There’s Safety in Numbers.” Nature 379, no. 6565 (February 8, 1996): 486-

487. Presents the evidence of the chlorofluorocarbon-ozone depletion link.Maduro, Rogelio A. The Holes in the Ozone Scare: The Scientific Evidence That the Sky Isn’t

Falling. Washington, D.C.: 21st Century Science Associates, 1992. Argues that scientificevidence does not support the theory of ozone depletion. Suggests a conspiracy theory.

Prather, Michael, et al. “The Ozone Layer: The Road Not Taken.” Nature 381, no. 6583 (June13, 1996): 551-555. Predictions stated indicate that if chlorofluorocarbons had not beenlinked to ozone depletion as early as 1974, the current state of the ozone layer would bemore greatly compromised.

Roan, Sharon. Ozone Crisis: The Fifteen-Year Evolution of a Sudden Global Emergency. New York:Wiley, 1990. Good coverage of the early days of the controversy.

760 • Ozone layer and ozone hole debate

PACIFIC ISLAND AGRICULTURE


Agricultural practices in the Pacific Islands have an unsettled history. Typhoons, storms, drought, and volcanic activitycan cause havoc to these islands’ agriculture. For some, the rise of sea level can be devastating.

Throughout history, attempts by foreigners toencourage commercial agriculture in the Pa-

cific Islands through monoculture (single cropping)have threatened fragile island environments. Large-scale land clearing and the use of fertilizers andpesticides have caused erosion, pollution, loss ofbiodiversity, and depletion of precious soils. Intro-ducing new crops to these communities has re-sulted in the loss of native crops, harm to nativespecies, and the elimination of traditional mixedfarming methods.

The Pacific Islands have unique and fragile eco-systems, many of which are endangered by modernagricultural practices. The islands can be dividedinto two main types: the high islands, which aregenerally volcanic islands, and the atolls, or low is-lands. Volcanic lava wears down rapidly and pro-vides fertile soil for cultivation. Atolls are low-lyingcoral reefs, which generally have an inadequatesupply of fresh water and poor soils.

Coconuts, which can grow in poor soil and ripenthroughout the year, are one of the few crops thatthrive on the low islands. Coconuts are an impor-tant source of nutritious food and are easy to store.The meat of the coconut can be dried into a productcalled copra, which is pressed to make multipur-pose oils.

Islands vary greatly in the amounts of rainfallthey receive, the steepness of their slopes, and theirvarieties of plant life. Differences in rainfall andvegetation also exist on different parts of the sameisland. For example, one side of the island of Ha-waii has one of the driest deserts on earth, whereasa few miles away on the other side of the island is atropical rain forest.

Melanesian IslandsThese fairly large islands are located to the

northeast of Australia. They are quite damp, have a

hot climate, and display a mountainous terrain cov-ered with dense vegetation. Papua New Guinea isthe largest island in Melanesia, slightly larger insize than California. It has a mountainous interiorwith rolling foothills that are surrounded by low-lands along the coastal areas. Its highest point isMount Wilhelm, which rises to 14,795 feet (4,509meters). Permanent crops occupy only 1 percent ofthe land. Crops are terraced in areas having steepslopes and extreme vegetation. Irrigation water isoften brought to the crops through bamboo pipes.Approximately 64 percent of the labor force is in-volved in subsistence agriculture. Products growninclude coffee, cocoa, coconuts, palm kernels, tea,rubber, fruit, sweet potatoes, vegetables, poultry,and pork. Palm oil, coffee, and cocoa are exported.In 1997 droughts brought on by the El Niñoweather cycle caused extreme damage to coffee, co-coa, and coconut production.

Vanuatu, which includes eighty islands in theSouth Pacific due east of Australia’s Cape York Pen-insula, covers a total area a bit larger than the stateof Connecticut. Its mostly mountainous terrain pro-vides minimal arable land. Approximately 2 percentof the land is arable, and another 2 percent is usedfor pasture. About two-thirds of the population isinvolved in subsistence or small-scale agriculture.The main agricultural products are coconuts, co-coa, yams, coffee, fruits, vegetables, fish, and beef.Copra, beef, cocoa, and coffee are exported.

New Caledonia, located east of Australia in theSouth Pacific, is almost the size of New Jersey. Itconsists of coastal plains with interior mountainsthat range up to 5,340 feet (1,628 meters) in height.New Caledonia, which is known for its nickel re-sources, imports much of its food supply. A fewvegetables are grown, but raising livestock is morecommon. Of New Caledonia’s land, 12 percent is inpermanent pasture, used for raising beef cattle.

761

The Fiji Islands include 332 islands, 110 of whichare inhabited, located east of Vanuatu in the SouthPacific. These islands are volcanic in origin. Ap-proximately 10 percent of the land is arable, and 10percent is in permanent pasture. About 67 percentof the labor force is involved in subsistence agricul-ture. Sugarcane is an important crop in Fiji and con-stitutes 32 percent of Fiji’s exports. Other productsgrown in Fiji are coconuts, cassava, rice, sweet pota-toes, cattle, pigs, and goats.

The Solomon Islands are a cluster of small is-lands that collectively cover an area almost the sizeof Maryland. They are located in the Solomon Seabetween Papua New Guinea and Vanuatu. Some ofthe islands have rugged mountainous terrain; oth-ers are low coral atolls. Only 1 percent of the land is

arable and 1 percent devoted to pastures. Approxi-mately 24 percent of the working population is in-volved in agriculture, forestry, or fishing. Beans,cocoa, coconuts, palm kernels, rice, potatoes, fruit,and vegetables are grown on the islands. Cattle andpigs are the primary livestock raised there. Palmoil, cocoa, copra, and tuna are exported.

Micronesian IslandsMicronesia comprises thousands of relatively

small islands located along the equator in the cen-tral Pacific Ocean and up to 1,200 miles (2,000 kilo-meters) north of the equator in the western PacificOcean. The region covers 1.54 million square miles(4 million square kilometers) and is subdivided intofour areas: the Kiribati group, which lies around the

intersection of the equator (0 degrees lati-tude) and the international date line (lon-gitude 180 degrees); the Marshall Islands,which are about 900 miles (1,500 kilome-ters) to the northwest of Kiribati; the Fed-erated States of Micronesia, which extendwestward from the Marshall Islands foranother 900 miles; and Guam, a U.S. terri-tory located within the western cluster ofislands of the Federated States of Micro-nesia.

The Kiribati group includes the Gil-bert, Line, and Phoenix Islands, which areprimarily low-lying atolls encircled byextensive living reefs. Of the thirty-threeislands in the group, twenty are inhab-ited. Agriculture is mainly subsistence,with copra being one of Kiribati’s few ex-ports. Grown on the island for local con-sumption are taro, breadfruit, sweet pota-toes, vegetables, and coconuts. Taro is oneof the oldest cultivated plants in PacificIsland history and was once a staple foodof the island people. This starchy, edibletuber can be cultivated by clearing or par-tially clearing a patch in the tropical rainforest and planting the taro in the moistground.

The Marshall Islands contain two is-land chains of 30 atolls and 1,152 islands.Agriculture exists as small farms that pro-vide commercial crops of tomatoes, mel-ons, coconuts, and breadfruit. Coconuts,cacao, taro, breadfruit, pigs, and chickensare produced for local consumption.

762 • Pacific Island agricultureP

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Tropical fruits like these on display at a market are a major exportfrom many Pacific islands.

The Federated States of Micronesia includePohnpei, the Truk Islands, the Yap Islands, andKosrae. There are a total of 607 diverse islands,some high and mountainous, others low-lyingatolls. Volcanic outcroppings are found on Kosrae,Pohnpei, and Truk. Agriculture on the islands ismainly subsistence farming. Products grown in-clude black pepper, coconuts, tropical fruits andvegetables, cassava, and sweet potatoes. Pigs andchickens are the main livestock raised. Bananas andblack pepper are exported.

Guam, the largest island in the Mariana archipel-ago, is of volcanic origin and surrounded by coralreefs. It has a flat coral limestone plateau that servesas a source of fresh water for the islands. In Guam,15 percent of the land is used as permanent pasture.and another 11 percent is arable. Although fruits,vegetable, copra, eggs, poultry, pork, and beef areraised on the island, much of its food is importedbecause the economy relies heavily on U.S. militaryspending and the tourist trade.

PolynesiaPolynesia comprises a diverse set of islands ly-

ing within a triangular area having corners at NewZealand, the Hawaiian Islands, and Easter Island.French Polynesia is at the center of the triangle, 15degrees south latitude and longitude 140 degreeswest, and includes 118 islands and atolls.

The five archipelagoes of French Polynesia in-clude four volcanic island chains (the Society Is-lands, the Marquesas, the Gambiers, and theAustrals) and the low-lying atolls of the Tuamotus.The mountainous volcanic islands contain fertilesoils along their narrow coastal strips. The atollshave little soil and lack a permanent water sup-ply. Permanent pasture covers 5 percent of FrenchPolynesia, and 6 percent of the land is used forpermanent crops. Agricultural products raised in-clude coconuts, vegetables, fruits, vanilla, poultry,beef, and dairy products. Coconut products and va-nilla are exported. Thirteen percent of the popula-tion is involved in agriculture. On Tahiti, the largestisland in French Polynesia, less than 10 percent ofthe land is arable. Exports include vanilla and cof-fee, but the main export from French Polynesia ispearls.

Samoa is a chain of seven islands lying severalthousand miles to the west of Tahiti. Collectively,the islands are almost the size of Rhode Island, andthey are covered with rugged mountains with anarrow coastal plain. Approximately 24 percent ofthe land sustains crops of bananas, taro, yams, andcoconuts. Taro is a crop that can be used in land rec-lamation by building mud ridges or mounds inswampy ponds between the beach rampart and thefoothills. Coconuts grown on the island are pro-cessed into creams and copra for export.

Pacific Island agriculture • 763

Leading Agricultural Crops of Pacific Island Nations

Country ProductsPercent

Arable Land

Fiji Sugarcane, coconuts, cassava, rice, sweet potatoes, bananas 10

Micronesia Black pepper, tropical fruits and vegetables, coconuts, cassava, sweet potatoes —

New Zealand Wheat, barley, potatoes, pulses, fruits, vegetables 9

Palau Coconuts, copra, cassava, sweet potatoes —

Papua New Guinea Coffee, tea, cocoa, coconuts, palm kernels, tea, rubber, sweet potatoes, fruit,vegetables

—

The Philippines Rice, coconuts, corn, sugarcane, bananas, pineapples, mangoes 19

Polynesia Coconuts, vegetables, fruits, vanilla —

Solomon Islands Coconuts, palm oil, rice, cocoa, yams, vegetables, timber, beans, potatoes 1

Vanuatu Copra, cocoa, coffee, coconut, taro, yams, fruits, vegetables 2

Source: Data are from Time Almanac 2000. Boston: Infoplease, 1999.

Tuvalu is a group of nine coral atolls located al-most halfway between Hawaii and Australia. Thesoil is poor on these islands, and there are nostreams or rivers. Water is captured in catchmentsystems and put into storage. Islanders live bysubsistence farming and fishing. Coconut farmingallows the islanders to export copra.

The Cook Islands comprise a combined area al-most the size of Washington, D.C. Located halfwaybetween Hawaii and New Zealand, the northern is-lands are primarily low coral atolls, and the south-ern Cook Islands are hilly volcanic islands. Perma-nent crops occupy about 13 percent of the land; 29percent of the labor force is involved in agriculture.The Cook Islands produce a wide diversity ofcrops, including pineapple, tomatoes, beans, papa-yas, bananas, yams, taro, coffee, and citrus fruits.Agriculture is an important part of the economy,and copra, fresh and canned citrus fruit, and coffeeare major exports.

The Hawaiian Islands are a volcanic chain ofmore than 130 islands, centered near 25 degreesnorth latitude and longitude 160 degrees west. Ha-waii, with more than one million people, has thelargest population in the Polynesian Island group.Lanai, one of the seven largest islands in Hawaii, isprivately owned, and almost all its cultivated landis planted in pineapples. There are more than fifty-five hundred farms in Hawaii, and more than fortycrops are grown commercially.

Because of the absence of adequate water onsome sides of the Hawaiian Islands, water isbrought through aqueducts from the wet sides ofthe islands to the dry sides, then kept in lined res-ervoirs to be used for irrigation, ranching, and

tourism. Sugarcane requires enormous amounts ofwater to grow, and most pineapple crops are grownunder irrigation. Hawaii is the second-largest pro-ducer of macadamia nuts in the world. The islandsof Hawaii, Kauai, Maui, Molokai, and Oahu pro-duce 7.6 million pounds of green coffee a year.Hawaii is also a prime producer of pineapple, canefrom sugar, greenhouse and nursery plants, anddairy products. The main exports are fruits, coffee,and nuts.

New ZealandAbout the size of Colorado, New Zealand is di-

vided into two large islands and numerous smallerislands. About 50 percent of its land is in permanentpasture. A mountainous country with large coastalplains, 9 percent of the land in New Zealand is ara-ble, and 5 percent is planted in permanent crops.Agriculture accounts for 9 percent of the gross na-tional product and employs approximately 10 per-cent of the labor force.

In New Zealand, livestock outnumber peopletwenty-three to one. Wool, lamb, mutton, beef, anddairy products account for more than one-third ofNew Zealand’s exports, and New Zealand is one ofthe world’s top exporters of these products. NewZealand exports 90 percent of its dairy products.Other exports include cheese, fruits, and vegeta-bles. Industry in New Zealand is primarily basedon food products, including processed fruit, wines,and textiles.

Toby R. Stewart

See also: Caribbean agriculture; Monoculture; Pa-cific Island flora.

Sources for Further StudyCampbell, Ian C. AHistory of the Pacific Islands. Berkeley: University of California Press, 1996.

Describes the development of various cultures in the Pacific Islands and their experiencewith intensive European contact during the last two hundred years.

Clarke, W. C., and R. R. Thaman, eds. Agroforestry in the Pacific Islands: Systems for Sustainabil-ity. New York: United Nations University Press, 1993. Covers agroforestry in Melanesia,Polynesia, and Micronesia as well as Pacific Island urban agroforestry. Includes appen-dix: One hundred Pacific Island agroforestry trees.

Gibson, Arrell Morgan. Yankees in Paradise: The Pacific Basin Frontier. Albuquerque: Univer-sity of New Mexico Press, 1993. The story of Americans searching for whales and market-able commodities. Includes a chapter titled “The Agrarian Frontier.”

Mackenzie, John M., and Herman Hiery, eds. European Impact and Pacific Influence: British andGerman Colonial Policy in the Pacific Islands and the Indigenous Response. New York: TaurusAcademic Studies, 1997. Studies the effects of British and German ambitions on the Pa-cific Islanders. Part 2 examines the ecological effect of European intervention.

764 • Pacific Island agriculture

Thawley, John. Austalasia and South Pacific Islands Bibliography. Lanham, Md.: ScarecrowPress, 1997. Directed toward the student who needs specific information about Aus-tralasia or the South Pacific. Each section is defined by key topics including flora andfauna.

PACIFIC ISLAND FLORA


Pacific Island ecosystems are unique and fragile, but with extensive wildlife management, many native species can besaved from extinction.

T he vast region of the Pacific Ocean collectivelycalled Oceania holds thousands of islands.

Oceania spreads across the Pacific from 20 degreesnorth latitude to 50 degrees south latitude and fromlongitude 125 degrees east to 130 degrees west. Themajor groupings of islands are Melanesia, Micro-

nesia, Polynesia, and New Zealand. Melanesia(“black islands”) is a group of large islands imme-diately north and east of Australia, from NewGuinea to New Caledonia. Micronesia (“little is-lands”) is made up of hundreds of tiny atolls in thewestern Pacific. Polynesia (“many islands”) covers

Pacific Island flora • 765D

igital

Stock

Hawaiian palm trees.

a huge region in the central Pacific. New Zealandlies east and south of Australia. For botanical pur-poses, these islands can be categorized by climateand formation type. Climates range from tropical tosub-Antarctic, dry to very rainy. Types include vol-canic (Fiji, Guam, and Hawaii), tectonic (New Zea-land and New Guinea), and low coral atolls (nearlyall of Micronesia’s islands).

Unique EcosystemsOrganisms have a hard time reaching islands

across the broad expanses of the Pacific Ocean. Theislands’ isolation leads to trends in the number ofspecies on any given island. Bigger islands havemore species; those farthest from continents havefewer species. To reach the islands, plants must becarried by animals or rely on wind or water cur-rents. Birds are usually the first visitors, bringingwith them hitchhiking insects and plant seeds intheir digestive tracts.

Island plants and animals evolve together, af-fected by difficult conditions. Soil is often poor andfood limited. Harsh environments and isolationcontribute to the formation of new and uniquespecies. Island ecosystems are sensitive to distur-bances, whether from natural causes, such as severestorms, or human activities, such as construction,agriculture, logging, and introduced species.

Introduced species (exotics), both accidental anddeliberate, are a serious problem. Rats and feral ani-mals (domestic animals that have gone wild) candevastate island ecosystems. Pigs, rats, and goatsare particularly devastating to vegetation. Intro-duced plants may overgrow native ones. Exoticsalso bring diseases or other problems to which na-tive plants and animals have no resistance. For ex-ample, a Hawaiian bee crawls headfirst into native,barrel-shaped flowers, gathers the nectar, and thenbacks out. Aplant that was introduced in Hawaii bylandscapers attracted bees, but the flowers weresmaller than those of the native plants. Once a beecrawled in, it became stuck like a cork in a bottle.This led to thousands of these plants, each withhundreds of flowers, becoming stoppered withdead bees.

Many tropical and temperate islands havecoastal wetlands and mangrove swamps growingat the edge of the sea. Mangroves are low-growing,salt-tolerant trees that form dense tangles virtuallyimpenetrable to humans. Wetlands and mangroveswamps are important breeding grounds for many

types of fish and crabs and also trap sediment, sta-bilize shorelines, and protect coastlines fromstorms. When humans fill in the wetlands and cutdown the mangroves, it causes coastal erosion andthe loss of food fish.

Fiji IslandsThe Fiji Islands are mostly volcanic in origin and

lie in the South Pacific Ocean between longitudes175 degrees east and 178 degrees west and 15 de-grees and 22 degrees south latitudes, about 1,300miles (2,100 kilometers) north of Auckland, NewZealand. Some parts of the islands receive up to 13feet (4 meters) of rain per year, while other parts re-main dry. A range of volcanic peaks divides the is-lands. The differences in weather and elevation cre-ate a variety of habitats—dense rain forests, grassysavanna, and mangrove swamps—and a large di-versity of species.

Human disruption on Fiji has been moderate.About half the total area is still forested, and lessthan one-fourth of the land is suitable for agricul-ture. Trees include mahogany, pine, pandanus, co-conut palms, mangoes, guava, and figs. Banyan figsare difficult to cut down and are responsible forsome of the lack of forest clearing. The figs are animportant food for many birds and animals. Otherrain-forest plants include orchids, ferns, and epi-phytes (plants that grow upon other plants).

There are nearly fifteen hundred endemic Fijianplant species, including ten species of palm tree onthe island of Viti Levu. Grassy savannas are foundhigher on the volcanic slopes and in the dry zones.They are often planted with coconut palms andtaro, a plant with potato-like tubers that grows onmany Pacific Islands.

There are twelve reserve areas in the Fijian is-lands, but several are being logged and providelittle sanctuary to native plants and animals. Thegovernment is interested in increased logging ofmahogany and pine. The Fiji Pine Commissionhopes to encourage the development of pine for-ests. Pines grow quickly and could form a sus-tainable logging industry, unlike the valuable butslow-growing mahogany trees. Increased world in-terest in herbal remedies has created a market forFiji’s traditional crop, kava root, and for gingerprocessing. The University of the South Pacific islocated in Fiji and is a center of serious researchinto South Pacific species. Tourism is important toFiji’s economy and, with management, could be a

766 • Pacific Island flora

source of income to Fijians while preserving nativewildlife.

New GuineaThe world’s largest tropical island, New Guinea

is located north of Australia, just south of the equa-tor. It is tectonic in origin, with large changes in ele-vation and many different habitats. Because of itssize and varied terrain, New Guinea has a greatervariety of habitats than any similar-sized land areain the world. In fact, New Guinea is so rugged thatit is one of the least explored or developed places onearth. It provides the best remaining example of thetypes of organisms that can develop in island isola-tion.

New Guinea habitats include cold tundra, tropi-cal rain forests, grassy savannas, coastal zones,montane rain forests, cloud forests, and bogs. Thereare at least twenty thousand species of floweringplants, including more than twenty-five hundredspecies of orchids, and hundreds of birds and ani-mals. Many New Guinea species are unusual. En-demic Klinki pines are the tallest tropical trees inthe world, reaching 295 feet (90 meters) tall. Manyforests host “ant-plants,” warty looking epiphytesthat have hollow mazes inside their tissues. Antslive in the maze, safe from predators. The ants pro-vide nutrition for the plant in the form of drop-pings, scraps of food, and dead ants.

Even though New Guinea is rugged and iso-lated, human impact is increasing. The populationis rising, which means that more forests are beinglogged and grasslands plowed for agriculture,roads, and development. Humans have brought infood plants such as the sago palm, which can be cul-tivated in areas where traditional crops will not sur-vive. They also cultivate pandanus trees, severalvarieties of fruiting vine, breadfruit, fungi, tubers,sugarcane, bananas, taro, and yams. Gold, silver,and copper have been discovered, which encour-ages destructive mining.

It has proven difficult to develop New Guineaeconomically without destroying the unique lifeof the island. It is hoped that lessons learned onother islands, such as Guam and New Zealand,may be applied to New Guinea. The governmenthas tried incentives to keep wild areas wild, includ-ing encouraging ecologically friendly businesses,such as crocodile and butterfly farms and eco-tourism. The National Park reserve that includesMount Jaya is the only place in the world where it

is possible to visit a glacier and a coral reef in thesame park.

New ZealandLocated off the eastern edge of Australia, New

Zealand has a fairly moderate climate that comesfrom conflicting warm, humid Pacific and colderAntarctic weather. It is similar to New Guinea, withrugged terrain, high mountains, and habitats fromgrassy open plains to dense forests, wet areas tonear-deserts. Unlike New Guinea, however, NewZealand has been occupied and developed by hu-mans for hundreds of years. Before large-scale agri-culture, about half of New Zealand was coveredwith forests and one-third with grassland commu-nities. Now half is pasture for grazing, and one-quarter is forest, mostly introduced species. Muchof the remaining native forest is maintained as na-tional parks and reserves. Pastureland usually con-sists of a single species of grass and does not sup-port the wide variety of bird and animal life of theoriginal grassland communities. There are morethan four thousand species of beetles, two thou-sand species of flies, and fifteen hundred species ofbutterflies and moths. In a reversal of the usual eco-logical concerns, some native insects are destroyingintroduced pasture grasses.

Coral AtollsThe Federated States of Micronesia consist

mainly of small atolls. Coral atolls are found onlyin tropical latitudes because coral (small, colonialanimals) grow only in warm water. Coral reefs sup-port a tremendous variety of fish, crabs, and mol-lusks. Atolls tend to have porous, infertile soil andto be very low in elevation. The inhabited state ofTokelau, three small islands located at 9 degreessouth latitude, longitude 172 degrees west, has amaximum elevation of 16 feet (5 meters). Due to thelow profile, poor soil, and occasional scouring bytyphoons, flora are mostly limited to hardy rootcrops and fast-growing trees such as coconut andpandanus. Other vegetation may include nativeand introduced species such as papaya, banana,arrowroot, taro, lime, breadfruit, and pumpkin.Common fauna are lizards, rodents, crabs, andother small creatures. Pigs, ducks, and chickens areraised for food.

Human disturbances on coral atolls often havebeen particularly violent; several nuclear testbombs were exploded on Bikini Atoll and other

Pacific Island flora • 767

islands in the 1940’s and 1950’s. Kwajalein, the larg-est atoll in the world, is used by the U.S. military forintercontinental ballistic missile target practice.Johnston Atoll, about 820 miles (1,320 kilometers)southwest of Honolulu, is a U.S. military base andstorage facility for radioactive and toxic sub-stances. It is also designated as a protected area andbird-breeding ground.

Future ProspectsIsland ecosystems are unique and fragile. Some,

like those in Guam and New Zealand, can never bereturned to their original state, but with extensivewildlife management, many native species can be

saved from extinction. New Guinea, Fiji, and manysmaller islands are in earlier stages of development,and wildlife destruction can still be controlled.Conservationists sometimes do not realize that is-lands are not small, geographical zoos and gardens;people live there and want to improve their lives.Development cannot be stopped, but it can be man-aged so that the humans can improve their stan-dard of living as they wish and the original, amaz-ing island dwellers can still survive.

Kelly Howard

See also: Biological invasions; Caribbean flora; In-vasive plants; Pacific Island agriculture.

Sources for Further StudyHargreaves, Dorothy. Tropical Blossoms of the Pacific. Kailua, Hawaii: Hargreaves, 1970. Illus-

trated identification guide to flora.Kirch, Patrick V., and Terry L. Hunt, eds. Historical Ecology in the Pacific Islands: Prehistoric En-

vironmental and Landscape Change. New Haven, Conn.: Yale University Press, 1997. Re-search findings challenge earlier views that the Pacific Islands underwent dramatic envi-ronmental change only after Western contact.

Stuessy, Tod F., and Mikio Ono, eds. Evolution and Speciation of Island Plants. New York: Cam-bridge University Press, 1998. Covers issues in oceanic island plant biology, focusing onpatterns and processes in the Pacific.

Whistler, W. Arthur. Flowers of the Pacific Island Seashore. Honolulu: Isle Botanica, 1992. Illus-trated book on tropical flowers identifies not only cultivated ornamental plants but alsoplants found growing all over the islands of Polynesia, Melanesia, and Micronesia.

_______. Wayside Plants of the Islands: A Guide to the Lowland Flora of the Pacific Islands. Hono-lulu: Isle Botanica, 1995. Guidebook to the identification of the most common plant spe-cies encountered in the lowlands of the Pacific Islands.

PACLITAXEL

Categories: Economic botany and plant uses; medicine and health

Also known by the brand name Taxol, paclitaxel is a potent cancer-fighting drug originally derived from the bark of thePacific yew tree, a small- to medium-sized understory tree that occupies Pacific coastal forests from southwestern Alaskato California.

Development of paclitaxel as a drug began in1962 with the collection in Washington State of

the reddish-purple bark of the Pacific yew tree(Taxus brevifolia nutt) by Kurt Blum, then a techni-cian with the National Cancer Institute (NCI). TheNCI was employing a “shotgun” approach to can-cer research: A wide variety of plant parts of var-

ious species were being screened for anticanceractivity. Thereafter, several scientists, includingMonroe Wall and M. C. Wani at Research TriangleInstitute in North Carolina and Susan Horwitz andPeter Schiff of the Albert Einstein College ofMedicine in New York, recognized the potential ofpaclitaxel and became intensely interested.

768 • Paclitaxel

After years of delay, the pharmaceutical com-pany Bristol-Myers Squibb continued tests and pro-duction on a larger scale. Paclitaxel was found to ar-rest the growth of cancer cells by attaching to theirmicrotubules, thus preventing cell division. By thelate 1980’s Taxol had become the drug of choice, de-spite its high cost, for the treatment of a wide rangeof cancers, especially ovarian and breast cancer.

In spite of Taxol’s prominence as a success storyin the “herbal renaissance” of the twentieth cen-tury, several problems involved in production andmedicinal use have persisted. For one, the cost ofTaxol treatment has been prohibitive for many whodesperately need it. The large amount of bark re-quired (all the bark from a one-century-old treeyields only enough paclitaxel for a 300-milligramdose) raised fears among conservationists that con-tinued harvest could threaten the species. While oc-curring over a wide area, the Pacific yew tree existsonly in relatively small numbers. Furthermore, it isa slow-growing species that rarely reaches a heightof more than 18 meters (60 feet); stripping the bark

kills the tree. Plantations of the Pacific yew treecould be established, but it would take years forthem to become productive.

Several means of producing paclitaxel withoutthe destruction of wild yew trees have been pro-posed. Attempts have been made to produce pac-litaxel from tissue cultures. Efforts to identify otherTaxus species that may contain paclitaxel have beenonly marginally successful. In 1993 Bristol-MyersSquibb announced that it had found a semisyn-thetic method of producing paclitaxel that does notrequire yew bark. Paclitaxel-like compounds havebeen found in extracts from needles of the Euro-pean yew tree (Taxus baccata) and those of severalyew shrub species. An important advantage is thatneedles can be harvested without killing the treesor shrubs. Similar compounds have also been foundin a fungus that grows on Taxus species.

Thomas E. Hemmerly

See also: Endangered species; Medicinal plants;Mitosis and meiosis.

Paclitaxel • 769

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Sources for Further StudyGeorg, Gunda, ed. Taxane Anticancer Agents: Basic Science and Current Status. Washington,

D.C.: American Chemical Society, 1995. Presents an overview of the development ofpaclitaxel as an anticancer drug, including its discovery, initial development, supply is-sues, and approaches to its total synthesis.

Goodman, Jordan, and Vivien Walsh. The Story of Taxol: Nature and Politics in the Pursuit of anAnti-Cancer Drug. New York: Cambridge University Press, 2001. Recounts a longstand-ing argument of environmentalists for preserving threatened habitats: that the plantswithin them may contain cures for ailments. This proved to be true with the discovery ofthe compound in Pacific yew bark, which pitted environmentalists against pharmaceuti-cal companies.

Suffness, Matthew, eds. Taxol: Science and Applications, Pharmacy and Toxicology, Basic andClinical Aspects. Boca Raton, Fla.: CRC Press, 1995. Brings together all aspects of Taxol re-search, development, and clinical use.

PALEOBOTANY

Categories: Classification and systematics; disciplines; evolution; paleobotany

The study of plants in the fossil record, in order to understand both the evolution of plant life and the ecology of ancienteras, is known as paleobotany.

Only a small percentage of the plants that everlived left a record of their existence, surviving

as fossils: mineralized wood, flowers in amber, leafimprints in coal, or other indicators of life in an ear-lier era. Paleobotanists document this fossil recordand use it to interpret the past evolution of plants.

Importance of Plant FossilsPaleontology (or paleobiology) is the science con-

cerned with fossils, the physical evidence of prehis-toric life—including plants, animals, and microor-ganisms—on the earth. Paleobotany focuses onplant fossils, including algae, fungi, and related or-ganisms, as well as mosses, ferns, and seed plants.As most organisms decompose rapidly after death,their preservation in nature is a rare event. Most in-dividuals are not represented in the fossil record,and even many species that must have existed havevanished without a trace.

As a branch of botany, paleobotany is of impor-tance primarily because the record of fossil plantshelps scientists understand the long process ofplant evolution. Especially since the 1940’s, fossilevidence has helped to explain the origin of majorclasses of organisms, such as algae and fungi. Re-

searchers now also have evidence for the origin ofthe earliest vascular plants and the formation of re-productive structures, such as cones of gymno-sperms (evergreen trees and relatives) and flowersof angiosperms (flowering plants). The location offossils, including both their temporal (age) andtheir spatial (geographical) arrangement, is used todetermine past climates.

The climates of the world have changed continu-ously as continents have shifted over the earth’ssurface. For example, the location of coal deposits(which are the remains of giant tree ferns) in what isnow Pennsylvania indicates the warmer climatethat must have existed then. Although perhapsmost of the contributions to paleobotany have beenmade by professionally trained scientists with asolid background in geology, botany, and relatedsciences, amateurs have also made significant dis-coveries. Many valuable specimens of universityand museum collections were made by people in-terested in paleobotany as a hobby.

How Fossils FormThe formation of a fossil is an exceptional event,

one that requires a special combination of favorable

770 • Paleobotany

environmental conditions. In the most commonfossilization process, the plant becomes covered bya soft sediment that then hardens to form a sedi-mentary rock. This type of rock forms gradually,over long periods of time, as particles produced byerosion are compacted on the bottom of the body ofwater.

The large-scale process by which plant parts be-come impregnated with minerals produces whathas traditionally been called petrified wood. Themodern term for this process is permineralization.Soluble carbonates, silicates, and other compoundsinfiltrate plant cells and the spaces between them.Eventually, the mineral deposits may completelyreplace the naturally occurring organic matter, pre-serving the details of the plant’s microscopic archi-tecture. Well known are the petrified forests ofwestern United States, many of which are protectedwithin national parks, such as Petrified Forest Na-tional Park in Arizona.

Being trapped in a sedimentary rock does notautomatically guarantee that the organism will bepreserved. The environment must be an anaerobicone—that is, one in which oxygen is excluded—thus preventing the decay that would otherwise re-sult. The process may be interrupted by the actionof waves or other erosive forces which re-exposethe developing fossil before the process of fossiliza-tion is completed. Even after the process is com-pleted, the well-preserved specimen may becomedistorted or altered in appearance because of thecombined effects of time, pressure, and high tem-peratures that convert sedimentary rocks intometamorphic rocks.

As one would expect, the harder cells and tissuesof plants are more likely to be preserved as fossilsthan are softer ones. For example, the thick-walledcells of wood and bark (called xylem) are more oftenpreserved than are those of the pith (center of a stem)or cortex (found in stems and roots beneath the barkor outer covering). Other cells that are often fossil-ized are pollen grains and spores, both of whichhave outer shells that are highly resistant to decay.

Limestone and dolomite are among the mostcommon types of rocks that form in such a way thatthey trap plants and form fossils. Coal, a combusti-ble sedimentary rock, is formed in much the sameway as other rocks but is distinctive because thesediment involved is of plant, rather than mineral,origin. Within this matrix of plant-derived materialis often embedded a variety of plant parts.

Special Types of FossilsTwo special kinds of rock that may contain plant

and animal fossils are diatomite and amber. Diato-mite is a rock that forms from the silica cell walls ofa group of unicellular algae known as diatoms. Be-cause silica is the same material that sand andquartz are composed of, it is unusually permanent.Diatoms are found in both fresh water and saltwater in great numbers and diversity. When theydie, their cell walls accumulate underneath thewater and become compacted over time into diato-mite. The rock, itself formed by fossilization, mayhave fossil remains of various kinds of plants andanimals preserved within it.

Amber, considered a semiprecious stone bygemologists and valued because of its beauty anddistinctive appearance, is also of interest to pa-leobotanists. Amber is basically the fossilized resinproduced by ancient cone-bearing evergreen trees.Sticky resins ooze from trees in response to injuries.Before such resins harden, various small animals,floral parts, pollen grains, fungal spores, and otherplant parts may become trapped and be preservedintact. Deposits of amber valued for their use as jew-elry and as fossils are recovered mainly from twoworld areas: the Baltic region of northern Europeand the Dominican Republic in the Caribbean Sea.

Paleobotanists are sometimes challenged bypuzzling specimens. Outright fakes are sometimespresented by pranksters, but more common arevarious mineral structures that bear a superficialresemblance to a plant. Such specimens are calledpseudofossils. Mineral deposits called dendritesfound in rock crevices bear a resemblance to fernleaves. A coprolite (fossilized feces) from the upperCretaceous period in Alabama was initially mis-taken for the cone of a conifer (cone-bearing ever-green tree); these specimens may be referred to aspseudo-plant fossils, as they are true fossils ofanimals. During the formation of flint, bands aresometimes formed that suggest fossil mollusks orcoral. Suspicious specimens require careful analy-sis by a specialist. In general, plants and animals,and therefore their remains, possess details and acharacteristic regularity of form absent in pseudo-fossils.

Naming and Classifying FossilsIn order to prevent confusion, fossils, like living

species, need to be named and classified in a consis-tent, systematic fashion recognized by paleobiol-

Paleobotany • 771

772 • Paleobotany

The Geologic Time ScaleMYA Eon Era Period Epoch Developments

0.01

Phan

eroz

oic

Eon

(544

mya

-tod

ay)

Cen

ozoi

c(6

5m

ya-t

oday

)

Quaternary(1.8 mya-today)

Holocene(11,000 ya-today)

Ice Age ends. Human activities begin to impact biosphere.

1.8

Pleistocene(1.8 mya-11,000 ya)

Ice Age in northwest Europe, Siberia, and North America. Plantsmigrate in response. New speciations lead to modern plants. Modernhumans evolve.

5Tertiary(65-1.8 mya)

Pliocene(5-1.8 mya)

Cooling period leads to Ice Age.

23Miocene(23-5 mya)

Erect-walking human ancestors

38Oligocene(38-23 mya)

Primate ancestors of humans

54Eocene(54-38 mya)

Intense mountain building occurs (Alps, Himalaya, Rockies); modernmammals appear (rodents, hoofed animals); diverse conferous forests

65

Paleocene(65-54 mya)

Cretaceous-Tertiary event leads to dinosaurs’ extinction c. 65 mya;seed ferns, bennettites and caytonias die off; ginkgoes decline;deciduous plants rise.

146

Mes

ozoi

c(2

45-6

5m

ya)

Cretaceous (146-65 mya) Breakup of supercontinents into today’s form; birds appear. Cycads,other gymnosperms still widespread, but angiosperms dominate by90 mya; animal-aided pollination begins to evolve.

208

Jurassic (208-146 mya) Earliest mammals; ginkgoes thrive in moister areas; drier climates inNorth and South America, parts of Africa, central Asia; rise of moderngymnosperms, such as junipers, pine trees. Earliest angiosperm fossil(from China) dates from end of this period.

245

Triassic (245-208 mya) Diminished land vegetation, with lack of variation reflecting globalfrost-free climate; gymnosperms dominate, bennettites andgnetophytes appear; dinosaurs develop.

286

Pale

ozoi

c(5

44-2

45m

ya)

Permian (286-245 mya) Permian extinction event initiates drier, colder period; supercontinentPangaea has formed; tree-sized lycophytes, club mosses, cordaites dieoff; horsetails, peltasperms, cycads, conifers dominate.

325Carboniferous(360-286 mya)

Pennsylvanian(325-286 mya)

Gymnosperms, ferns, calamites, lycopods thrive (first seed plants);reptiles appear. Plant life diverse, from small creeping forms to tallforest trees. Coal beds form in swamp forests from the dominantseedless vascular plants.

360Mississippian(360-325 mya)

410Devonian (410-360 mya) Club mosses, early ferns, lycophytes, progymnosperms; amphibians,

diverse insects; horsetails, gymnosperms present by end of period.

440

Silurian (440-410 mya) Early land plants: nonvascular bryophytes (mosses, hornworts),followed by seedless vascular plants in now-extinct phyla Rhyniophyta,Zosterophyllophyta, Trimerophytophyta.

505 Ordovician (505-440 mya) Life colonizes land; earliest vertebrates appear in fossil record.

544Cambrian(544-505 mya)

Tommotian(530-527 mya)

Cambrian diversification of life

900

Prec

ambr

ian

Tim

e(4

,500

-544

mya

)

Prot

eroz

oic

(250

0-54

4m

ya) Neoproterozoic

(900-544 mya)Vendian(650-544 mya)

Age of algae, earliest invertebrates

1600 Mesoproterozoic (1600-900 mya) Eukaryotic life established.

2500Paleoproterozoic (2500-1600 mya) Transition from prokaryotic to eukaryotic life leads to multicellular

organisms.

3800 Archaean (3800-2500 mya) Microbial life (anaerobic and cyanobacteria) as early as 3.5 bya

4500 Hadean (4500-3800 mya) Earth forms 4.5 bya.

Notes: bya = billions of years ago; mya = millions of years ago; ya = years ago.Source: Data on time periods in this version of the geologic time scale are based on new findings in the last decade of the twentieth century as presented

by the Geologic Society of America, which notably moves the transition between the Precambrian and Cambrian times from 570 mya to 544 mya.

ogists throughout the world. The branch of biologydevoted to the naming and classification of organ-isms is called taxonomy. The same system is appliedto both living and fossilized plants.

According to the system of binomial nomencla-ture, each species is given a scientific name consist-ing of two parts: the genus name followed by thespecies name. The former is capitalized, whereasthe latter is written in lower case; both are italicized.Often a name follows that belongs to the personwho assigned that name. As example, the scien-tific name of an extinct redwood tree is Sequoiadakotensis Brown, while that of a stemless palm isNipa burtinii Brongniart. The ginkgo tree, Ginkgobiloba L., is considered a “living fossil.” Knownfrom the fossil record, it persists as a commonlyplanted shade tree. The initial following its scien-tific name is that of Carolus Linnaeus, the Swedishbotanist who established this binomial system ofnomenclature in 1753.

Species, living or dead, are classified using a hi-erarchical system (also by Linnaeus), which reflectsdegrees of similarity or dissimilarity to other spe-cies. All three trees already mentioned, becausethey are (or were) vascular plants, are assigned tothe division Tracheophyta. Within that division theredwood and ginkgo, because they produce uncov-ered seeds, are placed into the class Gymnosper-mopsida (naked seeded plants), whereas the palm,a flowering plant, is assigned to the class Angio-spermophytina.

The plant fossil record is often used to establishnatural relationships among various extant plantspecies and other taxa at higher levels. This is es-pecially true of the vascular plants. In fact, thedivision Tracheophyta was established by A. J.Eames in 1936 to show the natural relationshipbetween seed plants and ferns. The basis for thisnew category was the discovery, earlier in the twen-tieth century, of Devonian fossils of a group ofprimitive vascular plants known as psilotophytes.They were recognized as ancestral to both ferns andseed plants. Previously, ferns and seed plants hadbeen assigned to a separate division of the plantkingdom.

As the plant fossil record becomes more com-plete, further revision of the classification systembecomes necessary to allow the system to morenearly reflect the true or natural relationshipsamong the various categories. This is, at least, thegoal of both paleobotanists and those who studymodern plants.

Thomas E. Hemmerly

See also: Angiosperm evolution; Cladistics; Coal;Cycads and palms; Dendrochronology; Diatoms;Evolution of plants; Ferns; Fossil plants; Ginkgos;Gymnosperms; Paleoecology; Petrified wood;Psilotophytes; Seedless vascular plants; Speciesand speciation; Systematics and taxonomy; Sys-tematics: overview; Tracheobionta.

Sources for Further StudyAgashe, Shripad N. Paleobotany: Plants of the Past, Their Evolution, Paleoenvironment, and Ap-

plication in Exploration of Fossil Fuels. Enfield, N.H.: Science Publishers, 1997. Describes thegeologic history of the earth and the origin and evolution of different groups of plants onthe earth’s surface. Covers principles and applications of paleobotany. With illustrations.

Cleal, Christopher J., and Barry A. Thomas. Plant Fossils: The History of Land Vegetation. NewYork: Boydell Press, 1999. For palaeobotany students or anyone interested in the originand evolution of land plants. Describes methods of study and reviews major figures inpaleobotany since the field’s inception in the early 1800’s. With illustrations.

Crane, Peter R. “Paleobotany: Back to the Future.” American Journal of Botany 87, no. 6 (June,2000): S2. Twentieth century advances in paleobotany have emphasized thorough inte-gration of data with information of living plants, meaning scientists know more about thepast than they did in the past.

Gensel, Patricia G. “Plants, Fossils, and Evolution: Lessons from the Fossil Hunters.” Ameri-can Journal of Botany 87, no. 6 (June, 2000): S1. Discusses the importance of and provides atemplate for recording and sharing information about plant biology and diversity and thepatterns and processes of evolution.

Gensel, Patricia G., and Dianne Edwards, eds. Plants Invade the Land: Perspectives in Paleo-biology and Earth History. New York: Columbia University Press, 2001. Contributors pre-

Paleobotany • 773

sent fossil record-based understanding of the evolution of plants on land. With summarychart of time period covered (500 million to 360 million years ago).

Pigg, Kathleen B. “Paleobotany.” Geotimes 45, no. 7 (July, 2000): 36-37. A description of theadvances in the field of paleobotany in 1999.

PALEOECOLOGY

Categories: Disciplines; ecology; evolution; paleobotany

Paleoecology is the study of ancient environments. As a field of science, paleoecology is most closely related to paleontol-ogy, the study of fossils. It is also related to paleobotany and a number of other areas of study dealing with the distantpast.

Because paleoecology and its related disciplines(paleontology, paleobotany, paleoclimatology,

paleogeography, and others) deal with the past, sci-entists are unable to apply the usual scientific crite-ria of direct observation and measurement of phe-nomena. In order to make any conclusions aboutthe past, scientists must assume at least one state-ment to be true without direct observation: The pro-cesses that exist in the modern universe and on themodern earth existed in the past.

Uniformitarianism and CatastrophismPaleontologists must assume that ancient plants

and animals had tolerances to temperature, mois-ture, and other environmental parameters similarto those of modern organisms. The belief that thepresent is the key to the past is called uniform-itarianism. It has been a key concept of the biologicaland earth sciences since the early nineteenth cen-tury. Uniformitarianism does not include the beliefthat the ancient earth was like the modern earth inits life-forms or geography.

During the early part of the nineteenth century,another worldview dominated: catastrophism, thebelief that the earth is relatively young and wasformed by violent upheavals, floods, and other ca-tastrophes at an intensity unlike those of modernearth. Many catastrophists explained the presenceof fossils at high elevations by the biblical flood ofNoah. The uniformitarian viewpoint prevailed and,although admitting that local catastrophes may beimportant, their long-term, earthwide importancewas denied. Catastrophism was revived in the1980’s to explain certain important events. The rapid

extinction of the large dinosaurs at the close of theMesozoic era has been attributed to the climaticchanges associated with an alleged encounter be-tween the earth and a comet—certainly a cata-strophic event.

Climatic CyclesOne of the most intensively investigated paleo-

ecological problems has been the changing envi-ronments associated with the ice ages of the pastmillion years. Analysis of pollen from bogs in manyparts of the world indicates that there have been atleast four advances and retreats of glaciers duringthat period. Evidence for this is the changing pro-portions of pollen from tree species found at thevarious depths of bogs. In North America, for ex-ample, spruces (indicators of cool climate) formerlylived much farther south than they do now. Theywere largely replaced almost eight thousand yearsago by other tree species, such as oaks, which are in-dicative of warmer climates. This warming trendwas a result of the latest glacial retreat.

Dendrochronology (tree-ring analysis) not onlyenables paleoecologists to date past events such asforest fires and droughts but also allows them tostudy longer-term cycles of weather and climate,especially those of precipitation and temperature.In addition, trees serve as accumulators of pastmineral levels in the atmosphere and soil. Lead lev-els of tree wood showed a sharp increase as the au-tomobile became common in the first half of thetwentieth century because of lead additives in gas-oline. Tree rings formed since the 1970’s haveshown a decrease in lead because of the decline in

774 • Paleoecology

use of leaded fuels. Tree-ring analysis has also beena valuable tool for archaeologists’ study of climaticchanges responsible for shifting patterns of popula-tion and agriculture among American Indians ofthe southwestern United States.

Traces of Ancient EnvironmentsFossil evidence is the chief source of paleoeco-

logical information. Afossil bed of intact clamshellswith both valves (halves) present in most individu-als usually indicates that the clams were preservedin the site in which they lived (autochthonous deposi-tion). Had they been transported by currents ortides to another site of deposition (allochthonous de-position), the valves would have been separated,broken, and worn. Similarly, many coal beds haveyielded plant fossils that indicate that their ancientenvironments were low-lying swamp forests withsluggish drainage periodically flooded by watercarrying a heavy load of sand. The resulting fossilsmay include buried tree stumps and trunks withroots still embedded in their original substrate andnumerous fragments of twigs, leaves, and barkwithin the sediment.

Certain dome- or mushroom-shaped structurescalled stromatolites are found in some of the mostancient of earth’s sedimentary rocks. These struc-tures may be several meters in diameter and consistof layers of material trapped by blue-green algae(cyanobacteria). Such structures are currently beingformed in shallow, warm waters. Uniformitarianinterpretation of the three-billion-year-old stromat-olites is that they were formed under similar condi-tions. Their frequent association with mud cracksand other shallow- and above-water features leadsto the interpretation that they were formed in shal-low inshore environments subject to frequent expo-sure to the air.

Relative oceanic temperature can be estimatedby observing the direction in which the shells ofcertain planktonic organisms coil. The shell ofGlobigerina pachyderma coils to the left in cool wa-ter and to the right in warmer water. Globigerinamenardii shells coil in an opposite fashion—to theright in cool water and to the left in warmer water.Uniformitarian theory leads one to believe thatancient Globigerina populations responded to watertemperature in a similar manner. Sea-bottom coresamples showing fossils with left- or right-coilingshells may be used to determine the relative watertemperature at certain periods. Eighteen-thousand-

year-old sediments taken from the Atlantic Oceanshow a high frequency of left-handed pachydermaand right-handed menardii shells. Such observa-tions indicate that colder water was much farthersouth about eighteen thousand years ago, a datethat corresponds to the maximum development ofthe last ice age.

Interpreting CluesFossil arrangement and position can be clues to

the environments in which the organisms lived orin which they were preserved. Sea-floor currentscan align objects such as small fish and shells. Notonly can the existence of the current be inferred, butalso its direction and velocity can be determined.Currents and tides can create other features in sedi-ments which are sometimes indicators of environ-ment. If a mixture of gravel, sand, silt, and clay isbeing transported by a moving body of water suchas a stream, tide, or current, the sediments will of-ten become sorted by the current and be depositedas conglomerates—sandstones, siltstones, andshales. Such graded bedding can be used to deter-mine the direction and velocity of currents. Largerparticles, such as gravel, would tend to be depos-ited nearer the sediment source than smaller parti-cles such as clay. Similarly, preserved ripple marksindicate current direction. Mud cracks in a rocklayer indicate that the original muddy sedimentwas exposed to the atmosphere at least for a time af-ter its deposition.

Certain minerals within fossil beds or within thefossil remains themselves can sometimes be used tointerpret the paleoenvironment. The presence ofpyrite in a sediment almost always indicates thatthe sedimentary environment was deficient in oxy-gen, and this, in turn, often indicates deep, stillwater. Such conditions exist today in the Black Seaand even in some deep lakes, with great accumula-tions of dead organic matter.

The method of preservation of the remains of thefossilized organism can be an indication of the envi-ronment in which the creature lived (or died). Am-ber, a fossilized resin, frequently contains the em-bedded bodies of ancient insects trapped in theresin like flies on flypaper. This ancient environ-ment probably contained resin-bearing plants(mostly conifers) and broken limbs and stumps thatoozed resin to trap these insects. Mummified re-mains in desert areas and frozen carcasses in thenorthern tundra indicate the environments in

Paleoecology • 775

which the remains were preserved thousands ofyears ago.

Fossils and FuelsOne of the most immediately important applica-

tions of paleoecological data is in the search for in-creasingly scarce fossil fuels: petroleum, natural gas,and coal. Reconstruction of ancient environmentsand their paleogeographical distributions is proba-bly the most accurate way to predict the presence ofreservoirs of these natural resources. Because thesesubstances are formed in environments that are bi-ologically highly productive, with abundant plantand animal life, they are commonly found associ-ated with reefs (petroleum, gas) and swamps (coal).

Index fossils and fossil assemblages can indicatesuch environments with a high degree of accuracy.Petroleum and gas reservoirs must be porous andpermeable in order for the material to accumulatein high concentrations. Reef material and poroussandstones formed from ancient sandbars meet

such criteria. Most of the historically famous oil-producing region of Pennsylvania lies within anarea 40 to 100 kilometers west of a former shorelinenear present-day Pittsburgh. Within this area, mostoil pools are within the ancient offshore sandbarbelt. Sediments forming this “Pocono formation”came from the east, from an area near present At-lantic City, New Jersey. These sediments, all depos-ited within the same time frame, grade from coarse,nonmarine conglomerates and sandstones in theeast near their source to fine-grained sandstonesand shales to the west in the oil-producing region.

P. E. Bostick

See also: Anaerobic photosynthesis; Archaea; Bacte-ria; Coal; Dendrochronology; Evolution: conver-gent and divergent; Evolution: gradualism vs.punctuated equilibrium; Evolution: historical per-spective; Evolution of cells; Evolution of plants;Fossil plants; Paleobotany; Petrified wood; Pro-karyotes; Stromatolites.

Sources for Further StudyAgashe, Shripad N. Paleobotany: Plants of the Past, Their Evolution, Paleoenvironment, and Ap-

plication in Exploration of Fossil Fuels. Enfield, N.H.: Science Publishers, 1997. Describes thegeologic history of the earth and the origin and evolution of different groups of plants onthe earth’s surface. Covers principles and applications of paleobotany. With illustrations.

Arduini, Paolo, and Giorgio Teruzzi. Simon and Schuster’s Guide to Fossils. New York: Simon& Schuster, 1986. An excellent introduction to the subject of fossils and fossilization. Gooddiscussion of paleoecology. Well illustrated with color photographs and symbols indicat-ing the time periods and habitats of the organisms.

Bennett, K. D. Evolution and Ecology: The Pace of Life. New York: Cambridge University Press,1997. Presents paleoecological evidence that evolutionary processes visible in ecologicaltime cannot be used to predict macroeveolutionary trends, contrary to Charles Darwin’sthesis.

Cowen, Richard. History of Life. 3d ed. Malden, Md.: Blackwell Science, 2000. A brief,easily understood introduction to evolution and paleontology. Good description of thepaleogeography of various continents and regions. Includes a brief list of further read-ings.

Davis, Richard A. Depositional Systems: An Introduction to Sedimentology and Stratigraphy. En-glewood Cliffs, N.J.: Prentice-Hall, 1992. This is an advanced but understandable text-book about sediments and their environmental aspects of deposition. Includes glossary,extensive bibliography.

Dodd, J. Robert, and Robert J. Stanton, Jr. Paleoecology, Concepts and Applications. 2d ed. NewYork: Wiley, 1990. This revised edition reflects developments and changing emphasis inthe field of paleoecology; includes references on all topics.

National Research Council. Commission on Geosciences, Environment, and Resources.Board on Earth Sciences and Resources. Effects of Past Global Change on Life. Washington,D.C.: National Academy Press, 1995. Recalls how particular environmental changes havecaused certain species to become extinct or give rise to other species and shows that somesystems are more fragile than others. Provides a scientific framework for policymakers.

776 • Paleoecology

Shipman, Pat. Life History of a Fossil: An Introduction to Taphonomy and Paleoecology. Cam-bridge, Mass.: Harvard University Press, 1993. Perhaps the best reference available on thefossilization process (taphonomy). Written at the advanced college level, this is a well-illustrated book with graphs, maps, line drawings, and photographs. Includes glossaryand further references.

PARASITIC PLANTS

Categories: Fungi; poisonous, toxic, and invasive plants

Parasitic plants are those symbiotic life-forms that form mutualistic relationships with other plants at their hosts’ ex-pense.

The more is learned about plants and other or-ganisms, the more apparent it is that many, if

not most, plants are involved in symbiotic associa-tions in which one or more other organisms live inclose interaction with the plant—almost as if theyare one. Symbiosis is frequently thought of as beinga mutualistic relationship in which both the plantand its symbiont benefit. Typical examples includethe alga and fungus that make up a lichen and thebacteria living in a legume root nodule. Commen-salistic relationships, such as Spanish moss hangingfrom a live oak, are less commonly thought of; inthis relationship, one plant obviously benefits,while the other seems to be little affected.

Parasitism is the third symbiotic condition, inwhich one plant benefits, but the other is harmed.Fungi are by far the most common parasites ofplants; the study of these fungi is a major compo-nent of the science of plant pathology. Less com-mon, but frequently well known, are some flower-ing plants, such as mistletoe, that parasitize otherplants.

HaustoriumThe defining feature of parasitic plants is the

presence of a haustorium. The concept of a haus-torium is broadly defined as a structure formedby the parasite to connect it physiologically to itshost—a morphological/physiological bridge. Thehaustorial organ may be a simple outgrowth andproliferation of tissue, or it may be an elaborate,highly organized structure. Its appearance is oftena useful diagnostic feature. Typically, the hausto-rium serves three functions: to attach the parasite to

the host (adhesion), to penetrate into the host tissueor even into individual host cells (intrusion), and toconduct water and nutrients from the host to theparasite (conduction).

Fungal ParasitesFungal parasites are among the most destructive

diseases of plants. (Some, such as athlete’s foot, canaffect animals.) Parasitic fungi occur in each of thefungal divisions. They can have tremendous eco-nomic impacts. For instance, rust fungi, membersof the Basidiomycetes, infect every species of cerealgrain and can reduce yields by as much as 25 per-cent.

In addition to their direct economic impact, par-asitic fungi have historically had profound socialeffects. In the 1800’s outbreaks of Phytophthorainfestans, late blight, decimated the potato crops ofEurope just prior to harvest. This oomycete firstturned the potato leaves and stems to mush andthen infected the tubers. As a result, millions of peo-ple who depended on potatoes as their primarysource of food starved to death. Ireland was partic-ularly hard-hit, and millions emigrated from there.

Some fungal parasites have a mixed impact onhumans. The genus Claviceps, an ascomycete, infectscereal grains, producing structures called ergots. Avariety of alkaloids are produced in the ergot tissuethat causes ergotism in animals that eat infectedgrain. Symptoms include convulsions, psychoticdelusions, and gangrene. It has been suggested thatthe Salem “witches” executed in colonial Massa-chusetts or their “victims” were suffering from theeffects of eating ergotted grain. Today ergot is the

Parasitic plants • 777

source of vasoconstricting drugs used in medicineto control bleeding and relieve migraine head-aches.

Angiosperm ParasitesAmong vascular plants, parasites are limited to

about twelve families of dicots. The best-knownexamples are the Christmas mistletoes, Viscum al-bum in Europe and Phoradendron serotinum in NorthAmerica. While most mistletoes are tropical, thetwo mentioned above are common in the temperateregions. Unlike many parasitic flowering plants,mistletoes are green and photosynthetic and canthus produce their own food. Many species, includ-ing the Christmas mistletoes, produce fleshy, rigidmature leaves, but in other species the leaves are re-duced to scales. Most species are epiphytic and growin the branches of a host tree. Some tropical species

are terrestrial, and at least one forms a 30-foot-talltree. Even these tree species form a characteristichaustorium connecting the parasite to the host. Mis-tletoes cause severe economic losses in many areas.For instance, dwarf mistletoe (Anceuthobium) at-tacks many gymnosperms in the southwesternUnited States, particularly ponderosa pine.

Another well-known flowering plant parasiteis dodder (Cuscuta). Dodder is distributed world-wide. It is easily recognizable because its rapidgrowth can quickly cover a host plant with a net-work of yellowish stems and scalelike, yellowleaves. Although dodder seeds germinate on theground, the root disintegrates as soon as haustoriamake connections with a host, at which time thedodder becomes completely dependent on the hostfor nutrients. Dodder is designated as a noxiousweed throughout the continental United States.

778 • Parasitic plants

Image Not Available

Origin of ParasitismIt is generally believed that parasites evolved

from “normal” plants. As haustoria developed andbecame more specialized, structural parts andphysiological processes normally associated withsupport and nutrient assimilation were reduceduntil they were no longer capable of supporting theplant, which became dependent on its parasitichabit. For instance, within the mistletoe familythere is a complete gradient from terrestrial root-parasites, in which the roots of the tree-mistletoeparasitize the roots of host plants, to epiphyticshoot parasites, which are completely dependenton their host. This gradient suggests that theepiphytic forms evolved from terrestrial speciesthrough increasing specialization of the haustorialorgans and reduction of roots and leaves.

The visible effects of a parasite on its host ranges

from spectacular malformations, such as “witches’brooms” (a proliferation of short branches by thehost at the site of parasite attachment), to nodiscernable effect. The physiological effects are alsovariable. While all parasites weaken their host tosome degree, in some cases the effect is hardlydiscernable. This makes sense because it would notbe advantageous for the parasite to so weaken itshost that the host dies. Nevertheless, this extreme isalso evident, as described above for late blight ofpotato.

Marshall D. Sundberg

See also: Adaptations; Ascomycetes; Bacteria;Basidiomycetes; Basidiosporic fungi; Chytrids; Co-evolution; Deuteromycetes; Diseases and disorders;Evolution of plants; Fungi; Mycorrhizae; Oomy-cetes; Protista; Rusts; Ustomycetes; Zygomycetes.

Sources for Further StudyBhandari, N. N., and K. G. Mukerji. The Haustorium. New York: Wiley, 1993. From the re-

search studies in botany series, a close look at the defining feature of parasites. Includesreferences, index.

Kuijt, Job. The Biology of Parasitic Flowering Plants. Berkeley: University of California Press,1969. Technical but comprehensive treatment of parasitic angiosperms.

Large, E. C. Advance of the Fungi. New York: Dover, 1962. This classic book describes the so-cial and economic importance of a number of interesting fungi.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Brief introduction to various aspects of plant parasitism inan introductory-level college text.

PEAT

Category: Economic botany and plant uses

Peat is an unconsolidated accumulation of partly decomposed plant material, used as a fuel source, as a mulch, and forother uses.

Peat has been burned for heating and cooking orused as soil since the New Stone (Neolithic)

Age, at least fifty-five hundred years ago. Peat hasmany uses in agriculture, industry, and energy gen-eration because of its organic chemical content andcombustion properties. Although abundant in themiddle latitudes of the Northern Hemisphere, ithas been exploited as fuel primarily in northwest-ern Europe.

Like crude oil and coal, peat is composed of theremains of dead organisms compressed under-ground, and it can be burned in home stoves andfireplaces or in factories and public power plants.Because peat is lightly compressed plant matter, italso works well as soil for agriculture and horticul-ture. Worldwide, reserves of peat are comparable tothose of other fossil fuels. For example, according tosome estimates, resources in the United States sur-

Peat • 779

pass the combined potential energy yield of the na-tion’s petroleum and natural gas.

Peat forms in bogs, fens, sedge meadows, andsome swamps as the debris of peat mosses (sphag-num), grasses, and sedges falls to the wet earth andbecomes water-soaked. In the absence of oxygenunderwater, the plant matter and microorganismscompact without completely decomposing, form-ing soft, usually fibrous soils that are tan to black incolor. The organic component, which includes cellu-lose, lignin, and some humus, is greater than 20 per-cent, and in most peat soils plant fragments are visi-ble. The ash content is less than 50 percent, usually aslow as 10 percent. Although the rate varies widely,in general a peat field increases in depth about threecentimeters yearly. The bottoms of large peat fieldsare typically about ten thousand years old and canbe as much as 50 meters below the surface, al-though 3-meter to 6-meter fields are common.

Geographical DistributionMost deposits lie between 40 and 65 degrees lati-

tude of the Northern Hemisphere. World reservesof exploitable peat exceed 200 billion tons, of whichmore than half is in Russia. Canada, the UnitedStates, Great Britain, Ireland, Finland, Norway,Sweden, Germany, Iceland, France, and Polandalso have substantial peat fields. In the UnitedStates, Alaska contains more than half the reserves,but peat is also abundant in Minnesota, Washing-ton, Michigan, Wisconsin, Maine, New York, NorthCarolina, Florida, and Louisiana. Some countrieswell below the fortieth meridian have exploitablepeat reserves, especially Indonesia, Cuba, and Is-rael.

Energy Potential and UsesIn northern Europe, peat has fueled fires since

the New Stone Age. It provides one-half to two-

780 • Peat

Agricultural use Fuel use

10.06.0

4.51.0

0.558.0

2.80.18

0.256.4

4.50.5

4.50.2

60.04.0

20.01.0

2010 30 40 50 60Millions of Metric Tons

Belarus

Estonia

Finland

Germany(western states)

Ireland

Latvia

Lithuania

Russia

Ukraine

Leading Peat-Producing Countries, 1994

Note: Venezuela and the eastern German states are also major peat producers, but output is not reported, so no reliable estimates canbe made concerning production. Reported world 1994 peat production was about 139 million metric tons.

Source: U.S. Bureau of Mines, Minerals Yearbook, 1994. U.S. Government Printing Office, 1996.

thirds as much energy as coal, orabout 3.8 megajoules per dry kilo-gram, yet gives off far fewer pollut-ants such as sulfur and ash. It can beconverted into coke, charcoal, or asynthetic natural gas.

Only in Ireland, Russia, Finland,and Great Britain is peat employedprimarily as a fuel, where in fact itis a traditional domestic resource.Dried and pressed into briquettes,peat burns easily in fireplaces, stoves,and braziers. During the twentiethcentury the four countries burnedincreasing amounts of peat to gener-ate electricity. Because it has verylimited wood and fossil fuel re-sources, Ireland has consumed aboutthree times as much peat for powergeneration as for domestic heating,whereas the other countries primar-ily rely on coal for that purpose.

Agricultural and HorticulturalUses

The United States and Canada, aswell as some European countries,process most of their peat as pottingsoil, lawn dressing, and soil conditioners. Becausethey are much lighter and fluffier than mineralsoils, peat preparations let water and oxygen pene-trate easily and increase water retention, and socan be soil supplements or mulch. Throughout theUnited States commercial nurseries and homeown-ers apply such products to gardens and tree beds.Farmers have raised grasses, clover, wild rice, cran-berries, blueberries, strawberries, Christmas trees,and root and leafy vegetables on peat fields, andranchers have used them for hay and grazing.However, peat fields are difficult to drain and clearand often remain wet, promoting rot and disease.They can be low in nutrients.

Other UsesDuring the energy crisis of the 1970’s, research-

ers investigated peat as an alternative to petroleum,although few of the efforts resulted in commercialproducts because oil again became cheaper thanpeat for industrial chemicals in the 1980’s. Peatyields such mineral and organic substances as dyes,paraffin, naphtha, ammonium sulfate, acetic acid,

ethyl and methyl alcohol, waxes, and phenols.Combined with clay, it forms lightweight blocks forconstruction. It can remove heavy metals from in-dustrial waste and can be turned into coke for ironprocessing or into charcoal for purifying water.With its mildly antibiotic properties, peat servedas a lightweight surgical dressing during WorldWar I. Another of peat’s well-known functions—and one of its oldest—is giving the smoky flavor toScotch and Irish whiskeys as their malts slowly dryover open peat fires.

Because peat fields, once harvested, regenerateonly after thousands of years, peat is not a renew-able resource in any practical sense. Accordingly,intensive peat “mining” has caused concern amongenvironmentalists. They worry that the rapid ex-ploitation of peat fields, especially in Ireland andGreat Britain, may permanently destroy bogs andfens and thereby threaten the wildlife dependentupon wetland habitats.

Roger Smith

See also: Coal; Wetlands.

Peat • 781

Image Not Available

Sources for Further StudyCharman, Daniel J., and Barry G. Warner. Peatlands and Environmental Change. New York:

J. Wiley, 2001. Examines peatland conservation and rehabilitation. Focuses on peatlandsas a system.

Crum, Howard. A Focus on Peatlands and Peat Mosses. Ann Arbor: University of MichiganPress, 1992. Surveys the types, ecology, and uses of peat in detail with many maps, draw-ings, and photographs.

Fuchsman, Charles H. Peat, Industrial Chemistry, and Technology. New York: Academic Press,1980. Explains the chemicals contained in peat that have commercial value.

Godwin, Harry. The Archives of the Peat Bogs. New York: Cambridge University Press, 1981.Examines the biochemistry, ecology, stratification, and resource availability of peatworldwide. Also discusses the importance of peat to Stone Age cultures.

McQueen, Cyrus B. Field Guide to the Peat Mosses of Boreal North America. Hanover, N.H.: Uni-versity Press of New England, 1990. Color photographs accompany these descriptions ofsphagnum species.

Moore, P. D., and D. J. Bellamy. Peatlands. New York: Springer-Verlag, 1974. Examines thebiochemistry, ecology, stratification, and resource availability of peat worldwide.

Parkyn, L., H. A. Ingram, and R. E. Stoneman, eds. Conserving Peatlands. New York: CAB In-ternational, 1997. Describes processes and influences involved in sustaining the remain-ing examples of these fragile ecosystems.

Wright, H. E., Barbara Coffin, and Norman E. Aaseng, eds. The Patterned Peatlands of Minne-sota. Minneapolis: University of Minnesota Press, 1992. Minnesota has more peat thanany other U.S. state except Alaska, and these peatlands exhibit a remarkable display ofthe complex adjustment of living organisms to their environment.

PEROXISOMES


Peroxisomes are distinct membrane-bound organelles within plant cells. A versatile microbody, the peroxisome playskey roles in photorespiration and in the conversion of stored fats to sucrose during the germination process in manyseeds.

Peroxisomes are small, vesicle-like organellesthat are found in virtually all eukaryotic plant

cells. They are surrounded by a single lipid bilayermembrane and are spherically shaped microbodieswith a granular interior containing proteins thatusually function as enzymes. Many of these en-zymes are important in the process of glycolic acidmetabolism. Because peroxisomes contain no ge-nome or ribosomes, their protein is imported fromthe cytosol. Peroxisomes reproduce by cycles ofgrowth and fission and are not members of the en-domembrane system.

Typically, peroxisomes vary in diameter from 0.2to 2 micrometers. They are generally distributed

throughout the cytosol, usually with more abun-dance around the nucleus and where the cell wallsof two cells abut. Many researchers believe thatperoxisomes are of primitive origin, predating theappearance of the mitochondria, and developed inresponse to the increased levels of oxygen in the en-vironment that was generated by cyanobacteria.

FunctionsIn plants, peroxisomes perform many important

roles in plant tissues and development. Their vari-ous functions are dictated by their enzyme content,which is constantly modified throughout the devel-opment of a plant. One function of peroxisomes is

782 • Peroxisomes

to remove hydrogen atoms from organic substratesby using oxygen, resulting in the production of hy-drogen peroxide, a strong oxidizing agent.

In green leaves, peroxisomes play a vital role inphotosynthesis by oxidizing glycolate to glyoxy-late, a critical reaction that fixes carbon dioxidefrom the air into carbohydrate. The process isknown as photorespiration. It provides a pathway forenergy transfer to the bonds of sugar molecules.Photorespiration takes place during daylight, and,like normal cellular respiration, it requires oxygenand produces carbon dioxide and water.

In addition to photorespiration, a second func-tion of peroxisomes that is unique to plants is theconversion of fatty acids to sugars by oxidation. Itprovides a vital pathway for young germinatingseeds, particularly sunflower, watermelon, castorbean, and soy bean seeds, for the use of fatty acids

as a source of food energy for germination andgrowth until they can carry out photosynthesis.

In seedlings and cotyledons, the specializedperoxisomes that carry out this function are calledglyoxysomes. During this oxidation process gly-oxysomes generate jasmonic acid, an importantsignaling molecule in plants. In leguminous plants,peroxisomes are vital in the formation of ureides,compounds that transport nitrogen to cellular lo-cations, where it can be used in organic combina-tion. Peroxisomes also function as sites of defenseagainst activated oxygen species that produce oxi-dative stress in plants.

Alvin K. Benson

See also: Cell theory; Cytosol; Eukaryotic cells;Microbodies; Plant cells: molecular level; Proteinsand amino acids; Respiration.

Sources for Further StudyGunning, Brian E. S., and Martin W. Steer. Plant Cell Biology: Structure and Function. Boston:

Jones and Bartlett, 1996. Comprehensive overview of plant cells. Includes illustrations,index.

Hopkins, William G. Introduction to Plant Physiology. 2d ed. New York: John Wiley and Sons,1999. Develops the foundations of plant physiology, covering cellular organelles and theendoplasmic reticulum. Includes illustrations, bibliographical references, and index.

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology. 4th ed. Pacific Grove, Calif.:Brooks/Cole, 1999. A fundamental textbook that covers plant cell biology. Includes illus-trations, bibliographical references, and index.

PESTICIDES

Categories: Agriculture; environmental issues; pests and pest control

Pesticides are substances designed to kill unwanted plants, fungi, or animals that interfere, directly or indirectly, withhuman activities.

While the use of pesticides has mushroomedsince the introduction of monoculture (the ag-

ricultural practice of growing only one crop on alarge amount of acreage), the application of toxinsto control pests is by no means new. The use of sul-fur as an insecticide dates back before 500 b.c.e.Salts from heavy metals such as arsenic, lead, andmercury were used as insecticides from the fif-teenth century until the early part of the twentiethcentury, and residues of these toxic compounds are

still being accumulated in plants that are grown insoil where these materials were used. In the seven-teenth and eighteenth centuries, natural plant ex-tracts, such as nicotine sulfate from tobacco leavesand rotenone from tropical legumes, were used asinsecticides. Other natural products, such as pyre-thrum from the chrysanthemum flower, garlic oil,lemon oil, and red pepper, have long been used tocontrol insects. Biopesticides are beneficial microbes,fungi, insects, or animals that kill pests.

Pesticides • 783

The major types of pesticides in common use areinsecticides (to kill insects), nematocides (to kill nema-todes), fungicides (to kill fungi), herbicides (to killweeds), and rodenticides (to kill rodents). Herbicidesand insecticides make up the majority of the pesti-cides applied in the environment. In 1939 the dis-covery of dichloro-diphenyl-trichlorethane (DDT)as a strong insecticide opened the door for the syn-thesis of a wide array of synthetic organic com-pounds to be used as pesticides. Chlorinated hydro-carbons such as DDT were the first group ofsynthetic pesticides. Other commonly used chlori-nated hydrocarbons have in the past includedaldrin, endrin, lindane, chlordane, and mirex. Be-cause of their low biodegradability and persistence inthe environment, the use of these compounds wasbanned or severely restricted in the United Statesafter years of use.

Organophosphates such as malathion, parathion,and methamidophos have replaced the chlorinated

hydrocarbons. These compounds biodegrade in afairly short time but are generally much more toxicto humans and other animals than the compoundsthey replaced. In addition, they are water-solubleand therefore more likely to contaminate watersupplies. Carbamates such as carbaryl, maneb, andaldicarb have also been used in place of chlorinatedhydrocarbons. These compounds rapidly biode-grade and are less toxic to humans than organo-phosphates, but they are less effective in killinginsects.

Herbicides are classified according to theirmethod of killing rather than their chemical com-position. As their name suggests, contact herbi-cides such as atrazine and paraquat kill when theycome in contact with a plant’s leaf surface. Contactherbicides generally disrupt the photosyntheticmechanism. Systemic herbicides such as diuronand fenuron circulate throughout the plant after be-ing absorbed. They generally mimic the plant hor-

784 • PesticidesP

ho

toD

isc

Chemical pesticides are sprayed on this field of tulips.

mones and cause abnormal growth to the extentthat the plant can no longer supply sufficient nutri-ents to support growth. Soil sterilants such as tri-flurain, diphenamid, and daiapon kill microorgan-isms necessary for plant growth and also act assystemic herbicides.

Pesticide UseIn the United States, approximately 55,000 dif-

ferent pesticide formulations are available, andAmericans apply about 500 million kilograms (1.1billion pounds) of pesticides each year. Fungicidesaccount for 12 percent of all pesticides used byfarmers, insecticides account for 19 percent, andherbicides account for 69 percent. These pesticideshave been used primarily on four crops: soybeans,wheat, cotton, and corn. Approximately $5 billionis spent each year on pesticides in the United States,and about 20 percent of this is for nonfarm use. On aper-unit-of-land basis, homeowners apply approx-imately five times as much pesticide as do farmers.On a worldwide basis, approximately 2.5 tons(2,270 kilograms) of pesticides are applied eachyear. Most of these chemicals are applied in devel-oped countries, but the amount of pesticide used indeveloping countries is rapidly increasing. Ap-proximately $20 billion is spent worldwide eachyear, and this expenditure is expected to increase inthe future, particularly in the developing countries.

Pesticide use has had a beneficial impact on thelives of humans by increasing food production andreducing food costs. Even with pesticides, pests re-duce the world’s potential food supply by as muchas 55 percent. Without pesticides, this loss would bemuch higher, resulting in increased starvation andhigher food costs. Pesticides also increase the profitmargin for farmers. It has been estimated that forevery dollar spent on pesticides, farmers experi-ence an increase in yield worth three to five dollars.

Pesticides appear to work better and faster thanalternative methods of controlling pests. Thesechemicals can rapidly control most pests, are cost-effective, can be easily shipped and applied, andhave a long shelf life compared to alternative meth-ods. In addition, farmers can quickly switch toanother pesticide if genetic resistance to a given pes-ticide develops.

Perhaps the most compelling argument for theuse of pesticides is the fact that pesticides havesaved lives. It has been suggested that since the in-troduction of DDT, the use of pesticides has pre-

vented approximately seven million premature hu-man deaths from insect-transmitted diseases suchas sleeping sickness, bubonic plague, typhus, andmalaria. Perhaps even more lives have been savedfrom starvation because of the increased food pro-duction resulting from the use of pesticides. It hasbeen argued that this one benefit far outweighs thepotential health risks of pesticides. In addition, newpesticides are continually being developed, andsafer and more effective pest control may be avail-able in the future. In spite of all the advantages ofusing pesticides, their benefit must be balancedagainst the potential environmental damage theymay cause.

Environmental ConcernsAn ideal pesticide should have the following

characteristics: It should not kill any organismother than the target pest; it should in no way affectthe health of nontarget organisms; it should de-grade into nontoxic chemicals in a relatively shorttime; it should prevent the development of resis-tance in the organism it is designed to kill; and itshould be cost-effective. Since no currently avail-able pesticide meets all of these criteria, a numberof environmental problems have developed, one ofwhich is broad-spectrum poisoning. Most, if not all,chemical pesticides are not selective; they kill awide range of organisms rather than just the targetpest. Killing beneficial insects, such as bees, ladybird beetles, and wasps, may result in a range ofproblems. For example, reduced pollination andexplosions in the populations of unaffected insectscan occur.

When DDT was first used as an insecticide,many people believed that it was the final solutionfor controlling many insect pests. Initially, DDTdramatically reduced the number of problem in-sects; within a few years, however, a number ofspecies had developed genetic resistance to thechemical and could no longer be controlled with it.By the 1990’s there were approximately two hun-dred insect species with genetic resistance to DDT.Other chemicals were designed to replace DDT, butmany insects also developed resistance to thesenewer insecticides. As a result, although many syn-thetic chemicals have been introduced to the en-vironment, the pest problem is still as great as itever was.

Depending on the type of chemical used, pes-ticides remain in the environment for varying

Pesticides • 785

lengths of time. Chlorinated hydrocarbons, for ex-ample, can persist in the environment for up to fif-teen years. From an economic standpoint, this canbe beneficial because the pesticide has to be appliedless frequently, but from an environmental stand-point, it is detrimental. In addition, when manypesticides are degraded, their breakdown prod-ucts, which may also persist in the environment forlong periods of time, can be toxic to other organ-isms.

Pesticides may concentrate as they move up thefood chain. All organisms are integral components ofat least one food pyramid. While a given pesticidemay not be toxic to species at the base, it may havedetrimental effects on organisms that feed at theapex because the concentration increases at eachhigher level of the pyramid, a phenomenon knownas biomagnification. With DDT, for example, somebirds can be sprayed with the chemical without anyapparent effect, but if these same birds eat fish thathave eaten insects that contain DDT, they lose theability to metabolize calcium properly. As a result,they lay soft-shelled eggs, which causes deaths ofmost of the offspring.

Pesticides can be hazardous to human health.

Many pesticides, particularly insecticides, are toxicto humans, and thousands of people have beenkilled by direct exposure to high concentrations ofthese chemicals. Many of these deaths have beenchildren who were accidentally exposed to toxicpesticides because of careless packaging or storage.Numerous agricultural laborers, particularly in de-veloping countries where there are no stringentguidelines for handling pesticides, have also beenkilled as a result of direct exposure to these chemi-cals. Workers in pesticide factories are also a high-risk group, and many of them have been poisonedthrough job-related contact with the chemicals.Pesticides have been suspected of causing long-term health problems such as cancer. Some of thepesticides have been shown to cause cancer in labo-ratory animals, but there is currently no direct evi-dence to show a cause-and-effect relationship be-tween pesticides and cancer in humans.

D. R. Gossett

See also: Agriculture: modern problems; Agricul-ture: world food supplies; Biopesticides; Fertil-izers; Green Revolution; Herbicides; Hormones;Monoculture.

Sources for Further StudyAltieri, Miguel A. Agroecology: The Science of Sustainable Agriculture. 2d ed. Boulder, Colo.:

Westview Press, 1995. Describes alternative methods for restoring soil fertility, soil con-servation, and the biological control of pests.

Carson, Rachel. Silent Spring. 1962. Reprint. Thorndike, Maine: G. K. Hall, 1997. Somewhatoutdated but excellent dramatization of the potential dangers of pesticides. The book ishistorically significant: It served as a major catalyst for bringing the hazards of using pes-ticides to the attention of the general public.

Mannion, Antionette M., and Sophia R. Bowlby. Environmental Issues in the 1990’s. Chich-ester, N.Y.: John Wiley & Sons, 1992. Another good discussion of the environmental im-pact of agricultural chemicals.

Milne, George W. A., ed. Ashgate Handbook of Pesticides and Agricultural Chemicals. Burling-ton, Vt.: Ashgate, 2000. A guide to the eighteen hundred most commonly used agricul-tural chemicals, organized by function and covering the common, chemical, and tradenames, molecular formulas, physical properties and uses, acute toxicity, and manufac-tures and suppliers.

Nadakavukaren, Anne. Man and Environment: A Health Perspective. 2d ed. Prospect Heights,Ill.: Waveland Press, 1990. A very good discussion on human health issues associatedwith the use of pesticides.

Pierce, Christine, and Donald VanDeVeer. People, Penguins, and Plastic Trees. 2d ed. Belmont,Calif.: Wadsworth, 1995. Good discussions of environmental ethics, including the ethicsof using agricultural chemicals.

786 • Pesticides

PETRIFIED WOOD

Categories: Evolution; paleobotany

Petrified wood is formed by the fossilized remains of ancient trees that were saturated with mineral-filled water which,over time, converted the woody tissues into stone.

Petrified wood is studied by scientists interestedin prehistoric plants and their environments.

The Latin word petros, meaning rock, is the sourcefor the scientific term petrification. Petrified wood isactually the stone remnant of a prehistoric tree.

FormationDuring the Triassic period, gymnosperms—seed-

producing trees without flowers, such as gingkosand conifers—grew over much of the earth’s land-mass. Volcanic eruptions triggered tremors, light-ning, and heavy rains, which washed trees fromhigher elevations down to swampy valleys. As theywere pushed downhill, the trees were stripped oftheir bark, branches, and roots from the force of thewater’s impact and broke into pieces. Under nor-mal circumstances, trees soaking in deep, muddywater would decay, but silt rapidly and completelycovered these trees, preventing exposure to oxygenand inhibiting aerobic decomposition. Volcanic ashin the floodwater consisted of inorganic com-pounds such as magnesium carbonate and ironsulphide, and the trees also absorbed silicon diox-ide (silica) that had dissolved in groundwater. Theminerals filled the spaces between cells in the treetrunks and branches. Molecules of these inorganicmaterials replaced molecules of organic tissues.During the next millions of years, wood graduallybecame stone in the process of silicification. As-sisted by extreme pressure and temperatures, thesilica that was lodged in the wood was transformedinto quartz. Plants that have undergone petrifica-tion are also referred to as being permineralized.

The trees remained preserved under the soil formillions of years until soil erosion and shiftingplates exposed them. Manganese, lithium, copper,and iron created patterns of bright colors as thewood fossilized. Some petrified wood displays vary-ing rings of vivid colors, resembling agates. Otherpieces are brown and look like driftwood. More sig-

nificant than its beauty, information about the his-tory of plants on earth is revealed by petrifiedwood. Unlike other fossils that are seen as an im-pression or compression, petrified wood is a three-dimensional representation of its organic materialthat preserves its external shape and internal struc-ture. The preserved tree trunk sections also indicatethe size of the Triassic forest. Scientists have evenseen chromosomes and stages of nuclear divisionin petrified cells. A termite nest was discovered inone petrified log, offering clues about that insect’scommunal evolution.

Identifying Petrified WoodGeologists and paleobotanists analyze petrified

wood samples to specify the type of ancient treethat became fossilized. The cell structure of hard-wood fossils is more diverse than that of softwoodtrees, causing its source to be more easily identified.Scientists choose pieces that exhibit an intact cellstructure and examine the specimen with a micro-scope to scrutinize how the wood’s bands andpores are arranged, both of which are crucial identi-fying characteristics. Softwood samples requiregreater magnification of thin slivers only one or twocells thick in order to permit enough light to shinethrough when mounted on a slide.

Wood anatomists then describe the sample’s cellstructure, which they compare to records of previ-ously identified petrified wood and existing trees.North Carolina State University and the Interna-tional Association of Wood Anatomists created acomputer database, the General Unknown Entryand Search System (GUESS), which can identifymatching cell patterns. Other databases contain in-formation about existing hardwood and softwoodtrees; one has information on at least 1,356 types oftrees of more than twelve hundred species.

Three types of petrified wood are found in theTertiary strata. Nondescript silicified wood has under-

Petrified wood • 787

gone silicification but still appears to be woodystructurally. Difficult to identify because of its ge-neric structure, nondescript silicified wood re-quires expert authentication for accurate labeling.Petrified palm wood has rod structures that rein-forced the tissue strength before the wood grain be-came silicified and that look like spots or lines whenthe wood is cut. Popular among rock collectors, thistype of petrified wood is both Arizona’s state fossiland state rock. Massive silicified wood is difficult torecognize because the tree’s grain was destroyedduring silicification, thus making identification re-liant on awareness of the area in which the tree waslocated and comparison to other petrified wood inadjacent territory.

Petrified Wood in the United StatesWood in Arizona’s Petrified Forest National

Park originated from a forest of giant conifer-liketrees that grew from Texas to Utah. The park’s93,533 acres are home to one of the world’s largestassortments of petrified wood. This desert area isdotted with stone log fragments. Although some

visitors expect to see rock trees standing in clumpssimilar to a natural forest, these petrified trees restwhere they fell individually or in groups.

Because the ancient forest lived simultaneouslywith the dinosaurs, archaeologists look for dino-saur fossils near petrified wood in the Arizonapark. Although collection and thefts have greatlyreduced the number of petrified logs, authoritiesbelieve that some areas of the park may shelter pet-rified wood buried as much as 100 meters beneaththe surface.

By the twentieth century, prospectors and tour-ists had taken the most beautiful pieces; as a result,local residents sought government protectionagainst further theft. In 1906 the petrified wood sitewas declared a national monument and was nameda national park in 1962. The U.S. National Park Ser-vice tries to protect petrified wood by preventingthe nearly one million yearly visitors from seizingsamples. Rangers patrol sites and ask tourists to re-port any thefts they witness. Despite these pre-cautions, several tons of petrified wood disappearannually from the park. Privately owned sites adja-

788 • Petrified wood

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cent to the park offer collectors opportunities tosearch for petrified wood without any restrictions.Stores sell small pieces.

Petrified wood has been discovered in manyother regions of the United States, especially inwestern areas where volcanic activity occurred, suchas Yellowstone National Park, and in areas whererivers and streams deposited large amounts of sand,such as Louisiana and Texas. Washington State ishome to the Ginkgo Petrified Forest State Park,which contains petrified logs that began fossilizingduring the Miocene epoch. This petrified wood isunique because it includes petrified ginkgo, an in-digenous tree that no longer grows naturally there.An unusual type of petrified wood that resemblespebbles and that originated in the Chehalis Valley issometimes seen on the state’s beaches.

The Calistoga, California, petrified forest is con-sidered one of the best sources of Pliocene fossilssimilar to existing redwoods. Measuring more than2 meters in diameter, the fossil logs reveal graystone veins of quartz. Petrified wood in New Mex-ico’s Bisti Badlands is not as colorful as neighboringArizona’s fossil wood. In Utah, petrified wood hasbeen found near the Escalante River and the CoyoteButtes region near the Paria River. Petrified woodhas also been found in Mississippi and Alabama.

Petrified Wood WorldwideA petrified forest on the Greek island of Lesbos

was named a protected site by presidential decree.Scientists have determined that the petrified woodbegan fossilizing during the Late Oligocene toLower-Middle Miocene epochs. Unlike other sitesof petrified forests, the Lesbos fossil trunks are erectand still have intact roots penetrating into fossil-ized soil, branches, leaves, cones, and seeds.

These trees were fossilized where they grew, of-fering insight into the environment and climate ofancient Lesbos. The ancient trees were well pre-served during the petrification process, and suchdetails as rings indicating age and growth patternsare visible. Both gymnosperms and angiosperms(flowering plants) grew in the ancient forests, along

with pteridophytes such as ferns. Many of these an-cient species no longer grow in the Mediterranean.Instead, they are found in Asian and American trop-ical and subtropical regions, indicating that Lesbos’spetrified forest presents information about how lifeon earth evolved as the continents moved apart.

Other sites of fossilized wood also reveal detailsabout the planet’s development. Argentina’s Pet-rified Forest on the Central Steppes was created af-ter the formation of the Andes Mountains. Largerthan American wood fossils, Argentinean petrifiedwood includes pieces 27 meters long and 3 metersin diameter. The petrified forest in Namibia con-tains giant tree trunks as long as 30 meters. Petrifiedwood has also been discovered in Australia, India,England, Turkey, and Switzerland.

Petrified Wood’s LegacyPaleobotanists research petrified wood to deter-

mine how the earth and the plants that have grownon its surface have changed since ancient times.They study how plants are related and descendedfrom similar ancestors. Throughout history, manymore tree species have existed than are docu-mented, and many species, existing and extinct,await discovery. Pieces of fossil wood are often theoldest known specimens of a tree species and mightbe the predecessors of living trees.

Petrified wood helps researchers comprehendhow trees have evolved to adapt to environmentaland climatic conditions. For example, researchershave hypothesized that the cell structure of tree xy-lem has not changed as much as fruit and leaves.Fossil wood sometimes reveals the reason for thedemise of extinct trees. The study of petrified woodhas altered how scientists perceive both ancientgeological ecosystems and modern environmentsby offering perspective on which plants lived onthe earth and survived, adapted, or died accordingto changes in the atmosphere and crust.


See also: Fossil plants; Paleobotany; Paleoecology;Plant tissues; Wood.

Sources for Further StudyDaniels, Frank, Brooks B. Britt, and Richard D. Dayvault. Petrified Wood: The World of Fossil-

ized Wood, Cones, Ferns, and Cycads. Grand Junction, Colo.: Western Colorado, 1998. Beau-tiful photographic presentation of petrified wood.

Long, Robert A., Rose Houk, and Doug Henderson. Dawn of the Dinosaurs: The Triassic in Pet-rified Forest. Petrified Forest, Ariz.: Petrified Forest Museum Association, 1988. Written by

Petrified wood • 789

former Petrified Forest National Park paleontologist Long and veteran natural historywriter Houk, with moody, atmospheric paintings by Henderson.

Lubick, George M. Petrified Forest National Park: A Wilderness Bound in Time. Tucson: Univer-sity of Arizona Press, 1996. Text speeds the reader on an ancient ecological journey, fromthe time of the forest’s early life to the discovery of its fossils, and through the politicalmanuevering to create a place for the forest in American history.

PHEROMONES

Categories: Cellular biology; physiology

Pheromones are any chemical or chemical mixture that, when released by one member of a species, affects the physiologyor behavior of another member of the same species.

Pheromones are semiochemicals that carry in-formation between members of a single spe-

cies. To do this, the pheromone must be releasedinto the atmosphere or placed on some structure inthe organisms’ environment. It is thus made avail-able to other members of the species for interpreta-tion and response. It is also available to membersof other species, however, so it is a potential allelo-chemic.

Types of PheromonesIn complex interactions, a pheromone may also

be acting as a kairomone, passing messages betweenspecies to the benefit of the recipient; an allomone,passing messages between species to the benefit ofthe sender; or a hormone, passing messages within asingle organism. One possible example of a hor-mone as a pheromone is the plant hormone ethyl-ene, which is produced by an individual plant tostimulate ripening of fruit, loss of leaves, and otherphysiological changes. Some evidence suggeststhat ethylene, produced in response to damagecaused by insects feeding on the plant, stimulatesproduction of chemicals that are detrimental to theinsects, thus acting as a hormone. It also passesthrough the atmosphere to surrounding plants andstimulates their production of defensive chemicals,thus acting as a pheromone. Not everyone is con-vinced by the evidence that has been presented forthis phenomenon, but the possibility is intriguing.

There are two general types of pheromone: thosethat elicit an immediate and predictable behavioralresponse, called releaser or signal pheromones, and

those that bring about a less obvious physiologicalresponse, called primer pheromones because theyprime the system for a possible behavioral response.Pheromones are also categorized according to themessages they carry. There are trail, marker, aggre-gation, attractant, repellant, arrestant, stimulant,alarm, and other pheromones. Their functions aresuggested by the terms used to name them.

The chemical compounds that act as phero-mones are numerous and diverse. Most are lipidsor chemical relatives of the lipids, including manysteroids. Even a single pheromonal message mayrequire a number of different compounds, eachpresent in the proper proportion, so that the activepheromone is actually a mixture of chemical com-pounds.

Functions and SourcesDifferent physical and chemical characteristics

are required for pheromones with different func-tions. Attractant pheromones must generally bevolatile to permit atmospheric dispersal to their tar-gets. On the other hand, many marking phero-mones need not be especially volatile because theyare placed at stations which are checked periodi-cally by the target individuals. Some pheromonesare exchanged by direct contact, and these need nothave any appreciable volatile component.

Pheromones are widespread in nature, occur-ring in most, if not all, species. Most are poorly un-derstood. The best-known are those found in in-sects, partly because of their potential use in thecontrol of pest populations and partly because the

790 • Pheromones

relative simplicity of insect behavior allowed forrapid progress in the identification of pheromonesand their actions. Despite these advantages, muchremains to be learned even about insect phero-mones.

Pheromones and Pest ControlPheromones and other semiochemicals are of in-

terest from the standpoint of understanding com-munication among living things. In addition, theyhave the potential to provide effective, safe agentsfor pest control. The possibilities include sex-attractant pheromones to draw insects of a particu-lar species to a trap (or to confuse the males andkeep them from finding females) and repellantpheromones to drive a species of insect away from avaluable crop. One reason for the enthusiasm gen-erated by pheromones in this role is their specificity.Whereas insecticides generally kill valuable insectsas well as pests, pheromones may target one or afew species.

These chemicals were presented as a panacea forinsect and other pest problems in the 1970’s, butmost actual attempts to control pest populationsfailed. Lack of understanding of the particular pestand its ecological context was called the most com-mon cause of failure. Some maintain that pest-control applications must be made with extensiveknowledge and careful consideration of pest char-acteristics and the ecological system. In this con-text, pheromones have become a part of integratedpest management (IPM) strategies, in which theyare used along with the pest’s parasites and preda-tors, resistant crop varieties, insecticides, and otherweapons to control pests. In this role, pheromoneshave shown great promise.

Carl W. Hoagstrom

See also: Allelopathy; Biopesticides; Hormones; In-tegrated pest management; Metabolites: primaryvs. secondary.

Sources for Further StudyAgosta, William. “Using Chemicals to Communicate.” Journal of Chemical Communication 71,

no. 3 (March, 1994): 242-247. Discusses biologists’ exploration of plant pheromone con-centrations through observations of reactions produced when the pheromone is released.

Rhoades, David F. “Pheromonal Communication Between Plants.” In Chemically MediatedInteractions Between Plants and Other Organisms, edited by Gillian A. Cooper-Driver, TonySwain, and Eric E. Conn. New York: Plenum Press, 1985. Written for botanists but mostlyaccessible to nonspecialists. Presents arguments for an alarm pheromone in plants. In-cludes many references; one index for the book.

Schiestl, Florian P., et al. “Orchid Pollination by Sexual Swindle.” Nature, June 3, 1999, 421-422. The odor of Orphrys sphegodes is almost identical to the sex pheromone of female beesand thus prompts almost equally intense reactions. Discusses innovation of this orchidfor attracting and deceiving pollinators.

PHOSPHORUS CYCLE

Categories: Biogeochemical cycles; cellular biology; ecology; nutrients and nutrition

The constant exchange of a mineral or elemental nutrient between organisms and the physical environment is called abiogeochemical cycle. Along with the carbon cycle and the oxygen cycle, one of the most important biogeochemical cyclesis that of the element phosphorus. The phosphorus cycle involves the movement of the element phosphorus as it circulatesthrough the living and nonliving portions of the biosphere.

Many of the chemical elements found on theearth are vital to the processes and systems of

living organisms. Unlike oxygen and carbon, phos-phorus follows complex pathways. It circulates

Phosphorus cycle • 791

through the earth’s soils, rocks, waters, and atmo-sphere and through the organisms that inhabitthese many ecosystems.

Elements or minerals are stored in discrete partsof the earth’s ecosystems called compartments. Ex-amples of compartments include all the plants in aforest, a certain species of tree, or even the leaves orneedles of a tree. Chemical elements reside withinthe compartments in certain amounts, or pools. Abasic description of biogeochemical cycles involvesfollowing nutrients in the form of minerals or ele-ments from pool to pool through the multitudes ofecosystem compartments.

Phosphorus and PlantsPhosphorus compounds reside primarily in

rocks. Phosphorus does not go through an atmo-spheric phase, but rather, phosphorus-laden rocksrelease phosphate (PO4

–3) into the ecosystem as theresult of weathering and erosion. To plants, phos-phorus is a vital nutrient (second only to nitrogen).Plants absorb phosphates through their root hairs.Phosphorus then passes on through the food chainwhen the plants are consumed by other organisms.Phosphorus is an essential component of many bio-logical molecules, including deoxyribonucleic acid(DNA) and ribonucleic acid (RNA). Adenosine

792 • Phosphorus cycle

Assimilationby plant cells

Weathering of rock

Incorporation into sedimentaryrock; geologic uplift moves this

rock into terrestrial environments

Phosphatesin solution

Loss indrainage

Phosphatesin soil

Decomposition byfungi and bacteria

Urine

Animal tissuesand feces

Planttissues

The Phosphorus Cycle

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The biogeochemical phosphorus cycle is the movement of the essential element phosphorus through the earth’s ecosys-tems. Released largely from eroding rocks, as well as from dead plant and animal tissues by decomposers such as bacteriaand fungi, phosphorus migrates into the soil, where it is picked up by plant cells and is assimilated into plant tissues. Theplant tissues are then eaten by animals and released back into the soil via urination, defecation, and decomposition ofdead animals. In marine and freshwater aquatic environments, phosphorus is a large component of shells, from which itsediments back into rock and can return to the land environment as a result of seismic uplift.

triphosphate (ATP), one of the nucleotides thatmake up DNA and RNA, is also the main energytransfer molecule in the multitude of chemical reac-tions taking place within organisms.

Because phosphorus is a major plant nutrient,massive amounts of phosphate-based fertilizers areeither derived from natural sources (in the form ofbat or bird guano) or chemically manufactured foruse by agriculture. As late as the early 1970’s, phos-phates were a major constituent of household de-tergents, until it was discovered that large amountsof phosphates were being released into the environ-ment. In aquatic systems such as rivers and lakes,where such runoff eventually appears, an infusionof phosphates can cause algal blooms (rapidly form-ing, dense populations of algae). When the algaedie, they are consumed by bacteria. Decomposi-tion by bacteria requires large amounts of oxygen,which soon depletes the available oxygen in thewater. If the process is allowed to continue un-checked, fish and other organisms die from lack ofoxygen. Both phosphates and nitrates contribute tocultural eutrophication.

Phosphates not taken up by plants go into thesedimentary phase, where they are very chemi-cally reactive with other minerals. Some of thesereactions produce compounds that effectively re-move phosphates from the active nutrient pool.This sedimentary phase is characterized by itslong residence time compared to the rapid cyclingthrough the biological phase. Phosphates can re-main locked up in rocks for millions of years be-fore being exposed and broken down by weather-

ing, which once again makes them available toplants.

Phosphorus and the EnvironmentBecause the phosphorus cycle is so complex, its

interactions with other biogeochemical cycles arenot completely understood. The study of these in-teractions is emerging as a vital field among the en-vironmental sciences. Excessive phosphates in aeutrophic lake disrupt the carbon cycle by reactingwith bicarbonates, thus increasing the pH. Manyfreshwater organisms depend on a neutral pH levelfor their survival. The presence of phosphorus un-der these oxygen-depleted conditions can also indi-rectly affect the sulfur cycle, leading to the conver-sion of sulfate to sulfide. When sulfide combineswith hydrogen to form the gas hydrogen sulfide, ittakes on the familiar “rotten egg” smell.

One of the keys to preventing environmentaldegradation through the altering of global chemi-cal cycles lies in recognizing the effects of such al-terations. With the perception of an environmentalcrisis in the early 1970’s, more attention was paid tothe role of human activity in these cycles. Test lakeswere studied to determine why freshwater fisher-ies were becoming oxygen-depleted at acceleratedrates. Dramatic progress has been made in elimi-nating the problem of algal blooms and oxygen de-pletion by limiting the phosphorus-laden effluentsbeing discharged into lakes.

David M. Schlom, updated by Bryan Ness

See also: Eutrophication; Fertilizers; Nutrients.

Sources for Further StudyKrebs, Charles J. Ecology: The Experimental Analysis of Distribution and Abundance. 5th ed. San

Francisco: Benjamin/Cummings, 2001. Extensive coverage of the biogeochemical cyclesin chapter dealing with nutrient cycling. Referenced, with an index including many en-tries on phosphorus. The nutrient cycle is discussed throughout the book. This is an excel-lent college-level text with some mathematical models of nutrient cycles and algaegrowth. Numerous illustrations, figures, graphs, and diagrams.

Lasserre, P., and J. M. Martin, eds. Biogeochemical Processes at the Land-Sea Boundary. NewYork: Elsevier, 1986. Acollection of very accessible technical papers dealing with nutrientcycling interactions at the continental-ocean boundary. Contains excellent discussions ofecosystem and nutrient-cycle modeling. Suitable for the advanced student or college-level reader with an interest in land-sea interactions. No index.

Pomeroy, Lawrence, ed. Cycles of Essential Elements. Stroudsburg, Pa.: Dowden, Hutchinson& Ross, 1974. Ahistorical perspective of the topic, this volume is a collection of classic pa-pers on nutrient cycling from 1855 to 1973. Part of the “Benchmark Papers in Ecology” se-ries, this book offers insight into the evolving science of ecology. Chapter 4 is a classicpaper by Juday, Birge, and Kemmerer titled “Phosphorus Content of Lake Waters in

Phosphorus cycle • 793

Northeast Wisconsin.” Other articles include discussions of particle analysis and cyclesin various bodies of water.

Tiessen, Holm, ed. Phosphorus in the Global Environment: Transfers, Cycles, and Management.New York: Wiley, 1995. Traces the global phosphorus cycle and discusses policy issuesconcerning phosphorus management for global food security and pollution control.

PHOTOPERIODISM

Categories: Physiology; reproduction

The reproductive cycles of many organisms, both plant and animal, are regulated by the length of the light and dark pe-riod, called the photoperiod. In flowering plants (angiosperms), flowers are organs for sexual reproduction, andphotoperiodism refers to the process by which these plants flower in response to the relative lengths of day and night.

Along the equator, the lengths of day and nightremain constant because the sun rises and sets

at the same time throughout the year. The lengths ofthe day and night are also equal (each is six monthslong) at the exact North and South Poles due to the

fact that the sun remains below the horizon for sixmonths each year and above the horizon for theother six months. Anyplace else in the world, thedays become longer in the summer and shorter inthe winter. The reproductive cycles of many organ-

isms, both plant and animal,are regulated by the length ofthe light and dark period,called the photoperiod. In flow-ering plants (angiosperms),flowers are organs for sexualreproduction, and photoperi-odism refers to the process bywhich these plants flower in re-sponse to the relative lengthsof day and night.

The synchronization of re-production with seasonal timeis a very important aspect ofplant physiology. Reproduc-tion in many angiosperms isdependent on cross-pollination,the process of pollen beingtransferred from one flower toanother. Hence, it is importantfor all of the plants of the samespecies in a given region toflower at the same time. Evenin nonflowering plants such asmosses, ferns, and some algae,it is usually beneficial for re-productive structures to beformed in a given season. The

794 • Photoperiodism

DarkLight

24 hours

Long-dayplant

Short-dayplant

Critical photoperiod

Photoperiodism

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Short-day plants flower when the length of the day is less than the criticalphotoperiod—that is, when days are relatively short, as in the late fall or earlywinter. Long-day plants flower when the days are longer than the criticalphotoperiod—as in late spring and early summer.

ability to detect the length of the day or night orboth makes it possible to synchronize the reproduc-tive event to a particular time of year. While therehave been hundreds of studies which show thatmany plants respond to changes in the photoperiod,there have been no broad sweeping generalities toprovide a better understanding of this phenome-non. Each species, and often each cultivar or varietywithin a species, appears to have its own photo-periodic response.

Photoperiodic ClassificationThe photoperiodic classification of plants is usu-

ally made on the basis of flowering, but other as-pects of their development may also be affected byday length. Based on their flowering response,plants are classified as short-day plants (SDPs),long-day plants (LDPs), intermediate-day plants,ambiphotoperiodic, or day-neutral plants.

Short-day plants flower when the days are rela-tively short (generally nine hours or less), such as inthe late fall or early winter. In some SDP speciesflowering is qualitative, meaning that short days areabsolutely required, while in other SDP speciesflowering is quantitative, which means flowering isaccelerated under short days, but short days are notan absolute requirement. Some examples of SDPsinclude rice, cocklebur, and soybean.

Long-day plants flower when the days are rela-tively long (generally fifteen hours or greater), aswould occur in late spring and early summer. Aswith SDPs, there are qualitative and quantitativespecies of LDPs.

Intermediate-day plants require quite narrow daylengths (between twelve and fourteen hours) in or-der to flower, and flowering is inhibited by eithershort or long days. Sugarcane is an example of anintermediate-day plant.

Ambiphotoperiodic plants are a specialized groupof plants that will flower in either short days or longdays, but flowering is inhibited by intermediateday lengths.

In day-neutral plants, flowering is not regulatedby day lengths. In other words, day-neutral plantsflower regardless of the day length. There are alsomany interesting interactions between photoperiod

and temperature. A plant may respond to a certainday length at one temperature but exhibit a differ-ent response at another temperature. For example,both the poinsettia and morning glory are absoluteSDPs at high temperature; however, they are abso-lute LDPs at low temperature and day-neutral atintermediate temperatures.

Chemical ControlFlowering is regulated by chemicals produced in

the plant, and a variety of plant hormones, includ-ing auxins, ethylene, gibberellins, cytokinins, andabscisic acid, have been shown to influence flower-ing in different species. The critical aspect of photo-periodism, however, is the measurement of sea-sonal time by detecting the lengths of day andnight. The discovery of the night break phenomenon,which showed that interruption of the night periodwith light inhibited flowering in SDPs, establishedthat the length of the dark period is the most criticalfor initiating a photoperiodic response.

The chemical phytochrome is responsible for mea-suring the dark period. Phytochrome, found in theleaves of plants, exists in two forms, Pr and Pfr. Pr ab-sorbs red light during the day and is converted toPfr. Pfr absorbs far-red light during the night and isconverted to Pr. The prevailing hypothesis is that Pfr

inhibits flowering, and the length of the dark pe-riod has to be sufficient for the Pfr to fall below somecritical level. When the Pfr falls below this level,chemical messages are sent to the floral regions,and flowering is initiated.

While phytochrome definitely has been shownto trigger the flowering response, it is not the onlychemical involved. It has been shown that a blue-light photoreceptor may also play a role in photo-periodism. In addition, phytochrome is not trans-located in the plant. It remains in the leaves. Hence,other chemicals which have not been positivelyidentified are responsible for signaling the photo-periodic response.

D. R. Gossett

See also: Circadian rhythms; Flower structure;Flowering regulation; Hormones; Pollination; Re-production in plants.

Sources for Further StudyHalevy, A. H., ed. CRC Handbook of Flowering. Boca Raton, Fla.: CRC Press, 1985. An excellent

source of information dealing with all aspects of flowering, for the more advanced stu-dent. Includes bibliographical references.

Photoperiodism • 795

Hopkins, William G. Introduction to Plant Physiology. 2d ed. New York: John Wiley and Sons,1999. This general text for the beginning plant physiology student contains a very cleardiscussion of photoperiodism. Includes diagrams, illustrations, and index.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. A very good general botany text with a simplified discus-sion of photoperiodism. Includes diagrams, illustrations, and index.

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology. 4th ed. Belmont, Calif.: Wadsworth,1992. One of the standard texts for the more advanced plant physiology student, with anexcellent discussion on photoperiodism. Includes biographical references, diagrams, il-lustrations, and index.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology. 2d ed. Sunderland, Mass.: Sinauer Asso-ciates, 1998. A text written for the advanced plant physiology student, with an outstand-ing discussion of photoperiodism. Includes biographical references, diagrams, illustra-tions, and index.

PHOTORESPIRATION

Categories: Cellular biology; photosynthesis and respiration; physiology

Photorespiration is a biochemical process in plants in which, especially under conditions of water stress, oxygen inhibitsthe Calvin cycle, the carbon fixation portion of photosynthesis.

Photorespiration results in the light-dependentuptake of oxygen and release of carbon dioxide

and is associated with the synthesis and metabo-lism of a small molecule called glycolate. Photores-piration takes place in green plants at the same timethat photosynthesis does. Because in photosynthe-sis carbon dioxide is taken in, and in photorespira-tion carbon dioxide is given off, these two processeswork against each other. The end result is that pho-torespiration decreases the net amount of carbondioxide which is converted into sugars by a photo-synthesizing plant. By interfering with photosyn-thesis in this way, photorespiration may signifi-cantly limit the growth rate of some plants.

PhotosynthesisIn green plants, photosynthesis takes place in the

special energy-storing molecules called chloroplasts.Photosynthesis can be divided into two parts: thelight reactions and the dark reactions. In the light reac-tions, light energy from the sun is captured by theplant and converted into chemical energy in theform of chloroplasts. An additional feature of thelight reactions is that a molecule of water is split sothat its oxygen is released. In the dark reactions, a

series of steps called the Calvin cycle converts car-bon dioxide from the air into organic moleculessuch as sugars and starch. The Calvin cycle requiresenergy in order to operate, and this is provided bythe energy-storing molecules (such as adenosinetriphosphate) formed in the light reactions. The car-bohydrates thus formed can serve as food for theplant or for an animal that eats the plant.

The overall pattern of gas exchange in photosyn-thesis, therefore, is the release of oxygen and theuptake of carbon dioxide. It has been found that, toa lesser extent, light can also cause plants to do justthe opposite—that is, to consume oxygen and re-lease carbon dioxide. This phenomenon was dis-covered in the 1950’s and is termed photorespira-tion. If net photosynthesis is defined as the totalamount of carbon dioxide taken in minus theamount given off, it is apparent that increasing therate of photorespiration will decrease net photo-synthesis. In terms of agricultural plants, this trans-lates into a decrease in the productivity of the crop.

Rubisco and GlycolateEach of the reactions of the Calvin cycle must be

catalyzed by an enzyme. The first reaction of the

796 • Photorespiration

cycle, in which carbon dioxide is taken up, utilizesan enzyme popularly known as Rubisco, an ab-breviation for ribulose bisphosphate carboxylase/oxygenase. The normal function of Rubisco is totake carbon dioxide from the atmosphere and com-bine it with another molecule in the chloroplast,ribulose bisphosphate (RuBP). The resulting com-pound is then acted upon by other enzymes whicheventually convert it into the simple sugar gly-ceraldehydes 3-phosphate, which is used in thesynthesis of more complex sugars and other com-pounds.

Rubisco, however, does not always behave in itsnormal fashion. It is sometimes unable to distin-guish between molecules of carbon dioxide andoxygen. Rubisco will sometimes “mistakenly” in-corporate an oxygen molecule into RuBP ratherthan the carbon dioxide that would normally havebeen used. The oxygen may come from the atmo-sphere, or it may originate from the oxygen that iscontinually being produced by the splitting ofwater during the light reactions of photosynthesis.

The result of this metabolic error is that, ratherthan forming compounds that can be convertedinto sugar, the plant forms a substance known asglycolate. Understanding what happens to glycolateis the key to understanding the process of photores-piration. The utilization of oxygen in the formationof glycolate accounts for part of the oxygen uptakethat is observed during photorespiration. Insteadof being used in sugar synthesis, the glycolate en-ters a different metabolic pathway, where it is actedupon by a different series of enzymes with differentconsequences. These sequential reactions, referredto as the glycolate pathway, result in the conversionof glycolate into a series of different compounds.This pathway constitutes the remainder of the pro-cess of photorespiration.

The Glycolate PathwayAn unusual feature of photorespiration is that

it involves three separate cellular organelles: thechloroplast, the peroxisome, and the mitochondrion.The first stage of photorespiration involves the for-mation of glycolate in the chloroplast. The glycolatedoes not undergo further reactions in the chloro-plast but instead is transported to the peroxisome.

Once inside the peroxisome, the glycolate entersa series of reactions, one of which causes oxygen tobe converted into hydrogen peroxide. This repre-sents a second point in the process at which oxygen

is consumed. If hydrogen peroxide were present inlarge quantities it could have a toxic effect upon thecell, so the peroxisome also contains the enzymecatalase, which destroys most of the hydrogen per-oxide thus formed.

In subsequent steps of the glycolate pathway,one of the compounds formed is glycine, which en-ters the mitochondrion and loses a carbon atom inthe form of carbon dioxide. This, then, accounts forthe observed release of carbon dioxide during pho-torespiration. If the carbon dioxide is lost to theatmosphere, it represents a decrease in the netamount of carbon taken up by the plant and, there-fore, a decrease in net photosynthesis.

Further reactions of the glycolate pathway occurin the mitochondrion and peroxisome, and eventu-ally a compound is formed which is returned to thechloroplast, where the process began. This com-pound is capable of reentering the Calvin cycle andcan actually be used for the synthesis of sugars. Thecritical point, however, is that not all the carbonatoms which left the chloroplast in the form ofglycolate are returning to it. Part of the carbon waslost in the form of the carbon dioxide that was re-leased in the mitochondrion. Furthermore, certainsteps in the glycolate pathway require the expendi-ture of energy. The process is, therefore, doublywasteful in that it results in the loss of both carbondioxide and energy storage molecules.

To summarize the process, oxygen is utilized inthe chloroplast to form the two-carbon compoundglycolate. The glycolate then enters a series of reac-tions that occur in the peroxisome and mitochon-drion and that take up additional oxygen and re-lease a portion of the carbon in the form of carbondioxide. The remaining carbon is converted into 3-phosphoglycerate, which can be returned to thechloroplast and reenter the Calvin cycle, where it isone of the normal intermediate compounds. Thisaccounts for the light-dependent uptake of oxygenand release of carbon dioxide, which constitutephotorespiration.

Factors That Increase PhotorespirationAlthough some amount of photorespiration oc-

curs in many plants regardless of conditions,photorespiratory rates increase any time that car-bon dioxide levels are low and oxygen levels arehigh. Such conditions occur whenever stomata(specialized pores for gas exchange) remain closed,or partially closed, while photosynthesis is under

Photorespiration • 797

way. Under most conditions plants are able to keeptheir stomata open, so photorespiratory rates re-main low. When plants become water stressed, theyclose their stomata to prevent further water loss bytranspiration. Water stress is most likely under hot,dry conditions. Under these conditions, the sto-mata close as far as needed to conserve water, thusrestricting normal gas exchange. Carbon dioxidelevels slowly rise as water is split during the lightreactions. Consequently, photorespiratory rates ac-celerate, and photosynthetic efficiency drops to aslow as 50 percent of normal.

In dry tropical and desert environments waterstress, and thus photorespiration, can significantlyreduce plant growth potential. Some plants haveevolved solutions to this problem by modifying theway they carry out photosynthesis. One common

adaptation is called C4 metabolism. This modifica-tion involves a different leaf anatomy, called Kranzanatomy, as well as a different enzyme pathway forinitially fixing carbon dioxide that is not prone toproblems with oxygen. The Calvin cycle still func-tions in C4 plants, as they are called, but it is pro-tected from photorespiration by the C4 adaptations.Many tropical grasses, including corn and sugar-cane, use this approach. Unfortunately, many cropplants, including wheat, soybeans, spinach, and to-matoes, do not possess this adaptation.

The other major adaptation is called CAM metab-olism, which is short for crassulacean acid metab-olism. This adaptation is common in succulents(plants that store excess water in their stems andleaves, making them very juicy), such as pineap-ples, cacti, and stonecrops. Stonecrops are in the

798 • PhotorespirationP

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In dry tropical and desert environments water stress, and thus photorespiration, can significantly reduce plant growthpotential. Some plants have evolved solutions to this problem by modifying the way they carry out photosynthesis. Onecommon adaptation is called C4 metabolism. The Calvin cycle still functions in C4 plants, but it is protected from photo-respiration by the C4 adaptations. Many tropical grasses, including sugarcane, use this approach.

family Crassulaceae, which is the source of the namefor this adaptation. CAM plants, as they are called,only open their stomata at night, when transpira-tion rates are low and water loss is minimal. Carbondioxide enters the leaf at night and is attached toorganic molecules in a different pathway that doesnot require light as an energy source. Then duringthe day, when the stomata are closed, this carbondioxide is released and enters the Calvin cycle. Pho-torespiration is prevented because carbon dioxidelevels can be maintained at appropriate levels, eventhough the stomata are closed.

Why Photorespiration?Several theories have been proposed to explain

why plants photorespire. One possibility is thatwhen plants first evolved, conditions on the primi-tive earth were very different from what they arenow. The early atmosphere contained little oxygen,so the inability of Rubisco to distinguish betweenoxygen and carbon dioxide was not a problem. Asthe oxygen level in the atmosphere gradually in-creased, the formation of glycolate during photo-synthesis began to occur, and this led to the prob-lem of photorespiration. The glycolate pathwaythen developed as a mechanism for salvaging someof the material that leaves the Calvin cycle in theform of glycolate, ultimately returning a portion ofit to the cycle.

Seen in this context, the real culprit in photores-

piration is the formation of glycolate by Rubisco,while the glycolate pathway is an evolutionary ad-aptation for making the best of a bad situation. Per-haps, millions of years in the future, plants willevolve a form of Rubisco that can more effectivelydistinguish between these two gases, and photores-piration will diminish or cease.

An alternative theory about why plants pho-torespire is that the process does, in fact, perform animportant function: protecting the plant from theharmful effects of very high internal concentrationsof oxygen or energy storage molecules. This highconcentration could occur when the plant is ex-posed to high light intensities, causing photosyn-thesis to generate these substances very rapidly.Photorespiration would then consume some of theexcess oxygen and energetic molecules, depletingthem to levels that would not be harmful to theplant. It has not yet been conclusively shown, how-ever, that photorespiration really does play such aprotective role. Further research will be requiredbefore scientists know whether photorespiration isbeneficial to the plant.

Thomas M. Brennan, updated by Bryan Ness

See also: ATP and other energetic molecules; C4

and CAM photosynthesis; Calvin cycle; Gas ex-change in plants; Glycolysis and fermentation;Photosynthesis; Photosynthetic light absorption;Photosynthetic light reactions; Respiration.

Sources for Further StudyDennis, David T., et al., eds. Plant Metabolism. 2d ed. New York: Longman, 1997. A well-

balanced overview. Revised edition contains new chapters on protein synthesis and themolecular biology of plant development and on the manipulation of carbon allocation inplants.

Foyer, Christine H. Photosynthesis. New York: John Wiley & Sons, 1984. A college-levelmonograph dealing with all aspects of photosynthesis. Chapter 9 gives a very readablediscussion of photorespiration and the glycolate pathway. Earlier chapters of the bookprovide a detailed discussion of chloroplast structure and the light and dark reactions ofphotosynthesis. Technical references provided at the end of each chapter.

Kaufman, Peter B., and Michael L. Evans. Plants: Their Biology and Importance. New York:Harper & Row, 1989. An introductory-level college textbook suitable for the nonsciencemajor interested in plant biology. Chapter 8 includes a very lucid discussion of photores-piration, its effect on plant productivity, and the relationship of these processes to variousenvironmental factors. Chapter 9 deals briefly with some of the interconnections betweenrespiration, photorespiration, and photosynthesis.

Kozaki, Akiko, and Go Takeba. “Photorespiration Protects C3 Plants from Photooxidation.”Nature 384, no. 6609 (December, 1996): 557-561. Discusses photorespiration as a defensemechanism against the harmful effects of light in C3 plants, such as wheat and rice, andways in which the protective role of photorespiration is supported.

Photorespiration • 799

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology. 4th ed. Belmont, Calif.: Wadsworth,1992. An intermediate-level college textbook which presupposes a moderate knowledgeof chemistry. The chapter on photorespiration and photosynthesis includes some back-ground information on how the process of photorespiration was discovered. Also in-cludes a good discussion of the anatomical and physiological features found in plantsthat do not photorespire.

Stumpf, Paul, and E. E. Conn, eds. Metabolism and Respiration. Vol. 2 in The Biochemistry ofPlants: A Comprehensive Treatise. New York: Academic Press, 1980. Chapter 12 of this vol-ume contains a comprehensive and detailed but clearly written discussion of all aspectsof photorespiration, including the methodology used in studying the process. The authortakes a strongly chemical approach to the material; intended for readers with some priorknowledge of plant physiology and biochemistry.

PHOTOSYNTHESIS

Category: Cellular biology

Photosynthesis is the process by which organic sugar molecules are synthesized from an inorganic carbon source (carbondioxide or bicarbonate), using sunlight as the energy source to drive the process. Although most often associated withplants (in which the reactions of photosynthesis occur within compartments called chloroplasts), algae and certain typesof bacteria are also capable of photosynthesis.

From an ecological perspective, photosynthesisis significant because the conversion of inor-

ganic carbon to organic carbon represents the entrypoint of carbon atoms into biological systems. Pho-tosynthesis is also significant because it is themeans whereby oxygen is released into the atmo-sphere. The atmospheric concentration of oxygen isapproximately 21 percent, and most of this oxygenoriginates from photosynthesis. In addition, solarenergy absorbed during photosynthesis serves asthe ultimate source of energy for almost all non-photosynthetic organisms.

Nature of LightLight from the sun is composed of various types

of radiation. Only a portion of this solar radiationcan be used by plants for photosynthesis. Thisphotosynthetically active radiation (PAR) ranges inwavelength from 400 to 700 nanometers and corre-sponds approximately to the visible light perceivedby the human eye. The energy content of light de-pends on its wavelength, with shorter wavelengthshaving a higher energy content than longer wave-lengths.

Role of PigmentsFor light energy to drive photosynthesis, it first

must be absorbed. Several types of photosyntheticpigments are found in plants. When these pigmentsabsorb light, some of the pigments’ electrons are el-evated to a high energy level. These high-energyelectrons are used to drive the reactions of photo-synthesis, thus converting light energy into chemi-cal energy. The most common photosynthetic pig-ment in plants is the green-colored chlorophyll. Twotypes of chlorophyll are found in plants, chloro-phyll a and chlorophyll b, with other types of chlo-rophyll found in various types of algae and photo-synthetic bacteria.

Additional plant accessory pigments, such as ca-rotenoids, which are yellow or orange, play a minorrole in the absorption of wavelengths of light notabsorbed by chlorophyll. Carotenoids also helpprotect chlorophyll from damage that may occur asa result of absorbing excess light energy. As themost abundant plant pigment, chlorophyll givesplants their green color and usually masks the othercolored pigments. In deciduous trees and shrubs,however, chlorophyll is degraded during the au-

800 • Photosynthesis

tumn, revealing a spectacular display of colorsfrom carotenoids and other pigments.

Reactions of PhotosynthesisThe process of photosynthesis is complex, in-

volving many biochemical reactions. Historically,the reactions of photosynthesis have been dividedinto the light reactions and the dark reactions. Thelight reactions include the absorption of light andthe conversion of light energy to chemical energy.The dark reactions use the chemical energy pro-duced in the light reactions to incorporate (or fix)carbon dioxide molecules into organic molecules(sugars). Within the chloroplast, the light reactionsare localized in the internal network of membranescalled thylakoid membranes.

The dark reactions occur in the aqueous regionof the chloroplast called the stroma. The term “darkreactions” is somewhat misleading because severalphotosynthetic enzymes are not active in the dark,so these reactions will not occur without light. Al-though it is common to separate the light and darkreactions when describing photosynthesis, itshould be noted that these reactions are tightly cou-pled and occur simultaneously in the plant.

Light ReactionsChlorophyll and other accessory pigments that

absorb light energy, along with certain proteins, areorganized into structures called photosystems. Twotypes of photosystems occur in plants, photo-system I and photosystem II, and both are embed-

Photosynthesis • 801

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The processes of photosynthesis and cellular respiration are complementary: Oxygen released into the atmosphere, a by-product of photosynthesis, is breathed in by animals, which in turn breathe out carbon dioxide, essential to photosynthe-sis. Likewise, photosynthesis is at the beginning of the carbon cycle and the food chain: Algae and plants generate organic(biochemical) energy from nonorganic sunlight, water, and minerals; animals eat plants and one another; decomposers(fungi and bacteria) return the essential elements to the ecosystem; and the cycle begins again.

ded in the thylakoid membranes. When light is ab-sorbed by photosystems, the energy is transferredto special chlorophyll molecules, called reactioncenter chlorophylls, where the energy is trans-ferred to electrons. High-energy electrons are re-leased from the reaction centers and are passedalong the thylakoid membranes by a series of elec-tron transport molecules. Energy is extracted fromthe electrons as they are passed along, and the en-ergy is used to transport protons (H+) across thethylakoid membrane to the thylakoid interior. Thisprocess establishes a proton concentration gradientthat is used to make ATP (adenosine triphosphate)from ADP (adenosine diphosphate) and inorganicphosphate (Pi) in a process called photophosphoryla-tion. The acceptor molecule NADP+ (nicotinamideadenine dinucleotide phosphate, oxidized form) fi-nally accepts the high-energy electrons and com-bines them with protons (H+) to form the high-energy molecule NADPH (nicotinamide adeninedinucleotide phosphate, reduced form). This pro-cess is called noncyclic electron flow, and the ATPand NADPH produced are forms of chemical en-ergy that will be utilized by the dark reactions.

Many of the functions of photosytems I and II aresimilar. However, only photosystem II is able tosplit apart water molecules in the thylakoid interiorinto electrons, protons, and oxygen in a processcalled photolysis. The electrons released from waterduring photolysis replace electrons lost by chloro-phyll molecules during the electron transport reac-tions. The protons released from water accumulatein the thylakoid interior, adding to the concentra-tion gradient established by noncyclic electronflow. Oxygen produced from the photolysis ofwater is released as a gas to the atmosphere. There-fore, oxygen gas may be considered a by-product ofplant photosynthesis. Algae and some bacteria (cy-anobacteria) also release oxygen during photosyn-thesis, but other photosynthetic bacteria do notsplit water molecules and thus do not releaseoxygen.

The proton gradient created across the thylakoidmembranes represents a source of potential energyused in making ATP. Protons are unable to diffuseacross the thylakoid membranes unless permittedto do so by a special enzyme complex called theATP synthase. The energy required to generate ATPis provided by protons as they move through theATP synthase from the thylakoid interior, wherethere is a high concentration of protons to the

stroma, where there is a lower concentration of pro-tons. The energy associated with the proton gradi-ent is analogous to a reservoir of water held back bya dam. Water may be allowed to pass through thedam by way of a turbine, thus using water to pro-duce electrical power. The use of a proton gradientacross a membrane as the energy source for the syn-thesis of ATP by ATP synthase is called chemios-mosis, and it occurs in both the chloroplast and themitochondria. In chloroplasts, during photosyn-thesis, the process is called photophosphorylation.In mitochondria, ATP is synthesized during theprocess of oxidative phosphorylation, a componentof cellular respiration. ATP, like NADPH, is a high-energy molecule produced by the light reactionsthat is consumed during the dark reactions.

In a process called cyclic electron flow, the elec-trons can travel within the electron transport sys-tem as described above but are diverted to an ac-ceptor in the electron transport chain betweenphotosystems I and II. Passing through the chainback to photosystem I, the electrons enable thetransport of protons across the thykaloid mem-brane, thus supplying power for the generation ofATP.

Dark ReactionsCarbon dioxide gas is a normal, but minor, com-

ponent of the atmosphere and enters leaves whenair diffuses through stomata, small pores on theplant surfaces. The first reaction in converting car-bon dioxide to sugar molecules occurs when the en-zyme Rubisco (also known as ribulose bisphosphatecarboxylase/oxygenase, and reportedly the mostabundant protein on earth) combines ribulosebisphosphate (RuBP) containing five carbon atomswith a carbon dioxide molecule to produce twoidentical molecules of a simple sugar, each contain-ing three carbon atoms. These three-carbon sugarmolecules are then subsequently metabolizedthrough a series of reactions leading to the produc-tion of a three-carbon sugar called glyceraldehyde3-phosphate (G3P). Some of the G3P is used tomake glucose and other organic molecules, and theremaining G3P is used to regenerate RuBP so theprocess can continue. This cyclic pathway is knownas the Calvin cycle (or the Calvin-Benson cycle).

Sugar products may be stored within the chloro-plast as starch, or they may be transported as su-crose to other parts of the plant as needed. ATP andNADPH from the light reactions are required for

802 • Photosynthesis

several of the reactions of the Calvin cycle. Becausethe first molecule produced by this pathway is asimple sugar with three carbon atoms, the pathwayis known as the C3 pathway. Plants using this path-way are known as C3 plants, and common examplesinclude many trees and the majority of agriculturalcrops.

The Rubisco enzyme, in addition to catalyzingthe uptake of carbon dioxide during the Calvincycle, can take up oxygen, initiating another meta-bolic pathway called photorespiration. In photores-piration, when oxygen is attached to RuBP insteadof carbon dioxide, a product results that cannot beused in the Calvin cycle, and that product must gothrough a different set of complex reactions. Forthis reason photorespiration is often described as awasteful process that competes with photosynthe-sis. The relative amounts of carbon dioxide andoxygen gases inside the chloroplast determine therelative rates of photosynthesis and photorespira-tion. Experiments in which the carbon dioxide con-centration of air has been altered have demon-strated that the rate of photosynthesis increases andthe rate of photorespiration decreases when theconcentration of carbon dioxide is increased. Insome agricultural and horticultural greenhouseoperations, carbon dioxide amounts in the atmo-sphere are elevated to stimulate photosynthesis,leading to increases in plant production and yield.

Some plants have an adaptation whereby carbondioxide is initially fixed by a pathway other thanthe Calvin cycle. This adaptation involves the en-zyme phosphoenolpyruvate carboxylase (PEPcarboxylase), an enzyme that lacks the oxygenaseactivity of Rubisco. PEP carboxylase actually at-taches bicarbonate to phosphoenolpyruvate (PEP)in mesophyll cells that are in contact with air spacesin the leaf. PEP is then converted into a series of or-ganic acids and, in the process, is transported into aspecialized set of cells called bundle sheath cells thatare separated from the air spaces in the leaf. In thebundle sheath cells carbon dioxide is released fromthe last organic acid in the series, which raises thecarbon dioxide level in these cells where the carbondioxide is used in the C3 cycle. Raising the carbondioxide concentration within the chloroplast in-creases photosynthesis while reducing photorespi-ration. The initial product of the pathway in themesophyll cells is an organic acid with four carbonatoms, and thus the pathway is called the C4 path-way. Plants possessing this pathway are known as

C4 plants. Examples include most grasses and a fewcrops, including corn and sugarcane.

A second adaptation that circumvents photo-respiration is the CAM (crassulacean acid metabo-lism) pathway, named after the family of plantsin it was first observed, Crassulaceae, or the stone-crop family. CAM photosynthesis is similar to C4 pho-tosynthesis in that it is another adaptation for rais-ing the concentration of carbon dioxide inside thechloroplast. CAM plants accomplish photosynthe-sis using a biochemical process essentially the sameas that of C4 plants, but instead of carrying out thesereactions in separate cells, they carry out certain re-actions at night. CAM plants open their stomataonly at night, and the carbon dioxide is transferredto PEP, which is converted into another organicacid that is stored throughout the night. Duringthe day, the stomata remain closed, and the carbondioxide needed for the C3 cycle is supplied by re-leasing carbon dioxide from the last organic acidin the CAM cycle. Examples of CAM plants in-clude cactus and pineapple. Both C4 and CAMplants typically require less water than do C3 plantsand may be found in warmer and drier environ-ments. C4 plants tend to have high rates of pho-tosynthesis, whereas CAM plants have low pho-tosynthetic rates because the CAM cycle is lessefficient than the C4 cycle. C3 plants typically haveintermediate photosynthetic rates under optimalconditions.

Photosynthesis and the EnvironmentSeveral environmental factors affect the rate of

photosynthesis. For example, temperature extremesand water stress inhibit photosynthesis. As light in-tensity increases, so do photosynthetic rates. How-ever, when photosynthesis becomes light-saturated,further increases in light intensity will not result ingreater rates of photosynthesis. Leaves of plantsthat grow in full-sun conditions are smaller andthicker, with more extensive vascular systems thanthose found in shade plants. Although so-calledsun leaves and shade leaves have similar photosyn-thetic rates in low light, shade leaves have muchlower rates of photosynthesis at high light intensi-ties and can be damaged when exposed to suchconditions.

As mentioned above, atmospheric carbon diox-ide concentrations can also regulate photosynthe-sis. At the present time the concentration of carbondioxide in the atmosphere is less than 0.04 percent,

Photosynthesis • 803

but scientific data show that this concentration isincreasing. Higher concentrations of atmosphericcarbon dioxide may stimulate plant photosynthesisand plant growth but may have other undesirableclimatic effects.

William J. Campbell

See also: Anaerobic photosynthesis; C4 and CAMphotosynthesis; Chemotaxis; Calvin cycle; Chloro-plasts and other plastids; Energy flow in plant cells;Eukaryotic cells; Membrane structure; Photosyn-thetic light absorption; Photosynthetic light reac-tions; Pigments in plants; Plant cells: molecularlevel; Respiration.

Sources for Further StudyBuchanan, Bob B., et al. Biochemistry and Molecular Biology of Plants. Rockville, Md.: Ameri-

can Society of Plant Physiologists, 2000. A comprehensive and up-to-date book present-ing the current understanding of the functioning of plants. Includes diagrams, photo-graphs, bibliographical references, and index.

Hopkins, William G. Introduction to Plant Physiology. 2d ed. New York: John Wiley & Sons,1999. An overview of the processes and functions of plants. Includes diagrams, photo-graphs, bibliographical references, and index.

Rao, K. K., and D. O. Hall. Photosynthesis. 6th ed. Cambridge, England: Cambridge Univer-sity Press, 1999. Covers all aspects of photosynthesis. Includes diagrams, photographs,bibliographical references, and index.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology. 2d ed. Sunderland, Mass.: Sinauer Asso-ciates, 1998. Thorough coverage of the physiology of flowering plants. Includes dia-grams, photographs, bibliographical references, and index.

PHOTOSYNTHETIC LIGHT ABSORPTION


Photosynthetic light absorption involves plants’use of pigments to facilitate the conversion of light energy into chemicalenergy.

Photosynthesis occurs in green plants, algae, andcertain types of bacteria. There is considerable

variation among the types of pigments found inthese different groups of organisms, but the basicmechanisms by which they absorb light are similar.Photosynthetic pigments are always attached tomembranes within a cell. In algae and higherplants, the photosynthetic pigments are located inthe chloroplast, where photosynthesis takes place.The pigment molecules are not dispersed ran-domly within the chloroplast but are arrayed on thesurface of the thylakoid membranes. The chloroplastsbecome oriented in such a way as to present a largesurface area to the sun or other light source, therebymaximizing the ability of the pigments to absorblight energy. In photosynthetic bacteria, the light-

absorbing pigments are not organized in chloro-plasts but are located on membranes that are dis-persed throughout the cell.

Chlorophylls and CarotenoidsThe two primary types of pigments utilized by

most photosynthetic organisms are the greenchlorophylls and the yellow to orange carotenoids.There are several forms of chlorophyll that differfrom one another in small details of their molecularstructures. The forms are designated chlorophyll a,b, c, and so on. Chlorophyll a is found in all plantsand algae, while the other forms are dispersedamong various taxonomic groups. The chlorophyllmolecule consists of two parts: an elaborate ringstructure that actually absorbs the light, and a long

804 • Photosynthetic light absorption

tail-like section that anchors the molecule in themembrane. Photosynthetic bacteria contain similarpigments called bacteriochlorophyll a and b.

Carotenoids, the other major group of photosyn-thetic pigments, also occur in various forms and arefound in all types of photosynthetic plants, algae,and bacteria. The xanthophylls, which are oxygen-ated carotenoids, form another widespread anddiverse subgroup of pigments. Carotenoid mole-cules have elongated structures and, like thechlorophylls, are embedded in the photosyntheticmembranes. It is a popular misconception that thechange in leaf color that takes place in the fall is theresult of the formation of new yellow or orangecarotenoid pigments. Actually, the carotenoids arepresent all the time but are masked by the presenceof the green chlorophyll. In fall, the chlorophyll be-gins to decompose more rapidly than the caroten-oids, whose yellow colors are then exposed.

Chlorophyll a, or something very similar to it,occurs almost universally in photosynthetic organ-isms, from bacteria to higher plants, because it is anessential component of photosynthetic reactioncenters. All of the other chlorophylls and caroten-oids involved in light absorption are referred to asaccessory pigments. Another kind of accessory pig-ment found in some groups of algae and photosyn-thetic bacteria are the phycobiliproteins, which mayimpart a red or blue color to the cells in which theyoccur. These molecules consist of a light-absorbingportion bound to a protein. In fact, all types of pig-ment molecules seem to be bound to proteinswithin the photosynthetic membranes. These pig-ment-protein associations are sometimes referredto as light-harvesting complexes, a term that accu-rately describes their function.

Properties of LightTo understand the functioning of photosynthetic

pigments, it is necessary to consider first the physi-cal nature of light. Visible light is only a small por-tion of the electromagnetic spectrum, which rangesfrom very short wavelength radiation, such as Xrays, to extremely long wavelength radiation, suchas radio waves. The visible portion of the spectrumis intermediate in wavelength and ranges from blue(at the short end) to red (at the long end). Sunlightcontains a mixture of all the visible wavelengths,which humans perceive as white light. The energyof light is inversely proportional to its wavelength;blue light has more energy than an equivalent

amount of red light. Light may be thought of as con-sisting of either waves or particles. For purposes ofstudying light absorption by pigments, it is easierto think of light as particles, referred to as photons orquanta.

When a photon is absorbed by a pigment mole-cule, the photon’s energy is transferred to one of theelectrons of the pigment. The electron is thus said toenter an excited state and contains the energy origi-nally associated with the photon of light. A specifickind of pigment is not capable of absorbing all thephotons it encounters. Only photons of certain en-ergy (and therefore wavelength) can be absorbedby each pigment. For example, chlorophyll primar-ily absorbs light in the blue and red wavelengthsbut not in the green portion of the spectrum. Conse-quently, the green light to which chlorophyll is ex-posed is either transmitted through it or reflectedfrom it, with the result being that the pigment ap-pears green. The color of the pigment results fromthe wavelengths of light that are not absorbed. Ca-rotenoids do not absorb light in the yellow to or-ange portion of the spectrum and, therefore, areseen as being that color.

The process of light absorption begins when aphoton of appropriate energy strikes a chlorophyllor carotenoid molecule located on a thylakoid orother photosynthetic membrane, thus causing anelectron in the pigment to be raised to an excitedstate. If two pigment molecules are situated adja-cent to each other in exactly the right orientationand are separated by a very small distance, it is pos-sible for the energy of excitation to be transferredfrom one molecule to the next. This transfer process(referred to as Forster resonance) enables the excita-tion energy to migrate throughout the array of pig-ment molecules that are attached to the photosyn-thetic membrane.

In addition to pigment molecules, the mem-branes also contain a smaller number of specialstructures called reaction centers, which consist ofspecial chlorophyll and protein molecules ar-ranged in a very specific fashion. The excitation en-ergy migrating throughout the pigment array willeventually find its way to one of the reaction cen-ters, and there it is utilized to form new energy-containing molecules. All of this occurs with a largenumber of photons and pigment molecules simul-taneously. The array of pigment molecules feedingexcitation energy into the reaction centers containsboth chlorophylls and carotenoids and is some-

Photosynthetic light absorption • 805

times referred to as an antenna, to indicate its role inlight absorption. This overall process constitutesthe light reactions of photosynthesis. The energy-containing molecules thus formed will then be usedin the Calvin cycle to convert carbon dioxide intocarbohydrates: Light energy has been convertedinto chemical energy.

Pigment FunctionsChlorophyll a, as well as the other chlorophylls,

makes up a major portion of the antenna pigmentsthat absorb light energy and transfer it to the reac-tion centers. The carotenoids, which are also part ofthe antenna, seem to contribute in two ways to theeffectiveness of the light absorption process. First,they increase the range of wavelengths that can beabsorbed. Chlorophyll absorbs mainly in the blueand red portions of the spectrum but is not effectiveat absorbing other wavelengths. Carotenoids areable to absorb some of the green light that would beunusable if chlorophyll were the only pigmentpresent, so having a combination of different pig-ments makes the organism more effective at usingthe various wavelengths that occur in sunlight.

A second function of the carotenoids has to dowith their ability to protect chlorophyll from dam-age by intense light. Under conditions of high lightintensity, chlorophyll has a tendency to decompose

through a process called photooxidation. The pres-ence of carotenoids prevents this decompositionfrom occurring and enables the chlorophyll to con-tinue to function effectively at light intensities thatwould otherwise cause damage.

Although virtually all photosynthetic organismsutilize some form of chlorophyll in light absorp-tion, an exception is found in the halobacteria.These bacteria live in conditions of very high saltconcentration, such as the Dead Sea or the GreatSalt Lake. In these bacteria is found a purple mem-brane containing molecules of a pigment calledbacteriorhodopsin which, remarkably, is very similarto rhodopsin, the pigment found in the visual sys-tems of higher animals. When bacteriorhodopsinmolecules absorb light, they cause a hydrogen ion(a proton of H+) to be ejected across the cell mem-brane, and this leads to the formation of energy-containing molecules that the bacterium can utilizefor its various metabolic requirements. Researchwith these unique photosynthetic organisms mayalso lead to a better understanding of the molecularbasis and evolution of vision in animals.

Thomas M. Brennan

See also: Anaerobic photosynthesis; Calvin cycle;Chloroplasts and other plastids; Photosynthesis;Photosynthetic light reactions; Pigments in plants.

Sources for Further StudyCurtis, Helena, and N. Sue Barnes. Biology. 5th ed. New York: Worth, 1989. An introductory

college or advanced high-school-level textbook. Chapter 10, “Photosynthesis, Light, andLife,” covers the absorption of light by photosynthetic pigments and its biological signifi-cance. Includes suggestions for further reading.

Häder, Donat-Peter, and Manfred Tevini. General Photobiology. Elmsford, N.Y.: PergamonPress, 1987. Acollege-level reference book that introduces the reader to the general princi-ples of photobiology as well as a number of specific processes, including photosynthesis.Contains sections on the physical nature of light, absorption and energy transfer, andmethods of study.

Hall, David O., and Krishna Rao. Photosynthesis. 6th ed. New York: Cambridge UniversityPress, 1999. Provides a clear, concise, and vivid account of the process of photosynthesis,including details at the macro and molecular levels.

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology. 4th ed. Belmont, Calif.: Wadsworth,1992. An intermediate-level college textbook which presupposes a moderate knowledgeof chemistry. The chapter “Photosynthesis: Chloroplasts and Light” includes a good dis-cussion of photosynthetic pigments and the concepts of absorption and action spectra.

Wilkins, Malcolm. Plantwatching: How Plants Remember, Tell Time, Form Partnerships, andMore. New York: Facts on File, 1988. This book is intended to introduce the nonscientist tovarious aspects of plant biology. It is clearly written and well illustrated. Chapter 9, “Pho-tosynthesis: The Capture of Solar Energy,” deals with light absorption by photosyntheticpigments.; photosynthesis and respiration; physiology.

806 • Photosynthetic light absorption

PHOTOSYNTHETIC LIGHT REACTIONS


Photosynthetic light reactions involve the absorption of light energy by plant pigments and the conversion of light en-ergy into adenosine triphosphate (ATP).

Photosynthesis is the process by which plants, al-gae, and certain types of bacteria use the energy

of sunlight to manufacture organic molecules fromcarbon dioxide and water. The process may be di-vided into two parts: the light reactions and the darkreactions. In the light reactions of photosynthesis,light energy coming from the sun or from an ar-tificial light source is absorbed by pigments andused to boost electrons into higher energy levels sothey can be used to do cellular work. In the darkreactions (also called the Calvin cycle), energy-containing molecules from the light reactions areused to convert carbon dioxide into carbohydrates.All living organisms ultimately depend upon thisprocess as their source of food.

In algae and higher plants, the light reactions ofphotosynthesis take place on thylakoid membraneslocated within chloroplasts. The surfaces of thethylakoids are covered with molecules of the greenpigment chlorophyll as well as yellow carotenoid pig-ments. Also located on the thylakoids, though fewerin number, are special structures called reaction cen-ters. The process begins when a unit of light energy(referred to as a photon or quantum) strikes a pig-ment molecule and causes one of its electrons to beraised to a higher energy level, or an excited state.Many chlorophyll and carotenoid molecules are lo-cated adjacent to one another on the thylakoids,and the energy of the excited electrons may be trans-ferred from one to the next until it reaches a reactioncenter. Overall, a very large number of pigmentmolecules are absorbing light, becoming excited,and passing the excitation energy to reaction centers.It is in the reaction centers that the central events ofthe photosynthetic light reactions take place.

Photosystem IHigher plants contain two different types of

reaction centers, referred to as photosystem I and

photosystem II. (The numbers I and II have no func-tional significance; they simply reflect the order inwhich they were discovered.) What the two typesof reaction centers actually consist of are groups ofspecial proteins complexed with several chloro-phyll molecules and structured in a very specific ar-rangement within the thylakoid membrane. Alsoembedded in the thylakoid membrane are a seriesof proteins and other molecules that are capable oftransporting electrons from photosystem II tophotosystem I. This process is referred to as the elec-tron transport system. The two photosystems and theelectron transport system work together to formthe energy-containing molecules produced in thephotosynthetic light reactions.

The process is best understood by looking first atwhat happens in photosystem I. As describedabove, light energy is absorbed by a pigment mole-cule and transferred to the reaction center, causingtwo photosystem I electrons to be raised to an ex-cited state. The excited electrons are then passedfrom the reaction center to a primary electron ac-ceptor. The primary electron acceptor passes theelectrons to the first of a series of electron trans-port proteins in the inner thykaloid membrane, thelast of these proteins being ferredoxin, an iron-containing protein. Ferredoxin passes the electronsto a coenzyme called nicotinamide adenine dinucle-otide phosphate (NADP+), which then becomes re-duced and, after joining with a free proton (H+), be-comes NADPH. The important point is that some ofthe energy of the excited electrons is incorporatedinto the molecule of NADPH, thereby convertinglight energy into chemical energy. The NADPH isnot attached to the thylakoid membrane but re-mains in the stroma (the region inside both the outermembranes of the chloroplast and surrounding thethykaloids) of the chloroplast, where it will be usedin the Calvin cycle.

Photosynthetic light reactions • 807

Photosystem IIHowever, photosystem I, by itself, could not con-

tinue to function in the manner described abovewithout having some way of replacing the electronswhich are being removed. This is where photo-system II comes in. The basic operation of pho-tosystem II is similar to photosystem I insofar aslight energy is absorbed by pigment molecules andtransferred to the reaction center. However, the ex-cited electron is not used to form NADPH; insteadit enters the electron transport system and is passedfrom one molecule to the next until it eventuallyreaches photosystem I, where it replaces the elec-tron previously lost in the formation of NADPH.

The electron transport system does much morethan merely replace electrons in photosystem I. Asan electron moves through the electron transportsystem, its energy is used to transfer protons (hy-drogen ions) from the stroma to the thykaloid space(the region inside the thykaloids). The thylakoidsare not simply flat sheets of membrane but arefolded in such a way as to form numerous saclikecompartments within the chloroplast. The result isthat a proton gradient is established across thethykaloid membrane.

Many electrons are carried through the electrontransport system, resulting in the accumulation of ahigh concentration of protons within the thylakoidcompartments. This high concentration of protonson one side of the membrane and relatively lowconcentration on the other represents a source of po-tential energy, somewhat analogous to the energycontained in a body of water held back by a dam.The protons are then allowed to move back acrossthe membrane through special proteins called ATPsynthase. ATP synthase is an enzyme capable of cat-alyzing the joining of adenosine diphosphate (ADP)with inorganic phosphate (Pi) in the stroma to formadenosine triphosphate (ATP). The passage of pro-tons through ATP synthase results in a change inshape in the protein, which brings about the reac-tion between ADP and Pi. The formation of ATPby this mechanism is called photophosphorylation.Once again, light energy has been transformed intochemical energy.

Electron ReplacementIf electrons from photosystem II are used to re-

place those from photosystem I, then the electronsfrom photosystem II must be replaced as well. Thisproblem is solved by the use of water as a source of

electrons. Photosystem II is capable of splittingapart molecules of water to extract electrons, usingthe electrons to replace the ones used by the elec-tron transport system. Water is always available ina functioning photosynthetic plant cell, and thisrepresents a virtually limitless source of electrons.Water molecules contain hydrogen and oxygen,and when they are split apart in this manner, theprotons left over from hydrogen simply go into so-lution, but the oxygen forms oxygen gas and is re-leased into the atmosphere. Although the oxygen isreally no more than a by-product as far as photo-synthesis is concerned, it is of profound signifi-cance to all higher organisms.

The overall flow of electrons in the light reac-tions is from water to photosystem II, through theelectron transport system where ATP is formed, tophotosystem I and finally to NADPH. This ar-rangement was hypothesized by Robin Hill andFay Bendall in 1960. Because of the way in whichthe process was diagrammed in their paper, it is of-ten referred to as the Z scheme. In energy terms,what has been accomplished is the conversion oflight energy to chemical energy in the form of ATPand NADPH. These energy-rich molecules are es-sential to the Calvin cycle as energy for the conver-sion of carbon dioxide into carbohydrates. The pro-cess is known as noncyclic electron flow.

Alternatively, in cyclic electron flow, the electronscan travel within the electron transport system asdescribed above but are diverted to an acceptor inthe electron transport chain between photosystemsI and II. Passing through the chain back to pho-tosystem I, the electrons enable the transport ofprotons across the thykaloid membrane, thus sup-plying power for the generation of ATP. This pro-cess is called cyclic photophosphorylation, because itinvolves a cyclic flow of electrons. In this way,photosystem I can work independently of pho-tosystem II. Apparently, this is the manner in whichsome bacteria carry out photosynthesis.

Bacterial PhotosynthesisCertain types of bacteria also carry on photo-

synthesis. These organisms do not contain chloro-plasts, so the light-absorbing pigments and reac-tion centers are located on membranes spreadthroughout the cell. In one group, the cyanobacte-ria, the photosynthetic process is similar to thatdescribed for algae and higher plants in that thereare two photosystems, water serves as the pri-

808 • Photosynthetic light reactions

mary electron donor, and oxygen is released.The other types of photosynthetic bacteria, how-

ever, are more primitive. They have only one pho-tosystem and use inorganic compounds such ashydrogen sulfide instead of water as a source ofelectrons. The cyanobacteria possess chlorophyll,and the other photosynthetic bacteria contain asimilar pigment known as bacteriochlorophyll. In ad-

dition, these organisms contain a variety of acces-sory pigments that also are involved in the photo-synthetic light reactions.

Thomas M. Brennan, updated by Bryan Ness

See also: Active transport; Anaerobic photosynthe-sis; Calvin cycle; Photosynthesis; Photosyntheticlight absorption; Pigments in plants.

Sources for Further StudyCurtis, Helena, and N. Sue Barnes. Biology. 5th ed. New York: Worth, 1989. An introductory

college or advanced high-school-level textbook. Chapter 10, “Photosynthesis, Light, andLife,” includes a well-written and illustrated discussion of both the light and dark reac-tions of photosynthesis, with suggestions for further reading. Chapter 54, “Ecosystems,”addresses the role of photosynthesis in energy flow through ecosystems.

Hall, David O., and Krishna Rao. Photosynthesis. 6th ed. New York: Cambridge UniversityPress, 1999. Provides a clear, concise, and vivid account of the process of photosynthesis,including details of the processes at the macro and molecular levels.

Nakatani, Herbert Y. Photosynthesis. Burlington, N.C.: Carolina Biological Supply Company,1988. A brief discussion of the ecological significance of photosynthesis and the mecha-nisms of both the light and dark reactions. Well-written and illustrated, with suggestionsfor further reading.

Ort, Donald R., Charles F. Yocum, and Iris F. Heichel, eds. Oxygenic Photosynthesis: The LightReactions. Boston: Kluwer, 1996. Describes reactions in eukaryotic thykaloid membranes;topics include photosystems, plastocyanin, ferredoxin, light-harvesting complexes, andthe coupling factor.

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology. 4th ed. Belmont, Calif.: Wadsworth,1992. An intermediate-level college textbook which presupposes a moderate knowledgeof chemistry. The chapter “Photosynthesis: Chloroplasts and Light” covers all aspects ofthe light reactions and chloroplast structure and also includes an interesting summary ofthe history of research on photosynthesis.

Wilkins, Malcolm. Plantwatching: How Plants Remember, Tell Times, Form Partnerships, andMore. New York: Facts on File, 1988. A unusually interesting and well-written book deal-ing with many aspects of plant life at a level understandable by the nonscientist. Chapter9, “Photosynthesis: The Capture of Solar Energy,” covers the light reactions clearly and ef-fectively, with good illustrations.

PHYTOPLANKTON

Categories: Algae; microorganisms; water-related life

The term “plankton,” from Greek planktos for “wandering,” is applied to any organism that floats or drifts with themovement of the ocean water. Most plankton are microscopic and are usually single-celled, a chain of cells, or a loosegroup of cells. Algal and cyanobacterial plankton are referred to as phytoplankton. The heterotrophic crustaceans andlarvae of animals are referred to as zooplankton.

The group of organisms known as phytoplank-ton (literally, “plant” plankton) do not consti-

tute a taxonomic group but rather refer to a collec-tion of diverse, largely algal and cyanobacterial,

Phytoplankton • 809

microorganisms that live in water and are at thebase of the food chain. The phytoplankton, includ-ing diatoms, unicellular cyanobacteria and cocco-lithophorids in nutrient-poor waters, and crypto-monads, manufacture organic material from carbondioxide, usually through photosynthesis. Phyto-plankton are responsible for one-half of the world’sprimary photosynthesis and produce one-half ofthe oxygen in the atmosphere.

Eighty to ninety percent of the weight of phyto-plankton is water, with the rest made up of protein,fat, salt, carbohydrates, and minerals. Some specieshave compounds of calcium or silica that make uptheir shells or skeletons. Phytoplankton includemany of the algal phyla: Chrysophyta (chrysophytes),Phaeophyta (golden-brown algae), coccolithophores,silicoflagellates, and diatoms. The most commontype of phytoplankton is the diatom (phylum Bacil-

lariophyta), a single-celled organism that can formcomplex chains. Dinoflagellates (phylum Dinophyta)are the most complex of the phytoplankton. Theyare unicellular and mobile. Green algae (phylumChlorophyta) are usually found in estuaries or la-goons in the late summer and fall. Some speciescan cause toxic algal blooms associated with coastalpollution and eutrophication. Cyanobacteria (oftencalled blue-green algae but not true algae) are prom-inent near shore waters with limited circulationand brackish waters.

PhotosynthesisPhytoplankton are primary producers, responsi-

ble for a half the world’s primary photosynthesis:the conversion of light energy and inorganic matterinto bioenergy and organic matter. Each year, 28 bil-lion tons of carbon and 250 billion to 300 billion tons

810 • Phytoplankton

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of photosynthetically produced materials are gen-erated in the oceans by phytoplankton. All animalorganisms eliminate carbon dioxide into the atmo-sphere, and plants remove carbon dioxide from theair through photosynthesis. In the oceans’ carboncycle, carbon dioxide from the atmosphere dis-solves in the ocean. Photosynthesis by marineplants, mainly phytoplankton, converts the carbondioxide into organic matter. Carbon dioxide is laterreleased by plants and animals during respiration,while carbon is also excreted as waste or in the deadbodies of organisms. Bacteria decompose organicmatter and release the carbon dioxide back into thewater. Carbon may be deposited as calcium carbon-ate in biogenous sediments and coral reefs (made ofskeletons and shells of marine organisms).

Food ChainBecause they are primary producers of organic

matter through photosynthesis, phytoplanktonplay a key role in the world’s food chain: They areits very beginning. Sunlight usually penetratesonly 200 to 300 feet deep into ocean waters, a regioncalled the photic zone. Most marine plant and ani-mal life and feeding take place in this zone.Phytoplankton, the first level in the marine foodchain, are the primary food source for zooplanktonand larger organisms. These microscopic plants usethe sun’s energy to absorb minerals to make basicnutrients and are eaten by herbivores, or plant eaters.Herbivores are a food source for carnivores, the meateaters. In temperate zones, phytoplankton increasegreatly in the spring, decline in the summer, and in-crease again in the fall. Zooplankton (animal plank-ton) are at their maximum abundance after thespring increase, and their grazing on the phyto-plankton causes a decrease in phytoplankton pop-ulation in the summer. Fish and invertebrates thateat zooplankton become more abundant and so on,up the food chain. Krill, planktonic crustaceans,and larvae commonly eaten by whales, fish, seals,penguins, and seabirds feed on diatom phyto-plankton.

Red TidesThe term red tide is applied to red, orange, brown,

or bright-green phytoplankton blooms, or even toblooms that do not discolor the water. Red tides arepoorly understood and unpredictable. No one iscertain what causes the rapid growth of a singlespecies of phytoplankton, although they can blos-

som where sunlight, dissolved nutrient salts, andcarbon dioxide are available to trigger photosyn-thesis. Dense phytoplankton blooms occur in stablewater where lots of nutrients from sewage and run-off are available. Natural events, such as stormsand hurricanes, may remobilize populations bur-ied in the sediment. These nuisance blooms, usu-ally caused by dinoflagellates, which turn the watera reddish brown, and cyanobacteria, are becomingmore frequent in coastal waters, possibly becauseof increased human populations and sewage. Inshallower bodies of water, such as bays and estuar-ies, nutrients from winter snow runoffs, springrains, tributaries, and sewage bring about springand summer blooms.

Some of the poisons produced during red tidesare the most powerful toxins known. The release oftoxins by dinoflagellates may poison the higherlevels of the food chain as well as suppress otherphytoplankton species. These toxins cause highmortality in fish and other marine vertebrates.They can kill the whales and seabirds that eat con-taminated fish. Dinoflagellates produce a deadlyneurotoxin called saxitoxin, which is fifty timesmore lethal than strychnine or curare. Commercialshellfish, such as mussels, clams, and crabs, canstore certain levels of the toxin in their bodies.

People who eat contaminated shellfish may ex-perience minor symptoms, such as nausea, diar-rhea, and vomiting, or more severe symptoms suchas loss of balance, coordination, and memory, tin-gling, numbness, slurred speech, shooting pains,and paralysis. In severe cases, death results fromcardiac arrest. When the toxins are blown ashore insea spray, they can cause sore throats or eye andskin irritations.

Toxic blooms costs millions of dollars in eco-nomic losses, especially for fisheries which cannotharvest some species of shellfish. Smaller fish farmscan be devastated. Additionally, coastal fish deathsfoul beaches and shore water with decaying bodies,which can cripple tourism in the coastal regions.

Not all blooms are harmful, but they do affect themarine environment. Even when no toxins are re-leased, massive fish kills can result when the largeblooms of phytoplankton die. When the bloomingphytoplankton population crashes, bacterial de-composition depletes the oxygen in the water,which in turn reduces water quality and condi-tions, and fish and other marine animals suffocate.

Virginia L. Hodges

Phytoplankton • 811

See also: Algae; Aquatic plants; Bacteria; Brownalgae; Chlorophyceae; Chrysophytes; Diatoms; Di-noflagellates; Eutrophication; Food chain; Green al-

gae; Invasive plants; Marine plants; Photosynthe-sis; Red algae.

Sources for Further StudyCastro, Peter, and Michael Huber. Marine Biology. 3d ed. Boston: McGraw-Hill, 2000. Well or-

ganized, basic textbook with summary and bibliography with each chapter. Includes il-lustrations, glossary, and index.

Cousteau, Jacques. The Ocean World. New York: Harry N. Abrams, 1985. Discusses all ma-rine life. Includes color photographs, glossary, bibliography, and index.

Levinton, Jeffrey S. Marine Biology: Function, Biodiversity, Ecology. New York: Oxford Uni-versity Press, 1995. Well-organized, advanced textbook, with a good section on phy-toplankton. Bibliography and review questions with each chapter. Includes glossary,index.

Sumich, James L. An Introduction to the Biology of Marine Life. 5th ed. Dubuque, Iowa: WilliamC. Brown, 1992. Textbook format with basic explanations, review and discussion ques-tions, and bibliography with each chapter. Includes glossary and taxonomic and subjectindexes.

PIGMENTS IN PLANTS

Categories: Animal-plant interactions; economic botany and plant uses; photosynthesis and respiration;physiology

Photosynthetic pigments color plants and participate in photosynthesis. Other plant pigments are important in flowersand fruits to attract pollinators and seed dispersers. Humans use plant pigments in vitamins and dyes.

Plant pigments can be classified as either nitroge-nous or non-nitrogenous, that is, either nitrogen-

containing or non-nitrogen-containing.

Non-nitrogenous pigmentsNon-nitrogenous forms are widely distributed

and include the carotenoids and the quinones. Carot-enoids are yellow, orange, or red pigments often in-volved as accessory pigments in photosynthesis.They are insoluble in water but soluble in a varietyof nonpolar solvents. They are easily bleached bylight or oxygen. Carotenoids occur as hydrocarboncarotenes and oxygenated xanthophylls. They aremade by bacteria, fungi, algae, and other plants.Variations of their occurrence and their multiplicitymake carotenoids useful in taxonomic differentia-tion of these organisms.

The quinones include benzoquinones, naphtho-quinones, and anthraquinones. Benzoquinones oc-cur in fungi and higher plants as yellow, orange,

red, or violet pigments. Yellow coenzyme Q vari-ants (ubiquinones) and plastoquinones are foundin most plants. Because of their low tissue levels,they do not affect plant color. Naphthoquinones ofbacteria and leaves, seeds, and woody parts ofhigher plants have yellow, orange, red, or purplepigments, soluble in nonpolar solvents. A familiarexample is vitamin K. Brightly colored anthraqui-nones also occur in many plants.

Many other complex plant quinones are water-soluble flavonoids, all containing a common fifteen-carbon skeleton, flavone (2-phenylben-zopyrone).The various flavonoids differ in how many hy-droxyl or methoxyl groups they contain. They oc-cur as sugar-containing substances called gly-cosides, which is the basis of their water solubility.The members of one plentiful group of flavonoidsare the anthoxanthins, which are yellow pigments.Another important flavonoid group, anthocyanins,are orange, red, crimson, or blue.

812 • Pigments in plants

Nitrogenous PigmentsThe nitrogenous (nitrogen-containing) pigments

are tetrapyrrole porphyrins and their derivatives,indigoids and flavins. The porphyrins are water-soluble, cyclic, nitrogen-containing substances. Thebasic structural unit of all porphyrins is a largetetrapyrrole ring made up of four smaller connectedpyrrole rings. Porphyrins combine with metal ionsand proteins. In plants they are represented bygreen chlorophylls. The chlorophylls contain magne-sium ions and are associated with water-insolubleproteins. Related bilins are a group of yellow, green,red, or brown compounds that have linear struc-tures composed of four connected pyrrole rings.The bilins include red bilirubin, green biliverdin,and the phycobilins of red algae or green plants.Examples are blue phycocyanobilin and phyto-chrome as well as red phycoerythrin. Plant bilinsbind water-soluble and water-insoluble proteins.

Indigoids, a group of indole pigments, derivefrom the amino acid tryptophan. They are red, blue,

or purple. The flavins (lyochromes) are pale yellow,water-soluble pigments widely distributed inplants. The most plentiful flavin is riboflavin (vita-min B2). Flavins are made by bacteria, yeasts, andgreen plants.

FunctionsNon-nitrogenous benzoquinones and ubiqui-

nones are involved in electron transport processesimportant in respiration. Naphthoquinones havesimilar functions in photosynthesis and are alsorepresented by K vitamins. Anthroquinones are thebrilliantly colored compounds seen in the colors offlowers. Similarly, flavonoid glycosides color manyflowers. For example, the anthoxanthins producethe yellow color of buttercups.

Anthocyanins make flower petals orange-red,crimson, and blue. They cause much of the red colorseen in some plant buds and shoots, the colors ofautumn leaves, fruits (such as berries), or roots(such as beets). A typical anthocyanin is red in acid

Pigments in plants • 813D

igital

Stock

Carotenoids are yellow, orange, or red pigments often involved as accessory pigments in photosynthesis. In the fall, de-struction of the green pigment chlorophyll is accompanied by increased visibility of the yellow or orange pigments of thecarotenoids, which were previously masked by chlorophyll in these maple leaves.

solution, violet in neutral solution, and blue in alka-line solution. Blue or red cornflowers and violetdahlias contain the same anthocyanin, their colordifferences simply being the result of pH (acidity oralkalinity) differences of the cell sap.

The function of flavonoids is still unclear. How-ever, it is believed that they are essential in flowersfor attracting bees and other pollinators, encourag-ing cross-pollination. They are also thought to at-tract larger animals to bright-colored, edible fruits,enhancing seed dissemination, and to protectplants from damage by ultraviolet light.

Nitrogen-containing tetrapyrrole plant porphy-rins are best known because of the key role they playin photosynthesis. Chlorophylls enable photosyn-thetic conversion of sunlight to chemical energy,which is used by plant cells to use carbon dioxideto make organic molecules. Chlorophylls includechlorophylls a and b of higher plants and green al-gae and the bacteriochlorophylls in photosyntheticbacteria. The various chlorophylls differ in only mi-nor ways as a result of side chain groups attached totheir pyrroles. In higher plants, chlorophylls bindto proteins and lipids in the thylakoid membranes ofchloroplasts, where they function in photosynthe-sis. Some of the phycobilins of blue algae, red algae,and green plants act as accessory photosyntheticpigments. Another function of bilins is as phyto-chromes, essential to photoperiodic processes.

Economic UsesMany plant pigments meet human nutritional

needs. The carotenes, derived from carotenoids, areused in the biosynthesis of vitamin A, essential to

vision and growth. Furthermore, napthoquinonephotosynthetic pigments lead to another importantvitamin group: K vitamins, essential to blood clot-ting. Yet another vitamin derived from plant pig-ments is a nitrogen-containing flavin called ribofla-vin, better known as vitamin B2.

Many plant pigments are used as dyes or asmodel compounds from which other dyes havebeen synthesized. The naphthoquinones fromleaves, seeds, and woody parts of higher plants areisolated as yellow, orange, red, or purple materialssoluble in organic solvents and used as fabric dyes.Indigo, a blue indigoid which occurs in manyplants of Asia, Africa, and South America, has beenused as a blue dye and the model for many industri-ally synthesized dyes. Similarly, anthraquinones,brightly colored plant pigments, are widely used.Moreover, because their colors change in acidic, ba-sic, and neutral solutions, anthraquinones are usedas acid-base indicators. Flavonoid anthocyanins (aswas mentioned) are also used as acid-base indica-tors. A variety of edible plant pigments are alsoused to add color to foods.

Sanford S. Singer

See also: Algae; Archaea; Bacteria; Angiospermevolution; Angiosperms; Animal-plant interac-tions; Biochemical coevolution in angiosperms;Chloroplasts and other plastids; Chromatography;Coevolution; Flower types; Flowering regulation;Fruit: structure and types; Leaf abscission; Metabo-lites: primary vs. secondary; Photoperiodism; Pho-tosynthesis; Photosynthetic light absorption; Pho-tosynthetic light reactions; Vacuoles.

Sources for Further StudyGross, Jeana. Pigments in Vegetables: Chlorophylls and Carotenoids. New York: Van Nostrand

Reinhold, 1991. Provides information on chlorophylls and carotenoids in vegetation. In-cludes illustrations, bibliography.

Mazza, G., and Enrico Miniati. Anthocyanins in Fruits, Vegetables, and Grains. Boca Raton, Fla.:CRC Press, 1993. Describes anthocyanin types, their structures, pH effects, biosynthesis,presumed functions, distribution in numerous vegetables, fruits, and grains. Includes anexcellent bibliography and illustrations.

Proctor, John, and Susan Proctor. Color in Plants and Flowers. New York: Everest House, 1978.An interesting, brief, nicely illustrated book on plant and flower color.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. This well-illustrated text presents many aspects of flowerstructure, also identifying flower structure, color, and means of pollination.

Singhal, G. S, ed. Concepts in Photobiology: Photosynthesis and Photomorphogenesis. Boston:Kluwer Academic, 1999. Topics related to pigments include photobiology, photosynthe-sis principles, chlorophylls, phycobiliins, and phytochromes. It has a fine bibliography.

814 • Pigments in plants

Young, Andrew, and George Britton, eds. Carotenoids in Photosynthesis. London: Chapmanand Hall, 1993. The illustrated text discusses carotenoid distribution in plants, their pro-tein complexes, biosynthesis, and photosynthetic functions. Includes bibliography, illus-trations.

PLANT BIOTECHNOLOGY

Categories: Biotechnology; economic botany and plant uses; environmental issues; genetics

Plant biotechnology may be defined as the application of knowledge obtained from study of the life sciences to create tech-nological improvements in plant species. By this very broad definition, plant biotechnology has been conducted for morethan ten thousand years.

The roots of plant biotechnology can be tracedback to the time when humans started collect-

ing seeds from their favorite wild plants and begancultivating them in tended fields. It appears thatwhen the plants were harvested, the seeds of themost desirable plants were retained and replantedthe next growing season. While these primitive ag-riculturists did not have extensive knowledge ofthe life sciences, they evidently did understand thebasic principles of collecting and replanting theseeds of any naturally occurring variant plantswith improved qualities, such as those with thelargest fruits or the highest yield, in a process thatwe call artificial selection. This domestication andcontrolled improvement of plant species was thebeginning of plant biotechnology. This very simpleprocess of selectively breeding naturally occurringvariants with observably improved qualitiesserved as the basis of agriculture for thousands ofyears and resulted in thousands of domesticatedplant cultivars that no longer resembled the wildplants from which they descended.

The second era of plant biotechnology began inthe late 1800’s as the base of knowledge derivedfrom the study of the life sciences increased dra-matically. In the 1860’s Gregor Mendel, using dataobtained from controlled pea breeding experi-ments, deduced some basic principles of geneticsand presented these in a short monograph mod-estly titled “Versuche über Pflanzenhybriden” (inVerhandlungen des naturforschenden Vereins, 1866;Experiments with Plant-Hybridisation, 1910). In thispublication, Mendel proposed that heritable ge-

netic factors segregate during sexual reproductionof plants and that factors for different traits assortindependently of each other. Mendel’s work sug-gested a mechanism of heritable factors that couldbe manipulated by controlled breeding of plantsthrough selective fertilization and also suggestedthat the pattern of inheritance for these factorscould be analyzed or, in some cases, predicted bythe use of mathematical statistics.

These findings complemented the work ofCharles Darwin, who expounded the principles ofdescent with modification and selection as the chieffactor of evolutionary change in his 1859 book Onthe Origin of Species by Means of Natural Selection. Theapplication of these principles to agriculture re-sulted in deliberately produced hybrid varieties fora large number of cultivated plants via selective fer-tilization. These artificially selected hybrids soonbegan to benefit humankind with tremendous in-creases in both the productivity and the quality offood crops.

Genetic EngineeringThe third era of plant biotechnology involves a

drastic change in the way crop improvement maybe accomplished, by direct manipulation of geneticelements (genes). This process is known as geneticengineering and results in plants that are called ge-netically modified organisms (GMOs), to distin-guish them from plants that are produced by con-ventional plant-breeding methods. Geneticallymodified plants can contribute desirable genesfrom outside traditional breeding boundaries.

Plant biotechnology • 815

Even genes from outside the plant kingdom cannow be brought into plants. For example, animalgenes, including human genes, have been trans-ferred into plants, a feat not replicated in nature.

Public ConcernIt is perhaps this lack of natural boundaries for

genetic exchange that seems so foreign to conven-tional scientific thought and that makes plant ge-netic engineering controversial. The thought of tak-ing genes from animals, bacteria, viruses, or anyother organism and putting them into plants, espe-cially plants consumed for food, has raised a host ofquestions among concerned scientists and publicalike. Negative public perception of geneticallymodified crops has affected the development andcommercialization of many plant biotechnologyproducts, especially food plants. While there aredozens of genetically engineered plants ready for

field production, public pressure has delayed therelease of some of these plants and has caused thewithdrawal of others from the marketplace.

This public concern also appears to be drivingincreased government review of products and de-creased government funding for plant biotechnol-ogy projects in Europe. Negative public percep-tions do not seem to be as strong in Asia, since thepressures of feeding large populations tend to out-weigh the perceived risks. The social climate of theUnited States toward biotechnology, althoughguarded, appears to be less apprehensive than thatof most European countries. Therefore, many agri-cultural biotechnology projects have moved fromEuropean countries to U.S. laboratories.

Economic GoalsTo what end are humans genetically engineering

plants? This is an essential question for researchers,

816 • Plant biotechnology

Time Line of Plant BiotechnologyYear Event

1838 German scientists Matthias Schleiden and Theodor Schwann presented their cell theory: that all life-forms are made up of cells.

1858 Biologist Rudolf Virchow adds to the cell theory, proposing that, “where a cell exists, there must havebeen a preexisting cell.”

1902 Austrian botanist Gottlieb Haberlandt completes the cell theory with his idea of totipotency: Cells mustcontain all the genetic information necessary to create an entire, multicellular organism. Therefore,every plant cell is capable of developing into an entire plant.

1939 R. J. Gautheret demonstrates the first successful culture of isolated plant tissues as a continuouslydividing callus tissue.

1953 James Watson and Francis Crick make their landmark proposal for the double-helical structure ofdeoxyribonucleic acid (DNA), the molecule that carries genetic material.

1954 The first whole plant is regenerated, or cloned, from a single adult plant cell by W. H. Muir andcolleagues.

1967 DNA ligase, the enzyme that joins DNA molecules, is discovered.

1970 Daniel Nathans, Hamilton Smith, and others discover restriction endonucleases.

1972 Researchers at Stanford University construct the first recombinant DNA molecules, and the followingyear DNA is inserted into Escherichia coli cells.

1980 The U.S. Supreme Court rules that genetically altered life-forms can be patented, allowing Exxon topatent a microorganism that eats oil.

1983 The National Institutes of Health permit scientists at the University of California at Berkeley to releasegenetically engineered bacteria designed to retard frost formation on crop plants.

1983 The U.S. Patent Office begins to issue a series of patents for genetically modified plants.

executives of biotechnology companies, and con-sumers at large. Before addressing technical ques-tions about how to apply biotechnology, the de-sired goals must be clearly defined. The generalgoals of plant biotechnology appear to be (1) eco-nomic improvement of existing products, (2) im-provement of human nutrition, and (3) develop-ment of novel products from plants.

Economic improvements include increases inyield, quality, pest resistance, nutritional value,harvestability, or any other change that adds valueto an established agricultural product. Examples ofthis category include insect-protected tomatoes,potatoes, cotton, and corn; herbicide-resistant ca-nola, corn, cotton, flax, and soybeans; canola andsoybeans with genetically altered oil compositions;virus-resistant squash and papayas; and improved-ripening tomatoes. All these examples were intro-duced to agriculture in the later half of the 1990’s.

Nutritional GoalsAdditionally, some products appearing in the

scientific literature but awaiting commercializa-tion have the potential to dramatically improve hu-man nutritional deficiencies, which are especiallyprevalent in developing countries. These productsinclude “golden rice,” genetically modified ricethat produces carotenoids, a dietary source of vi-tamin A. Golden rice has the potential to preventvitamin Adeficiency in developing countries, wherethis vitamin deficiency is a leading cause of blind-ness.

Researchers are also using genetic engineeringto increase the amount of the iron-storing proteinferritin in seed crops such as legumes. Iron defi-ciency, which affects 30 percent of the human popu-lation, can impair cognitive development andcause other other health problems. This proposedenhancement of iron content in consumable plant


Year Event

1984 The Plant Gene Expression Center, a collaborative effort between academia and the U.S. Department ofAgriculture, is established to research plant molecular biology, sequence plant genomes, and developgenetically modified plants.

1985 Field testing of plants genetically modified to resist plant pathogens and disease vectors begins.

1986 The first release of a genetically modified crop, genetically engineered tobacco plants, is approved bythe Environmental Protection Agency.

1987 Calgene receives a patent for a DNA sequence that extends the shelf life of tomatoes.

1987 Advanced Genetic Sciences, Inc., field-tests a recombinant organism designed to inhibit frost instrawberries.

1990 Calgene conducts field experiments with herbicide-resistant cotton plants.

1990 At the Plant Gene Expression Center, biologist Michael Fromm announces the use of a high-speed “genegun” to transform corn. Gene guns are used to shoot genetic material directly into cells via DNA-coatedmicroparticles.

1993 Kary Mullis wins the Nobel Prize in Chemistry for development of polymerase chain reactiontechnology, a technique he invented in 1981 for quickly multiplying DNA sequences in vitro.

1993 The U.S. Food and Drug Administration (FDA) announces its finding that genetically modified foodsare not “inherently dangerous” and not in need of regulation. The following year, the FDA approves thefirst genetically modified whole food crop, Calgene’s Flavr Savr tomato.

1996 The genomes of Saccharomyces cerevisiae (baker’s yeast), with 12 million base pairs, and of ancientarchaea cells, which live near thermal vents at the ocean bottom, advance biologists’ understanding ofthe evolution of life.

2000 Researchers complete the full genomic sequence for the model flowering plant Arabidopsis thaliana.

2001 Researchers complete the genomic sequence for rice, Oryza sativa.

products could help more than a billion people whosuffer from chronic iron deficiency.

Novel ProductsNovel products include those not traditionally

associated with plants and are limited only by imag-ination and currently available techniques. Theseinclude the production of plastics, vaccines, anti-bodies, human blood proteins, and new pharma-ceuticals. One project has involved the productionof hepatitis B vaccine in transgenic tomatoes. Thisproject, which underwent clinical trials in the late1990’s, has the potential to provide a simple andinexpensive means of vaccinating people againsthepatitis B. By oral administration of tomato juicecontaining the vaccine protein, humans are thoughtto develop an immune response that may protectthem from infection by the hepatitis B virus. Hepa-titis B is epidemic in Asia and increasing at analarming rate in the rest of the world. The diseaseultimately causes liver disease, cancer, and death inmillions of infected people.

Plant Tissue CulturesCentral to plant biotechnology is the use of in vi-

tro methods. Researchers use plant tissue cultures,for example, to grow plant cells on sterile nutrientmedia. Countless recipes for these nutrient mediaexist. The choice of which one to use is based on theplant species and the tissue type to be grown. Allsuch media contain at least some of the importantnutitional elements, such as nitrogen, potassium,calcium, magnesium, sulfur, phosphorus, iron, bo-ron, manganese, zinc, iodine, molybdenum, cop-per, and cobalt, usually in the form of inorganicsalts or as metal chelates, and an organic energysource, such as sucrose. The media may also con-tain vitamins, hormones, and other ingredients, de-pending on the intended use.

To initiate plant tissue culture, a piece of a livingplant is excised and disinfected using a chemicaldisinfectant. This piece of plant tissue, called anexplant, is placed on a sterile plant tissue culturemedium to grow. Many plant tissues may be usedto obtain explants for plant tissue culture, includingthose from leaves, petioles, shoots, tubers, roots,and meristematic regions. When an explant isplaced in the sterile tissue culture medium, cellsthat are not terminally differentiated will grow anddivide. If plant hormones are included in the recipe,the plant cells can be coaxed to develop into differ-

ent types of tissues or organs. By using a successionof media containing different hormones, it is possi-ble to regenerate whole plants from single cells. Thechoice of tissue used for the explant and the choiceof hormones included in the tissue culture mediumdepend on the desired result.

MicropropagationMicropropagation, another biotechnology tech-

nique, is the production of many clonal plants us-ing tissue culture methods. By means of micro-propagation, it is possible to generate manythousands of plant clones using tissue explants ob-tained from a single parent plant. The main advan-tage to micropropagation is the potential of produc-ing thousands of exact copies of a plant withdesirable traits. Micropropagation is especially im-portant for rare plants, genetically engineeredplants, and plants that have sexual reproductiveproblems. Many plant species are now routinelypropagated by micropropagation methods, includ-ing orchids, ferns, many flowering ornamentals,and vegetable plants.

Steps in Genetic EngineeringThe first genetically engineered plants, tobacco

plants, were reported in the scientific literature in1984. Since 1984 there have been thousands of ge-netically engineered plants produced in laborato-ries worldwide. The process of genetically engi-neering a plant involves several key steps:

• isolating the genetic sequence (gene) to beplaced from its biological source

• placing the gene in an appropriate vehicle tofacilitate insertion into plant cells

• inserting the gene into the plant in a processknown as plant transformation

• selecting the few plant cells that contain thenew gene (transformed cells) out of all theplant cells in the explant

• multiplying the transformed cells in sterile tis-sue culture

• regenerating the transformed cells into awhole plant that can grow outside the tissueculture vessel

The gene or genes to be placed in the plant maybe obtained from virtually any biological source:


animals, bacteria, fungi, viruses, or other plants.Placing genes into an appropriate vehicle for trans-fer into a plant involves using various molecularbiology techniques, such as restriction enzymesand ligation, to essentially “cut and paste” the geneor genes of interest into another DNA molecule,which serves as the transfer vehicle (vector).

Plant Transformation MethodsCurrently plant transformation with foreign genes

may be accomplished by several proven methods,including bacteria-mediated transfer, microparticlebombardment, electroporation, microinjection,sonication, and chemical treatment.

By far, the most often utilized method of planttransformation involves the use of naturally oc-curing plant pathogenic bacteria from the genus

Agrobacterium. In nature, this bacterium infectsplants and transfers some of its own bacterial DNAinto the plant. Through the action of proteins pro-duced by the bacteria, bacterial DNA is made to in-tegrate permanently into the plant’s own genomicDNA. Expression of the bacterial DNA in the plantcauses the plant to produce unusual quantities ofplant hormones and other compounds, calledopines, which provide food for the bacteria. Theunusual quantities of plant hormones around theinfection site cause the plant cells to grow abnor-mally, producing characteristic tumors. Scientistshave harnessed this pathogenic bacterium to insertgenes into plants by deleting the bacterial genesthat cause tumors in the plant and then insertingdesirable genes in their place. When the modifiedAgrobacterium infects a plant, it transfers the desir-


Image Not Available

able genes into the plant genome instead of caus-ing tumors. The desirable genes become a perma-nent part of the plant genome, and expression ofthese genes in plant cells produces desirable prod-ucts.

One major drawback of the Agrobacteriummethod is that insertion of bacterial DNA into theplant genome is essentially random. The gene maynot be efficiently transcribed at its location, or theinsertion of bacterial DNA may knock out an im-portant plant gene by inserting in the middle of it—or both may occur. Therefore, the fact that a cell isgenetically transformed does not guarantee that itwill perform as desired.

Microparticle bombardment is the introduction offoreign DNA constructs into plant cells by attach-ing the DNA to small metal particles and blastingthe particles into plant cells using either a com-pressed air gun or a gun powered by a 0.22 calibergun cartridge. This is truly a “brute force” methodof introducing DNA into a cell that inadvertentlycauses many lethal casualties among the bom-barded plant cells. However, some plant cellsblasted with the DNA-containing metal particleswill recover and survive. The plant cells may ex-press the DNA for only a short time (transient ex-pression), because the DNA does not readily inte-grate into the plant genome, but occasionally theforeign DNA may spontaneously recombine intothe plant genome and become permanent.

Other ways of introducing foreign DNA intoplant cells include electroporation, microinjection,sonication, and chemical treatment. These methodsare not used extensively, because they generally re-quire the production of protoplasts (plant cells thatlack their cell walls) from plant cells before trans-formation. To create protoplasts, the plant cell wallis removed by digestion with the enzymes cellulaseand pectinase. Protoplasts are fragile structures,but the absence of a cell wall is desirable because itleaves only the plasma membrane as a barrier toforeign DNA entering a plant cell.

Electroporation uses very brief pulses of high-voltage electrical energy to create temporary holesin the plasma membrane through which the foreignDNA can pass. Microinjection involves physicallyinjecting a small amount of DNA into a plant cellusing a microscope and an extremely fine needle.Sonication uses ultrasonic waves to punch tempo-rary holes in the plasma membrane; this methodis therefore similar to electroporation. Chemical

treatment involves the use of polyethylene glycol torender the plasma membrane permeable to foreignDNA.

All the transformation procedures produce onlya few transformed cells out of the millions of cells inan explant, so selection of transformed cells is es-sential.

Selection of Transformed Plant CellsSelecting the few transformed plant cells out of

all the plant cells in an explant requires some ad-vance planning. Most foreign DNA constructs in-troduced into a plant are designed and built to con-tain additional genes that function as selectablemarkers or reporter genes. Selectable markers in-clude genes for resistance to antibiotics or herbi-cides. Plant cells containing and expressing thesegenes will be tolerant of antibiotics or herbicidesadded to the plant tissue culture media, while thenontransformed plant cells will be killed off. Thesurviving cells in the tissue culture media aremostly transformed.

Instead of selectable markers, reporter genes maybe used. Reporter genes induce an easily observ-able trait to transformed plant cells that facilitatesthe physical isolation of these cells. Reporter genesinclude beta-glucuronidase, luciferase, and plantpigment genes. Beta-glucuronidase (commonlyknown as GUS) allows the plant cells expressingthis gene to metabolize colorigenic substrates whilenontransformed plant cells cannot. To use this test,researchers treat a small amount of plant tissuewith the colorigenic chemical substrate. If the cellturns color (blue) it is known to be transformed andexpressing the GUS gene. If the cell does not turncolor, it probably is not transformed. Another re-porter gene is luciferase, an enzyme isolated fromfireflies. Luciferase makes plant cells glow in thepresence of certain chemicals if the gene is present;hence, transformed cells glow, whereas nontrans-formed cells do not glow. Plant pigment genes,such as anthocyanin pigment genes, occur natu-rally in plants and produce pigments that impartcolor to flowers. Inclusion of these pigment genesas reporter genes will allow transformed plantcells to be selected by their color. Transformedcells have color, while nontransformed cells re-main colorless. Both selectable markers and re-porter genes allow selection of cells into whichgenes have been successfully inserted and are oper-ating properly.


Regenerating Whole Transformed PlantsAfter successfully getting a gene construct into a

plant cell and selecting the transformed cells, it ispossible to get the plant cells to multiply in tissueculture. Also, by treating the plant cells with combi-nations of plant hormones, the cells are made to dif-ferentiate into various plant organs or wholeplants.

For example, treating transformed plant cellswith a high concentration of the plant hormonecytokinin causes shoots to develop. Transferringthese shoots to another medium, one that is high inthe plant hormone auxin, will cause roots to de-velop on the shoots. In this way a whole transgenicplant may be regenerated from transformed plantcells. Once a transformed plant is regenerated intissue culture, the plant may be transferred to aclimate-controlled greenhouse, where it can growto maturity.

Future generations of transgenic plants maythen be propagated sexually via seeds or asexuallyvia vegetative propagation methods. Often trans-genic plants must be grown in containment green-houses to prevent accidental release into the envi-ronment. In such high-tech greenhouses, all factorscontributing to optimal plant growth—lighting,temperature, humidity, nutrients, and other envi-ronmental conditions—are tightly controlled. Of-ten hydroponic systems, which use a solution ofplant nutrients as a growth medium in place ofsoil, are employed to control all aspects of plantnutrition.

Robert A. Sinnott

See also: Biotechnology; Cloning of plants; DNA:recombinant technology; Environmental biotech-nology; Genetically modified bacteria; Geneticallymodified foods.

Sources for Further StudyDixon, Richard A., and Robert A. Gonzales, eds. Plant Cell Culture: A Practical Approach. 2d

ed. New York: IRL Press at Oxford University Press, 1994. A handbook for researchers, il-lustrated, with bibliography and index.

Gamborg O. L., and G. C. Phillips, eds. Plant Cell, Tissue and Organ Culture: FundamentalMethods. New York: Springer Verlag, 1995. A substantial (358-page) laboratory manualfor researchers. Bibliography, index.

Kyte, Lydiane, and John G. Kleyn. Plants from Test Tubes: An Introduction to Micropropagation.3d ed. Portland, Oreg.: Timber Press, 1996. A laboratory manual with illustrations, bibli-ography, index.

Owen, Meran R. L., and Jan Pen, eds. Transgenic Plants: A Production System for Industrial andPharmaceutical Proteins. New York: John Wiley, 1996. Geared for commercial purposes. Il-lustrations, bibliography, index.

PLANT CELLS: MOLECULAR LEVEL

Categories: Cellular biology; physiology

Water, ions, salts, and gases all are types of inorganic molecules that are essential to cellular function. The chemical prop-erties of water make it an ideal solvent and buffer for the chemistry that occurs inside cells. The capillary action that helpswater travel up plant tissues from the roots is a direct consequence of the polarity of the water molecule.

The chemistry of life on earth is carbon andwater chemistry. Water is the most abundant

compound in living cells and makes up as muchas 90 percent of the weight of most plant tissues.

Many of the molecules that are part of larger macro-molecules in cells are linked together chemically bydehydration synthesis, or the loss of water. Thesemacromolecules are broken up into their compo-

Plant cells: molecular level • 821

nent units by the addition of a water molecule be-tween the units, a process known as hydrolysis. Thechemical properties of water make it an ideal sol-vent and buffer for the chemistry that occurs insidecells.

Because the electrons of the covalent bondswithin the water molecule are more often orbitingthe oxygen atom, the oxygen atom gains a slightlynegative charge. The hydrogen atoms are slightlypositive. This separation of charge across the watermolecule is said to make it polar. Because of its po-lar nature, water is able to dissolve, or ionize, a vari-ety of molecules. This gives water its buffering ca-pacity.

Water molecules are attracted to one another be-cause of this polarity. This weak attraction, which

occurs in the form of hydrogen bonds, has greatchemical consequences when many molecules ofwater are involved. Hydrogen bonding allowswater to have surface tension. The capillary actionthat helps water travel up plant tissues from theroots is a direct consequence of the polarity of thewater molecule. Water is also able to absorb heatwithout vaporizing (changing from a liquid to a gasstate) quickly. Therefore, physiological tempera-tures can be maintained as water molecules absorbthe heat from metabolic reactions.

Water, ions, salts, and gases all are types of inor-ganic molecules that are essential to cellular function.Inorganic molecules are chemical molecules that donot contain carbon. The remainder of the moleculeswithin cells are built around the unique properties

822 • Plant cells: molecular level

Middle lamella

Ribosomes

Primary pit fieldwith plasmodesmata

Cytoplasmic strands

Peroxisome

Vacuole

Intercellular space

Plasma membrane

Chloroplasts

Cell wall

Nucleolus

Mitochondrion

Nucleus

Golgi body

Parts of a Plant Cell

Cytosol

Peroxisome

of the carbon atom and are called organic mole-cules.

Organic MacromoleculesThere are four major classes of organic molecules

in cells: carbohydrates, lipids, nucleic acids, and pro-teins. All of these molecules contain carbon back-bones, and almost all of them contain oxygen andhydrogen as well as other elements. Some or all ofthe members of each class of organic moleculesoccur as very large molecules, called macromole-cules, that are polymers of smaller molecules joinedtogether by covalent bonds. For example, starchand cellulose are carbohydrate polymers of simplercarbohydrates called sugars. Likewise, fats and oilsare lipid polymers composed of smaller lipidscalled fatty acids and the sugar alcohol called glyc-erol.

CarbohydratesCarbohydrates are molecules that consist of pri-

marily carbon, hydrogen, and oxygen atoms. Car-bohydrates are the primary source of stored energyin most living organisms. They can also serve asstructural molecules in cell walls and as markers onsome cell membranes, identifying different types ofcells.

Simple sugars, or monosaccharides, are sugarsthat are small molecules composed of a chain of co-valently bonded carbon atoms with associated hy-drogen and oxygen atoms. These molecules alwayshave a ratio of one carbon atom to two hydrogen at-oms to one oxygen atom (CH2O). The monosaccha-ride glucose is the primary sugar produced fromsimpler sugars made in photosynthesis.

When two simple sugars are covalently linkedtogether, they form a disaccharide. In plants, thedisaccharide sucrose, which is composed of onefructose molecule and one glucose molecule, is themost common sugar. Sucrose is the same thing asso-called table sugar, which is harvested fromsugar cane or sugar beets.

Many sugars can be linked together to form acarbohydrate polymer, or polysaccharide. Starch iscomposed of many glucose molecules linked to-gether and is the major form of carbohydrate stor-age in plants. When energy is required, the individ-ual sugars of the polysaccharides are hydrolyzed(broken down to simpler molecules), and the glu-cose that is released is used by the mitochondria togenerate energy. Polysaccharides are also impor-

tant structural molecules in plants. The most abun-dant polysaccharide in nature is cellulose, anotherpolymer of glucose and a major component of plantcell walls.

LipidsLipids are diverse group of unrelated molecules

which includes fats, oils, steroids and sterols,waxes, and other water-insoluble molecules.Lipids are characterized by their hydrophobic, or“water-fearing,” chemical behavior, which is whatmakes them insoluble in water. Unlike other mole-cules that ionize and are dissolved by water, lipidmolecules are nonpolar. They are repelled by thepolar nature of water and tend to aggregate inaqueous solutions. Lipids also are used to storeenergy and are especially abundant in seeds be-cause lipids contain more energy by weight thancarbohydrates.

Examples of lipids commonly found in biologi-cal systems include fats and oils that are storagemolecules known as triglycerides. A triglycerideconsists of glycerol (a three-carbon molecule) andthree fatty acid molecules, long-chain hydrocarbonmolecules that are attached to each of the threeglycerol-carbon atoms by ester linkages.

The long chain of carbon atoms of the fatty acidcan be saturated or unsaturated with respect tohydrogen content. Saturated fatty acids contain asmany hydrogen atoms as allowed bonded toeach carbon atom. Saturated fats tend to be solidat room temperature and include substances suchas butter and lard. Unsaturated fatty acids do nothave the maximum number of hydrogen atomsbecause some of the carbon atoms form doublebonds with adjacent carbon atoms in the chain. Un-saturated fats tend to be liquid at room tempera-ture and include substances such as corn oil andolive oil.

Plants have many lipids that are unique to them.For instance, cutin and suberin are two lipid poly-mers that form structural components of manyplant cell walls. These two molecules form a mesh-work that secures another type of lipid polymerfound in plants, wax. Waxes are long-chain lipidcompounds that are integrated into the cutin andsuberin meshwork and are important in preventingwater loss for plants. Waxes give apple peels theircharacteristic shiny appearance.

Phospholipids are a type of lipid molecule that isfound in all living organisms. They are structurally


similar to triglycerides, except instead of havingthree fatty acids attached to glycerol, they haveonly two. Replacing the third fatty acid is a chargedphosphate group. This unique structure results inone end of the molecule being hydrophilic (thephosphate end, often called the head) and the otherbeing hydrophobic (the end with the two fatty ac-ids, often called the tail). Consequently, phospho-lipids will spontaneously form an oily layer at thewater surface, orienting their charged phosphateheads toward the water and their fatty acid tailsaway from the water and toward the air. This is thebasis for the phospholipid bilayer structure that un-derlies the formation of all cellular membranes. Inthe case of a lipid bilayer, because there is water onboth sides, the two layers are tail to tail, with theirheads oriented to the inside and outside of themembrane, where they come into contact withwater.

Nucleic AcidsThe information that directs all cellular activity

is contained within the chemical structure of the nu-cleic acids. Nucleic acids are polymers of smallermolecules called nucleotides. Nucleotides, in turn,are composed of three types of covalently linkedmolecules: a ribose sugar, a phosphate group, and anitrogen-containing base. The two major nucleo-tides that are found in cells are deoxyribonucleic acid(DNA) and ribonucleic acid (RNA).

DNA contains the genetic information that di-rects the development and activity of the organism.In eukaryotic cells DNA resides in the nucleus inlinear molecules of repeating nucleotide units, al-though there are circular molecules of DNA foundin the mitochondria and chloroplasts of eukaryoticcells. DNA nucleotides are composed of a five-carbon deoxyribose sugar, a phosphate group, andone of four possible bases: adenine (A), thymine(T), cytosine (C), and guanine (G). The informationof the DNAmolecule is found in the sequence of thenitrogenous bases along its length. Any region ofDNA that directs a cellular function or encodes an-other molecule is called a gene. Not all DNAregionsencode proteins. Some regions encode the instruc-tions for RNA molecules that are used as catalystsand for protein synthesis reactions. Some genes areregulatory, controlling the time and place wherecertain genes are expressed. In many eukaryotes,genes only account for 10 percent of the DNA. Al-though some of the remaining 90 percent carries

various structural functions, most of it is of uncer-tain function.

In 1953 Francis Crick and James Watson con-structed a molecular structure for the DNA mole-cule, relying heavily on the experimental data gen-erated by Rosalind Franklin. The structure theyproposed, which has since been supported by addi-tional experimental data, was that of a double helix.The DNA molecule can be envisioned as a ladder.The sugars and phosphates of the nucleotides alter-nate with each other to form the backbone, the out-side vertical support, and the bases form the indi-vidual rungs of the ladder. The ladder is twisted tocreate a helical structure. DNA can exist as singlestrands and in other confirmations in the cell, butthe “B-form” of the DNA double helix is the mostcommon form in the cell.

RNA molecules are also polymers of nucleo-tides, but the nucleotides of the RNA molecule dif-fer slightly from those of the DNA molecule. RNAnucleotides contain a five-carbon ribose sugar, aphosphate group, and one of four bases. Three ofthe four bases are the same as found in DNA: ade-nine, guanine, and cytosine. Instead of thymine,RNA uses the base uracil. RNA bases can pair in es-sentially the same way as DNA bases, but mostoften RNA exists as single-stranded molecules incells. These long strands of RNAcan often pair withother bases in short regions, causing the RNA tofold up into highly complex, three-dimensionalstructures important for RNA function.

RNA is found throughout cells. Messenger RNA(mRNA) is made by the cell using the DNA se-quence in genes as a template for making a comple-mentary strand of RNA in a process called tran-scription. In After being transcribed and modifiedin certain complex ways, most mRNA is trans-ported to the cytoplasm where it is used to directthe synthesis of proteins. Ribosomal RNA (rRNA)is a major component of ribosomes, which are re-sponsible for coordinating protein synthesis, alongwith transfer RNA (tRNA). Some RNA molecules,like protein molecules, can also catalyze chemicalreactions. Catalytic RNA molecules are calledribozymes, and they play roles in gene expressionand protein synthesis.

Single nucleotides and compounds that aremade from them are involved in many cellular pro-cesses. The universal unit of “energy currency” inthe cell is adenosine triphosphate (ATP). Guanosinetriphosphate (GTP) is a molecule that is involved in

824 • Plant cells: molecular level

relaying signals received at the cell membrane tothe nucleus of the cell. Compounds, such as NADHand NADPH, that are involved in metabolic reac-tions in the mitochondria and in energy capture re-actions in the chloroplasts also contain nucleotides.

ProteinsProtein molecules are large, complex molecules

with a huge variety of structures and functionswithin cells. Most chemical reactions in cells are cat-alyzed by proteins called enzymes. Proteins formthe basis of the cytoskeleton of cells, providingstructure and motility. Proteins are also essential forthe communication between cells and within cells.In plants, the largest concentration of proteins canbe found in some seeds.

Proteins are polymers of nitrogen-containingmolecules called amino acids. The amino acids aremuch simpler molecules than the nitrogenousbases found in nucleic acids. The same twentyamino acids is are used in the manufacture of pro-teins in the cells of all living organisms. An aminoacid is built around a single carbon atom called thealpha carbon. Bonded to the alpha carbon are a hy-drogen atom (H), a carboxyl group (COOH), and anamino group that contains nitrogen (NH2). A spe-cialized “R” group is attached at the last site. The R-groups are different for each of the twenty aminoacids, and their chemical properties, such as charge,hydrophilic or hydrophobic nature, and size, dic-tate protein function and shape.

The order and number of amino acids that arelinked together to form a protein are determined bythe order of the codons in the DNAthat encode thatprotein. The order and number of the amino acidsin a protein is called the primary structure, and it ulti-mately determines the shape of the protein. Pro-

teins can have secondary structures formed by hy-drogen bonding between the peptide bonds thatlink the amino acids together. The two commonsecondary structures in proteins are the alpha helixand the beta pleated sheet. The amino acid chain (alsocalled a peptide chain) can fold up on itself to formglobular structures. This is known as tertiary struc-ture. Tertiary structure is determined by the num-ber and order of amino acids in the protein and isformed when molecules in the R-groups of theamino acids interact with one another. When twoor more peptide chains interact to form a singlefunctional molecule, the protein is said to have qua-ternary structure.

Michele Arduengo

See also: ATP and other energetic molecules; Cal-vin cycle; Carbohydrates; Cell-to-cell communica-tion; Cell wall; Cells and diffusion; ChloroplastDNA; Chloroplasts and other plastids; Chromatin;Chromosomes; Cytoplasm; Cytoskeleton; Cytosol;DNA: historical overview; DNA in plants; DNAreplication; Eukaryotic cells; Fluorescent stainingof cytoskeletal elements; Gene regulation; Geneticcode; Genetics: mutations; Glycolysis and fermen-tation; Hormones; Krebs cycle; Lipids; Liquidtransport systems; Membrane structure; Metabo-lites: primary vs. secondary; Microbodies; Mito-chondria; Mitochondrial DNA; Mitosis and meio-sis; Molecular systematics; Nuclear envelope;Nucleic acids; Nucleolus; Nucleus; Nutrients; Oilbodies; Osmosis, simple diffusion, and facilitateddiffusion; Oxidative phosphorylation; Peroxi-somes; Pheromones; Plasma membranes; Prokary-otes; Proteins and amino acids; Ribosomes; RNA;Sugars; Vacuoles; Vesicle-mediated transport; Vi-ruses and viroids.

Sources for Further StudyAmerican Society for Cell Biology. Exploring the Cell. Available on the World Wide Web at

http://www.ascb.org/pubs/exploring.pdf. Features excellent images of cells and cellu-lar processes.

Lodish, Harvey, et al. Molecular Cell Biology. 4th ed. New York: W. H. Freeman, 2000. One ofthe most authoritative cell biology reference texts available. Provides excellent graphics,CD-ROM animations of cellular processes, and primary literature references.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Introductory botany text provides a chapter introducing eu-karyotic cells in general and plant cells specifically.


PLANT DOMESTICATION AND BREEDING

Categories: Agriculture; economic botany and plant uses

Plant domestication and breeding are the processes by which wild plants are intentionally raised to meet human food, fi-ber, shelter, medicinal, or aesthetic needs.

No one knows exactly when the first crop wascultivated, but most authorities believe that it

occurred at some time between eight and ten thou-sand years ago. For centuries prior to that time, hu-mans had known that some wild plants and plantparts (such as fruits, leaves, and roots) were edible.These plants appeared periodically (usually annu-ally) and randomly throughout a given region.Eventually humans discovered that these wildplants grew from seeds and that the seeds from cer-tain wild plants could be collected, planted, andlater gathered for food. This most likely occurred atabout the same time in both the Sumerian regionbetween the Tigris and Euphrates Rivers and inMexico and Central America. While the earliest at-tempts at domesticating plants were primarily tosupplement the food supply provided by huntingand gathering, people soon improved their abilityto domesticate and breed plants to the point thatthey could depend on an annual supply of food.This food supply allowed the development of per-manent settlements.

Early Crop DomesticationBy six thousand years ago, agriculture was

firmly established in Asia, India, Mesopotamia,Egypt, Mexico, Central America, and South Amer-ica. Before recorded history, these areas had domes-ticated some of the world’s most important food(corn, rice, and wheat) and fiber (cotton, flax, andhemp) crops. The place of origin of wheat is un-known, but many authorities believe that it mayhave grown wild in the Tigris and Euphrates Val-leys and spread from there to the rest of the OldWorld. Wheat was grown by Stone Age Europeansand was reportedly produced in China as far backas 2700 b.c.e. Wheat is now the major staple forabout 35 percent of the people of the world. The ear-liest traces of the human utilization of corn date

back to about 5200 b.c.e. It was probably first culti-vated in the high plateau region of central or south-ern Mexico and represented the basic food plant ofall pre-Columbian advanced cultures and civiliza-tions, including the Inca of South America and theMaya of Central America.

Botanists believe that rice originated in South-east Asia. Rice was being cultivated in India as earlyas 3000 b.c.e. and spread from there throughoutAsia and Malaysia. Today rice is one of the world’smost important cereal grains and is the principalfood crop of almost half of the world’s people.Hemp, most likely the first plant cultivated for itsfiber, was cultivated for the purpose of makingcloth in China as early as the twenty-eighth centuryb.c.e. It was used as the cordage or rope on almostall ancient sailing vessels. Linen, made from flax, isone of the oldest fabrics. Traces of flax plants havebeen identified in archaeological sites dating backto the Stone Age, and flax was cultivated in Meso-potamia and Egypt five thousand years ago. Cottonhas been known and highly valued by peoplethroughout the world for more than three thousandyears. From India, where a vigorous cotton indus-try began as early as 1500 b.c.e., the cultivation ofcotton spread to Egypt and then to Spain and Italy.In the West Indies and South America, a differentspecies of cotton was grown long before the Euro-peans arrived. Other important plants that havebeen under domestic cultivation since antiquity in-clude dates, figs, olives, onions, grapes, bananas,lemons, cucumbers, lentils, garlic, lettuce, mint,radishes, and various melons.

Modern Plant BreedingGenetic variability is prevalent in plants and

other organisms that reproduce sexually andthereby produce spontaneous mutants. Through-out most of history, plant domestication and breed-

826 • Plant domestication and breeding

ing were primarily based on the propagation ofmutants. When a grower observed a plant with apotentially desirable mutation (such as a changethat produced bigger fruit, brighter flowers, or in-creased insect resistance), the grower would collectseeds or take cuttings and produce additionalplants with the desirable characteristic. Advancesin the understanding of genetics in the early part ofthe twentieth century made it possible to breedsome of the desirable characteristics resulting frommutation into plants that previously had lacked thecharacteristic.

The obvious advantages of producing plantswith improved characteristics such as higher yieldmade plant breeding very desirable. As humanpopulations continued to grow, there was a need toselect and produce higher-yielding crops. The de-velopment and widespread use of new high-yield

varieties of crop plants in the 1960’s is often referredto as the Green Revolution. Basic information sup-plied by biological scientists allowed plant breed-ers to fuse a variety of characteristics from differentplants to produce new, higher-yielding varieties ofnumerous crops, particularly seed grains.

When a plant characteristic is identified as desir-able, it is studied both morphologically and bio-chemically to determine the mechanism of inheri-tance. If it is determined that the mechanism istransferable, attempts are made to incorporate thetrait into the target plant. If the plants are closely re-lated, traditional breeding techniques are used tocrossbreed the plant with the desirable trait withthe plant that lacks the characteristic. Although thisprocess is often tedious, it is based on a fairly simpleconcept. Basically, pollen from one of the planttypes is used to fertilize the other plant type. This

Plant domestication and breeding • 827

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process often requires specialized handling tech-niques to ensure that only the pollen from the plantwith the desired characteristic is allowed to fertilizethe eggs of the recipient plant.

Sometimes this process involves the use of bagsor other materials to isolate the recipient flowers,which are then pollinated by hand. Another tech-nique involves the introduction of a gene for malesterility into the recipient plant. In these cases, onlypollen from another plant can be used to fertilizethe egg. Once plants with the desirable characteris-tics are developed, the lines are often inbred tomaintain large numbers of progeny with the de-sired traits. In many cases, inbred lines will losevigor after several generations. When this occurs,two inbred lines may be crossed to produce hy-brids. A majority of the hybrid offspring will stillcontain the desired characteristics but will be morevigorous.

Recombinant TechnologyUntil recently, the use of traditional breeding

techniques between two very closely related spe-cies was the only means of transferring heritablecharacteristics from one to the other. The adventof recombinant technologies in the manipulation ofdeoxyribonucleic acid (DNA), however, made it

possible to transfer genetic characteristics from anyplant (or from any organism) to any other. The sim-plest method for accomplishing this transfer in-volves the use of a vector, usually a piece of circu-lar DNA called a plasmid. The plasmid is removedfrom a microorganism such a bacterium and cutopen by an enzyme called a restriction endonu-clease, or restriction enzyme. A section of DNAfrom the plant donor cell that contains the gene foran identified desirable trait is cut from the donorcell DNA by the same restriction endonuclease. Thesection of plant donor cell DNA with the gene forthe characteristic of interest is then combined withthe open plasmid DNA, and the plasmid closeswith the new gene as part of its structure. The re-combinant plasmid (DNA from two sources) isplaced back into the bacterium, where it will repli-cate and code for protein just as it did in the donorcell. The bacterium is then used as a vector to trans-fer the gene to another plant, where it will also betranscribed and translated.

D. R. Gossett

See also: Agricultural revolution; Agriculture: tra-ditional; Agronomy; DNA: recombinant technol-ogy; Genetically modified foods; Green Revolu-tion; High-yield crops; Hybridization; Plant fibers.

Sources for Further StudyAmerican Chemical Society. Genetically Modified Foods: Safety Issues. Washington, D.C.:

American Chemical Society, 1995. A thorough overview of the technology, applications,and risks associated with genetically engineered foods.

Fennema, O. R., ed. Principles of Food Science. New York: Dekker, 1975- . Covers topics infood science, including the harvesting, preservation, and marketing of a variety of horti-cultural food crops.

Metcalfe, Darrel S., and Donald M. Elkins. Crop Production: Principles and Practices. 4th ed.New York: Macmillan, 1980. One of the most valuable sources available on the practicalaspects of crop production.

Rissler, Jane. The Ecological Risks of Engineered Crops. Cambridge, Mass.: MIT Press, 1996. Acarefully reasoned science and policy assessment showing that the commercializationand release of transgenic crops on millions of acres of farmland can pose serious environ-mental risks. The authors propose a practical, feasible method of conducting precom-mercialization evaluations that will balance the needs of ecological safety with those ofagriculture and business, and that will assist governments seeking to identify and protectagainst two of the most significant risks.

828 • Plant domestication and breeding

PLANT FIBERS


Plants are the natural sources of many raw materials used to produce textiles, ropes, twine, and similar products.

The major fiber crops are cotton, flax, and hemp,although less important plants, such as ramie,

jute, and sisal, are grown in small amounts. With atotal annual production of more than 13 milliontons, cotton is by far the most important fiber cropin the world. Because humans heavily rely on cot-ton for clothing and other textiles, it enters the dailylives of more people than any other product exceptsalt.

CottonCotton (Gossypium) fiber has been known and

highly valued by people throughout the world formore than three thousand years. The early historyof cotton is obscure. Avigorous cotton industry waspresent in India as early as 1500 b.c.e. From India,the cultivation of cotton spread to Egypt and then toSpain and Italy. In the New World, a different spe-cies of cotton was being grown in the West Indiesand South America long before Europeans arrived.In the United States, cotton is grown from theEast Coast to the West Coast in the nineteen south-ernmost states.

Botanically, cotton is in the mallow family, whichalso includes okra, hollyhock, hibiscus, and althea.Cotton has a taproot and branching stems. Flowersform at the tips of fruiting branches, and the ovarywithin each flower develops into a boll, which con-tains the seed, fiber, and fuzz. The fiber, most com-monly referred to as lint, develops from epidermalcells in the seed coat of the cottonseed. The fiberreaches its maximum length in twenty to twenty-five days, and an additional twenty-five days arerequired for the fiber to thicken. Fiber length from2.0 to 2.4 centimeters is referred to as short-staplecotton, and fiber length from 2.4 to 3.8 centimetersis called long-staple cotton.

The boll normally opens forty-five to sixty-fivedays after flowering. Cotton is native to tropical re-gions but has adapted to the humid, subtropicalclimate, where there are warm days (30 degrees

Celsius), relatively warm nights, and a frost-freeseason of at least 200 to 210 days. There are eightspecies of cotton in the genus Gossypium, but onlythree species are of commercial importance.Gossypium hirsutum, also known as upland cotton,has a variable staple length and is produced pri-marily in North and Central America. Gossypiumbarbadense, a long-staple cotton, is primarily pro-duced in South America and Africa. Gossypiumherbaceum is a shorter-staple cotton native to Indiaand eastern Asia.

Cotton is one of the more labor-intensive and ex-pensive crops to produce. The most opportune timeto plant cotton is at least two weeks after the lastkilling-frost date of the region. Prior to seeding, thefield is prepared by plowing to a depth of 2.5 centi-meters. Fertilizer, which is applied before seedingor at the same time the seeds are planted, is placedto the side and below the cotton seed. Once theseeds germinate and emerge from the soil, they of-ten have to be thinned, and shortly afterward theproducer begins to apply irrigation water asneeded. After the plants have developed a stand,weed control becomes crucial. Weeds are controlledboth by cultivation and herbicides.

Cotton plants are subject to invasion by a varietyof insect pests, such as the boll worm and boll wee-vil; therefore considerable attention is given to in-sect control, typically using a number of differentinsecticides.

When the bolls ripen with mature fiber, the leavesof the plant are removed by the application of achemical defoliant, and the fiber is harvested. Har-vesting was once done almost entirely by hand, buttoday mechanical pickers harvest almost all the cot-ton produced in the United States. The picked cottonis ginned to remove the seed and compressed intobales. The bales are transported to a cotton mill,where the cotton is cleaned and spun into yarn,which is then woven into fabric. One pound of fiberis sufficient to produce up to 6 square yards of fabric.

Plant fibers • 829

FlaxFlax (Linum usitatissimum) is the fiber used to

make linen. While some flax is still grown for thepurpose of producing this fabric, much of the flax,particularly that grown in the United States, is usedto produce the flaxseed, from which linseed can beextracted. Linen made from flax is one of the oldestfabrics. Flax was cultivated in Mesopotamia andEgypt five thousand years ago, and traces of flaxplants have been identified in archaeological sitesdating back to the Stone Age. Flax was one of thefirst crops brought to North America by Europeansettlers. Today, most of the flax produced in theUnited States is grown in the north-central states.

An annual plant, flax grows to a height of 60 to100 centimeters and bears five-celled bolls or cap-sules with ten seeds each at the ends of fertilebranches. Because the flax fiber is found in thestems from the ground to the lowest branches, vari-eties that are long-stemmed with little branchingare grown for fiber production. Selection of quality,

disease-free seed is essential in flax production.Flax fields are usually prepared in the fall to allowthe soil to settle before planting. Flax is usuallysown in early spring, two to three weeks prior to thedate of the last killing frost of the region. Consider-able attention is given to controlling weeds in a flaxfield. When the crop is harvested for fiber, theplants are pulled from the soil, the seeds are re-moved, and the flax straw is “retted” to separate thefiber from the woody part of the stem. When thestraw is completely retted, it is dried and then bro-ken apart to remove the 50-centimeter fibers whichcan be woven into fabrics.

HempHemp (Cannabis sativa), a term used to identify

both the plant and the fiber it produces, is used tomake the strongest and most durable commercialfibers available. Hemp was most likely the firstplant cultivated for its fiber. It was cultivated for thepurpose of making cloth in China as early as the

830 • Plant fibers

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twenty-eighth century b.c.e. It was also used as adrug by the ancient Persians as early as 1400 b.c.e.and was used as the cordage or rope on almost allancient sailing vessels. Today hemp is commer-cially produced for heavy textiles in numerouscountries, but less than 1,000 acres is devoted tocommercial hemp production in the United States.Hemp production is problematic in the UnitedStates because it is illegal to grow Cannabis sativa,the source of marijuana.

Hemp is an annual plant in the mulberry family.The plant is dioecious, meaning that it has stami-nate or “male” flowers and pistillate or “female”flowers. It has a rigid stalk, which can reach a thick-ness of more than 2.5 centimeters in diameter, and aheight of 5 meters. The plant has a hollow stem, andthe bark or “bast” located outside the woody shellis used to make the bast fiber, which is then used tomake hemp twine, ropes, and other textiles wherestrength and durability are desired.

Humid climates with moderate temperaturesand a period of at least 120 frost-free days are neces-sary for hemp production. Unlike flax, hemp re-

quires that the soil be plowed and thoroughlydisked or harrowed prior to planting. The entireaboveground portion of the plant is harvestedwhen the male plants are in full flower. After two tothree days the plants are tied in bundles and set inshocks. Hemp fiber is retted and prepared for themills in a manner very similar to that of flax exceptthat heavier machines are used to handle the stron-ger hemp stalks.

Minor CropsRamie (Boehmeria nivea) is produced primarily in

Asia and is used to make strong cloth, such as Chi-nese linen. Jute (Corchorus capsularis) is grown pri-marily in India and Pakistan and is used to manu-facture burlap for bags and sacks. Sisal (Agavesisalana) is produced in East Africa and the West In-dies and is used to make different types of cordage,such as baler twine.

D. R. Gossett

See also: Plant domestication and breeding; Tex-tiles and fabrics.

Sources for Further StudyDillman, A. C. “Flaxseed Production.” In Farmer’s Bulletin Number 1747. Washington, D.C.:

U.S. Department of Agriculture, 1935. Old but very useful publication on flax production.Kipps, Michael S. The Production of Field Crops. 6th ed. New York: McGraw-Hill, 1970. Excel-

lent discussions of the importance and production of cotton, hemp, and flax.Lester, D. H. “Hemp,” In Yearbook. Washington, D.C.: U.S. Department of Agriculture, 1913.

Old but very useful publication on hemp.Mauney, J. R., and J. M. Stewart, eds. Cotton Physiology. Memphis, Tenn.: Cotton Foundation,

1986. Contains excellent discussions of the physiology of the cotton plant.Metcalfe, Darrel S., and Donald M. Elkins. Crop Production: Principles and Practices. 4th ed.

New York: Macmillan, 1980. One of the most valuable sources available on the practicalaspects of cotton production.

PLANT LIFE SPANS

Category: Reproduction and life cycles

The cycle of a plant’s life, from seed germination to death, is referred to as its life span. Some plants have short life spans(less than one year), whereas others have life spans that are measured in centuries.

The longest-lived organisms are plants. For ex-ample, one bristlecone pine tree in eastern Cali-

fornia is forty-nine hundred years old, and some

creosote bushes, also in California, are estimated tobe about twelve thousand years old. People havelong recognized this variation in plant longevity,

Plant life spans • 831

but the understanding of plant life spans improvedgreatly after research during the 1960’s.

Types of Life SpansThe life span of an individual plant depends

upon two factors. The first is the innate, geneticallydetermined potential for longevity. The second isthe effects of the environment, including soil andweather conditions, competing plants, disease-causing microbes, and herbivores.

Historically, people have classified the life spansof plants into three categories: annuals, biennials,and perennials. Annual plants live for up to one year.Biennials live for approximately two years. Peren-nials live for more than two years, often for severaldecades, even centuries.

While this categorization is useful in manyways, botanists have come to recognize that it is in-accurate, especially for plants that grow under nat-ural conditions. Plant life histories are now classi-fied mainly according to the number of times thateach individual normally reproduces before it dies.Two main categories are recognized using this sys-tem: monocarpic plants and polycarpic plants.Monocarpic plants reproduce once before they die(mono means “one”; carpic means “fruits”). Poly-carpic plants reproduce several or many times be-fore they die (poly means “many”). Some bota-nists have defined a third group, the paucicarpicplants, that are intermediate between the two.Paucicarpic plants reproduce up to five times (paucimeans “few”).

Monocarpic PlantsMonocarpic plants have a general life history

that involves four separate stages: germination,vegetative growth, reproduction, and death. Theperiod of vegetative growth is very important tothe monocarpic plant because during this time theplant manufactures and stores starch, which is richin energy. When reproduction occurs, all that storedenergy is devoted to producing flowers, fruits, andseeds; none is saved for the following year. Theplant literally reproduces itself to death. Monocar-pic plants vary greatly in their longevity, and it ispossible to recognize several subcategories: ephem-erals, annuals, obligate biennials, facultative biennials,and long-lived monocarpic perennials.

Ephemerals are plants that germinate, grow, re-produce, and die within a few weeks or months.They are typically found in environments in which

conditions favor active plant growth for only ashort period of time during the year, such as adesert. Desert ephemerals spend most of the year asseeds. When a heavy rainstorm occurs, the seedsgerminate, and the new plants grow quickly and re-produce before the soil dries out. One species in theSahara Desert can complete its life cycle in as littleas ten days.

Annuals are plants that progress from germina-tion to true seed within a six- to twelve-month pe-riod. Botanists recognize two major subcategoriesof annual plants. One is the summer annual, whichgerminates in the spring: The plant grows vegeta-tively during the summer and reproduces duringthe autumn. Examples of summer annuals includetouch-me-not, common ragweed, and goosefoot.The second subcategory is the winter annual, inwhich germination occurs in the fall, vegetativegrowth occurs in the winter, and reproduction oc-curs in the spring. Daisy fleabane and winter wheatare examples of winter annuals.

Obligate biennials are monocarpic plants thatgerminate and grow vegetatively over the course ofone year and throughout much of a second year. Atthe end of the second year, the plant always repro-duces, sets seed, and dies (hence the designation“obligate”). During the 1960’s and 1970’s, somebotanists doubted that obligate biennials existed innature. Studies conducted during the 1970’s and1980’s demonstrated that some plants, such as thewhite and yellow sweet clovers, are indeed obligatebiennials.

Facultative biennials are monocarpic plants thathave the ability to germinate, grow, and reproducewithin two years. They can behave as biennialsonly when they grow under favorable conditions,with adequate moisture, light, and soil nutrients.More commonly, these plants grow under stressfulconditions—either infertile soils or high competi-tion. On such sites, they grow vegetatively forthree, four, or even five years before they repro-duce. Examples of facultative biennials includewild carrot, foxglove, burdock, teasel, and thistle.

Long-lived monocarpic perennials are able tolive for many years or a few decades before they re-produce—once—and then die. Well-known exam-ples include bamboo and plants from the aridsouthwestern United States, such as species ofYucca and the century plant Agave. These may re-produce only after they attain an age of sixty, eighty,or even one hundred years.

832 • Plant life spans

Polycarpic and Paucicarpic PlantsPaucicarpic and polycarpic plants normally re-

produce more than once before they die. They areable to survive for at least one year following repro-duction and hence are true perennials. Paucicarpicplants are short-lived herbs that may die after re-production but more commonly live to reproducetwo, three, or four times before dying. Paucicarpicplants are therefore intermediate between the truemonocarps and the true polycarps. Examples in-clude the common and English plantains, whichare weeds found in lawns and fields throughouttemperate North America and Europe.

True polycarpic plants survive to reproducemany times during their lifetimes and usually re-main alive for at least ten years. Unlike the mono-carps, polycarps do not expend all of their energy inreproduction. They save some of their energy andmaintain part of the plant for the post-reproductiveperiod. In seasonal climates, some of that energy

must be directed to forming structures that allowthe plant to survive the unfavorable season—a coldwinter or a rainless period. These structures arecalled perennating buds, and they differ from plantto plant in their location relative to the ground sur-face. In some plants, called cryptophytes (the prefixcrypto means “hidden”), the perennating buds areburied several centimeters under the ground. Ex-amples of cryptophytes include milkweed, iris, andonion. Conversely, hemicryptophytes (hemi means“partial”) have their perennating buds at the soilsurface; a good example is the dandelion. Bothcryptophytes and hemicryptophytes are herba-ceous plants, never producing an abovegroundwoody structure.

Phanerophytes are polycarpic plants that do pro-duce an aboveground woody structure—the peren-nating buds are borne above the ground surface.Some phanerophytes are shrubs that have severalshoots. Examples of shrubs include lilac, blueberry,

Plant life spans • 833D

igital

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Extreme examples of seed longevity can be seen in species of lotus which have survived for more than fifteen hundredyears.

hawthorn, hydrangea, rhododendron, and manydogwoods and willows. A second category ofphanerophytes is the trees, which typically have asingle woody stem emerging from the rootstock.

In theory, most species of polycarpic plants canlive for decades, if not centuries, under ideal condi-tions. Many do not appear to have a maximum lifespan because they rejuvenate their tissues witheach reproductive period, as in some polycarpicherbs or because the tissues that they accumulatedo not put much of an added strain on the plant, asin many phanerophytes.

In nature, such polycarpic plants are not killedby old age. External factors such as herbivory (con-sumption by animals), fire, severe weather, disease,and competition from other plants contributeheavily to die-off among individuals. Otherpolycarpic plants form senescent tissue that has-tens their death.

Potential and Real Life SpansThere have not been many studies of the longev-

ity of most polycarpic plant species. The logisticproblems involved and the consideration of mor-tality are more closely related to the size of the plantthan to its age. Knowledge of the longevity in manyspecies, particularly polycarpic herbs, is very poor.

Many herbs, such as buttercups and clovers, livefor five to twenty-five years. Other herbs, such asblazing star, milkweed, and some goldenrods, maylive for twenty-five to fifty years. Some shrubs, in-cluding blueberries and sumacs, can live for thirtyto seventy years. Trees such as gray birch, pincherry, and trembling aspen live for fifty to onehundred fifty years. Conifers such as hemlock,white pine, and red spruce have longevity in the

range of two hundred to three hundred years, withsome trees living five hundred to six hundredyears. Hardwood trees such as sugar maple, whiteoak, sycamore, and beech can live for a similar du-ration. The oldest trees are the redwoods, at thir-teen hundred years, and the bristlecone pines, atthree thousand to five thousand years.

Most plants do not live to their maximal poten-tial, succumbing to environmental factors. The av-erage age that plants attain is well below the maxi-mum life span. Beginning in the 1960’s, ecologistsbegan to examine the length of time that individualplants in a population remain alive. Although theactual patterns differ greatly from one species toanother, plants typically suffer heavy mortalityshortly after germination. For most species, fewerthan 10 percent of newly emergent seedlings sur-vive for two months. After that point, there are ad-ditional losses, although the death rate slows.

Some plants produce seeds that can remain dor-mant for many years. For example, seeds of manyweed species, including monocarpic and polycarpicherbs, can remain dormant for seventy years inabandoned farm soil. Under experimental condi-tions, seeds of some of these species were found tobe capable of germinating after one hundred years.Extreme examples of seed longevity can be seen inspecies of goosefoot and lotus, both of which havesurvived for more than fifteen hundred years. Thevariety of plant life cycles appears to be related tothe earth’s widely divergent habitats.

Kenneth M. Klemow

See also: Angiosperm life cycle; Dendrochronol-ogy; Growth and growth control; Hormones; Leafabscission; Seeds.

Sources for Further StudyBarbour, Michael G., Jack H. Burk, and Wanna D. Pitts. Terrestrial Plant Ecology. 3d ed. Menlo

Park, Calif.: Benjamin/Cummings, 1999. Awell-written text intended for undergraduatestudents. Excellent overview of most phases of plant ecology, including physiologicalecology, population biology, and community ecology. Chapter 4 focuses on the patternsof survival found in representative plants. Chapter 5 addresses plant life histories and in-cludes a good explanation of the interrelationships among different life spans and the en-vironments in which they are found.

Dennis, David T., and David H. Turpin, eds. Plant Physiology, Biochemistry, and Molecular Bi-ology. New York: John Wiley, 1990. Examines the assimilation and metabolism of carbonand nitrogen in an integrative fashion, assessing the physiology, biochemistry, and mo-lecular biology of each topic discussed, for beginning or advanced students.

Hardin, James W., Donald J. Leopold, and F. M. White. Harlow and Harrar’s Textbook of Den-drology. 9th ed. Boston: McGraw-Hill, 2001. This well-illustrated book describes the bio-

834 • Plant life spans

logical features of nearly all species of evergreen and deciduous trees in North America. Itincludes information on the life spans of most of the species as well as descriptions of veg-etative and reproductive features, habitat preferences, and forestry.

PLANT SCIENCE

Categories: Disciplines; history of plant science

Botany is the study of plants, stationary organisms with chlorophyll that are able to make their own food. Major catego-ries of the plant kingdom include algae, mosses, ferns, and seed plants. Plant science includes many subdisciplines ofboth botanical (nonapplied science) and applied studies of plants, especially agriculture.

TaxonomyOne of the basic subdisciplines of plant science

and life science in general, taxonomy (also known assystematics) is the study of relationships and organi-zation of plant species. The great diversity withinthe plant kingdom requires a system by whichplant species are named and classified. The modernsystem is a modification of the system first estab-lished in the eighteenth century by the Swedishbotanist Carolus Linnaeus. Each species is placedinto a hierarchy of groups that indicate its similarityand dissimilarity to other species. These categories(taxa) are from the most to the least inclusive: do-main, kingdom, phylum, class, order, family, ge-nus, species.

The species is the most natural and fundamen-tal unit. Similar species are grouped into a genus,similar genera into a family, families into orders,orders into classes, classes into phyla, and phylainto the kingdoms typically studied in plant sciencecourses: true plants, or Plantae, Fungi, and Protista(which include many unicellular organisms and al-gae). Each species is given a scientific name whichincludes the genus name followed by the speciesname. An example is the scientific name of thedwarf crested iris: Iris cristata.

MorphologyMorphology includes the study of the general

structure of plants. Morphologists study the parts ofa plant and how they are arranged and function. Forexample, when a seed of an angiosperm (floweringplant) germinates, the radicle of the seed embryodevelops downward to form a root system. Growth

in the length of the root occurs within the meristem(region of cell division). Branch roots form due tothe activity of pericycle cells within the root. Someepidermal cells develop root hairs as extensions ofthe cells. The shoot system, which includes the stemand leaves, develops from the epicotyl of the seedembryo. Stems are often branched, allowing for theattachment of leaves in such a manner as to permittheir maximum exposure to sunlight.

Also included in the study of morphology arethe reproductive parts of plants. The pollination offlowers causes the ovary of the flower to matureinto a fruit. At the same time, the one or moreovules inside the ovary become seeds.

AnatomyAnatomy is the study of plant tissues. New plant

cells are formed within meristems. There, the cellsbegin the process of becoming specialized (differ-entiated) for a particular function. As a result, threecategories of plant tissues are formed: dermal, vas-cular, and ground.

Dermal tissues, which form protective cover-ings, include the epidermis, which covers all partsof a young plant, and others that develop as a plantmatures. The periderm commonly replaces the epi-dermis and includes tissues found in bark.

Vascular tissues, derived from cambium cellswithin the meristem, conduct water and dissolvedcompounds within a plant. They include xylem andphloem.

Ground tissues are the less specialized tissues.Among their functions are storage, support, andphotosynthesis. A common type is parynchema.

Plant science • 835

CytologyCytology is the study life at the cellular

level; another name for this discipline iscell biology. Plant cells share many fea-tures with animal cells. Both forms of lifeare composed of eukaryotic cells, whichhave a distinct nucleus surrounded bya nuclear envelope which separates itfrom the cytoplasm. (By contrast, bacteriahave nucleus-free cells, called prokary-otic cells.) Inside the nucleus is chroma-tin, which becomes organized into chro-mosomes as a cell divides. Chromosomesare composed of functional units calledgenes, which serve as the control center ofthe cell. Genes are composed of nucleo-protein. The nucleus also contains a nu-cleolus.

The cytoplasm is differentiated into nu-merous organelles, each specializing in aparticular activity. Among those whichplant cells share with animal cells are mi-tochondria (which conduct cellular respi-ration), ribosomes (which conduct pro-tein synthesis), endoplasmic reticulum(for strengthening), Golgi apparatus (forpackaging), and a plasma membrane(which functions as the cell’s outer bound-ary). Not found in animal cells are chloro-plasts (which conduct photosynthesis).Surrounding each plant cell are severallayers of compounds (especially cellulose)that form the cell wall.

PhysiologyPhysiology is the study of the various

functions performed in and by living or-ganisms. Physiological processes of plantsinclude the flow of energy, movement ofsolutes, and control by hormones. Chemical reac-tions are mediated (their rate is controlled) by en-zymes.

For example, those studying plant physiologyare concerned with the way that plants trap lightenergy as light is absorbed by chlorophyll. As aresult of a series of chemical reactions, glucose, asix-carbon carbohydrate, is formed (preceding glu-cose are molecules of PGAL, a three-carbon sugar,which pair to form glucose). The glucose may beoxidized within the same cell or within another cellof the same plant (or utilized by an animal). This ox-

idation process produces adenosine triphosphate(ATP). As this compound is converted to adenosinediphosphate (ADP), energy is released, allowingorganisms to perform other essential energy-requiring life activities.

Plant physiologist would also be concerned withthe transport of nutrients and water throughout aplant. By means of their roots, plants absorb waterand dissolved materials from the soil, after whichthey are conducted upward, by means of xylem tis-sue, to all parts of the plant. This upward move-ment is called transpiration. The glucose formed in

836 • Plant science

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leaves is dissolved in water and transported, bymeans of phloem tissue, to all parts of the plant.This movement, called translocation, is commonlydownward, but also may be upward. By these pro-cesses, water, minerals, and sugars are transportedto all parts of a plant.

Another area of concern for plant physiologistsis the function of plant hormones (phytohor-mones); in fact, those specializing in this area havetheir own discipline, endocrinology. Phytohormonesare compounds produced within a plant. They aretransported to other parts of plant, where they reg-ulate growth and development. Early in the twenti-eth century, auxin was the first phytohormone to bediscovered. It promotes growth by causing cells toelongate but was found also to inhibit growth of lat-eral buds. Gibberellins, a second group of hor-mones, also stimulate growth by causing cell elon-gation. Among their activities is the promotion ofseed germination. Cytokinins are abundant in di-viding tissues, where they stimulate cell division.Abscisic acid is a growth-inhibiting hormone thatmaintains dormancy in buds and fruits and also isassociated with the falling of leaves in autumn. Eth-ylene causes fruits to ripen. Several hormones areused in agriculture for increasing growth rates ofcrops.

GeneticsGenetics, the study of heredity and the mecha-

nisms that control it, is an outgrowth of the studiesof Gregor Mendel. In the 1860’s, he performed ex-periments with garden peas which resulted in anew way of explaining how traits are passed fromgeneration to generation. Mendel’s ideas were re-vived in 1900 as other European investigators con-firmed his basic tenets. Heredity is due to discretehereditary particles which soon came to be calledgenes, which are located on located on paired chro-mosomes. The application of the principles of ge-netics, begun in the first few decades of the twenti-eth century, has resulted in the development ofgreatly improved varieties of crop plants.

Molecular BiologyGenetics today is in many ways the concern of

another discipline of plant science, molecular biol-ogy. The basic chemical nature of genes and howthey express themselves remained in question untilthe 1950’s, when James Watson and Francis Crickdeveloped the double-helix model of deoxyribonu-

cleic acid (DNA), explaining how genes occur ingreat variety, replicate (duplicate) themselves, andproduce phenotypes (observable traits). Today, ge-netics is largely concerned with studying the chem-ical reactions that control DNA replication. Manyresearchers are also at work mapping the genomes(identifying the genes responsible for expressedcharacteristics) of various organisms. In 2001, re-searchers completed a map of the genome of themodel plant Arabidopsis thaliana as well as the morecomplex genome of the rice plant, Oryza sativa.

Molecular biology has many practical applica-tions. A knowledge of the genetic control of cellshas already resulted in new crop plants. The term“genetic engineering” indicates that plants can bedesigned for specific purposes.

EcologyThe early Greek scientist Theophrastus, in the

third century b.c.e., recognized environmental ef-fects on plants. Much later, naturalists documentedthe geographical distribution of plants as deter-mined by various climatic factors. Such studieswere the roots of the scientific discipline ecology,which emerged in the late nineteenth century. Plantecologists of the early twentieth century were con-cerned largely with describing the nature and distri-bution of world plant communities and developinga “successional theory” as a means of understand-ing the dynamics of changing plant communities.Now, ecologists study plants as integral parts ofecosystems that also include animals and microor-ganisms. They are concerned with countering thethreat of loss of species as a result of human activi-ties such as pollution and habitat destruction.

PaleobotanyFossils have long been recognized as remnants

of plants and animals that lived and died many mil-lennia ago. The animal fossil record was an impor-tant factor in the development of Charles Darwin’stheory of evolution in the 1800’s. Paleobotany as asubdiscipline can be traced to the efforts of AlbertSeward of Cambridge University of England in thelate nineteenth and early twentieth centuries.Studies of plant fossils have resulted in a clearer un-derstanding of plant evolution.

Economic BotanyPeople have always relied on plants to provide

basic necessities of life: food, shelter, and clothing.

Plant science • 837

Economic botany developed as a specialty withinbotany to acquaint botanists with plant uses. Topicsconsidered in economic botany include plant do-mestication, food and beverage plants, essentialoils, oils and waxes, latexes and resins, medicines,fibers, tannins and dyes, wood products, and orna-mental plants.

Related DisciplinesCourses in botany and plant science often ad-

dress organisms that are not, in the strict sense,plants but that nevertheless are appropriately stud-ied in the same context. Hence, although bacteriadiffer from plants primarily because of their cells,which are prokaryotic (lacking a nucleus and mostcytoplasmic organelles), they are often studied inbotany courses. Bacteria are early and evolution-arily significant organisms. Some are closely re-lated to the protists known as algae and thereforeimportant in the study of photosynthesis.

Fungi, too, are often studied in the context ofplant science. These are mostly multicellular fila-mentous eukaryotic organisms lacking chlorophyll.Because they do not make their own food but livein or on the food provided by plant and animal tis-sues, fungi are heterotrophs (rather than autotrophs,like plants, which make their own food throughphotosynthesis). In some ways, therefore, fungi aremore similar to animals than they are to plants.Nevertheless, they are traditionally studied in thecontext of plant courses because they were onceconsidered to be plants, given their lack of move-ment and other gross similarities. Their world sig-nificance parallels that of bacteria: They, too, areimportant as decomposers, returning nutrients and

other elements to the environment. The study offungi is mycology.

Protists are unicellular eukaryotes, forming oneof the four kingdoms of Eukarya, the others beingfungi, plants, and animals. Included among theprotists are slime molds and protozoans which,lacking chlorophyll, are said to be heterotrophicprotists, obtaining their food from other sources,generally other organisms. Also included are the al-gae, which are autotrophic eukaryotes—many us-ing photosynthesis, like plants, to generate theirown food. For this reason, protists are often studiedin the context of plant science, and algae are almostalways included in such studies. The study of algaeis phycology.

Viruses, the study of which is virology, are non-cellular entities that can reproduce only inside spe-cific host cells. Not generally considered to be liv-ing, they are nevertheless important because of theinfections they cause. To the extent that they causeinfection in plants, they are important in the studyof plant science, particularly plant pathology.

Thomas E. Hemmerly

See also: Agriculture: history and overview;Agronomy; Algae; Biotechnology; Botany; Cell the-ory; Cladistics; Dendrochronology; Ecology: his-tory; Environmental biotechnology; Evolution: his-torical perspective; Fungi; Genetics: Mendelian;Genetics: post-Mendelian; History of plant science;Hormones; Hydroponics; Molecular systematics;Paleobotany; Paleoecology; Plant biotechnology;Population genetics; Protista; Systematics and tax-onomy; Systematics: overview; Viruses and vir-oids.

Sources for Further StudyMadigan, Michael T., et al. Brock Biology of Microorganisms. 9th ed. Upper Saddle River, N.J.:

Prentice Hall, 2000. This microbiology textbook serves as a reference for other microor-ganisms.

Magner, Lois N. AHistory of the Life Sciences. 2d ed. New York: Marcel Dekker, 1994. Aconcisetreatment of the history of biology.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman, 1999. An excellent textbook that gives an overview of plants and relatedgroups and provides rationales for the inclusion of various subdisciplines under the um-brella of plant science.

838 • Plant science

PLANT TISSUES

Categories: Anatomy; physiology

Plant tissues are the distinctive structural and functional units of a plant that carry out all its basic life functions, in-cluding growth, reproduction, support, metabolism, circulation, and protection from the environment.

The body plan of a plant is very different fromthat of most animals. Terrestrial plant bodies

are anchored in a growing medium, which has anenormous influence over the form and behavior ofplant tissues.

Growth and Protective TissueMeristematic tissues in plant bodies are responsi-

ble for the growth that results from an increase incell number. In the meristems, individual cells di-vide to produce pairs of daughter cells which havethe ability to divide further or to enlarge and differ-entiate. The meristems are located at the ends ofbranches and roots (shoot apical meristems androot apical meristems, respectively) and within thecambia of woody plants, which grow in girth. Theshoot and root apical meristematic tissues producecells that account for the lengthening of the shootsand roots.

The primary developmental tissues are in a re-gion called the zone of elongation. These develop-mental tissues are distinguished from meristematictissues by the larger size of their cells and by theirlocations. Three primary developmental tissues areproduced by the shoot and root apical meristems.They are the protoderm, the ground tissues, and theprocambium. As these primary developmental tis-sues mature, they will ultimately differentiate intothe metabolically more active portions of the plant.

In a region called the zone of maturation, the cellsbegin to take on the characteristics of mature, func-tioning tissues. The protoderm differentiates toform the epidermis, a mature tissue protecting thesurfaces of plant parts which do not have second-ary vascular tissues. The epidermis is made of cellswhich have one side in contact with the environ-ment (air, water, or soil). The other side is in contactwith other cells in the plant body.

Epidermal tissue in contact with air is usuallyprotected by a layer of wax called the cuticle. It may

also be covered with hairs, water-filled cells, poison-filled barbs, or even digestive glands. These spe-cialized structures provide protection from partic-ular environmental conditions and may even serveas paths for the absorption of nutrients in the case ofcarnivorous plants.

The underlying tissues must have access to at-mospheric gases for their metabolic activities. Toaccomplish this, the epidermal tissues are punctu-ated by pores which open and close (stomata) or arepermanently open (lenticels). Epidermal tissues incontact with the ground require a different kind ofprotection. These tissues may secrete mucus, whichprotects growing underground structures. Thereare epidermal cells that fall off the plant body toprovide a lubricating barrier between the rest of theplant and the soil. Finally, the epidermal tissuesnearest the root tip may be covered with longsubcellular hairs that contribute significantly to theroot’s ability to absorb water and minerals. Epider-mal tissues of plant organs that normally grow inwater are less likely to bear the specialized struc-tures of epidermal tissues from aerial or subterra-nean parts. These cells are often more like paren-chyma cells of ground tissues than they are likeepidermal cells of subterranean or aerial structures.

Ground TissuesThe ground tissues, the second of the primary

developmental tissues, differentiate in the zone ofmaturation to form tissues called parenchyma, col-lenchyma, or sclerenchyma. The parenchymous tis-sues are the primary site of cellular metabolism.The organelles of parenchyma cells in different partsof the plant vary so that they can accommodate dif-ferences in metabolic functions. Cells of leaf paren-chyma and some stem parenchyma have large num-bers of chloroplasts to carry out photosynthesis.Stem and root parenchyma cells have amyloplasts,organelles that store starch. Chromoplasts in the pa-

Plant tissues • 839

RootCortex

Epidermis

Phloem

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Xylem

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Epidermis

LeafStoma

Mesophyll

Phloem

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Apicalmeristem

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Node

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renchyma of flower petals contribute to the color ofthe flower petals. Parenchyma cells producing largequantities of protein have more ribosomes thanthose specialized for starch storage. Reproductiveparenchyma cells may have unusual nuclear char-acteristics that prevent these tissues from compet-ing with the developing embryos for nutrients orspace.

Parenchyma fill the inner parts of leaves, stems,and roots. These cells have large, water-filled vacu-oles. The water pressure from these vacuoles pro-vides much of the rigidity of the body of nonwoodyplants. When a leaf is limp, its parenchyma cells areusually depleted of water. Many of the chemicalsthat give plants their unique tastes or pharmaceuti-cal characteristics are produced and stored in pa-renchyma. For example, the bulk of a carrot root(especially outside the central core), the mass of apotato tuber (which is actually a unique form ofstem), and much of a lettuce leaf are all made of pa-renchyma.

Collenchyma cells are similar to parenchymacells in many ways. They use water pressure to pro-vide support. However, they are normally foundnear the surface of stems and leaves. Collenchymacells have a unique pattern of cell-wall thickeningthat allows expansion in diameter but not in length.This makes collenchyma especially suited to pro-viding support for soft-bodied plant parts that havecompleted much of their longitudinal growth. Col-lenchyma cells rarely provide bulk to plant struc-tures. Instead, they form thin sheets just below theepidermis and outside much of the parenchyma.Because collenchyma is thin, it has a smaller vol-ume than parenchyma and contributes less to themetabolism of the plant organs. It may neverthelesssupport some of the photosynthesis of the plant,and it provides textures to the organs as well.

Sclerenchyma cells occur throughout the body ofthe plant and include three types of cells: elongatedfibers; branched sclereids, resembling a three-dimensional jigsaw puzzle piece; and globularstone cells. All three cell types have heavy, second-ary cell walls and have lost many organelles.Sclerenchyma is a type of differentiated tissue thatfunctions when its cells are dead.

Fibers support plant organs in the same way asdoes collenchyma, but because the secondary cellwalls of sclerenchyma cells resist longitudinal andlatitudinal expansion, they are not common in grow-ing tissues. Their rigidity helps to supply support

even when tissues are water-stressed, but it alsolimits the potential for the organs to expand in girthor length. Fibers, sclereids, and stone cells all pro-vide protection against predation. The gritty tex-ture of a ripe pear, the shell of a nut, and the stringsof a coconut husk are all composed of sclerenchymacells and promote the wear and breakage of preda-tors’ teeth and other chewing structures.

ProcambiumThe procambium, the third of the primary devel-

opmental tissues, differentiates to form primaryxylem and primary phloem as well as the vascularcambium. The vascular cambium produces cells thatdifferentiate into secondary xylem and secondaryphloem. It also regenerates the supply of cells in thevascular cambium.

An example of xylem is the woody tissue at thecenter of most trees. (Palm trees are a notable excep-tion.) Smaller bundles of xylem are found in theroots, stems, and leaves of most plants, even whenthey are not woody. Xylem tissues are made of fourcell types: fibers and parenchyma cells (which alsooccur in sclerenchyma and parenchyma) and xylemvessel elements and tracheids (which are found onlyin the xylem). These cells work in concert to movewater upward through the plant. The xylem vesselelements and tracheids provide the actual channelsfor the movement, and the fibers serve largely asphysical supporting structures.

The parenchyma cells are responsible for somelateral movement in the xylem tissues. These pa-renchyma cells also have the ability to revert toa meristematic condition, providing a mechanismfor the xylem to replace damaged cells. Tracheidsand vessels have unusual, patterned secondary cellwalls that resist the physical stresses involved inmoving xylem sap. The cell organelles are lost be-fore the vessels and tracheids are functional. Thesap moves through a channel where the body of thecell had been before it was lost. Xylem is another tis-sue that contains cell types that function when theyare dead.

An example of phloem is the tissue on the insideof the bark of most trees (again, palm trees are anexception). Smaller bundles of phloem are found inthe roots, stems, and leaves of most plants, evenwhen they are not woody. Phloem tissues are madeof fibers, parenchyma cells, sieve tube elements,and companion cells. These four cell types work inconcert to move sugars, other organic molecules,

Plant tissues • 841

and some ions throughout the body of the plant.The sieve tube elements (or sieve cells, in some

plants) provide the actual channels for the move-ment. The fibers serve largely as physical support-ing structures. The parenchyma cells are responsi-ble for some lateral movement and also provide amechanism to replace damaged cells. Sieve tube el-ements and sieve cells have unusual, perforatedcell walls whose appearance indeed resembles asieve. Many of the cell organelles are lost before thesieve tube elements and sieve cells are functional.The phloem sap moves through living cells, butthey resemble no other cells in the plant.

The companion cells (which in some plants arecalled albuminous cells, to indicate a different devel-

opmental origin) are similar to parenchyma cells,but they provide substantial metabolic support tothe sieve tube elements and sieve cells. These cellsfunction together: The companion cells could liveindependently, while the sieve tube element couldnot, but the important function is carried out by thesieve tube element.

Most species that grow in girth are woody, andthe wood of woody plants is composed almost en-tirely of secondary xylem. The bark of woodyplants is made of phloem and corky layers. Thereare two principal cambia, the vascular cambiumand the cork cambium. Both contribute to the in-crease in girth. The vascular cambial tissues pro-duce the cells that will differentiate to form the sec-

842 • Plant tissues

Plant TissuesTissue Type Location in Plant Functions and Characteristics

MeristemsApical meristems Roots and shoot tips Site of primary growth; these cells eventually differentiate

into the plant’s primary tissues: dermal, vascular, andground.

Lateral meristems Stems Secondary growth of vascular and cork cambia, lateralbudding and branching limited by apical dominanceunless the apical meristem is cut as in pruning.

Dermal tissue (epidermis) Outer layer Retention and absorption of water and minerals,protection against herbivores, control of gas exchange.Includes stomata, trichomes, root hairs.

Vascular tissueXylem Throughout Conducts water through plant. Contains two kinds of

conducting cells: tracheids and vessel elements.

Phloem Throughout Transports dissolved organic materials throughout plant.Contains sieve cells, sieve-tube elements, other cellsresponsible for conducting nutrients and information.

Ground tissueParenchyma cells Throughout Food and water storage, sites for metabolism (respiration,

photosynthesis), healing.

Chlorenchyma cells Throughout Chloroplast-containing parenchyma cells specialized forphotosynthesis.

Collenchyma cells Beneath epidermis Support growing regions of shoots; common in petioles,elongating stems, expanding leaves.

Sclerenchyma cells Mature regions Rigid, producing thick secondary walls; usually dead atmaturity. Support and strengthen leaves, stems, roots.Two types: sclereids (short, compact, forming cores, seedcoats, other tough, gritty tissue) and fibers (long, slender,occuring in bundles).

ondary xylem and phloem of woody species. Thecork cambium produces the corky cells on the out-side of the bark.

Craig R. Landgren

See also: Angiosperm cells and tissues; Flowerstructure; Growth and growth control; Leaf anat-omy; Roots; Shoots; Stems; Wood.

Sources for Further StudyBurgess, Jeremy. An Introduction to Plant Cell Development. New York: Cambridge University

Press, 1985. A text for the advanced student, with difficult and technical language on oc-casion. Assumes significant knowledge of botany. Illustrated with both line drawingsand photographs.

Dennis, David T., and David H. Turpin, eds. Plant Physiology, Biochemistry, and Molecular Bi-ology. New York: John Wiley, 1990. Examines the assimilation and metabolism of carbonand nitrogen in an integrative fashion, assessing the physiology, biochemistry, and mo-lecular biology of each topic discussed, for beginning or advanced students.

Hartmann, Hudson T., Anton M. Kofranek, and Vincent E. Rubatzky. Plant Science: Growth,Development, and Utilization of Cultivated Plants. 2d ed. Englewood Cliffs, N.J.: PrenticeHall, 1988. An easily read introductory college botany text which places plant tissues incontext with the biology and economical uses of plants. Chapter 2, “Structure of HigherPlants,” describes the relationship between plant tissues and cell types and between planttissues and organs. Heavily illustrated (in black and white) with both line drawings andphotographs.

Kaufman, Peter B., et al. Plants: Their Biology and Importance. New York: Harper & Row, 1989.Illustrated with black-and-white and color drawings and photographs, this college-levelbotany text is easily understood. Chapter 2, “Plant Tissues and Their Functions,” pro-vides a good review of plant tissues but is not as well illustrated as it might be.

Raven, Peter H., and George B. Johnson. Biology. 5th ed. Boston: WCB\McGraw-Hill, 1999.Beautifully illustrated with many color drawings and photographs, this college-level bi-ology text places plant tissues in the context of the biology of plants and places the plantsin the context of the living and nonliving world. The chapter “Vascular Plant Structure”provides a simple review of plant cell types, tissues, and organs.

Taiz, Lincoln, and Eduardo Zeiger. 2d ed. Plant Physiology. Sunderland, Mass.: Sinauer Asso-ciates, 1998. Clear, comprehensive textbook covers the transport and translocation ofwater and solutes, biochemistry and metabolism, and growth and development.

PLANTAE

Categories: Plantae; Taxonomic groups

Life on earth is dependent on the ability of plants to capture the sun’s energy. Directly or indirectly, members of the greenkingdom, Plantae, provide food and shelter for nearly all other organisms, including humans. Plants also generate muchof the earth’s oxygen. The biosphere would not exist without plants.

Most plants are multicellular, autotrophic or-ganisms, that is, able to produce their own

food from inorganic elements by converting waterand carbon dioxide to sugar. Plants are sessile, sta-tioned in one spot throughout their lives. Most

plants have a complex life cycle called alternation ofgenerations between diploid and haploid forms. Thediploid generation, in which the plant body is madeup of diploid cells, is called the sporophyte. Thesporophyte produces haploid spores by meiosis.

Plantae • 843

These haploid spores then grow mitotically to pro-duce the haploid generation, called the gametophyte,which produces haploid gametes. Gametes fuse toform a diploid zygote, a fertilized reproductive cellthat marks the beginning of a new sporophyte gen-eration.

Various methods may be used to group mem-bers of Plantae, which comprises about 300,000 spe-cies. Based upon where plants live, they can bedivided into terrestrial (land) and aquatic plants.Members of these two groups vary widely in theirsize, body structure, and level of complexity as a re-sult of their interactions with their environments.Plants that live in water usually lack true roots,stems, and leaves as well as complex reproductivestructures, such as flowers. Because they are sur-rounded by water, aquatic plants also lack the rigidsupporting substances required by terrestrial plants.Terrestrial plants are more complex and may bebroadly grouped into vascular (plants with vessels)and nonvascular (plants without vessels). Vascularplants are divided into various phyla, based upontheir adaptive features and complexity of structure.

Algae vs. PlantaeThose plants known as algae are not members of

kingdom Plantae but instead are primarily mem-

bers of another group of eukaryotic life,Protista, which also produce their ownfood by photosynthesis. Algal pigmentsare red or brown, absorbing the green,violet, and blue lights that most readilypenetrate the water. The combination ofvarious pigments with chlorophyll re-sulted in the distinctive coloration of al-gae of the three phyla: red algae, brownalgae, and green algae.

Within the kingdom Protista, threemain algal phyla exist. Red algae (phy-lum Rhodophyta) are multicellular withred pigment (phycobilins) masking theirgreen chlorophyll. Brown algae (phy-lum Phaeophyta) are multicellular andinclude the largest and most complex ofthe marine algae. Green algae (phylumChlorophyta) are mostly multicellular in-habitants of freshwater environments,with about seven thousand species. Thealgae are thought to be primitive evolu-tionary precursors to the species whichnow make up the kingdom Plantae.

Transition from Water to LandThe move from water to land requires adapta-

tions and some new features. The advantages to lifeon land seem obvious, with plentiful access to car-bon dioxide and sunlight, reduced competition,fewer predators, and increased nutrient concentra-tions. The challenges are also plentiful: The sup-portive buoyancy of water is missing, and the airtends to dry things out.

Conditions on land favored the evolution ofstructures that support the plant body, vessels thattransport water and nutrients throughout the body,and structures that conserve water. Adaptationsto dry land called conducting vessels (collectivelyknown as the vascular system) emerged to trans-port water and minerals as well as products of pho-tosynthesis. Roots or rootlike structures evolved tohelp anchor the plant and absorb nutrients andwater from the soil. A stiffening substance calledlignin, made up of rigid polymer, enables plants tostand in wind, hence exposing maximal surfacearea to sunlight. Numerous small pores calledstomata in the leaves and stems open to allow gasexchange and close to conserve water when neces-sary. A waxy cuticle covering of the surfaces ofleaves and stems also reduces the loss of water.

844 • Plantae

Phyla of Kingdom PlantaePhylum Common Name Living Species

Bryophytes (nonvascular plants)Anthocerotophyta Hornworts 100Bryophyta Mosses 8,000Hepatophyta Liverworts 6,000

Seedless vascular plantsLycopophyta Lycopods (Lycophytes) 1,200Psilotophyta Whiskferns 2 generaPterophyta Ferns 12,000Sphenophyta Horsetails 15

GymnospermsConiferophyta Conifers 550Cycadophyta Cycads 150-200Ginkgophyta Ginkgo 1Gnetophyta Gnetophytes 90

AngiospermsAnthophyta Flowering plants 235,000

Source: Data on species are from Peter H. Raven et al., Biology of Plants, 6th ed.(New York: W. H. Freeman/Worth, 1999).

Based upon their structure, complexity, and dis-tribution over the globe, land plants can be classi-fied into three phyla of bryophytes and nine phyla ofvascular plants, which include seedless plants, gym-nosperms, and angiosperms.

BryophytesThree phyla of plants—the liverworts, horn-

worts, and mosses—have been commonly knownas bryophytes. The sixteen thousand species ofbryophytes are among the least complex terrestrialplants. They are the plant equivalent of amphibi-ans. Although they have rootlike anchoring struc-tures (rhizoids), they lack true roots, leaves, andstems. They are also nonvascular, lacking well-developed structures for conducting water and nu-trients. Because they must rely upon slow diffusionor poorly developed tissues for distribution ofwater and nutrients, their body size is limited. Mostbryophytes are less than 1 inch (2.5 centimeters)tall.

The liverworts, phylum Hepatophyta, comprisesix thousand species of small, inconspicuous plantsforming large colonies in moist, shaded soil orrocks, tree trunks, or branches. Due to the liver-shaped gametophyte in some genera and the fictionthat these plants might be useful in treating liver-related diseases, they were named liverworts manycenturies ago. Liverworts are the simplest of all liv-ing land plants, lacking cuticle, stomata, and vascu-lar tissue.

The hornworts, phylum Anthocerophyta, are asmall phylum of plants consisting of about onehundred species. By appearance, many species ofhornworts have a remarkable resemblance to greenalgae. Hornworts, however, have stomata, an im-portant structure for land plants. Like liverworts,hornworts lack specialized conducting tissue.

The mosses, phylum Bryophyta, constitute a di-verse group of some ninety-five hundred species ofsmall plants. Many species have both stomata andspecialized conducting tissue, resembling the re-maining phyla of land plants. Mosses usuallythrive in relatively moist areas, where a variety ofspecies can be found. Some mosses are used tomonitor air pollution because of their acute sensi-tivity to air pollutants such as sulfur dioxide. Thereare three classes of mosses: Bryidae (the “true”mosses), Sphagnidae (the peat mosses), and Andre-aeidae (the granite mosses), each with distinctivefeatures.

Seedless Vascular PlantsThe overall pattern of plant diversification may

be explained in terms of the successive rise to domi-nance of each of four major plant groups. Earlyvascular plants, including Rhyniophyta, Zosterophyl-lophyta, and Trimerophyta, were primitive in mor-phology yet dominant during a period from about420 million to 370 million years ago. Ferns, lyco-phytes, sphenophytes, and progymnosperms weredominant from about 380 million to 290 millionyears ago. Seed plants arose about 360 million yearsago, with gymnosperms dominating the globe until100 million years ago. Finally, the angiosperms, orflowering plants (phylum Anthophyta) appearedabout 127 million years ago and have been the mostdominant and diverse group for the past 100 mil-lion years.

The seedless vascular plants dominated thelandscape during the Carboniferous period. Theyare the primary source of coal, which was formedthrough gradual transformation of plant bodies un-der high pressure and heat. The three dominantphyla—the Rhyniophyta, Zosterophyllophyta, andTrimerophyta—had become extinct by about 360million years ago. The modern representatives ofseedless vascular plants have become reduced insize and importance. The landscape once domi-nated by seedless plants has largely been replacedby the more versatile seed plants. Five phyla ofseedless plants have living representatives: Psilo-tophyta, Lycophyta, Sphenophyta, Pterophyta, andProgymnospermophyta.

PsilotophytesCommonly called the whiskferns, the phylum

Psilotophyta includes two living genera, Psilotumand Tmesipteris. Both are very simple plants.Psilotum is widely known as a greenhouse weedthat prefers tropical and subtropical habitats. In theUnited States, it is found in Arizona, Florida, Ha-waii, Louisiana, Puerto Rico, and Texas. Tmesipterisis restricted to South Pacific regions such as Austra-lia, New Caledonia, and New Zealand.

Psilotum is unique among vascular plants in thatit lacks both true roots and leaves. The under-ground portion of Psilotum forms a system of rhi-zomes with many rhizoids that are a result of a sym-biotic relationship between fungi and Psilotum.Psilotum is homosporous, meaning the male and fe-male gametophytes are produced through the ger-mination of the spores of same origin. Psilotum

Plantae • 845

sperm require water to swim to and fertilize theegg.

Tmesipteris grows as an epiphyte on tree fernsand other plants and in rock crevices. The leaflikeappendages of Tmesipteris are larger than those ofPsilotum. Otherwise, Tmesipteris is very similar toPsilotum.

LycophytesThere are about one thousand living species of

phylum Lycophyta that belong to three orders of tento fifteen genera. At least three orders of Lycophytahave become extinct; these include small and largetrees, the dominant plants of the coal-forming for-est of the Carboniferous period. The three orders ofliving Lycophyta consist of herbs, each including asingle family: Lycopodiaceae, Selaginellaceae, andIsoetaceae.

Lycopodiaceae are commonly known as clubmosses. All except two genera of Lycophyta be-long to this family. Most of the estimated four hun-dred species of Lycopodiaceae are tropical. Theyrarely form conspicuous elements in any plantcommunity, except in some temperate forestswhere several species may form distinct mats onthe forest floor. Because they are evergreen, theyare most noticeable during winter months. Lyco-podiaceae are homosporous and require water forfertilization. Among the genera that grow in theUnited States and Canada are Huperzia (the firmosses, seven species), Lycopodium (tree clubmosses, five species), Diphasiastrum (club mossesand ground pines, eleven species), and Lycopodiella(six species).

Selaginella is the only living genus of the familySelaginellaceae. Among its seven hundred species,most are tropical, growing in moist habitats. A fewspecies occur in deserts, becoming dormant duringthe driest season. The well-known resurrectionplant, S. lepidophylla, grows in Mexico, New Mex-ico, and Texas. Selaginella are heterosporous, havingseparate male and female gametophytes.

The only member of the family Isoetaceae isIsoetes, commonly known as quillwort. Isoetes maybe aquatic or grow in pools that have alternate dryand wet seasons. Isoetes is also heterosporous. Theunique feature of some species of Isoetes is theirability to acquire carbon dioxide for photosynthesisfrom soil where they grow rather than from the at-mosphere. Their leaves have thick cuticles and lackstomata.

HorsetailsOnly one genus, Equisetum, consisting of fifteen

living species, makes up the phylum Sphenophyta.Members of this phylum are known as the horse-tails, believed to be the oldest surviving plants onearth. They are widely distributed in moist or dampplaces, by streams, and along the edges of woodsand roadsides. The leaves are reduced to tiny scaleson the branches. The ribs are tough and strength-ened with silicon deposits. Due to its abrasive tex-ture, Equisetum was used in past times to scour pots,pans, and floors. Hence, they were also called“scouring rushes.”

The roots of Equisetum are adventitious, emerg-ing at the nodes of the rhizomes. The aerial stems ofEquisetum arise from branching underground rhi-zomes. Although the plants may die back duringthe dry season, the rhizomes are perennial. Equi-setum is homosporous. Its gametophytes are greenand free-living, most being about the size of a pin-head. They become established mainly in mud thathas recently been flooded and is rich in nutrients.The gametophytes, which reach sexual maturity inthree to five weeks, are either bisexual or male. Thesperm require water to swim to the eggs. A fertil-ized egg develops into an embryo or young sporo-phyte.

PterophytaMembers of the phylum Pterophyta are com-

monly called ferns, the largest group of plants otherthan the flowering plants. About eleven thousandliving species of ferns are widely distributed on theearth, among which three-fourths of the species arefound in the tropics. There the greatest diversity offern species exists, and they are abundant in manyplant communities. In the small tropical country ofCosta Rica, 1,000 species of ferns have been identi-fied, whereas only 380 species of fern occur in theUnited States and Canada combined. In both formand habitat, ferns exhibit amazing diversity. Someare small and have undivided leaves, while otherscan reach up to 30 meters in height, with a trunk ofmore than 30 centimeters in diameter. Most livingferns are homosporous, except for two orders ofwater ferns.

Two genera of the order Ophioglossales, the grapeferns (Botrychium), and the adder’s-tongues (Ophio-glossum), are widespread in the north temperate re-gion. A single leaf is usually produced each yearfrom the rhizome. Each leaf consists of two parts:

846 • Plantae

the blade (the vegetative portion) and a fertile seg-ment that typically bears two rows of eusporangia,hence the name eusporangiate ferns.

The order Filicales consists of 35 families and 320genera, with more than 10,500 species. Most the fa-miliar ferns are members of this order, such as thegarden and woodland ferns of temperate regions.They have rhizomes that produce new sets of leaveseach year. The root system is primarily adventi-tious, arising from the rhizomes near the bases ofthe leaves. They are homosporous. With a high sur-face-to-volume ratio, their bodies capture sunlightmuch more effectively than those of the lycophytes.

The water ferns, Marsileales and Salviniales, arethe only living heterosporous ferns. Members ofMarsileales grow in mud, on damp soil, or oftenwith their four-leaf-clover-like leaves floating onthe surface of water. Their unique, drought-resistant, bean-shaped reproductive structures areable to germinate even after one hundred years ofdry storage. Members of Salviniales, genera Azollaand Salvinia, are small plants that float on the sur-

face of water. They are harvested and used as feedor fertilizer in some Asian countries.

GymnospermsGymnosperms, literally “naked seeds,” are the

earliest-evolved plants that produce seeds. Livinggymnosperms comprise four phyla: Cycadophyta,Ginkgophyta, Coniferophyta, and Gnetophyta. The lifecycles of all bear a remarkable resemblance: an al-teration of heteromorphic generations, with large,independent sporophytes and greatly reduced ga-metophytes. The ovules are exposed on the surfacesof the megasporophylls. At maturity the female ga-metophyte of most gymnosperms is multicellularin structure, with several archegonia. The male ga-metophytes develop as pollen grains. Except for theginkgo and cycads, the sperm cells of seed plantsare nonmotile.

In seed plants, water is not required for transferof sperm or fertilization of an egg. Pollen grains thatusually contain two sperm nuclei may be trans-ferred to the egg via various means, such as wind,

Plantae • 847D

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The smallest of the modern flowering plants, or angiosperms, is duckweed, a few millimeters in diameter, which floatson ponds.

insects, and animals. A pollen grain then germi-nates and sends one nucleus to fuse with the egg,which in turn develops into embryo. After fertiliza-tion, each ovule develops into a seed.

Among the four phyla, the conifers (Conifero-phyta) are the largest and most widespread gymno-sperms, with about 50 genera and 550 species. Theystill dominate many of the earth’s plant communi-ties, with pines, firs, spruces, and other familiarevergreen trees over wide stretches of the north.Living cycads (Cycadophyta) constitute 11 genera,and some 140 species grow primarily in tropicaland warm regions. Cycads are palmlike plants,with trunks and sluggish secondary growth. Onlyone living species of ginkgo (Ginkgophyta) exists.The phylum Gnetophyta consists of three generathat are close relatives of the angiosperms, withwhich they share many characteristics.

AngiospermsAs plants adapted to terrestrial environments,

more effective means of reproduction and distribu-tion emerged. Flowers, fruits, and seeds resulted inthe dominance by angiosperms: flowering plants,the most diverse group of plants. Modern flower-ing plants, or angiosperms, constitute the phylumAnthophyta, which are incredibly diverse, with morethan 230,000 species. They range in size from a fewmillimeters in diameter, such as the duckweed thatfloats on ponds, to more than 320 feet (about 100 me-ters) tall, such as the eucalyptus. From desert cactito tropical orchids to grasses to major food crops,angiosperms dominate the plant kingdom.

Anthophyta is divided into two large classes:Monocotyledons (65,000 species) and Eudicotyle-dones, or “true” dicots (170,000 species).

In addition to some features shared with gymno-sperms, angiosperms have a few unique character-istics. Within seeds, nutrients and food are usuallystored in a triploid tissue called an endosperm. The

presence of carpels makes flowers shiny and moreattractive for pollinators, enhancing their repro-ductive success. The nutritious fruit-encasing seedsoffer protection and ensure the wide distribution ofangiosperms as various animals eat fruits and dis-perse the seeds throughout the ecosystem. Angio-sperms have broad leaves, giving them the advan-tage of collecting more sunlight for photosynthesis,especially in warm, moist climates. The extra en-ergy gained in spring and summer allows trees todrop their leaves and enter a dormant period,which reduces water evaporation when water is inshort supply.

Pollination in angiosperms takes place by thetransfer of pollen from anther to stigma. Each pol-len grain contains sperm, typically with two orthree cells. One cell grows, sending pollen tubes tothe ovule, where one sperm nucleus unites with theegg and produces a diploid zygote. The other cell orcells fuse with two polar nuclei, giving rise to pri-mary endosperm nucleus. This phenomenon,called double fertilization, is a unique characteristicfor angiosperms. The zygote then develops into anembryo (sporophyte). The primary endospermgrows and matures into a nutritive endosperm.Both self-pollination and cross-pollination occur inflowering plants. The angiosperm domination ofearth began about 100 million years ago and haspersisted to the present.

Ming Y. Zheng

See also: Angiosperms; Angiosperm evolution;Aquatic plants; Bryophytes; Conifers; Cycads andpalms; Eudicots; Evolution of plants; Ferns; Gink-gos; Gnetophytes; Gymnosperms; Hornworts;Horsetails; Liverworts; Lycophytes; Monocots vs.dicots; Monocotyledones; Mosses; Plant life spans;Psilotophytes; Reproduction in plants; Rhyniophyta;Seedless vascular plants; Spermatophyta; Tracheo-bionta; Trimerophytophyta; Zosterophyllophyta.

Sources for Further StudyBeck, C. B., ed. Origin and Evolution of Gymnosperms. New York: Columbia University Press,

1988. An excellent collection of essays on every aspect of gymnosperms and their evolu-tion.

Buckles, M. P. The Flowers Around Us: A Photographic Essay on Their Reproductive Structures.Columbia: University of Missouri Press, 1985. Description and discussion on the diver-sity of flowers and fruits. Well illustrated with diagrams and photos.

Gifford, E. M., and A. S. Foster. Morphology and Evolution of Vascular Plants. 3d ed. New York:W. H. Freeman, 1989. Awell-written, general discussion on vascular plants. Illustrated byphotos.

848 • Plantae

Schofield, W. B. Introduction to Bryology. New York: Macmillan, 1985. An excellent analysis ofall three phyla of mosses, dealing with their morphology, physiology, ecology, and geog-raphy.

South, G. R., and A. Whittick. Introduction to Phycology. Oxford, England: Blackwell Scien-tific, 1987. Excellent account of algae, well written and illustrated.

PLANTS WITH POTENTIAL


One of the primary reasons humans cultivate plants is to satisfy an economic need for natural resources. In order for aplant to realize its full economic potential, it must not only fill an economic need but also do so in a cost-efficient manner.

There are several examples of plants which, be-cause of their unique products, appear to fulfill

an economic need. However, these plants may notdo so in a cost-effective manner. With developmentof improved agronomic practices and plant-processing methods, which lower the cost of pro-duction, some plants may eventually realize theirfull economic potential.

Another factor that may affect a plant’s eco-nomic potential includes the availability and priceof competing products on the world market. Thisfactor is beyond the control of domestic agrono-mists. Drastic changes in world conditions, such asin times of natural disaster, severe economic reces-sion, or wartime, can have a dramatic impact onthe economic feasibility of natural resources. Cropswith economic potential may move in and out ofeconomically favorable conditions as world mar-kets change and new markets develop.

GuayuleGuayule (Parthenium argentatum) is a shrubby

member of the Compositae family that is native tothe desert regions of the southwestern UnitedStates and northern Mexico. Other species of thisgenus are found in all regions of the Americas.Guayule is one of more than two thousand plantspecies that can potentially be used to produce latexand rubber. Only guayule and its chief competitor,Hevea brasiliensis, have been used to produce com-mercial quantities of rubber.

Although commercial production of guayule-derived rubber dates back to the 1920’s, when Con-tinental Rubber Company produced small quanti-

ties of latex and rubber from guayule plants grownin Arizona and California, it was not until theEmergency Rubber Project during World War IIthat large-scale production of guayule rubber com-menced. With the U.S. economy on a wartime foot-ing and with supplies of imported Hevea rubberbecoming uncertain, U.S. Code Title 7, Section 171,authorized the U.S. Department of Agriculture toacquire the technology of the Continental RubberCompany, plant up to 500,000 acres of guayule, anddevelop factories for the production of guayule-based rubber. With the end of World War II and re-establishment of the Hevea rubber supply fromAsia, guayule-based rubber was no longer econom-ically competitive. The unfavorable price differ-ence between guayule-based rubber and Hevea-based rubber has remained unchanged, eventhough many agronomic improvements have beenmade for guayule.

However, new life may be developing forguayule-based rubber in a large niche market: med-ical products. A method for producing hypoal-lergenic latex derived from guayule has been devel-oped and patented by the U.S. Department ofAgriculture. A private company, Yulex Corpora-tion, has licensed this technology and intends to useit to manufacture medical items, such as surgicalgloves. The gloves produced from guayule-basedlatex do not contain the allergenic proteins thatHevea-based latex contains, so they will not causeallergic reactions in the estimated twenty millionAmericans who are allergic to Hevea rubber prod-ucts. In this case, the cost disparity between Hevearubber and guayule rubber is offset by the technical

Plants with potential • 849

improvement that guayule latex brings to the high-value market for medical devices.

JojobaJojoba (Simmondsia chinensis) is a woody, ever-

green desert shrub that, despite its misleading spe-cies epitaph, chinensis, is native to the southwesternUnited States. In cultivation, jojoba may be irri-gated during its two- to three-year establishmentperiod after which, assuming that the roots findgroundwater, the plants do not require irrigation.During the plant’s initial production period of threeto ten years, the female plants may produce 350 ki-lograms of seeds per hectare. After ten years ofgrowth, the plants may yield 500 to 800 kilogramsper hectare for many decades.

Jojoba oil is extracted from jojoba seeds and com-prises approximately 40 to 60 percent of the mass ofthe seeds. Jojoba oil is not really an oil per se, as it isnot a triglyceride; jojoba oil is a plant wax similar toplant cuticular waxes, being composed of long-

chain alcohols and fatty acids. The value of jojobaoil comes from its desirable stability. Jojoba oil isstable up to 300 degrees Celsius and does not be-come rancid even after decades of storage. Also,jojoba oil is very similar chemically to the highlyprized sperm whale oil, so it is useful in cosmetics.

Jojoba oil was first produced in commerciallyimportant quantities during World War II, as ahigh-temperature lubricant and an extender forpetroleum-based lubricants. These jojoba-basedproducts were used for engines, machinery, vehi-cles, and guns. As seen with guayule, the econom-ics of jojoba oil production did not compare favor-ably with abundant petroleum products afterWorld War II, so production of jojoba oil decreasedsharply after the war.

In recent decades, the economics of jojoba oilproduction has gone through many fluctuations. Inthe 1970’s the Green Revolution reignited interestin renewable, natural resources, especially prod-ucts that could replace petrochemicals and animal-

850 • Plants with potential

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derived products. Growers took advantage of taxincentives to start farming jojoba. Then the disap-pearance of tax incentives and the decade-longproduction time to achieve commercially usefulquantities of jojoba seed proved to be economicallydisastrous for many growers. Many jojoba farmsshut down operations. In the 1990’s the price ofjojoba oil ranged from $40 per gallon to $200 pergallon, an unacceptable fluctuation in price thatdiscouraged many industries from becoming de-pendent on jojoba oil. The price of jojoba oil muststabilize before industries can once again exploreadding jojoba to their lines. Additionally, someunique, value-added products containing chemi-cally modified jojoba oil are becoming common inmany upscale health and beauty products.

HesperaloeHesperaloe (Hesperaloe funifera) is a plant in the

agave family that produces long, thin fibers thatmay be processed into exceptionally light andstrong paper. Long-term biomass production stud-ies began on hesperaloe in 1988, and since thenmany agronomic and processing improvementshave been made. The fibers of hesperaloe seem suit-able for the production of high-value products,such as ultralight coated papers.

KenafKenaf (Hibiscus cannabinus) is another fast-

growing fiber crop that is finding utility in nichemarkets. Kenaf may be used to produce brightwhite paper, building materials, and absorbent ma-terials. Additionally, the black lignin liquor, a by-product of kenaf processing, may add value to thecrop by functioning as a binder for animal feeds, afertilizer, or a termite-resistant coating.

LesquerellaLesquerella (Lesquerella fendleri) is an industrial

crop under development for its unique seed oils.Initial research indicates that domestically pro-duced lesquerella may eventually replace importedcastor oil in many cosmetics, pharmaceuticals, andindustrial products. Enhanced agronomic tech-niques, processing methods, conventional breed-ing, genetic engineering, and emerging niche mar-kets may one day push lesquerella and other plantswith potential into the realm of commercial via-bility.

Robert A. Sinnott

See also: Agricultural crops: experimental; Cul-turally significant plants; Green Revolution; Me-dicinal plants; Rubber.

Sources for Further StudyRicketts, Cliff, and Omri Rawlins. Introduction to Agribusiness. Albany, N.Y.: Delmar, 1999.

Introduction to economic aspects of agricultural industries in the United States.Simpson, Beryl, and Molly Ogorzaly. Economic Botany: Plants in Our World. 3d ed. New York:

McGraw-Hill, 2000. Introductory-level text for economic botany.Wiersema, John H., and Leon Blanca. World Economic Plants: A Standard Reference. Boca

Raton, Fla.: CRC Press, 1999. Acomprehensive review of ten thousand vascular plants, re-lating them to the global economy.

PLASMA MEMBRANES

Categories: Anatomy; cellular biology

The plasma membrane is a structure of the plant cell that forms a semipermeable, or selective, barrier between the interiorof the cell and the external environment; they also function in transport of molecules into and out of the cell.

In addition to forming the structural barrier be-tween the internal contents of a cell and the ex-

ternal environment, plasma membranes contain

proteins involved in the transport of moleculesand other substances into and out of the cell, andthey contain proteins and other molecules that are

Plasma membranes • 851

essential for receiving signals from the environ-ment and from plant hormones that direct growthand division. Carbohydrates associated with theplasma membrane are markers of cell type. Inplants, the plasma membrane is the site of cellulosesynthesis.

Lipid molecules provide the structure for theplasma membrane, which is described by the fluidmosaic model as a dynamic ocean of lipids in whichother molecules float. Phospholipids are the mostabundant lipid of plasma membranes, and they areorganized in a fluid phospholipid bilayer in which

852 • Plasma membranes

Mechanisms for Transport Across Cell MembranesType of Transport Mechanisms and Functions

Diffusion Passive movement of substances into and out of the cell.

Simple diffusion Dissolved gases (oxygen, carbon dioxide) and lipid-solublesubstances move down the concentration gradient throughthe lipid portion of the membrane.

Facilitated diffusion Specialized membrane proteins allow substances with lowlipid-solubility to move down the concentration gradientby binding to carrier (transport) proteins.

Osmosis Net movement of water molecules down the concentrationgradient across the membrane, driven by the difference insolute concentrations on either side.

Active transport Movement of substances against the concentration gradient (“uphill”), requiring anexpenditure of cellular energy, usually from adenosine triphosphate.

Cotransport (symport) Active transport involving a specialized protein moleculecalled a symport carrier.

Countertransport (antiport) Active transport that uses a specialized protein moleculecalled an antiport carrier.

Endocytosis(vesicle-mediated)

Movement of materials into the cell: The cell’s plasma membrane engulfs material andpacks it into saclike vesicles that detach from the plasma membrane and move into thecell.

Phagocytosis “Cell eating”: cellular intake of large and generallyinsoluble molecules and macromolecules (and little fluid)that cannot be taken into the cell using other membranetransport mechanisms.

Pinocytosis “Cell drinking”: cellular intake of fluids and dissolvedsolutes by the formation of membrane-bound vesicles atthe cell membrane surface, which are then taken into thecell interior and released.

Receptor-mediated Cellular intake of nutrients and other essentialmacromolecules at specific receptor sites. Receptor-ligandcomplexes combine to form clusters around which aportion of the cell membrane encircles, producing a vesiclethat invaginates inward to pinch off as a coated vesicle.Once inside, the vesicle dissolves as a result of changes inthe acidity of the cytoplasm.

Exocytosis(vesicle-mediated)

Transport of macromolecules and large particles outside the cell, the reverse ofendocytosis. Materials inside the cell are packed in a vesicle, which fuses to the plasmamembrane and expels its contents into the extracellular medium.

sterols, proteins, and other molecules are inter-spersed. Phospholipids are amphipathic molecules,containing water-loving (hydrophilic) regions andwater-fearing (hydrophobic) regions.

Each phospholipid consists of a three-carbonglycerol backbone; two of the carbons are attachedto long-chain fatty acid molecules, and the third car-bon is attached to a phosphate-containing group.Because the fatty acids are nonpolar and hydropho-bic, they tend to aggregate and exclude water. Thisaggregation allows the phospholipids to form abilayer structure that has the fatty acids of bothlayers in the middle and the charged, phosphate-containing groups toward the outside. This bilayerstructure allows one surface of the plasma mem-brane bilayer to interact with the aqueous externalenvironment, while the other interacts with theaqueous internal cellular environment.

Sterols are also found within the plasma mem-branes of plant cells. The major sterol found inplant cell plasma membranes is stigmasterol (as op-posed to cholesterol, which is found in animal cellplasma membranes). Sterols found in plant cells areimportant economically as the starting material forsteroid-based drugs such as birth control pills.

Membrane Proteins and CarbohydratesSome membrane proteins span the entire length

of the phospholipid bilayer and are called trans-membrane proteins. Transmembrane proteins aresometimes referred to as integral membrane pro-teins and have varied structures and functions.They may pass through the lipid bilayer only once,or they may be “multiple pass” transmembraneproteins, weaving into and out of the membranemany times.

The portion of a transmembrane protein thatpasses through the interior of the membrane oftenconsists of amino acids that have nonpolar sidechains (R-groups) and is known as the transmem-brane domain. The portion of the transmembraneprotein that is on the external surface of the mem-brane and interacts with the aqueous environmentoften contains charged, or polar, amino acids in itssequence.

Membrane proteins are often important for re-ceiving signals from the external environment asmembrane receptors. For instance, protein or pep-tide hormones interact with transmembrane pro-tein receptors on the plasma membrane. Membraneproteins are also involved in receiving signals such

as light photons. Membrane proteins form poresthat allow ions (charged particles) to pass throughthe interior of the membrane. Membrane proteinscalled carriers are essential for bringing nutrientmolecules such as simple sugars into the cell.

Not all proteins within the membrane aretransmembrane proteins. Some are only loosely as-sociated with the membrane, attached to other pro-teins, or anchored in the membrane by a lipid tail.These proteins, which do not span both sides of themembrane, are often called peripheral membrane pro-teins.

In addition to proteins, the plasma membranecontains carbohydrate molecules. Carbohydratemolecules are usually attached to membrane pro-teins or to lipid molecules within the bilayer. Car-bohydrates provide important information aboutcell type and identity.

Transport Across MembranesTransport of molecules into and out of the cells is

an important function of the plasma membrane.Hydrophobic molecules, such as oxygen, andsmall, uncharged molecules, such as carbon diox-ide, cross the membrane by simple diffusion. Thesemolecules use the potential energy of a chemicalgradient to drive their movement from an area ofhigher concentration on one side of the membraneto an area of lower concentration on the other side.Diffusion works best when this concentration gra-dient is steep. For example, in cells that do not havethe ability to carry out photosynthesis, oxygen isused almost as quickly as it enters the cell. Thismaintains a sharp gradient of oxygen moleculesacross the membrane, so that molecules continuallyflow from the area of greater oxygen concentrationoutside the cell to the area of lower concentrationinside the cell.

Molecules that are polar are excluded from thehydrophobic area of the bilayer. Two factors influ-ence the transport of these kinds of molecules: theconcentration gradient and the electrical gradient.Lipid bilayers separate differences in electricalcharge from one side of the membrane to the other,acting as a kind of biological capacitor. If the insideof the cell is more negative than the outside of thecell, negatively charged ions would have to movefrom the inside to the outside of the cell to travelwith the electrical gradient. The combination of theconcentration and electrical gradients is called theelectrochemical gradient.

Plasma membranes • 853

Transport of charged or polar molecules requiresthe assistance of proteins within the membrane,known as transporters. Channel proteins formpores within the membrane and allow small,charged molecules, usually inorganic ions, to flowacross the membrane from one side to the other. Ifthe direction of travel of the ion is down its electro-chemical gradient, the process does not requireadditional energy and is called passive transport.Carrier proteins change shape to deposit a smallmolecule, such as a sugar, from one side of a mem-brane to the other. Pumps are proteins within themembrane that use energy from adenosinetriphosphate (ATP) or light to transport moleculesacross the membrane. When energy is used intransport, the process is called active transport.

Cellulose BiosynthesisIn plants, the plasma membrane is the site for the

synthesis of cellulose, the most abundant biopoly-mer on earth. Electron microscope studies suggestthat the plant cell membrane contains rosette struc-

tures that are complexes of many proteins and arethe sites of cellulose synthesis. Studies in bacteria,cotton plants, and the weed Arabidoposis thalianahave allowed scientists to isolate the gene that ac-tually carries out the chemical reactions linkingglucose molecules together into the long cellulosemicrofibril structure. This gene encodes a proteincalled glycosyl transferase. Antibodies against thecatalytic, or active, subunit of glycosyl transferasespecifically label these rosette structures. Two ofthese transferase molecules act simultaneouslyfrom opposite sides to add two glucoses at a time tothe growing microfibril, accounting for the rotationof alternating glucoses in cellulose molecules.

Michele Arduengo

See also: Active transport; ATP and other energeticmolecules; Carbohydrates; Cell wall; Cells and dif-fusion; Endocytosis and exocytosis; Lipids; Liquidtransport systems; Membrane structure; Osmosis,simple diffusion, and facilitated diffusion; Plantcells: molecular level; Proteins and amino acids.

Sources for Further StudyCarpita, Nicholas, and Claudia Vergar. “Enhanced: A Recipe for Cellulose.” Science 279

(1998): 672. Ashort review of a research article describing cellulose biosynthesis. Includesuseful graphics with review.

Lodish, Harvey, et al. Molecular Cell Biology. 4th ed. New York: W. H. Freeman, 2000. This isone of the most authoritative cell biology reference texts available. Provides excellentgraphics, CD-ROM animations of cellular processes, and primary literature references.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Introductory botany text provides a chapter introducingeukaryotic cells in general and plant cells specifically.

PLASMODIAL SLIME MOLDS

Categories: Molds; Protista; taxonomic groups

The plasmodial slime molds, or myxomycetes, phylum Myxomycota, are a group of funguslike organisms usually pres-ent and sometimes abundant in terrestrial ecosystems. However, this group comprises about eight hundred species, re-lated neither to cellular slime molds nor to fungi.

The plasmodial slime molds have no cell wallsand exist as thin masses of protoplasm, which

appear to be streaming in a fanlike shape, underfavorable conditions. As these masses, calledplasmodia, travel, they absorb small particles of de-

caying plant and animal matter as well as bacteria,fungi, and yeasts. When mature, a plasmodiummay weigh 20-30 grams and take up an area of 1meter or more.

Myxomycetes have been known from their fruit-

854 • Plasmodial slime molds

ing bodies (often a sporangium sprouting from asmall mound that forms when the plasmodiumstops moving) since at least the middle of the seven-teenth century. Their life cycle has been understoodfor more than a century. The reproductive, spore-producing stage in the life cycle can achieve macro-scopic dimensions and be collected and preservedfor study in much the same way as mushrooms andother fungi or even specimens of bryophytes, li-chens, and vascular plants. However, most speciesof myxomycetes tend to be rather inconspicuous orsporadic in their occurrence and thus not alwayseasy to detect in the field. Moreover, fruiting bodiesof most species are relatively ephemeral and do notpersist in nature for very long. Myxomycetes alsospend a portion of their life cycle as true eukaryoticmicroorganisms, when their very presence in agiven habitat can be exceedingly difficult, if notimpossible, to determine.

Life CycleThe life cycle of a myxomycete involves two very

different trophic (or feeding) stages, one consistingof uninucleate (single-nucleus) amoeboid cells, withor without flagella, and the other consisting of adistinctive multinucleate structure, the plasmodium.Under favorable conditions, the plasmodium givesrise to one or more fruiting bodies containing spores.The fruiting bodies produced by myxomycetes aresomewhat suggestive of those produced by somefungi, although they are considerably smaller (usu-ally no more than 1-2 millimeters tall). The sporesof myxomycetes are for most species apparentlywind-dispersed and complete the life cycle by ger-minating to produce the uninucleate amoeboid cells.These cells feed and divide by binary fission tobuild up large populations in the various habitatsin which these organisms occur.

The transformation from one trophic stage to theother in the myxomycete life cycle is in most casesthe result of fusion between compatible amoeboidcells, which thus function as gametes. The fusion ofthe two cells produces a diploid zygote that feeds,grows, and undergoes repeated mitotic nuclear di-visions to develop into the plasmodium. Bacteriarepresent the primary food resource for bothtrophic stages, but plasmodia are also known tofeed upon yeasts, cyanobacteria, and fungal spores.

Myxomycete plasmodia usually occur in situa-tions in which they are relatively inconspicuous,but careful examination of the inner surface of dead

bark on a fallen log or the lower surface of a piece ofcoarse woody debris on the ground in a forest, es-pecially after a period of rainy weather, often willturn up an example or two. Most of the plasmodiaencountered in nature are relatively small, but somespecies are capable of producing a plasmodiumthat can reach a size of more than 1 meter across.Under adverse conditions, such as drying out ofthe immediate environment or low temperatures,a plasmodium may convert into a hardened, resis-tant structure called a sclerotium, which is capableof reforming the plasmodium upon the return offavorable conditions. Moreover, the amoeboid cellscan undergo a reversible transformation to dor-mant structures called microcysts. Both sclerotia andmicrocysts can remain viable for long periods oftime and are probably very important in the contin-ued survival of myxomycetes in some habitats,such as deserts.

Structure of Fruiting BodiesIdentification of myxomycetes is based almost

entirely upon features of the fruiting bodies pro-duced by these organisms. Fruiting bodies (alsosometimes referred to as “sporophores” or “sporo-carps”) occur in four generally distinguishableforms or types, although there are a number of spe-cies that regularly produce what appears to be acombination of two types. The most common typeof fruiting body is the sporangium, which may besessile or stalked, with wide variations in color andshape. The actual spore-containing part of the spo-rangium (as opposed to the entire structure, whichalso includes a stalk in those forms characterized bythis feature) is referred to as a sporotheca. Sporangiausually occur in groups, because they are derivedfrom separate portions of the same plasmodium.

A second type of fruiting body, an aethalium, is acushion-shaped, sessile structure. Aethalia are pre-sumed to be masses of completely fused sporan-gia and are relatively large, sometimes exceedingseveral centimeters in extent. A third type is thepseudoaethalium (literally, a false aethalium). Thistype of fruiting body, which is comparatively un-common, is composed of sporangia closely crowdedtogether. Pseudoaethalia are usually sessile, al-though a few examples are stalked. The fourth typeof fruiting body is called a plasmodiocarp. Almost al-ways sessile, plasmodiocarps take the form of themain veins of the plasmodium from which theywere derived.

Plasmodial slime molds • 855

A typical fruiting body consists of as many assix major parts: hypothallus, stalk, columella, peri-dium, capillitium, and spores. Not all of these partsare present in all types of fruiting bodies. The hypo-thallus is a remnant of the plasmodium sometimesfound at the base of a fruiting body. The stalk (alsocalled a stipe) is the structure that lifts the sporo-theca above the substrate. As already noted, somefruiting bodies are sessile and thus lack a stalk.The peridium is a covering over the outside of thesporotheca that encloses the actual mass of spores.It may or may not be evident in a mature fruitingbody. The peridium may split open along clearlydiscernible lines of dehiscence, as a preformed lid,or in an irregular pattern. In an aethalium, the rela-tively thick covering over the spore mass is referredto as a cortex rather than a peridium. The columella isan extension of the stalk into the sporotheca, al-though it may not resemble the stalk.

The capillitium consists of threadlike elementswithin the spore mass of a fruiting body. Many spe-cies of myxomycetes have a capillitium, either as asingle connected network or as many free elementscalled elaters. The elements of the capillitium maybe smooth, sculptured, or spiny, or they may ap-pear to consist of several interwoven strands. Someelements may be elastic, allowing for expansionwhen the peridium opens, while other types are hy-groscopic and capable of dispersing spores by atwisting motion. Spores of myxomycetes are quitesmall and range in size from slightly less than 5 tooccasionally more than 15 micrometers. Nearly allof them appear to be round, and most are orna-mented to some degree. Spore size and also colorare very important in identification. Spores can bedark or light to brightly colored.

Occurrence in NatureThere are approximately eight hundred recog-

nized species of myxomycetes, and these have beenplaced in six different taxonomic orders: Cerat-iomyxales, Echinosteliales, Liceales, Physarales,Stemonitales, and Trichiales. However, members of

the Ceratiomyxales are distinctly different frommembers of the other orders, and many modernbiologists have removed these organisms from themyxomycetes and reassigned them to anothergroup of slime molds, the protostelids. The majorityof species of myxomycetes are probably cosmopoli-tan, and at least some species apparently occur inany terrestrial ecosystem with plants (and thusplant detritus) present. However, a few species doappear to be confined to the tropics or subtropics,and others have been collected only in temperateregions. Compared to most other organisms, myx-omycetes show very little evidence of endemism,with the same species likely to be encountered inany habitat on earth where the environmental con-ditions suitable for its growth and developmentapparently exist.

Although the ability of a plasmodium to migratesome distance from the substrate upon or withinwhich it developed has the potential of obscuringmyxomycete-substrate relationships, fruiting bod-ies of particular species of myxomycetes tend to berather consistently associated with certain types ofsubstrates. For example, some species almost al-ways occur on decaying wood or bark, whereasothers are more often found on dead leaves andother plant debris and only rarely occur on wood orbark. In addition to these substrates, myxomycetesalso are known to occur on the bark surface of livingtrees, on the dung of herbivorous animals, in soil,and on aerial portions of dead but still-standingherbaceous plants. The myxomycetes associatedwith decaying wood are the best known, becausethe species typically occurring on this substratetend to be among those characteristically produc-ing fruiting bodies of sufficient size to be detectedin the field. Many of the more common and widelyknown myxomycete taxa, including various spe-cies of Arcyria, Lycogala, Stemonitis, and Trichia, arepredominantly associated with decaying wood.

Steven L. Stephenson

See also: Bacteria; Protista.

Sources for Further StudyFarr, Marie L. How to Know the True Slime Molds. Dubuque, Iowa: Wm. C. Brown, 1981. A

good general introduction to myxomycetes, with illustrated keys to many of the speciesfound in North America.

Ing, Bruce. The Myxomycetes of Britain and Ireland. Slough, England: Richmond, 1999. Alargely taxonomic treatment of the species known from Britain and Ireland but with in-formation on ecology and the techniques used in collecting and studying myxomycetes.

856 • Plasmodial slime molds

Katsaros, Peter. Illustrated Guide to Common Slime Molds. Eureka, Calif.: Mad River Press,1989. Essentially a collection of photographs of some of the more common species, withsome general information on structure and ecology.

Keller, Harold W., and Karl L. Braun. Myxomycetes of Ohio: Their Systematics, Biology, and Usein Teaching. Columbus: Ohio Biological Survey, 1999. A primarily taxonomic account ofthe species known to occur in Ohio.

Martin, George W., and Constantine J. Alexopoulos. The Myxomycetes. Iowa City: Universityof Iowa Press, 1969. The standard monograph on myxomycete taxonomy, now out of print.

Stephenson, Steven L., and Henry Stempen. Myxomycetes: A Handbook of Slime Molds. Port-land, Oreg.: Timber Press, 2000. Arelatively nontechnical but fairly comprehensive intro-duction to the biology, ecology, and taxonomy of the myxomycetes.

POISONOUS AND NOXIOUS PLANTS

Categories: Diseases and conditions; medicine and health; poisonous, toxic, and invasive plants

Poisonous plants have evolved toxic substances that function to defend them against herbivores and thereby better adaptthem for survival.

After evolving adaptations that facilitated colo-nization of terrestrial habitats, plants were

confronted with a different type of problem. Thiswas the problem of herbivory, or the inclination ofmany different types of organisms, from bacteria toinsects to four-legged herbivores, to eat plants.Pressures from herbivory drove many differenttypes of plants, from many different families, toevolve defenses. Some of these defenses includedchanges in form, such as the evolution of thorns,spikes, or thicker, tougher leaves. Other plantsevolved to produce chemical compounds thatmake them taste bad, interrupt the growth and lifecycles of the herbivores, make the herbivores sick,or kill them outright.

PhytochemicalsOne of the most interesting aspects of plants, es-

pecially prevalent in the angiosperms (floweringplants), is their evolution of substances called sec-ondary metabolites, sometimes referred to as phyto-chemicals. Once considered waste products, thesesubstances include an array of chemical com-pounds: alkaloids, quinones, essential oils, terpe-noids, glycosides (including cyanogenic, cardio-active, anthraquinone, coumarin, and saponinglycosides), flavonoids, raphides (also called oxa-lates, which contain needle-like crystals of calcium

oxalate), resins, and phytotoxins (highly toxic pro-tein molecules). The presence of many of thesecompounds can characterize whole families, oreven genera, of flowering plants.

Effects on HumansThe phytochemicals listed above have a wide

range of effects. In humans, some of these com-pounds will cause mild to severe skin irritation, orcontact dermatitis; others cause mild to severe gas-tric distress. Some cause hallucinations or psychoac-tive symptoms. The ingestion of many other types ofphytochemicals proves fatal. Interestingly, many ofthese phytochemicals also have important medicaluses. The effects of the phytochemicals are depen-dent on dosage: At low doses, some phytochemicalsare therapeutic; at higher doses, some can kill.

AlkaloidsAlkaloids are nitrogenous, bitter-tasting com-

pounds of plant origin. More than three thousandalkaloids have been identified from about fourthousand plant species. Their greatest effects aremainly on the nervous system, producing eitherphysiological or psychological results. Plant fami-lies producing alkaloids include the Apocynaceae,Berberidaceae, Fabaceae, Papaveraceae, Ranunculaceae,Rubiaceae, and Solanaceae. Some well-known alka-

Poisonous and noxious plants • 857

loids include caffeine, cocaine, ephedrine, mor-phine, nicotine, and quinine.

GlycosidesGlycosides are compounds that combine a sugar,

usually glucose, with an active component. Whilethere are many types of glycosides, some of themost important groups of potentially poisonousglycosides include the cyanogenic, cardioactive, an-thraquinone, coumarin, and saponin glycosides.

Cyanogenic glycosides are found in many mem-bers of the Rosaceae and are found in the seeds, pits,and bark of almonds, apples, apricots, cherries,peaches, pears, and plums. When cyanogenicglycosides break down, they release a compoundcalled hydrogen cyanide.

Two other types of glycosides, cardioactive gly-

cosides and saponins, feature a steroid molecule aspart of their chemical structure. Digitalis, a cardio-active glycoside, in the right amounts can strengthenand slow the heart rate, helping patients who sufferfrom congestive heart failure. Other cardioactiveglycosides from plants such as milkweed and ole-ander are highly toxic. Saponins can cause severeirritation of the digestive system and hemolyticanemia. Anthraquinone glycosides exhibit purga-tive activities. Plants containing anthraquinoneglycosides include rhubarb (Rheum species) andsenna (Cassia senna).

Household PlantsMany common household plants are poisonous

to both humans and animals. One family of popularhousehold plants that can cause problems is the

858 • Poisonous and noxious plants

Common Poisonous Plants and FungiCommon Name Scientific Name

Aconite Aconitum spp.Alfalfa Medicago sativaAmaryllis (bulbs) Hippeastrum puniceumAnemone Anemone tuberosumAngel’s trumpet Datura spp.Apple (seeds, leaves) Malus sylvestrisApricot (seeds, leaves) Prunus armeniaceaArrowgrass Triglochin maritimaAsparagus (berries) Asparagus officinalisAzalea Rhododendron spp.Baneberry Actaea spp.Belladonna Atropa belladonnaBirdsfoot trefoil Lotus corniculatusBitter cherry Prunus spp.Black cherry Prunus spp.Black locust Robinia pseudoacaciaBleeding heart Dicentra spp.Bloodroot Sanquinaria canadensisBouncing bet Saponaria spp.Bracken fern Pteridium aquiliniumBroad beans Vicia spp.Buckeye Aesculus spp.Buckwheat Fagoypyrum esculentumBuffalo bur Solanum spp.Buttercups Ranunculus spp.Caladium Caladium bicolorCaley pea Lathyrus spp.Cardinal flower Lobelia spp.Castor bean Ricinus communisCelandine Chelidonium majus


Choke cherry Prunus spp.Christmas rose Helleborus nigerClovers

(alsike, red, white)Trifolium spp.

Cocklebur Xanthium strumariumCorn cockle Agrostemma githagoCorn lily Veratrum californicumCow cockle Saponaria spp.Creeping charlie Glechoma spp.Croton Croton spp.Crowfoot Ranunculus spp.Crown-of-thorns Euphorbia miliiCrown vetch Coronilla variaDaffodil (bulbs) Narcissus pseudonarcissusDaphne Daphne spp.Datura Datura spp.Deadly nightshade Atropa belladonnaDeath angel mushroom Amanita spp.Death camas Zigadenus spp.Death cap mushroom Amanita spp.Delphiniums and

larkspursDelphinium spp.

Destroying angels Amanita vernaDevil’s trumpet Datura spp.Dock Rumex spp.Dogbane Apocynum spp.Dolls eyes Actaea spp.Downy thornapple Datura spp.Drooping leucothoe Leucothoe axillarisDutchman’s breeches Dicentra spp.

Araceae, the philodendron family, including plantssuch as philodendron and dieffenbachia. All mem-bers of this family, including these plants, containneedlelike crystals of calcium oxalate that, when in-gested, cause painful burning and swelling of thelips, tongue, mouth, and throat. This burning andswelling can last for several days, making talkingand even breathing difficult. Dieffenbachia is oftenreferred to by the common name of dumb cane, be-cause eating it makes people unable to talk for a fewdays.

Landscape PlantsMany landscape plants are also poisonous. For

example, the yew (genus Taxus), commonly plantedas a landscape plant, is deadly poisonous. Children

who eat the bright red aril, which contains the seed,are poisoned by the potent alkaloid taxine. Yewsare poisonous to livestock as well, causing death tohorses and cattle. Death results from cardiac or re-spiratory failure.

Other poisonous landscape and garden plantsinclude oleander, rhododendrons, azaleas, hya-cinths, lily of the valley, daffodils, tulips, and star-of-Bethlehem. Many legumes are also toxic, includ-ing rosary pea, lupines, and wisteria. Castor beanplant, a member of the family Euphorbiaceae, pro-duces seeds that are so toxic that one seed will kill achild and three seeds are fatal to adults. The toxinproduced by the seeds is called ricin, which manyscientists consider to be the most potent naturaltoxin known.



Eastern skunk cabbage Symplocarpus foetidusEggplant (leaves, stems) Solanum melongenaElderberry Sambucus canadensisErgot Claviceps spp.Everlasting pea Lathyrus spp.False hellbore Veratrum californicumFiddleneck Amsinckia intermediaFlax Linum usitatissimumFly agaric Amanita muscariaFoxglove Digitalis purpureaGill over the ground Glechoma spp.Gloriosa lily Gloriosa spp.Golden chain Laburnum anagyroidesGreat lobelia Lobelia spp.Ground ivy Glechoma spp.Groundsels Senecio spp.Halogeton Halogeton glomeratusHenbane Hyoscyanamus nigerHolly (berries) Ilex spp.Horse chestnut Aesculus spp.Horse nettle Solanum spp.Horsebrush Tetradymia spp.Horsetail Equisetum arvense

& other spp.Hyacinth (bulbs) Hyacinthus orientalisHydrangea Hydrangea spp.Indian tobacco Lobelia spp.Irises (leaves, rhizomes) Iris spp.Ivy (leaves, berries) Hedera helixJack in the pulpit Arisaema spp.Japanese pieris Pieris japonicaJessamine Gelsemium sempervirens


Jimsonweed Datura spp.Johnson grass Sorghum spp.Klamath weed Hypericum perforatumLaburnum Laburnum anagyroidesLamb’s quarters Chenopodium albumLantana Lantana camaraLarkspur Delphinium spp.Lily of the valley Convallaria majalisLobelia Lobelia cardinalisLocoweed Astragalus, Oxytropis spp.Lucerne Medicago sativaLupine Lupinus spp.Mandrake Podophyllum peltatumMarijuana Cannabis sativaMarsh marigold

or cowslipCaltha palustris

Mayapple Podophyllum peltatumMilkweed Asclepias spp.Mistletoe Phoradendron spp.Monkey agaric

mushroomAmanita spp.

Monkshood Aconitum spp.Moonseed Menispermum canadenseMorning glory (seeds) Ipomoea tricolorMountain fetterbrush Pieris spp.Mountain laurel Kalmia latifoliaNarcissus (bulbs) Narcissus spp.Nightshades Solanum spp.Oak trees Quercus spp.Oleander Nerium oleanderPanther Amanita pantherinaPanther cap mushroom Amanita spp.

(continued)

Arrow PoisonsToxic plant and animal products have been used

for thousands of years in hunting, executions, andwarfare. Usually the poisonous extracts weresmeared on arrows or spears. The earliest reliablewritten evidence for these uses comes from theRigveda from ancient India. Arrow poisons come inmany different varieties, and most rain-forest hunt-ers have their own secret blend. South American ar-row poisons are generically called curare. There aremore than seventy different plant species used inmaking arrow poisons. Two of the main arrow poi-son plants are woody vines from the Amazon:Strychnos toxifera and Chondodendron tomentosum.Some types of curare have proven medically useful.

They are used as muscle relaxants in surgery,which lessens the amount of general anestheticneeded. A plant called Strychnos nux-vomica fromAsia yields the poison strychnine, a stimulant of thecentral nervous system.

In ancient times, toxic plant products were alsocommonly used in executions. Many people wereexpert, professional poisoners in the ancient world.They could select a poison that would take days oreven months to take effect, thus ensuring, for exam-ple, that an unfaithful spouse or lover would notsuspect the reason for his or her lingering illness.On occasions when a more rapid result was re-quired, a strong dose or more powerful poisoncould be prescribed.

860 • Poisonous and noxious plants


Peach (leaves, seeds) Prunus persicaPhilodendron Philodendron spp.Pigweed Amaranthus spp.Pin cherry Prunus spp.Poinsettia Euphorbia spp.Poison hemlock Conium maculatumPoison ivy Toxicodendron radicansPoison oak Toxicodendron diversilobaPoison sumac Toxicodendron vernixPokeweed Phytolacca americanaPonderosa pine Pinus ponderosaPoppies (inc. opium) Papaver spp.Potato Solanum spp.Prickly (Mexican) poppy Argemone mexicanaPrivet (leaves, berries) Ligustrum japonicumRagworts Senecio spp.Red sage Lantana camaraRhodendron Rhodendron spp.Rhubarb (leaves) Rheum rhaponticumRosary pea Abrus precatoriusSenecio Senecio spp.Sensitive fern Onoclea sensibilisSierra laurel Leucothoe davisiaeSingletary pea Lathyrus spp.Snakeberry Actaea spp.Snow on the mountain Euphorbia spp.Sorghum or milo Sorghum spp.Spurges Euphorbia spp.


Squirrel corn Dicentra spp.St. John’s wort Hypericum perforatumStar of Bethlehem Ornithogalum umbellatumStinging nettle Urtica spp.Sudan grass Sorghum spp.Sweet pea Lathyrus spp.Tall fescue Festuca arundinaceaTangier pea Lathyrus spp.Tobacco Nicotiana spp.Tomato (leaves, stems) Lycopersicon lycopersicumTree tobacco Nicotiana spp.Tung oil tree Aleurites fordiiVetches Vicia spp.Virginia creeper

(berries)Parthenocissus quinquefolia

Water hemlock/cowbane

Cicuta spp.

West Indian lantana Lantana camaraWhite cohosh Actaea spp.White snakeroot Eupatorium rugosumWhite sweetclover Metilotus albaWild cherries Prunus spp.Wisteria (pods, seeds) Wisteria spp.Wolfbane Aconitum spp.Yellow sage Lantana camaraYellow star thistle Centaurea solstitialisYellow sweetclover Metilotus officinalisYew Taxus cuspidata

Note: spp. = species; This is a partial listing only: Parts of some plants that are particularly poisonous are identified in parentheses.However, it should not be assumed that other parts are necessarily benign or that plants not listed here are edible.

Source: Data are from Cornell University Poisonous Plants Informational Database, compiled by Dan Brown and staff, http://www.ansci.cornell.edu/plants/comlist.html (accessed April 11, 2002), and from Brian Capon, Botany for Gardeners: AnIntroduction and Guide (Portland, Oreg.: Timber Press, 1990), p. 96.

Common Poisonous Plants and Fungi (continued)

Poison IvyToxicodendron radicans, commonly known as poi-

son ivy, is well known for causing contact dermati-tis. Poison ivy is a member of the Anacardiaceae, orcashew family, and is a widespread weed in theUnited States and southern Canada. It grows in avariety of habitats: wetlands, disturbed areas, andthe edges of forests. It has many forms, appearingas either a shrub or a woody vine which will growup trees, houses, fences, and fence posts. It has al-ternate leaves with three leaflets, forming the basisof the old saying “Leaves of three, let it be.” Afterpoison ivy flowers, it develops clusters of white oryellowish-white berries. Related species are poisonoak, western poison oak, and poison sumac, whichsome scientists consider to be different types of poi-son ivy.

Roughly half the world’s population is allergicto poison ivy. Very sensitive people develop a se-vere skin rash; about 10 percent of the people whoare allergic require medical attention after expo-

sure. The chemical compound causing the allergicreaction is called urushiol, a resin found in all partsof the plant. Urushiol is so potent that in some indi-viduals, just one drop produces a reaction. Inhalingsmoke from burning poison ivy can result in eyeand lung damage. For some people, mere contactwith the smoke from burning poison ivy can triggera reaction. Urushiol lasts forever; in herbaria, driedplants one hundred years old have given unluckybotanists contact dermatitis.

Carol S. Radford

See also: Allelopathy; Animal-plant interactions;Ascomycetes; Bacterial resistance and super bacte-ria; Basidiomycetes; Basidiosporic fungi; Biochemi-cal evolution in angiosperms; Biological invasions;Biological weapons; Carnivorous plants; Coevolu-tion; Culturally significant plants; Deuteromycetes;Dinoflagellates; Horsetails; Legumes; Medicinalplants; Metabolites: primary vs. secondary; Mush-rooms; Parasitic plants.


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Sources for Further StudyBurrows, George E., and Ronald J. Tyrl. Toxic Plants of North America. Ames: Iowa State Uni-

versity Press, 2001. Comprehensive reference of toxic plants found in North America.Covers plant morphology and distribution, toxicants, treatments, and associate diseases.Includes illustrations and maps.

Levetin, Estelle, and Karen McMahon. Plants and Society. 2d ed. Boston: WCB/McGraw-Hill, 1999. An introductory text on the science of ethnobotany and uses of plants. Includessome basic botany.

Lewis, Walter H., and Memory P. F. Elvins-Lewis. Medical Botany: Plants Affecting Man’sHealth. New York: John Wiley and Sons, 1997. Athorough discussion and listing of medic-inal and toxic plants.

Simpson, Beryl B., and Molly Conner Ogarzaly. Economic Botany: Plants in Our World. 3d ed.Boston: McGraw-Hill, 2000. An introductory text in basic botany and ethnobotany.

POLLINATION

Categories: Physiology; reproduction

Pollination involves the transfer of pollen from anther to stigma in flowering plants, or from male cone to ovules in gym-nosperms. There are two different types of pollination: self-pollination and cross-pollination.

Pollination is the process, in sexually reproduc-ing plants (both angiosperms and gymno-

sperms), whereby the male sperm and female eggare joined via transfer of pollen (male microspore). Ifthe anthers and stigmas of the plants involved havethe same genetic makeup or they are produced onthe same plant, the type of pollination is called self-pollination. If anthers and stigmas are from plantswith different genetic makeups, the type of pollina-tion is called cross-pollination.

Self-pollination is efficient because pollen fromthe anther of a flower can be transferred easily ontothe stigma of the same flower, owing to the proxim-ity of the two parts. On the other hand, cross-pollination is risky because the transfer of pollen in-volves long distances and precise destinations,both of which depend on animal pollinators. In ar-eas with few animal pollinators, the opportunitiesfor cross-pollination may be greatly reduced (one ofthe many reasons that preserving biological diver-sity is an important ecological issue).

In spite of the risk associated with cross-pollination, most flowers have mechanisms thatpromote this kind of pollination. Cross-pollinationincreases the likelihood that offspring are vigorous,healthy, fertile, and able to survive even if the en-

vironment changes. Self-pollination leads to off-spring that are less vigorous, less productive, andmore subject to inbreeding depression (weakening ofthe offspring as a result of inbreeding).

When certain consumers forage among plantsfor food, they often come in contact with flowers.Many insects and other animals become dustedwith pollen, and in the course of their travel theyunintentionally but effectively bring about pollina-tion. Throughout the evolutionary history of flow-ering plants, many pollinators have coevolved withplants. Coevolution occurs when the floral parts ofa plant and the body parts and behavior of thepollinators become mutually adapted to each other,thereby increasing the effectiveness of their interac-tion. In many instances, the relationship betweenthe plant and pollinator has become highly special-ized, resulting in mutualism, which is interactionwhere both organisms benefit from each other.

In the case of pollination by animals, the polli-nator receives a reward from the flower in the formof food. When the pollinator moves on, the plant’spollen is transferred to another plant. The adapta-tions between the flower and its pollinators can beintricate and precise and may even involve force,drugs, deception, or sexual enticement. In flower-

862 • Pollination

ing plants, pollination is mostly due to insects orwind, but birds, bats, and rodents also act as polli-nators for a number of plants.

InsectsInsect pollination occurs in the majority of flow-

ering plants. There is no single set of characteris-tics for insect-pollinated flowers, because insectsare a large and diverse group of animals. Rather,each plant may have a set of reproductive fea-tures that attracts mostly a specific species of insect.The principal pollinating insects are bees, althoughmany other kinds of insects act as pollinators, in-cluding wasps, flies, moths, butterflies, ants, andbeetles.

Bees have body parts suitable for collecting andcarrying nectar and pollen. Their chief source ofnourishment is nectar, but they also collectpollen for their larvae. The flowers that beesvisit are generally brightly colored and pre-dominantly blue or yellow—rarely pure red,because red appears black to bees. The flow-ers they visit often have distinctive markingsthat function as guides that lead them to thenectar. Bees can perceive ultraviolet (UV) light(a part of the spectrum not visible to humans),and some flower markings are visible only inUV light, making patterns perceived by beessometimes different from those seen by hu-mans. Many bee-pollinated flowers are deli-cately sweet and fragrant.

Moth- and butterfly-pollinated flowers aresimilar to bee-pollinated flowers in that theyfrequently have sweet fragrances. Some but-terflies can detect red colors, and so red flow-ers are sometimes pollinated by them. Manymoths forage only at night; the flowers theyvisit are usually white or cream-colored be-cause these colors stand out against dark back-grounds in starlight or moonlight. With theirlong mouthparts, moths and butterflies arewell adapted for securing nectar from flowerswith long, tube-shaped corollas (the petals col-lectively), such as larkspur, nasturtium, to-bacco, evening primrose, and amaryllis.

The flowers pollinated by beetles tend tohave strong, yeasty, spicy, or fruity odors.They are typically white or dull in color, inkeeping with the diminished visual sense oftheir pollinators. Although some beetle-pollinated flowers do not secrete nectar, they

furnish pollen or other foods which are available onthe petals in special storage cells.

BirdsBirds and the flowers that they pollinate are also

adapted to each other. Birds do not have a highlydeveloped sense of smell, but they have a keensense of vision. Their flowers are thus frequentlybright red or yellow and usually have little, if any,odor. The flowers are typically large or are part ofa large inflorescence. Birds are highly active pol-linators and tend to use up their energy very rap-idly. Therefore, they must feed frequently to sustainthemselves. Many of the flowers they visit producecopious quantities of nectar, assuring the birds’continued visitation. The nectar is frequently pro-duced in long floral tubes, which prevent most

Pollination • 863P

ho

toD

isc

In a classic example of cross-pollination, this honeybee gathersnectar while inadvertently providing pollen from another plantto the ovule of this sunflower.

insects from gaining access to it. Examples of bird-pollinated flowers are red columbine, fuchsia, scar-let passion flower, eucalyptus, hibiscus, and poin-settia.

Bats and RodentsBat-pollinated flowers are found primarily in the

tropics, and they open only at night, when the batsare foraging. These flowers are dull in color, andlike bird-pollinated flowers, they are large enoughfor the pollinator to insert part of its head inside.The plants may also consist of ball-like inflores-cences containing large numbers of small flowerswhose stamens readily dust the visitor with pollen.

Bat-pollinated flowers includebananas, mangoes, kapok, and si-sal. Like moth-pollinated flowers,flowers that attract bats and smallrodents open at night. Mammal-pollinated flowers are usuallywhite and strongly scented, oftenwith a fruity odor. Such flowersare large, to provide the pollina-tors enough pollen and nectar tofulfill their energy requirements.The flowers are also sturdy, tobear the frequent and vigorousvisits of these small mammals.

Orchid PollinatorsThe orchid family has pollina-

tors among bees, moths and but-terflies, and beetles. Some of theadaptations between orchid flow-ers and their pollinators are ex-traordinary. Many orchids pro-duce their pollen in little sacscalled pollinia, which typicallyhave sticky pads at the bases.When a bee visits such a flower,the pollinia are usually depositedon its head. In some orchids, thepollinia are forcibly “slapped” onthe pollinator through a triggermechanism within the flower. Insome orchids, a petal is modi-fied so that it resembles a femalewasp or bee. Male wasps or beesemerge from their pupal stagebefore the females and can mis-take the orchids for potential

mates. They try to copulate with these flowers, andwhile they are doing so, pollinia are deposited ontheir heads. When the wasps or bees visit otherflowers, the pollinia are caught in sticky stigma cav-ities.

When moths and butterflies pollinate orchids,the pollinia become attached to their long tonguesby means of sticky clamps instead of pads. Thepollinia of certain bog orchids become attached tothe eyes of the female mosquitoes that pollinatethem. After a few visits, the mosquitoes are blindedand unable to continue their normal activities (agood example of a biological control within an eco-system).

864 • Pollination

FilamentAnther

Stamen

Locule

Ovary wall(pericarp)

Ovulecontainingegg cells

Style

Anther atopfilament(= stamen)

Pollengrains

Pollentube

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Pollination

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Pollination occurs as either self-pollination (shown here) or cross-pollination,in which the pollen from one plant is dispersed via wind, water, or animals topollinate another plant. In both cases, pollen lands on the stigma and grows apollen tube down through the style toward the ovule in the ovary, where it de-posits the sperm into an egg cell inside the ovule.

Among the most bizarre of the orchid pollina-tion mechanisms are those whose effects are todunk the pollinator in a pool of watery fluid se-creted by the orchid itself and then permit thepollinator to escape underwater through a trapdoor. The route of the insect ensures contact be-tween the pollinia and stigma surfaces. In other or-chids with powerful narcotic fragrances, polliniaare slowly attached to the drugged pollinator.When the transfer of pollinia has been completed,the fragrance abruptly fades away, and the insectrecovers and flies away.

Wind and WaterWind pollination is common in those plants with

inconspicuous flowers, such as grasses, poplars,walnuts, alders, birches, oaks, and ragweeds. Theseplants lack odor and nectar and are, hence, unat-tractive to insects. Furthermore, the petals are ei-ther small or absent, and the sex organs are oftenseparate on the same plant. In grasses, the stigmasare feathery and expose a large surface to catch pol-len, which is lightweight, dry, and easily blown bythe wind. Because wind-pollinated flowers do notdepend on animals to transport their pollen, theydo not invest in the production of rewards for their

visitors. However, they have to produce enormousquantities of pollen. Wind pollination is not effi-cient because most of the pollen does not end up onthe stigmas of appropriate plants but on theground, bodies of water, and in people’s noses (amajor cause of allergic reactions). Wind pollinationis successful in cases where a large number of indi-viduals of the same species grow fairly close to-gether, as in grasslands and coniferous forests.

Water pollination is rare, simply because fewerplants have flowers that are submerged in water.Such plants include the sea grasses, which releasepollen that is carried passively by water currents. Insome plants, such as the sea-nymph, pollen isthreadlike, thus increasing its chances of coming incontact with stigmas. In eelgrass, the entire maleflower floats.

Danilo D. Fernando

See also: Animal-plant interactions; Angiospermevolution; Angiosperm life cycle; Aquatic plants;Biochemical coevolution in angiosperms; Coevolu-tion; Flower structure; Flower types; Metabolites:primary vs. secondary; Orchids; Population genet-ics; Reproduction in plants; Reproductive isolatingmechanisms.

Sources for Further StudyBarth, Friedrich G. Insects and Flowers: The Biology of a Partnership. Princeton, N.J.: Princeton

University Press, 1991. Describes the relationships of plants and insects with interestingemphasis on the pollinators. Includes photographs.

Proctor, Michael, Peter Yeo, and Andrew Lack. The Natural History of Pollination. Portland,Oreg.: Timber Press, 1996. Describes in detail mechanisms of pollination. Includes photo-graphs.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Contains a chapter that integrates pollination with the evo-lution of angiosperms. Includes photographs.

POLYPLOIDY AND ANEUPLOIDY

Categories: Genetics; reproduction and life cycles

In aneuploidy, one or more whole chromosomes has been lost or gained from the diploid state. In polyploidy, one or morecomplete sets of chromosomes has been gained from the usual state (generally diploid), as in triploidy and tetraploidy.

For each species of higher plants and animals, thebase number of nuclear chromosomes is called

the haploid number, denoted as n. Individuals ofmost species are diploid, having double the haploid

Polyploidy and aneuploidy • 865

number of chromosomes (2n) in each somatic cell.Aneuploid and polyploid organisms have abnor-mal numbers of whole chromosomes.

AneuploidyStrictly speaking, aneuploidy refers to any num-

ber of chromosomes in a cell or organism that is notan exact multiple of the haploid number. However,in common practice the term is used to refer specifi-cally to situations in which an organism or cell hasonly one chromosome or a few chromosomesadded or missing. In animals, aneuploidy is usuallylethal and so is rarely encountered. In the plantkingdom, on the other hand, the addition or elimi-nation of a small number of individual chromo-somes may be better tolerated.

Nullisomy is the aneuploid condition in whichtwo homologous chromosomes are missing, so thatthe organism has 2n – 2 chromosomes. Monosomyrefers to the absence of a single chromosome, giv-ing 2n – 1 total chromosomes. In trisomy, one extrachromosome is present (2n + 1). An example ofaneuploidy in humans is the case of Down syn-drome, trisomy-21, in which the individual has oneextra copy of the twenty-first chromosome (thus,three total copies).

Aneuploidy is caused by nondisjunction, whichoccurs when a pair of homologous chromosomesfail to separate during cell division. If nondisjunc-tion occurs in the first stage of meiosis, all four re-sulting gametes will be abnormal. Two of them willhave no copy of the given chromosome, and two,correspondingly, will have one extra copy each. Ifnondisjunction occurs in the second stage of meio-sis, one of the four resulting gametes will have nocopy of the given chromosome, another will havean extra copy, and two will be normal.

PolyploidyPolyploidy is caused by the addition of one or

more complete chromosome sets to the normal dip-loid complement. In the animal kingdom poly-ploidy is lethal in nearly every case, but it is rela-tively common in plants. It is estimated thatbetween 30 percent and 70 percent of extant angio-sperms are polyploid. The process of sex determi-nation is more sensitive to polyploidy in animalsthan in plants, and because many plants undergoself-fertilization, those with an even number ofchromosome sets (such as those that are tetraploid)may still produce fertile gametes. This fact points to

the crucial factor that determines whether a poly-ploid plant may be fertile: whether it has an even oran odd number of chromosome sets. Plants with anodd number of chromosome sets are almost alwayssterile. Because they always have an unpaired chro-mosome of each type, it is extremely unlikely forthem to produce viable balanced gametes. On theother hand, there is potential for a polyploid plantwith an even number of chromosome sets to pro-duce a balanced gamete if multiple sets of conspe-cific chromosomes pair during meiosis.

Autopolyploidy and AllopolyploidyPolyploid plants exist in two categories. Auto-

polyploids have a genome comprising multiple setsof chromosomes that are all from one species. In al-lopolyploids, the multiple sets of chromosomes comefrom multiple (usually related) species. Auto-polyploidy can arise from situations in which adefect in meiosis creates a diploid or triploid ga-mete. If such a gamete is fused with a typical hap-loid gamete from the same species, the union leadsto a polyploid zygote.

The most common such pairing, of a diploid anda haploid gamete, produces an autotriploid. As theprevious section suggests, they are usually sterile.However, some sterile autotriploids that can be cul-tivated through vegetative propagation (by plant-ing cuttings) are attractive food crops because theylack robust fertile seeds. For example, the culti-vated banana is an effectively seedless (and there-fore sterile) autotriploid; it cannot reproduce with-out human intervention.

Allopolyploidy arises through the interbreedingof different species. An allodiploid formed by theunion of two haploid gametes from separate spe-cies will be sterile because there are no matchingchromosomes to pair at meiosis. However, if thetwo sets of chromosomes become doubled within acell, the result will be a potentially fertile allo-tetraploid. An organism in this condition is basi-cally a double-diploid, in which homologous pairsof conspecific chromosomes can join at meiosis toproduce a viable gamete. This type of polyploidyhas played an important role in the natural historyof many plants, including wheat, the most widelycultivated cereal in the world.

Allopolyploidy and SpeciationContemporary domesticated wheat species can

be identified as belonging to one of three groups,

866 • Polyploidy and aneuploidy

based on their number of chromosomes. One grouphas fourteen chromosomes, another twenty-eight,and a third has forty-two chromosomes. Thesegroups form a series of polyploids based on a hap-loid number, n, equal to seven chromosomes. It isbelieved that these groups of domesticated wheatevolved in two major steps. First, members of a dip-loid genus Triticum (2n = fourteen chromosomes)may have hybridized with one of the diploid goatgrasses Aegilops (2n = fourteen chromosomes) toform allotetraploid species of emmer and durumwheats (4n = twenty-eight chromosomes). Then, itis believed, these species underwent a secondround of hybridization with separate goat grassspecies to form the allohexaploid (6n = forty-twochromosomes) species that is now known as breadwheat, or Triticum aestivum. Bread wheat, whichprobably appeared around eight thousand yearsago, combines desirable qualities of all three of itsdiploid relatives, including nonshattering grains, ahigh protein content in the endosperm, and goodtolerance to various environmental conditions.Allohexaploid wheat can reproduce, thanks to nor-mal meiosis, in which homologous chromosomespair to form a triploid gamete with twenty-onechromosomes.

The possibility of speciation by allopolyplidy isone danger of introducing plants to new regions.The American species of salt marsh grass Spartinaalterniflora was introduced to the south coast of En-gland around 1870, probably in ships’ ballast water.It crossed with the native salt marsh grass Spartinamaritima to form an allotetraploid species, Spartinaanglica, which overran the native grass in the twen-tieth century and colonized coastal flats so aggres-sively as to create a floral monoculture that hasproved inadequate for wintering populations ofwading birds and wildfowl.

Agricultural ApplicationsLike wheat, many of today’s other crops are

polyploid. By creating polyploid lines, plant breed-ers can introduce desirable traits cumulatively.

Among major polyploid crops are dietary staples,such as the white potato (4n = forty-eight chro-mosomes), the domestic oat (6n = forty-two chro-mosomes), the peanut (4n = forty chromosomes),textile-producing plants such as cotton (4n = fifty-two chromosomes), and the cash crops tobacco(4n = forty-eight chromosomes) and coffee, of whichexisting species range from diploid to octoploid(8n = eighty-eight chromosomes). As well as theaforementioned domesticated banana, which, as anautotriploid, is sterile and correspondingly seed-less, some varieties of cultivated apple are triploidspecies.

Beyond engineering particular traits, anotherreason it is desirable to cultivate polyploid speciesis that polyploid plant cells are usually larger thanthe corresponding diploid cells. Consequently, thepolyploid plants themselves are usually larger.Species of particularly large watermelons, mari-golds, and snapdragons have been created throughcultivation of polyploid lines.

Plant polyploidy is induced in the laboratory bytreating dividing cells with the drug colchicine. Itprevents the formation of a spindle during mitosisby disrupting the microtubules, causing the dupli-cated chromosomes to fail to separate. The mostcommon method of application is to place the rootsof a plant in a colchicine solution. Because col-chicine inhibits the actual division of cells withoutaffecting the duplication of chromosomes, whenfull rounds of mitosis commence upon removingthe roots from the colchicine, the resulting cells con-tain an extra set of chromosomes.

Alistair Sponsel

See also: Agriculture: world food supplies; Bio-technology; Chromosomes; DNA in plants; Ge-netically modified foods; Genetics: Mendelian; Ge-netics: post-Mendelian; Grains; Green Revolution;High-yield crops; Hybrid zones; Hybridization;Mitosis and meiosis; Plant biotechnology; Popula-tion genetics; Species and speciation.

Sources for Further StudyKlug, William, and Michael Cummings. Concepts of Genetics. 5th ed. Upper Saddle River,

N.J.: Prentice Hall, 1997. College-level textbook contains excellent diagrams relating toaneuploidy, engineering of polyploid lines, and the use of colchicine. Includes good pho-tographs of the karyotypes and phenotypes of various polyploid and aneuploid organ-isms.

Levetin, Estelle, and Karen McMahon. Plants and Society. 2d ed. New York: WCB/McGraw-

Polyploidy and aneuploidy • 867

Hill, 1999. Has a useful section on the evolution and breeding of domesticated wheat asan example of allopolyploidy.

Russell, Peter J. Fundamentals of Genetics. New York: HarperCollins College, 1994. College-level textbook contains a particularly thorough section on aneuploidy and the variousforms of polyploidy.

POPULATION GENETICS

Categories: Evolution; genetics; reproduction and life cycles

Population genetics is concerned with analyzing the frequencies of the alleles, or forms, of genes in a population of indi-viduals within a species. It examines how gene frequencies change across generations in response to external forces, suchas mutation, natural selection, migration, or genetic drift.

The natural process that eliminates individualsof low fitness and advances those of high fit-

ness is termed natural selection. Natural selectioncauses changes in allele frequency in a population,which is a process called evolution. Thus, popula-tion genetics combines Charles Darwin’s ideas ofnatural selection and evolution with the basic prin-ciples of genetics set forth by Gregor Mendel.

The study of population genetics has many prac-tical uses: It can help reveal how new diseases ariseor why diseases persist in living organisms by ex-amining genetic variation within populations. Pop-ulation genetics principles also can be useful asguidelines for devising strategies for improvingcrop plants. Hardy-Weinberg equilibrium providesthe framework for population genetics.

Hardy-Weinberg TheoremThe starting point for population genetics is the

Hardy-Weinberg theorem. Derived from Mendelianprinciples, it states that in the absence of mutation,selection, migration, or genetic drift, the allele fre-quencies and genotype frequencies remain con-stant from generation to generation. This law ap-plies to populations in a state of Hardy-Weinbergequilibrium, meaning large populations that arerandom-mating. If a population is not in this state,one generation of random mating restores equilib-rium.

George H. Hardy, an English mathematician,and Wilhelm Weinberg, a German biologist andphysician, independently developed this conceptin 1908. In 1903 Harvard University professor Wil-

liam E. Castle also had shown that in the absence ofselection, the composition of the randomly bred de-scendants of a population would remain constantthereafter. Castle did not, however, generalize theconcept.

More specifically, Hardy-Weinberg equilibriumis maintained in a population as long as the follow-ing assumptions are true. First, there are a large(virtually infinite) number of individuals in thepopulation. Second, there is random mating amongindividuals. Third, there are no new mutations.Fourth, there is no migration (in or out of the popu-lation). Fifth, there are no genotype-dependent dif-ferences for survival to reproductive age and trans-mission of genes to the next generation. This fifthassumption is often stated more succinctly as:There is no natural selection.

The Hardy-Weinberg theorem can also be ex-pressed mathematically in the form of an equationfor calculating genotype and phenotype frequen-cies. If two alleles at a locus, A and a, occur in a pop-ulation with frequencies p and q, respectively, then(p + q) = 1. The proportion of individuals resultingfrom random matings will occur with the followingfrequencies:

AA = p2, Aa = 2pq, and aa = q2

Putting these terms together results in theHardy-Weinberg equation:

p2 + 2pq + q2 = 1

868 • Population genetics

This simple equation can be modified in a vari-ety of ways to mathematically model what happenswhen one or more of the Hardy Weinberg assump-tions are violated. Violation of one or more of theseassumptions will result in changes in allele fre-quencies, and thus, evolution. Changes in allele fre-quencies across a limited number of generations isoften referred to as microevolution. Extending suchchanges across thousands of generations or moreresults in more extensive change and is often calledmacroevolution.

The Gene PoolThe Hardy-Weinberg equation and its modifica-

tions are used by population geneticists to describechanges in allele frequencies in gene pools. A genepool represents all the genes carried and shared byindividuals of an interbreeding population, with theassumption that each parent contributes equally toa large pool of gametes (eggs and pollen or sperm).Another assumption is that the parent and off-spring generations are distinct, or nonoverlapping.

Species and SpeciationThe gene pool of a population, when divided or

isolated for a prolonged period of time, may form

distinct subgroups. If, through isolating mechanismssuch as genetic changes or geographical separation,the subpopulations are kept from interbreeding,new species may eventually arise after many gener-ations when the isolated subpopulations evolve indifferent directions. The entire process of the split-ting of a population into two or more reproduc-tively isolated populations is termed speciation.Thus, a species, according to the commonly ac-cepted biological species concept, is a group of inter-breeding individuals that are capable of producingfertile offspring but are unable to do so with othersuch populations. Sometimes hybridization be-tween two species can result in a fertile hybrid, thusexposing one of the weaknesses of the biologicalspecies concept. Speciation can be the result of ei-ther geographic separation or reproductive isola-tion at the cellular and molecular levels.

Manjit S. Kang

See also: Genetic drift; Genetics: Mendelian; Genet-ics: mutations; Genetics: post-Mendelian; Hardy-Weinberg theorem; Hybrid zones; Hybridization;Polyploidy and aneuploidy; Reproductive isolat-ing mechanisms; Species and speciation.

Sources for Further StudyBaradat, P., W. T. Adams, and G. Müller-Starck, eds. Population Genetics and Genetic Conserva-

tion of Forest Trees. Amsterdam: SPB Academic, 1995. Papers from the International Sym-posium on Population Genetics and Gene Conservation in Carcans-Maubuisson, France,August 24-28, 1992, organized by the International Union of Forestry Research Organiza-tions. Illustrations, maps.

Crow, J. F. Basic Concepts in Population, Quantitative, and Evolutionary Genetics. New York:W. H. Freeman, 1986. Chapters 1-4 and 7 contain excellent accounts of population ge-netics.

Gillespie, John H. Population Genetics: A Concise Guide. Baltimore, Md.: Johns Hopkins Uni-versity Press, 1998. Includes bibliographical references and index.

Hadrick, Philip W. Genetics of Populations. 2d ed. Boston: Jones and Bartlett, 2000. Containschapters on the diversity of genetic variation, measures of genetic variation, selection, al-ternative models to selection, inbreeding, genetic drift and effective population size, geneflow and population structure, mutation, molecular population genetics and evolution,multiple-gene models, and quantitative traits and evolution. Illustrated.

Hartl, Daniel L. A Primer of Population Genetics. 3d ed. Sunderland, Mass.: Sinauer Associ-ates, 2000. Includes bibliographical references and index.

Rédei, G. P. Genetics Manual. River Edge, N.J.: World Scientific, 1998. Provides detailed defi-nitions of the various forms of speciation and the Hardy-Weinberg theorem.

Roughgarden, Jonathan. Theory of Population Genetics and Evolutionary Ecology: An Introduc-tion. Upper Saddle River, N.J.: Prentice Hall, 1996. A thorough, detailed introduction atmore than 600 pages. Illustrated.

Population genetics • 869

PROKARYOTES

Categories: Bacteria; cellular biology; microorganisms

Prokaryotes are one of two types of cell that form living organisms. Prokaryotic cells lack a nucleus and other organellesfound in eukaryotic cells. Prokaryotes include the unicellular life-forms found in two of the three domains of life,Archaea and Bacteria, whereas all protists, algae, fungi, plants, and animals are eukaryotic organisms, together form-ing the domain Eukarya.

There are architecturally two distinct types ofcells of living organisms: prokaryotic cells and

eukaryotic cells. The defining difference betweenthese two types of cells is that prokaryotic cells lackany of the internal membrane-bound structures(organelles) found in eukaryotic cells, such as a nu-cleus, mitochondria, chloroplasts, endoplasmic re-ticulum, and Golgi apparatus. Bacterial and ar-chaeal cells are prokaryotes, while plants, animals,fungi, algae, and protozoa (protists) are composedof eukaryotic cells.

StructureAlthough prokaryotic cells do not contain mem-

brane-bound organelles, they do have a highlycomplex organization and structure. Like all cells,prokaryotes are surrounded by a cytoplasmic mem-brane. This membrane is composed of proteins andlipids and is semipermeable. This semipermeablelayer regulates the flow of material into and out ofthe cell.

For most prokaryotes, the cell membrane is sur-rounded by a cell wall. The cell wall of almost everybacterial cell contains peptidoglycan, a cross-linked structure consisting of chains of sugar mole-cules, with the chains attached to one anotherthrough bridges composed of amino acids. This cellwall protects the bacterial cell from osmotic shock.Some bacterial cells also have an outer membranelinked to the peptidoglycan layer by lipoproteins.The outer membrane is a lipid bilayer that containssugars and lipids and is known as lipopolysac-charide (LPS). LPS is often called endotoxin be-cause this molecule can induce fever, shock, anddeath in animals. Archaeal cells may have cell wallscomposed of pseudopeptidoglycan, which is verysimilar to the peptidoglycan layer found in bacte-

rial cells, or they may have cell walls composed ofprotein, polysaccharides, or other chemicals. Somebacteria and archaea lack cell walls entirely.

Some prokaryotic cells have structures externalto the cell wall. These structures include capsules,slime layers, and S layers. Capsules are usually com-posed of polysaccharides, although some cells haveproteinaceous capsules. Capsules are protectivelayers that are particularly important in allowingdisease-causing bacteria to evade attack by mam-malian immune systems. Slime layers are composedof polysaccharides and resemble less organizedcapsules. Slime layers help bacteria attach to sur-faces, prevent dessication, and assist in trappingnutrients near the cell. S layers are crystalline pro-tein layers of unknown function.

Many prokaryotic cells are motile due to thepresence of flagella. Some bacteria have flagella at-tached only at one or both ends of the cell. Theseflagella are known as polar flagella. Other bacteriahave flagella all around the cell, an arrangementknown as peritrichous. Each flagellum is an inflexi-ble, helical structure composed of molecules of theprotein flagellin. Flagella rotate like propellers,causing bacteria to move in a corkscrew fashion.

Some prokaryotes produce spores, specializedstructures that are extremely resistant to heat, cold,and desiccation. Spores are metabolically inert andcan survive for extended periods, possibly forthousands of years. Spores form within prokaryoticcells when environmental conditions become unfa-vorable for survival. Once the spore is formed, thecell that produced the spore breaks open, releasingthe spore. When the spore finds itself in favorablegrowth conditions, it germinates by swelling,breaking out of the spore coat, and resuming meta-bolic function.

870 • Prokaryotes

ReproductionProkaryotic cells reproduce by binary fission. The

cell cycle of prokaryotes has three parts: elongation,DNA synthesis, and cell division. During elonga-tion, the cell synthesizes and secretes cytoplasmicmembrane and cell wall material. Prokaryotes usu-ally possess a single, double-stranded, circular DNAchromosome attached to the cytoplasmic membraneat one point. DNAsynthesis, which occurs continu-ously in actively growing cells, results in two com-plete copies of the chromosome, each attached tothe cytoplasmic membrane. As new membrane ma-terial is inserted into the cytoplasmic membraneduring elongation, the two chromosomes are sweptaway from one another. During cell division, a sep-tum forms in the center of the cell which eventuallydivides the cell into two daughter cells.

Binary fission is a type of asexual reproduction.Each of the daughter cells is identical to the parentcell, and there is no exchange of genetic material.Some prokaryotes, however, do engage in geneticrecombination through a process called conjugation.Conjugation requires the pres-ence of extrachromosomal piecesof DNA called plasmids. Theseplasmids are small, circular DNAmolecules found in the cyto-plasm of many prokaryotic cells.Some of these plasmids containgenes that encode a special struc-ture, the F-pilus. The F-pilus is aproteinaceous rod that extendsfrom the surface of cells. Cellsthat have an F-pilus are donorcells and can attach, via the F-pilus, to recipient cells whichlack an F-pilus. Following attach-ment, the F-pilus contracts, draw-ing the donor and recipient closetogether. The donor then trans-fers DNA to the recipient. Al-though conjugation results intransfer of genes from one cell toanother, it is not itself a method ofreproduction.

MetabolismProkaryotes are metabolically

diverse. Two basic nutritionalpathways are found: autotrophyand heterotrophy. Autotrophic

prokaryotes are capable of synthesizing their ownenergy-yielding compounds from simple inorganiccompounds such as carbon dioxide and water. Someprokaryotic autotrophs, the cyanobacteria and thegreen and purple bacteria, utilize the energy fromsunlight, in a process known as photosynthesis, toconstruct food molecules. It has been hypothesizedthat chloroplasts in plant cells evolved from cyano-bacteria that were engulfed by a eukaryotic cellmore than one billion years ago. Other autotrophsextract energy from metabolizing inorganic com-pounds such as hydrogen sulfide, iron sulfide, andammonia.

Heterotrophic prokaryotes obtain energy fromthe metabolism of organic compounds. Variousprokaryotes are capable of metabolizing a wide va-riety of organic molecules, including sugars, lipids,proteins, petroleum products, antibiotics, andmethanol. Heterotrophs can metabolize food mole-cules using one of three methods: fermentation,aerobic respiration, and anaerobic respiration. Fer-mentation and anaerobic respiration do not require

Prokaryotes • 871

Photosyntheticmembranes

Nucleoid (regioncontaining DNA)

Cell wall

Cytoplasm

Cytoplasmicmembrane

A Prokaryotic Cell

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The simplest life-forms, archaea and bacteria, comprise prokaryotic cells.Based on an electron microscope image of one cell in a string forming acyanobacterium, this depiction shows the basic features of a prokaryote: cellwall, cytoplasmic membrane, cytoplasm, and in this organism photosyn-thetic membranes and a series of internal membranes housing photosyntheticpigments such as chlorophyll. Prokaryotic cells lack the defined nucleus andorganelles found in eukaryotic cells, and instead the genetic material is lo-cated in an unbound region called the nucleoid.

the presence of oxygen, while aerobic respirationdoes require oxygen. Fermentation often results inmetabolic end products that include acids, carbondioxide, alcohol, or a combination of these. The an-aerobic respiration process is similar to aerobic res-piration, except that molecules such as nitrate, sul-fate, and iron are used instead of oxygen. The endproducts of aerobic respiration are carbon dioxideand water; for anaerobic respiration they are nitrite,hydrogen sulfide, or other reduced compounds.

Roles in the Global EcosystemProkaryotes play important roles in the decay of

organic matter as well as in three vital cycles of na-ture: the carbon, sulfur, and nitrogen cycles. Themajor categories of biological macromolecules (car-bohydrates, lipids, proteins, and nucleic acids) areall carbon-containing compounds. Photosyntheticorganisms, including photosynthetic prokaryotes,take carbon dioxide and convert it into carbohy-drates. Those carbohydrates can be used for energyand biosynthesis by the photosynthetic organismsas well as by heterotrophs, which consume thephotosynthetic organisms. Both heterotrophs andautotrophs also metabolize carbon-containing mol-ecules, releasing carbon dioxide back into the atmo-sphere.

Sulfur is a component of certain amino acidsfound in proteins. As decomposers, prokaryotes de-compose proteins deposited in water and soil bydead organisms and release the sulfur from sulfur-containing amino acids, often in the form of hydro-gen sulfide. Some prokaryotes convert hydrogensulfide to sulfates during their metabolism. The sul-fates can then be taken up by plants, where they arereincorporated into sulfur-containing amino acids.

Nitrogen is an essential element in nucleic acidsand proteins. Some prokaryotes, particularly soilmicrobes, digest proteins and release ammonia.Denitrification occurs when groups of symbioticprokaryotes metabolize ammonia, first to nitrites,then to nitrates, then to atmospheric nitrogen. Ni-trogen fixation occurs when nitrogen-fixing pro-

karyotes in the soil trap atmospheric nitrogen andconvert it to ammonia that can be used by plants tosynthesize new proteins and amino acids.

DiseaseInfectious disease is a disturbance in normal

organismal function caused by an infecting agent.Although most prokaryotes do not cause disease,some bacteria are capable of parasitizing a host anddisrupting normal function. Prokaryotes capable ofproducing disease in plants are widely distributedand cause a number of diseases, including wilts,rots, blights, and galls. Some of these diseases arecaused by soil-dwelling prokaryotes, while othersare seedborne or are caused by obligate parasites,unable to survive outside plant tissue.

Commercial UsesProkaryotes are easily manipulated and there-

fore are useful for many commercial applications.Prokaryotes have been used for centuries in theproduction of food. Yogurt, sauerkraut, poi, kim-chee, dry and semidry sausages, and vinegar are allexamples of bacterially produced foods. Geneticengineering is more easily accomplished in pro-karyotes than in eukaryotes. Prokaryotes now pro-duce human insulin, antibiotics, plant hormones,and industrial solvents. Prokaryotes have been en-gineered to protect plants from frost damage, whileplants have been genetically engineered, using bac-terial vectors, to develop resistance to herbicidesand to produce toxins that destroy insect pests.

Lisa M. Sardinia

See also: Anaerobes and heterotrophs; Anaerobicphotosynthesis; Archaea; Bacteria; Cell wall; Che-motaxis; Chloroplasts and other plastics; Diseasesand disorders; Eukaryotic cells; Evolution of cells;Flagella and cilia; Gene regulation; Genetic code;Membrane structure; Microbial nutrition and me-tabolism; Mitosis and meiosis; Molecular systemat-ics; Paleoecology; Plasma membranes; Protista;Respiration; Systematics and taxonomy.

Sources for Further StudyAlcamo, I. Edward. Fundamentals of Microbiology. 6th ed. Boston: Jones and Bartlett, 2001.

This college textbook is written for introductory microbiology courses with an emphasison human disease, but it also contains sections on food and environmental microbiology.Illustrations, photographs, and “MicroFocus” boxes containing interesting facts.

Atlas, Ronald M. Principles of Microbiology. 2d ed. Boston: WCB/McGraw-Hill, 1997. De-tailed college textbook focusing on prokaryotic biology with little reference to human dis-

872 • Prokaryotes

eases. Illustrations, photographs, bibliographic references, and an appendix containingsystematic descriptions of groups of microorganisms.

Madigan, Michael T., John M. Martinko, and Jack Parker. Brock Biology of Microorganisms. 9thed. Upper Saddle River, N.J.: Prentice Hall, 2000. Comprehensive college textbook cover-ing all aspects of prokaryote biology, from metabolism to genetics, ecology, and diversity.Illustrations, micrographs, glossary, index.

Silverstein, Alvin, Robert Silverstein, and Virginia Silverstein. Monerans and Protists. Breck-enridge, Colo.: Twenty-First Century Books, 1996. Describes single-celled organisms andexplains their importance in maintaining the ecosystem. Aimed at middle schoolers.

PROTEINS AND AMINO ACIDS

Category: Cellular biology

Proteins are found in all cells and carry out a variety of important cellular functions. Within any one cell there may bethousands of different proteins having a variety of sizes, structures, and functions. Proteins are also important struc-tural components of the cell wall.

Proteins are the most complex and abundant ofthe macromolecules. Within cells, many pro-

teins function as enzymes in the catalysis of meta-bolic reactions, while others serve as transport mol-ecules, storage proteins, electron carriers, andstructural components of the cell. They are espe-cially important in seeds, where they make up asmuch as 40 percent of the seed’s weight and serve tostore amino acids for the developing embryo. Pro-teins are also important structural components ofthe cell wall. Because proteins and their buildingblocks, amino acids, form such a large componentof plant life, plants serve as an important dietarysource of the eight to ten essential amino acids forhumans and other animals.

Amino AcidsAmino acids are the molecular building blocks

of proteins. Amino acids all share a structure, with acentral carbon atom, the alpha carbon, covalentlybonded to a hydrogen atom, an amino group, acarboxylic acid group, and a group designated asan R group, which varies in structure from aminoacid to amino acid. It is the diverse nature of the Rgroup that provides the protein with many of itsstructural and functional characteristics. Some Rgroups are either polar or electrically charged atphysiological pH, making the R groups hydro-philic (water-loving). Other R groups are nonpolar

and hydrophobic (water-avoiding). The twentystandard amino acids the cell uses to synthesize itsproteins are alanine, arginine, aspartate (asparticacid), asparagine, cysteine, glutamate (glutamicacid), glutamine, glycine, histidine, isoleucine,leucine, lysine, methionine, phenylalanine, proline,serine, threonine, tryptophan, tyrosine, and valine.

Each of the twenty amino acids differs from theother nineteen in the structure of its R group. Onceincorporated into a protein, a standard amino acidmay undergo modification to create nonstandardamino acids and an even greater diversity of proteinstructures. One of the more common nonstandardamino acids found in proteins is hydroxyproline,which is commonly found in plant cell-wall pro-teins. In addition to the twenty amino acids thatbuild proteins, many nonstandard amino acids oc-cur free in the cell and are not found in proteins.Canavanine, for example, occurs in the seeds ofmany legumes.

Based on the information in cellular deoxyribo-nucleic acid (DNA), the cell joins the twenty stan-dard amino acids by peptide bonds in specific se-quences, resulting in chains ranging from as few astwo amino acids to many thousands. Shorter chainsof amino acids are referred to as peptides oroligopeptides, while longer chains are referred to aspolypeptides. The term “protein” is usually reservedfor those oligopeptides and polypeptides that have

Proteins and amino acids • 873

biological functions, because single polypeptidesoften do not have biological functions unless asso-ciated with other polypeptides.

Primary StructureProteins differ from one another in the sequences

of their amino acids. The sequence of amino acids ofa protein is called its primary structure. Mutationshave been shown to result in the change of as few asone amino acid in a protein. Because DNA specifiesa protein’s primary structure, protein sequence in-formation is often used to study the evolutionaryrelationships among organisms.

Proteins are often complexed with other com-pounds in their biologically active state. These pro-teins are called conjugated proteins. Proteins com-plexed with metals, lipids, sugars, and riboflavinare called metalloproteins, lipoproteins, glycoproteins,and flavoproteins, respectively. Glycoproteins (liter-ally, “sugar proteins”) are important constituents ofthe plasma membrane. These sugar molecules canoccur singly or in short, simple branched chains.

A protein chain may be folded into a variety ofthree-dimensional shapes. The three-dimensionalshape a protein assumes is called its conformationand is determined by its amino acid sequence. In or-der for a protein to be active, it must assume a cer-tain conformation. Any alteration in its conforma-tion may result in reduced activity. Denaturingagents alter the structure of a protein so that it losesits conformation, biological function, and activity.

Secondary StructureThe secondary structure refers to the local folding

or conformation of the polypeptide chain over rela-tively short (fifty amino acids or so) stretches. Twocommon secondary structures, the alpha helix andthe beta sheet, occur regularly in proteins. On aver-age, only about half of the polypeptide chain as-sumes the alpha or beta conformation, while the re-

mainder exists in turns and random structures.Some proteins show only alpha structure, othersonly the beta structure, while still others show ei-ther a mixture of the two structures or neither sec-ondary structure. Both the alpha and the beta struc-tures increase the structural stability of the protein.The amino acid sequence determines whether aparticular sequence of amino acids in a protein willassume the alpha or beta structure.

Tertiary StructureThe overall spatial orientation of the entire poly-

peptide chain in space is referred to as its tertiarystructure. Generally, two tertiary structures are rec-ognized. Fibrous or filamentous proteins are ar-ranged as fibers or sheets, while globular proteinsare arranged roughly as spherical or globular struc-tures. The amino acid sequence determines the over-all folding of the protein tertiary structure. Fibrousproteins are primarily involved with structural func-tions, whereas globular proteins function as en-zymes, transport molecules, electron carriers, andregulatory proteins.

Quaternary StructureSome proteins are composed of more than one

polypeptide chain. Aprotein composed of only onepolypeptide is called a monomer, while proteins com-posed of two, three, four, and so on are referred to asdimers, trimers, and tetramers, and so on, respectively.

Charles L. Vigue

See also: Carbohydrates; Cell wall; Chromosomes;Cytoskeleton; Cytoplasm; Cytosol; DNA in plants;DNA replication; Endomembrane system andGolgi complex; Endoplasmic reticulum; Energyflow in plant cells; Evolution of cells; Lipids; Mem-brane structure; Nuclear envelope; Nucleic acids;Nucleolus; Nucleoplasm; Nucleus; Oil bodies;Plasma membranes; RNA.

Sources for Further StudyDey, P. M., and J. B. Harborne, eds. Plant Biochemistry. San Diego: Academic Press, 1997. De-

scribes various topics in plant biochemistry, each covered by a different specialist. Illus-trated and referenced, the text emphasizes plant metabolism but also includes enzymes,functions, regulation, and molecular biology.

Mathews, Christopher K., K. E. Van Holde, and Kevin G. Ahern. Biochemistry. 3d ed. NewYork: Addison Wesley Longman, 2000. Covers all aspects of biochemistry for the ad-vanced student. Profusely illustrated and referenced.

Nelson, David L., and Michael M. Cox. Lehninger Principles of Biochemistry. 3d ed. New York:Worth, 2000. Covers general biochemistry, from structures to metabolism. Excellently il-

874 • Proteins and amino acids

lustrated and referenced. Comes with a CD of art and animations.Zubay, Geoffrey L. Biochemistry. 4th ed. Boston: Wm. C. Brown, 1998. General, advanced bio-

chemistry text has especially good coverage of proteins. Includes illustrations and refer-ences and comes with a CD of art and animations.

PROTISTA

Categories: Microorganisms; Protista; taxonomic groups; water-related life

The Protista form one of the four kingdoms of eukaryotic organisms and include the algae, protozoans, slime molds, andoomycota.

Included among the diverse organisms calledprotistans (kingdom Protista) are algae, protozo-

ans, slime molds, and the oomycota. Algae werelong considered to be simple plants and were as-signed to kingdom Plantae but lack the more highlydifferentiated tissues and organs characteristic of“higher plants” such as mosses, ferns, and seedplants. Protozoa, all of which are unicellular, wereassigned to the kingdom Animalia; consideredmore animal-like than plant-like, they are not con-sidered here in detail.

The two remaining groups of protists, the slimemolds and oomycota, were traditionally consideredfungi, as indicated by their names (mycoto means“fungus”), and accordingly were assigned to thekingdom Fungi, largely due to their being hetero-trophs (nonphotosynthetic). As information aboutthe true nature of these life-forms has accumulated,however, they were grouped together with the pro-tists. Nevertheless, Protista is a heterogeneous king-dom, composed of both the heterotrophic slimemolds and oomycetes and the autotrophic (photo-synthetic) algae. Thus, the kingdom Protista is a“catch-all” group, containing various organismsthat seem not to fit into any of the other kingdoms.

Biology of the ProtistaMost protistans are unicellular, but some, nota-

bly the large species of marine algae called kelpsand seaweed, are multicellular. All are eukaryotic(with cells that have nuclei, making them membersof the domain Eukarya, along with fungi, plants,and animals), in contrast to prokaryotic bacteria(which are divided between the domains Archaeaand Bacteria).

In addition to having a distinct nucleus sur-rounded by a nuclear membrane (envelope), the cy-toplasm of a eukaryotic cell contains various typesof organelles, each specialized for performing aparticular task (or related ones). Examples are mito-chondria (cellular respiration), chloroplasts (photo-synthesis), Golgi bodies (packaging of molecules),and the endoplasmic reticulum (to lend rigidity).As a result of this specialization, eukaryotic cellsare able to function more efficiently than do pro-karyotic cells, which must perform the same func-tions but within the cell as a whole. In this way,protistans are similar to plants, animals, and fungi.Furthermore, the evolution of eukaryotic cells fromprokaryotic ones was a pivotal event in the evolu-tion of life, leading not only to protistans but alsofrom them to the more highly evolved kingdoms.

As previously stated, protists exist in great vari-ety. Included are some that, due to the absence of arigid cell wall, are able to change shape rapidly.Other protists have cell walls surrounding their cellmembranes, resulting in a more permanent shape.Some possess fringelike cilia or whiplike flagellawhich they use to swim; others move by othermeans; many are nonmotile. Some protists live soli-tary lives, while others aggregate to form colonies.Some are parasitic, whereas most are free-living(nonparasitic).

In size there is also great variation: Many are uni-cellular and microscopic, while some algae, such asthe kelps and seaweeds, grow to many meters inlength. Furthermore, many protists have complexlife cycles, with the various stages possessing dif-ferent combinations of these characteristics.

Biologists generally agree that fungi, plants, and

Protista • 875

animals are derived from ancient protists. Thus, thestudy of protists, which continue to inhabit theearth, sheds light on the origin of these groups ofmore highly evolved organisms. Also, each of thesemodern protists plays an ecological role togetherwith the other organisms that occupy its ecosystem.Some protists affect humans more directly asagents of disease; some serve as a source (or poten-tial source) of medicines that can be used to combatvarious diseases.

Algae: ClassificationThe term “algae” (singular “alga”), when un-

qualified, refers generally to an organism, usuallyinhabiting water or a wet habitat, that is somewhatplantlike (photosynthetic) but that lacks the morespecialized tissues characteristic of plants. Further-more, nearly all algae produce reproductive cells,spores or gametes, which lack surrounding special-ized enclosures such as are typical of plants.

The blue-green “algae,” historically consideredto be algae, are now recognized as a separate groupof organisms. As they are prokaryotic, they are nowconsidered to be a special type of bacteria, the cya-nobacteria. They differ from other bacteria primarilybecause they possess chlorophyll and thereforehave the ability to photosynthesize.

Within the kingdom Protista, algae are currently

divided by phycologists (scientistswho study algae) among anywherefrom four to thirteen phyla. Here, asystem is used in which nine phylaof the kingdom Protista include al-gae. The nine phyla include thefollowing.

Euglenophyta (euglenoids). Thissmall group of protists (about ninehundred species) is a good startingpoint for studying algae, as eugle-noids combine traits characteristicof plants, fungi, and animals. Allare unicellular, and nearly all aremobile by means of two flagellathat emerge from a groove at theend of the cell. By means of aneyespot near the base of the fla-gella, euglenoids can detect lightand swim toward it. Most mem-bers of the phylum lack chloro-phyll and therefore must absorbfood from external sources. Oth-

ers, though, such as Euglena, have chloroplasts withchlorophylls a and b, along with carotenoids and ac-cessory pigments. They reproduce by fission (celldivision). Most are freshwater organisms, but a fewoccur in brackish or marine environments.

Cryptophyta (cryptomonads). These algae aresingle-celled flagellates that are commonly brown-ish, blue-green, or red. Their name (Greek kryptosmeans “hidden”) refers to their small size (3-50 mi-crometers), which makes them inconspicuous. Likeeuglenoids, they include both colorless (unpig-mented) and pigmented photosynthetic members.Pigments includes chlorophylls a and c and carot-enoids. Evidence exists to indicate that cryptomo-nads arose from the fusion of two different kinds ofeukaryotic cells. Of the two hundred known spe-cies, some are marine; others live in fresh water.

Rhodophyta (red algae). Red algae are primarilymulticellular marine organisms and are commonlyreferred to as seaweeds. However, about one hun-dred of the five hundred species are unicellular andlive in fresh water. The chloroplasts of red algaecontain chlorophyll a, but the presence of phy-cobilins (red pigments) usually masks the chloro-phyll, giving them a red or reddish appearance. Thered pigment aids in light absorption in deep water,where many red algae are found. As chloroplasts ofred algae resemble those of cyanobacteria, there is

876 • Protista

Phyla of ProtistaPhylum Common Name Approx. Species

Slime moldsDictyosteliomycota Cellular slime molds 50Myxomycota Plasmodial slime molds 700

Water moldsOomycota Oomycetes 700

AlgaeBacillariophyta Diatoms 100,000+Chlorophyta Green algae 17,000Chrysophyta Chrysophytes 1,000Cryptophyta Cryptomonads 200Dinophyta Dinoflagellates 4,000Euglenophyta Euglenoids 1,000Haptophyta Haptophytes 300Phaeophyta Brown algae 1,500Rhodophyta Red algae 6,000

Source: Data on numbers of species adapted from Peter H. Raven et al., Biology ofPlants, 6th ed. (New York: W. H. Freeman/Worth, 1999).

reason to believe that they probably evolved fromthese prokaryotic organisms.

One group of red algae, the coralline algae, depositcalcium carbonate in their cell walls, resulting instony formations in the ocean. They are often asso-ciated with coral reefs, which they help to stabilize.

Many red algae have complicated life histories.The simplest type is that in which a haploid game-tophyte alternates with a diploid sporophyte. Thispattern, known also throughout the plant king-dom, is known as alternation of generations. Inmost red algae, there are three generations: a hap-loid gametophyte, a carposporophyte, and atetrasporophyte (both diploid). It has now been rec-ognized that some red algae previously consideredto be different species are actually different stagesof the same species.

Dinophyta (dinoflagellates). Most dinoflagellatesare unicellular marine algae, each with two flagella.Of the two, one is within an equatorial groovearound the cell; the other passes down a longitudi-nal groove before extending outward. In additionto chlorophyll, they have accessory pigments thatgive them a golden-brown color. Many dinofla-gellates are associated symbiotically with variousinvertebrates; for example, many corals benefitfrom the food they derive from these algae. Otherdinoflagellates are nonphotosynthetic and live asparasites within other marine organisms.

Many dinoflagellates are bioluminescent; theyemit a faint light that can be seen in darkness from apassing ship. Others are responsible for fish killswhen they become superabundant in warm stag-nant water. The death of the fish within these “redtides” is often due to toxins produced by thedinoflagellates.

Haptophyta (haptophytes). This group, consist-ing of only about three hundred known species, in-cludes a diversity of primarily marine species, butsome freshwater and even terrestial species areknown. Included are both unicellular and colonialflagellates, along with others that are nonmotile.The distinctive feature of haptophytes is the hapto-nema, a threadlike structure that bends and coils asit apparently helps the cell to catch food particles. Itdiffers from a flagellum by lacking the 9 + 2 ar-rangement of microtubules, which is characteristicof flagella of eukaryotic cells. Most haptophytes arephotosynthetic and possess chlorophylls a and c to-gether with an accessory pigment such as fuco-xanthin. Although the phenomenon is not as well

documented as for dinoflagellates, haptophytesalso cause marine fish kills by releasing toxins.

Chrysophyta (chrysophytes). Some one thousandspecies of chrysophytes exist, including both uni-cellular and colonial organisms that are often abun-dant in both fresh and marine habitats. Some lackchlorophylls a and c. However, the golden color offucoxanthin, which they possess, usually masks thegreen pigments, giving them their characteristicgolden hue and accounting for their name (chrysosmeans “gold”). Some chrysophytes feed on bacte-ria. Some are responsible for “brown tides” thatcauses damage to shellfish and salmon fisheries.

Bacillariophyta (diatoms). The name “diatom”comes from the two overlapping shells that fit to-gether like the two parts of a candy box. Composedof silica (silicon dioxide), the shells persist long af-ter the living cell inside has died. Most species arephotosynthetic, possessing chlorophylls a and c aswell as fucoxanthin. The life cycles of diatoms in-clude both asexual (cell division) and sexual phases.

It would be difficult to overestimate the im-portance of diatoms. There are more than 100,000known species, and they are abundant in practicallyall aquatic and marine habitats. Due to the persis-tence of their shells, it is known that there aremanyextinct species also. Diatoms are responsible for asmuch as 25 percent of total world food production(photosynthesis). Especially in polar waters, theyare the primary food source for aquatic animals.Large accumulations of diatom shells, known as“diatomaceous earth,” are mined, cleaned, and usedin filters, in gas masks, in toothpaste, and for a vari-ety of other purposes.

Phaeophyta (brown algae). Found only in saltwater, this group includes large, conspicuous, mul-ticellular forms generally called kelps or seaweeds.Often seen on rocky shores, various of the fifteenhundred species inhabit the ocean, especially intemperate and cooler waters. Laminaria are calledkelps and often form “kelp forests” along the gentlysloping shores off the coast of California.

Although lacking true roots, stems, and leaves,kelps have the most highly differentiated bodies ofany of the algae. Some of their cells resemble thephloem (food-conducting cells) of vascular plants.The brown pigment fucoxanthin is present in addi-tion to chlorophylls a and c.

Chlorophyta (green algae). The seventeen thou-sand or more species of green algae are perhaps themost diverse group of the algae. Most live in fresh

Protista • 877

water, but some live in the ocean, and others live insoil or on tree trunks. Many form symbiotic associa-tions with sponges, protozoa, and other inverte-brates; others are associated with fungi within li-chens. As green algae resemble plants more than doany other group of algae, plants are believed tohave evolved from green algae. They, like plants,possess chlorophyll a and b; food is stored in spe-cialized cytoplasmic organelles called plastids.

The phylum is divided into three classes. In theclass Chlorophyceae are included a diversity offorms, nearly all of which are freshwater species.Chlamydomonas is a motile unicellular specieswhich, nevertheless, exhibits a complex life cycle.Volvox and several other large spherical colonialforms are composed of cells, each of which stronglyresembles Chlamydomonas. Other members of thisclass are filamentous.

The class Ulvophyceae includes primarily marinespecies. A common example is the Ulva species (sealettuce), composed of flat sheets of cells; it is foundin shallow seas around the world.

Included in the class Charophyceae is the familiarSpirogyra, a freshwater filamentous species withspiral chloroplasts.

Human Uses of AlgaeReferences have already been made to ways that

algae are involved in the overall “economy of na-ture.” Because algae are autotrophs (producers)and photosynthesize, they generate food that ismade available to the heterotrophic animals (con-sumers). At the same time, oxygen, which results asa by-product, is made available to these same ani-mals. As algae perform functions in marine andfreshwater environments that are similar to thoseperformed by grasses (and other plants) on land, al-gae have been called “the grasses of many waters.”

Algae are often involved also in human affairs inmore direct ways. In Asian countries especially,kelps and other multicellular algae have been usedfor food for centuries. Nori, a red alga of the genusPorphyra has been collected and eaten as a vegetablein Japan and China. It is now cultivated on a largescale, thus increasing its availability and popular-ity. Unfortunately, most seaweeds are not high infood value, although they do provide some neededminerals and vitamins. Another problem is theirtaste, which is not acceptable to many Westerners.

Of more commercial value in Europe and NorthAmerica are a number of products derived from

kelps and various seaweeds: alginates, carrageen-ans, and agar. Alginates are hydrophobic (water-attracting) compounds derived from various brownalgae such as Laminaria. After they are harvestedmechanically from the ocean, the algae are pro-cessed. The resulting products are salts of sodiumand potassium alginate. These alginates are used inthe paper industry as a sizing and polishing agentand in the manufacture of paints, cosmetics, and awide variety of foods. In each case, the role of thealginate is to improve the consistency of the prod-uct and to prevent the separation of its ingredients.

Carrageenans are obtained primarily from Irishmoss (Chondrus crispus), a red alga found off thecoast of New England. After processing, the result-ing carrageenans are used for some of the samepurposes as alginates. However, because of theirhigher melting point, they have been found supe-rior for uses in many kinds of foods, especiallydesserts.

The pioneer German bacteriologist Robert Kochpopularized the use of agar for the culture of bacte-ria. Agar is obtained from certain red algae. Afterprocessing and cleaning, agar is added in smallquantities (1-2 percent) to water along with nutri-ents required by the bacteria. The result is a solidmedium on which bacteria can be isolated from amixed culture.

Although Asians have long used certain algaefor folk medicinal purposes, their use in Westernmedicine has until recently been limited largely toserving as a binder in medicinal tablets or as a laxa-tive. Their potential as a source of therapeutic drugsis being pursued by an increasing number of re-searchers. Included are those from which antibiot-ics and anticancer drugs may be extracted.

Slime MoldsAs the name indicates, these organisms resemble

molds (kingdom Fungi) and thus have historicallybeen considered to be fungi. Like fungi, they areheterotrophic and are found growing on decayingorganic matter. However, the accumulation of moredata, including molecular information, indicatesthat they are a group distinct from fungi. Slimemolds are typically divided between two phyla.

Plasmodial slime molds (phylum Myxomycota)often exist as a conspicuous fan-shaped mass of pro-toplasm that creeps along a surface somewhat as doamoebas. From this stage, known as a plasmodium,are formed sporangia with spores inside. From the

878 • Protista

spores are formed the single-celled amoebalikestage; they converge to form the plasmodium.

Cellular slime molds (phylum Dictyosteliomy-cota) are amoeba-like organisms that combine atone stage to form “slugs,” or pseudoplasmodia.Like that in plasmodial slime molds, reproductionin cellular slime molds is both sexual and asexual,but, unlike plasmodial slime molds, no flagellatedcells are known. In both types of slime molds, par-ticulate food such as bacteria can be ingested (fungiabsorb only digested food).

OomycetesAlso previously considered to be fungi, oomy-

cetes (phylum Oomycota) are probably more nearlyrelated to certain algae than to fungi. Like algae,their cell walls are of cellulose. Some species areunicellular, whereas others are filamentous orhighly branched. Some of the filamentous forms arecoenocytic (no cell walls separate adjacent cells).The name of the group reflects the large female ga-mete or egg; this type of sexual reproduction iscalled oogamy.

Some terrestial oomycetes are plant pathogensof considerable importance. Downy mildew ofgrapes, caused by Plasmopara viticola, has oftenthreatened the wine industry of France. Species ofthe genus Phytophthora cause diseases of many fruitcrops and other plants of economic importance.Among these is P. infestans, which causes the potatolate blight. One particular outbreak of this parasitecaused the infamous Irish Potato Famine of themid-1840’s, during which more than a million peo-ple (some estimates say four million) were affected.

Saprolegnia is a prominent member of a group ofaquatic oomycetes called “water molds.” Most aresaprophytic on dead plants and animals, but a feware parasitic.

Thomas E. Hemmerly

See also: Algae; Brown algae; Cellular slime molds;Charophyceae; Chlorophyceae; Chrysophytes; Cryp-tomonads; Diatoms; Dinoflagellates; Euglenoids;Eukaryotic cells; Evolution of cells; Green algae;Haptophytes; Oomycetes; Phytoplankton; Plasmo-dial slime molds; Red algae; Ulvophyceae.

Sources for Further StudyBold, H. C., and Michael J. Wynne. Introduction to the Algae: Structure and Reproduction. 2d ed.

Englewood Cliffs, N.J. 1985. This classic by renowned phycologists provides detailedcoverage of this important group of organisms.

Purves, William K., et al. Life: The Science of Biology. 6th ed. New York: W. H. Freeman, 2001.This university-level textbook includes an excellent overview of the Protista and the originof eukaryotic cells (chapters 26 and 27).

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman, 1999. Chapters 16 and 17 include a detailed look at the various groups oforganisms classified as protists.

PSILOTOPHYTES

Categories: Evolution; paleobotany; Plantae; seedless vascular plants; taxonomic groups

Psilotophyte is the common name for members of the phylum Psilotophyta (from the Greek word psilos, meaning“bare”). Molecular evidence points to the likelihood of psilotophytes as being highly reduced (and therefore derived)ferns. If psilotophytes are indeed reduced ferns, they probably diverged from the fern lineage early, after ferns arose some400 million years ago during the Devonian period.

The family Psilotaceae is the only family ofpsilotophytes. There are two living genera:

Psilotum and Tmesipteris. Psilotum, the whiskfern, is

widespread throughout tropical and subtropicalregions. Lacking leaves and roots, Psilotum speciesgrow in a variety of soil conditions, including very

Psilotophytes • 879

warm soils near active volcanoes, or they may beepiphytic, growing on the trunks of host trees.Psilotum nudum, the best-known species of psiloto-phyte, has an upright growth habit and is easilymaintained in greenhouse culture. One epiphyticspecies of Psilotum has a pendulous, or hanging,growth habit.

Tmesipteris is found only in the South Pacific re-gion, including Australia, New Zealand, and someSouth Pacific islands. Terrestrial species of Tmesip-teris may be upright in growth habit, while somespecies are epiphytes growing on thestems of ferns and trees or in moundsof moist humus. Epiphytic Tmesipterisspecies express a pendulous growthhabit.

Although psilotophytes have a ratherrestricted range and do not appear to bea highly diverse group, they are impor-tant members of the ecosystems in whichthey are found. Psilotum has been culti-vated as a horticultural specimen formany years in Asia. One particular vari-ety, called Bunryu-zan, has been selectedto express no prophylls and to producesynangia (fused sporangia, the sites ofspore production) at the tips of the aerialbranches, which makes this Psilotum va-riety appear even more similar to plantsknown only from the fossil record.

Life CyclePsilotophytes exhibit a life cycle pat-

tern called alternation of generations.Haploid spores produced in the sporan-gia of the diploid sporophytes (the dip-loid, spore-producing, generation) arereleased when the sporangium splitsopen. The spores fall to the ground orinto rock or bark crevices, where theygerminate in complete darkness.

Haploid gametophytes (the haploid,gamete-producing generation) developfrom the spores. The nonphotosyntheticunderground gametophytes are com-pletely dependent on their endophyticfungal partners for energy. Male and fe-male sex organs develop on a single ga-metophyte. Usually, the male antheridiaand the female archegonia develop atslightly different times to reduce the

likelihood of self-fertilization. When gametes (eggand sperm) are mature and when liquid water ispresent, multiflagellated sperm are released fromthe antheridia and swim to a nearby archegonium.The sperm swim through the neck of the arche-gonium, where fertilization takes place in theventer. The resulting zygote develops into an em-bryo, which in turn develops into a young sporo-phyte. The young sporophyte stage is dependenton the gametophyte for support until it growsthrough the soil and begins photosynthesis.

880 • Psilotophytes

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Sporophyte AnatomyAs is the pattern in vascular plants, the diploid

sporophyte generation is the dominant phase of thelife cycle. Psilotophytes have many features that aresimilar to primitive vascular plants. Like fossil vas-cular plants, psilotophytes have a horizontal stemcalled a rhizome. Aerial branches grow from therhizome. The lower portions of the aerial stems inPsilotum are often five-sided, while the upper aerialstems are usually three-sided. Both the rhizomeand the aerial stem branch dichotomously, whichmeans they tend to produce two equal branches ateach node. Psilotum usually produces several di-chotomies, or branching series, while Tmesipterismay have only one dichotomy on a particular aerialstem. Dichotomous branching is considered aprimitive characteristic.

The outermost layer of the stem is composed ofepidermal tissue. A waxy cuticle covers the epider-mis of the aerial branches. Stomata are present inspaces between the ribs, which run lengthwisealong the branch. Interior to the epidermis is a widecortex region. The cortex is composed of paren-chyma cells. The cortex parenchyma is importantfor storage, as is evident from the presence of themany starch granules in each cell. Lying inwardfrom the cortex in Psilotum is a layer of cells calledthe endodermis. Endodermal cells have a layer ofsuberin (fatty material) called a Casparian strip em-bedded in part of their cell walls to restrict watermovement into and out of the vascular cylinder.The endodermis is well developed throughout theaerial part of the Psilotum plant body as well as theunderground parts. In Tmesipteris, the endodermisis present only in the rhizome. A layer of cells con-taining tannins and phenolic compounds lies be-tween the phloem cylinder and the cortex inTmesipteris stems. This layer may be the physio-logically active equivalent of an endodermis.

Although psilotophytes lack true roots, they dopossess rootlike epidermal extensions called rhi-zoids. The rhizoids aid in absorption of water andmineral nutrients and also act as the points of entryfor symbiotic fungi, whose presence may be essen-tial for the survival of the organism. The associationof mutualistic fungi with plant roots or rhizoids iscalled a mycorrhiza. Because the fungi invade thebody of the plant, they are sometimes referred to asendophytic fungi.

The xylem tissue of psilotophytes is composed oftracheids. Phloem tissue is made up of sieve cells,

along with some parenchyma helper cells called al-buminous cells. The stele, or vascular cylinder, ofthe psilotophyte rhizome is usually interpreted as atype of protostele. Protosteles have no pith; theyhave a solid core of vascular tissue. In the pro-tostelic actinostele of Psilotum, the solid core ofxylem has armlike extensions that reach into thesurrounding tissues. Phloem surrounds the xylem.In the lower portions of the aerial branches ofPsilotum, the stele is considered to be a type ofsiphonostele. In this case, the center of the stele isinterpreted as a pith made up of sclerotic (hard,thick-walled) cells. This is similar to the aerial stempattern in Tmesipteris. The tips of the aerialbranches in Psilotum are strictly actinosteles.

Psilotum lacks true leaves. The leaflike append-ages of the Psilotum shoot are called enations, orprophylls. The prophylls are composed of smallflaps of photosynthetic tissue. Small traces of vas-cular tissue end at the base of the prophyll and donot actually enter the structure. In contrast, the fo-liar appendages of Tmesipteris are larger and areconsidered to be a type of true leaf called a micro-phyll. Microphylls have a single vein extendinginto the blade of the leaf.

Spores are produced in sporangia located onvery short shoots on the sides of aerial branches.The lateral placement of sporangia in psilotophytescontrasts with the terminal placement of sporangiain primitive vascular plants such as Rhynia. Be-cause each sporangium appears to represent a fu-sion of two sporangia (as in Tmesipteris) or threesporangia (as in Psilotum), they are generally calledsynangia. Cells within the synangium undergomeiosis to produce haploid spores. The spores aredescribed as monolete, meaning that they have asingle ridge. The monolete character is considered aderived trait, which is different from the trilete, orthree-ridged, spores of primitive vascular plants.

Gametophyte AnatomyThe haploid psilotophyte gametophytes are

very small and grow underground. In general, theyare cylindrical, brown in color, covered with rhi-zoids, and are often branched. The branching maybe dichotomous, as in the sporophyte, or irregularin response to the wounding of the gametophyte’sapical meristems.

Gametophytes rely on endophytic fungi to ob-tain energy. Fungi invade nearly all of the cells ofthe gametophyte body except the apical meristems

Psilotophytes • 881

and the gametangia, or gamete-producing organs.Both male and female sex organs are found scat-

tered throughout a single plant. The antheridia(male organs) are small, multicellular, hemispheri-cal structures that protrude from the surface of thegametophyte. Cells inside the antheridia undergomitosis to produce multiflagellated sperm cells.The archegonia (female organ) has a swollen basecalled the venter. The venter is sunken below thesurface of the gametophyte.

A cell within the venter undergoes mitotic celldivision to produce an egg. Four rows of cells formthe neck of the archegonium, which surrounds aneck canal. Two cells initially fill the neck canal.These neck canal cells break down when the arche-gonium is ready for fertilization.

Darrell L. Ray

See also: Evolution of plants; Ferns; Horsetails; Ly-cophytes; Seedless vascular plants.

Sources for Further StudyBell, Peter R., and Alan R. Hemsley. Green Plants: Their Origin and Diversity. 2d ed. New York:

Cambridge University Press, 2000. Contains an introduction to psilotophytes. Includesphotos, diagrams.

Bold, Harold C., Constantine J. Alexopoulos, and Theodore Delevoryas. Morphology of Plantsand Fungi. 5th ed. New York: HarperCollins, 1987. A classic text on plant morphology, in-cluding a chapter on psilotophytes. Includes photos, diagrams.

Gifford, Ernest M., and Adriance S. Forster. Morphology and Evolution of Vascular Plants. 3ded. New York: W. H. Freeman, 1989. A standard plant morphology text. Devotes chapterto psilotophytes. Includes photos, diagrams.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. A standard text for introductory botany courses at the uni-versity level. Discusses structure and significance of major groups of seedless vascularplants, including psilotophytes. Includes photos, diagrams.

882 • Psilotophytes

RAIN-FOREST BIOMES

Categories: Biomes; ecosystems; forests and forestry

Tropical forests exist in frost-free areas between the Tropic of Cancer and the Tropic of Capricorn. Temperatures rangefrom warm to hot year-round. These forests are found in northern Australia, the East Indies, Southeast Asia, equatorialAfrica, and parts of Central America and northern South America.

Rain forests are complicated tropical ecosystemswith extremely high levels of biodiversity. Oc-

cupying only 6 percent of the earth’s surface, theycontain at least 50 percent of the world’s knownplant and animal species. The rain forests are beingdestroyed at such an unprecedented rate, however,that if the trend continues, no sustainable tropicalrain forests will remain by the middle of thetwenty-first century.

Tropical rain forests are the most complex eco-systems on earth, consisting of interacting systemsof vegetation and animal species, all interdepen-dent and coexisting in a concentrated cacophony oflife. Because species found in one area may differradically from those in another region only milesaway, rain forests are individualistic and highly di-versified.

The rain forests surrounding the Amazon basinin South America represent the last great contigu-ous expanse of tropical rain forest remaining in theworld. Containing at least 20 percent of the earth’shigher plant species and an equal percentage of theworld’s birds, Amazonia is being systematicallydevastated by human actions. About 20 percent hasalready been destroyed, and the rate has acceler-ated. In the mid-1990’s about 14,000 acres were be-ing cleared every day.

The environmental, economic, and social conse-quences of large-scale tropical deforestation are nu-merous and severe for the entire earth. The implica-tions of losing the rain forests’ biodiversity are dire.Moreover, rain forests act as giant solar-poweredengines that pump water, nutrients, and carbon di-oxide through the biosphere. Water captured by veg-etation is returned to the atmosphere by evapora-tion and then to the forest as rain. The forest alsostores large quantities of water in the vines androots of the plants, water that is slowly released to

streams and rivers. When large tracts of rain forestare cleared, the system’s ability to store water de-creases, disrupting the local climate and causing al-ternating periods of drought and floods.

Rain forests consist of lush, abundant growth,but the soil is deficient in nutrients and only mar-ginally fertile. Although organic materials decom-pose rapidly in warm, humid climates, heavy rainsleach nutrients from the soil. Plant life has adaptedby rapidly ingesting nutrients as they becomeavailable; thus most nutrients are stored in thevegetation itself, not in the soil. When the land iscleared for agriculture or livestock grazing, thenecessary crop-sustaining nutrients are depletedwithin a few growing seasons. Rain-forest destruc-tion also adversely impacts the environment andhuman life by reducing the conversion of atmo-spheric carbon dioxide into oxygen.

Food ResourcesTropical rain forests are important sources of ge-

netic material for improving crop plants or breed-ing new varieties. As the population size of a speciesshrinks, genetic diversity shrinks in direct propor-tion because as a species diminishes in number,genes disappear even if the species survives. A re-duction of the gene pool renders a species lessadaptable to changing environments and more sus-ceptible to extinction. Future generations will beunable to benefit from currently unidentified butpotentially useful properties of these species if theirgenetic diversity is extinguished.

Without periodic infusions of new germ plasm,crops bred specifically for humans, such as coffee,bananas, and cocoa, cannot continue to producehigh yields at low cost. As the products of genera-tions of selective breeding, these crops continuallyrequire the amalgamation of new genetic material

883

to maintain productivity and fla-vor, to counteract new diseasesand insect strains, and to endureenvironmental stresses such asunusual cold or drought.

In recent decades, a number ofimportant crops, including cocoa,coffee, bananas, and sugarcane,have been saved from viruses andother pathogens by being cross-bred with wild species havingnaturally acquired resistance. Al-though crop diseases can be con-tained or eliminated by apply-ing pesticides, the cost is morethan many farmers can afford. Itis much less expensive, and moreenvironmentally benign, to find a resistant strain ofthe same species in the wild.

Future contributions from wild germ plasm, inaddition to breeding new disease-resistant variet-ies of crops, might include the creation of hybridperennial varieties of annual crops, eliminating theneed for yearly plowing and sowing. Another pos-sibility may be new varieties of conventional cropswhich could survive in conditions or environmentsthat are currently unsuitable, extending the plants’cultivation range.

Rain forests also provide opportunities for hu-mans to develop and cultivate entirely new crops.Many staples, such as rice, peanuts, yams, and pine-apples, originated in ancient tropical forests. Thesestaples are not necessarily the best possible sourcesof nutrition and protein; they were merely the cropsmost easily cultivated by Neolithic humans.

While temperate forests produce fewer than twodozen edible fruits, more than twenty-five hundredpalatable fruits grow in the tropics. Only about 10percent of these are typically consumed, and onlyabout one dozen are exported. The variety of fruitsthat Americans purchase from grocery stores is buta small fraction of those potentially available.

Because Americans consume an enormous quan-tity of sugar annually, there is a need for a sweeten-ing agent lacking the potentially undesirable sideeffects of synthetic sweeteners. Natural sweetenersfound in common fruits are problematic becauseAmericans already consume more of these thanis healthy. However, a new class of nonfatteningnatural sweeteners made of protein compoundshas been identified. At least one thousand times

sweeter than sucrose, and with no known detri-mental side effects, these tropical sweeteners mayprove a viable replacement for the sucrose com-monly added to food items ranging from saladdressing to fish products.

Medicinal PlantsRain-forest plants contain a plethora of yet un-

investigated biodynamic compounds with undis-covered potential for use in modern medicine. Fu-ture generations may benefit from these substancesonly if the species containing them are preservedand studied. Although the medicinal propertiesof plants have been known for at least thirty-fivethousand years, modern science has reduced hu-man dependence on medicinal plants. Neverthe-less, approximately 50 percent of all U.S. prescrip-tion drugs contain substances of natural origin, andat least half of these contain active ingredients de-rived from plants.

Medicinal plants from the tropics aid modernpharmacologists in four ways. Plants may be usedas a direct source of therapeutic drugs that cannoteffectively be synthesized in the laboratory. Plantextracts may become the starting point for morecomplex compounds. Derived substances canserve as “blueprints” for new synthetic com-pounds. Finally, plants may assist as taxonomicmarkers for uncovering new healing compounds.Of the hundreds of thousands of plant species in-habiting the rain forests, only a small fraction havebeen identified and studied, and the potentiallybeneficial pharmacological properties of many ofthese are yet to be ascertained.

884 • Rain-forest biomes

African Rain ForestsAlthough the rain forests of central Africa are at lower risk of deforesta-tion than are tropical forests in some other parts of the world, they arestill in great danger. First, they contain many valuable timber trees thatare being exploited actively. Second, these forested areas are in demandfor other uses. In some areas, excessive timber harvesting and clearingto make way for oil palm and rubber plantations leaves little untouchedforest. Some of the worst damage has occurred in Liberia and the IvoryCoast, where populations are on the rise. Much research is being doneto develop sustainable and regenerative ways to harvest trees. In manyareas, certain parts of the forest are considered sacred by local tribesand are jealously guarded from all encroachment. Medicinal plants areobtained from these areas, and they also are used as burial grounds thatshelter the tribes’ ancestral spirits.

Endangered BiomeAlthough extinction is a natural process, tropical

rain forests are disappearing at a rate four hundredtimes faster than during the recent geological past.One prominent and direct cause of the destructionof the rain forest in Amazonia is the production andtransportation of oil. The fragile rain forest environ-ment is easily contaminated by leaks, spills, and theejection of effluents during pumping operations.Of even greater environmental impact is the oilcompanies’ practice of building roads from inhab-ited areas to the well sites. Pipelines are built alongthe roads to carry the oil out of the jungle, but un-employed urban residents often follow the roadsinto the jungle and become squatters on adjoiningland. They clear a small section of rain forest, usingthe slash-and-burn method, to eke out a living assubsistence farmers.

Unfortunately, when the rain forest is gone, soare most of the nutrients needed for local agricul-ture; the land cannot sustain crops for long. Thenthe farmers and their families must move on and

claim more of the forest, regardless of the effects onthe jungle or its native species. The tens of thou-sands of squatters engaging in this practice contrib-ute to the rapid rate of rain-forest destruction.

Although aware of the problem, the governmentsof most South American oil-exporting countrieshave a strong incentive to underplay or ignore thenegative impact of oil production in their rain for-ests: Their economies depend on oil, which is one ofSouth America’s largest exports. Ultimately the de-mand for oil, driven by high consumption rates inthe industrialized nations of the Northern Hemi-sphere, particularly the United States, is one of theprimary factors causing rain-forest destruction.

George R. Plitnik

See also: African flora; African agriculture; Asianflora; Asian agriculture; Australian flora; Austra-lian agriculture; Biomes: types; Endangered spe-cies; Plant domestication and breeding; Rain forestsand the atmosphere; Slash-and-burn agriculture;South American flora; South American agriculture.

Sources for Further StudyBranford, Susan, and Oriel Glock. The Last Frontier: Fighting Over Land in the Amazon. Totowa,

N.J.: Biblio Distribution Center, 1985. Explains the battles being fought and lost inAmazonia and presents cogent arguments on why it is happening.

Farnsworth, N. “Screening Plants for New Medicines.” In Biodiversity, edited by E. O. Wil-son. Washington, D.C.: National Academy Press, 1988. Presents arguments for preserv-ing the vast pharmacological potential of rain forests.

Myers, Norman. A Wealth of Wild Species: Storehouse for Human Life. Boulder, Colo.: WestviewPress, 1983. Discusses the importance of tropical rain forests as a viable and necessarysource of new genetic material important to the future of humanity.

Place, Susan, ed. Tropical Rainforests: Latin American Nature and Society in Transition. Rev. ed.Wilmington, Del.: Scholarly Resources, 2001. Offers comprehensive coverage of complexrain-forest issues.

RAIN FORESTS AND THE ATMOSPHERE

Categories: Biomes; ecosystems; environmental issues; forests and forestry; pollution

Because photosynthesis releases large amounts of oxygen into the air, a curtailment of the process by rain-forest defores-tation may have negative effects on the global atmosphere.

Rain forests are ecosystems noted for their highbiodiversity and high rate of photosynthesis. The

rapid deforestation of such areas is of great concernto environmentalists both because it may lead to

Rain forests and the atmosphere • 885

the extinction of numerous species and because itmay reduce the amount of photosynthesis occur-ring on the earth.

All living things on the earth—plants, animals,and microorganisms—depend on the “sea” of airsurrounding them. The atmosphere includes abun-dant, permanent gases such as nitrogen (78 percent)and oxygen (21 percent) as well as smaller, variableamounts of other gases such as water vapor andcarbon dioxide. Organisms absorb and use this airas a source of raw materials and release into it by-products of their life activities.

Cellular RespirationCellular respiration is the most universal of the life

processes. A series of chemical reactions begin-ning with glucose and occurring in cytoplasmic or-ganelles called mitochondria, cellular respirationproduces a chemical compound called adenosinetriphosphate (ATP). This essential substance fur-nishes the energy cells need to move, to divide, andto synthesize chemical compounds—in essence, to

perform all the activities necessary to sustain life.Cellular respiration occurs in plants as well as ani-mals, and it occurs during both the day and thenight. In order for the last of the series of chemicalreactions in the process to be completed, oxygenfrom the surrounding air (or water, in the case ofaquatic plants) must be absorbed. The carbon diox-ide that forms is released into the air.

For cellular respiration to occur, a supply ofglucose (a simple carbohydrate compound) is re-quired. Photosynthesis, an elaborate series ofchemical reactions occurring in chloroplasts, pro-duces glucose, an organic carbon compound withsix carbon atoms. Energy present in light must betrapped by the chlorophyll within the chloroplaststo drive photosynthesis. Therefore, photosynthesisoccurs only in plants and related organisms, such asalgae, and only during the daytime. Carbon diox-ide, required as a raw material, is absorbed from theair, while the resulting oxygen is released into theatmosphere. The exchange of gases typically in-volves tiny openings in leaves, called stomata.

886 • Rain forests and the atmosphere

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Oxygen CycleOxygen is required for the survival of the major-

ity of microorganisms and all plants and animals.From the surrounding air, organisms obtain theoxygen used in cell respiration. Plants absorb oxy-gen through the epidermal coverings of their rootsand stems and through the stomatal openings oftheir leaves.

The huge amounts of oxygen removed from theair during respiration must be replaced in orderto maintain a constant reservoir of oxygen in theatmosphere. There are two significant sources ofoxygen. One involves water molecules of the atmo-sphere that undergo a process called photodis-sociation: Oxygen remains after the lighter hydro-gen atoms are released from the molecule andescape into outer space.

The other source is photosynthesis. Chlorophyll-containing organisms release oxygen as they uselight as the energy source to split water moleculesin a process called photolysis. The hydrogen istransported to the terminal phase of photosynthe-sis called the Calvin cycle, where it is used as thehydrogen source necessary to produce and releasemolecules of the carbohydrate glucose. In the mean-time, the oxygen from the split water is releasedinto the surrounding air.

Early in the history of the earth, before certainorganisms evolved the cellular machinery neces-sary for photosynthesis, the amount of atmosphericoxygen was very low. As the number and sizes ofphotosynthetic organisms gradually increased, sodid the levels of oxygen in the air. A plateau wasreached several million years ago as the rate ofoxygen release and absorption reached an equilib-rium.

OzoneAnother form of oxygen is ozone. Unlike ordi-

nary atmospheric oxygen, in which each moleculecontains two atoms, ozone molecules have threeoxygen atoms each. Most ozone is found in thestratosphere at elevations between 10 and 50 kilo-meters (6 and 31 miles). This layer of ozone helps toprotect life on earth from the harmful effects of ul-traviolet radiation. Scientists, especially ecologists,are concerned as the amount of ozone has been re-duced drastically over the last few decades. Al-ready, an increase in the incidence of skin cancer inhumans and a decrease in the efficiency of photo-synthesis has been documented. Another concern

related to ozone is that of an increase in ozone levelsnearer to the ground, where living things areharmed as a result. The formation of ozone fromordinary oxygen within the atmosphere is greatlyaccelerated by the presence of gaseous pollutantsreleased from industrial processes.

Carbon CycleAll forms of life are composed of organic (carbon-

containing) molecules. Carbohydrates include glu-cose as well as lipids (fats, oils, steroids, and waxes),proteins, and nucleic acids. The ability of carbon toserve as the backbone of these molecules resultsfrom the ability of carbon atoms to form chemicalbonds with other carbon atoms and also with oxy-gen, hydrogen, and nitrogen atoms.

Like oxygen, carbon cycles in a predictable man-ner between living things and the atmosphere. Inphotosynthesis, carbon is “fixed” as carbon dioxidein the air (or dissolved in water) is absorbed andconverted into carbohydrates. Carbon cycles to ani-mals as they feed on plants and algae. As both greenand nongreen organisms respire, some of their car-bohydrates are oxidized, releasing carbon dioxideinto the air. Each organism must eventually die, af-ter which decay processes return the remainder ofthe carbon to the atmosphere.

Greenhouse EffectLevels of atmospheric carbon dioxide have fluc-

tuated gradually during past millennia, as revealedby the analysis of the gas trapped in air bubbles ofice from deep within the earth. In general, levelswere lower during glacial periods and higher dur-ing warmer ones. After the nineteenth century, lev-els rose slowly until about 1950 and then muchmore rapidly afterward. The apparent cause hasbeen the burning of increased amounts of fossil fu-els associated with the Industrial Revolution andgrowing energy demands in its wake. The globalwarming that is now being experienced is believedby most scientists to be the cause of increased car-bon dioxide levels. The greenhouse effect is the termgiven to the insulating effects of the atmospherewith increased amounts of carbon dioxide. Theearth’s heat is lost to outer space less rapidly, thusincreasing the earth’s average temperature.

Forest EcosystemsThe biotic (living) portions of all ecosystems in-

clude three ecological or functional categories: pro-

Rain forests and the atmosphere • 887

ducers (plants and algae), consumers (animals), anddecomposers (bacteria and fungi). The everyday ac-tivities of all organisms involve the constant ex-change of oxygen and carbon dioxide between theorganisms of all categories and the surrounding at-mosphere.

Because they release huge quantities of oxygenduring the day, producers deserve special atten-tion. In both fresh and salt water, algae are the prin-cipal producers. On land, this role is played by a va-riety of grasses, other small plants, and trees. Forestecosystems, dominated by trees but also harboringmany other plants, are major systems that producea disproportionate amount of the oxygen releasedinto the atmosphere by terrestrial ecosystems.

Forests occupy all continents except for Antarc-tica. A common classification of forests recognizesthese principal categories: coniferous (northern ev-ergreen), temperate deciduous, and tropical ever-green, with many subcategories for each. The des-ignation “rain forest” refers to the subcategories ofthese types that receive an amount of rainfall wellabove the average. Included are tropical rain forests(the more widespread type) and temperate rain for-ests. Because of the ample moisture they receive,both types contain lush vegetation that producesand releases oxygen into the atmosphere on a largerscale than do other forests.

Tropical and Temperate Rain ForestsTropical rain forests exist at relatively low eleva-

tions in a band about the equator. The Amazon ba-sin of South America contains the largest continu-ous tropical rain forest. Other large expanses arelocated in western and central Africa and the regionfrom Southeast Asia to Australia. Smaller areas oftropical rain forests occur in Central America andon certain islands of the Caribbean Sea, the PacificOcean, and the Indian Ocean. Seasonal changeswithin tropical rain forests are minimal. Tempera-tures, with a mean near 25 degrees Celsius, seldomvary more than 4 degrees Celsius. Rainfall eachyear measures at least 400 centimeters.

Tropical rain forests have the highest biodiver-sity of any terrestrial ecosystem. Included is a largenumber of species of flowering plants, insects, andanimals. The plants are arranged into layers, orstrata. In fact, all forests are stratified but not to thesame degree as tropical rain forests. Amature tropi-cal rain forest typically has five layers. Beginningwith the uppermost, they are an emergent layer (the

tallest trees that project above the next layer); a can-opy of tall trees; understory trees; shrubs, tall herbs,and ferns; and low plants on the forest floor.

Several special life-forms are characteristic of theplants of tropical rain forests. Epiphytes are plantssuch as orchids that are perched high in thebranches of trees. Vines called lianas wrap them-selves around trees. Most tall trees have trunks thatare flared at their bases to form buttresses that helpsupport them in the thin soil.

This brief description of tropical rain forestshelps to explain their role in world photosynthesisand the related release of oxygen into the atmo-sphere. As a result of the many layers of forest vege-tation, the energy from sunlight as it passes down-ward is efficiently utilized. Furthermore, the hugeamounts of oxygen released are available for usenot only by the forests themselves but also, becauseof global air movement, by other ecosystemsthroughout the world. Because of this, tropical rainforests are often referred to as “the earth’s lungs.”

Temperate rain forests are much less extensivethan tropical rain forests. Temperate rain forests oc-cur primarily along the Pacific Coast in a narrowband from southern Alaska to central California.Growing in this region is a coniferous forest but onewith higher temperatures and greater rainfall thanthose to the north and inland. This rainfall of 65 to400 centimeters per year is much less than that of atropical rain forest but is supplemented in the sum-mer by frequent heavy fogs. As a result, evapora-tion rates are greatly reduced. Because of generallyfavorable climatic conditions, temperate rain for-ests, like tropical ones, support a lush vegetation.The rate of photosynthesis and release of oxygen ishigher than in most other world ecosystems.

Ecologists and conservationists are greatly con-cerned about the massive destruction of rain for-ests. Rain forests are being cut and burned at a rapidrate to plant crops, to graze animals, and to providetimber. The ultimate effect of deforestation of thesespecial ecosystems is yet to be seen.

Thomas E. Hemmerly

See also: Acid rain; Air pollution; Biosphere con-cept; Calvin cycle; Carbon cycle; Deforestation;Ecosystems: overview; Ecosystems: studies; Green-house effect; Hydrologic cycle; Ozone layer andozone hole debate; Photorespiration; Photosynthe-sis; Rain-forest biomes; Respiration; Savannas anddeciduous tropical forests.

888 • Rain forests and the atmosphere

Sources for Further StudyLaurance, William F., and Richard O. Bierregaard, eds. Tropical Forest Remnants: Ecology,

Management, and Conservation of Fragmented Communities. Chicago: University of ChicagoPress, 1997. A compilation of articles on tropical forest habitat fragmentation and its ef-fects on global biology. Each section of the book deals with a different component of theproblems.

Shipp, Steve. Rainforest Organizations: A Worldwide Directory of Private and Governmental En-tities. Jefferson, N.C.: McFarland, 1997. Contains contact and mission information for 120environmental, governmental, nongovernmental, religious, and educational agenciesdealing with rain-forest issues.

Townsend, Janet G. Women’s Voices from the Rainforest. New York: Routledge, 1995. Discussesthe many issues of rain-forest management which are obfuscated by the language andmethods of international development policy.

Vandermeer, John. Breakfast of Biodiversity: The Truth About Rain Forest Destruction. Oakland,Calif.: Institute for Food and Development Policy, 1995. Covers human involvement intropical forests in general and ramifications of U.S. policy in the tropics.

RANGELAND

Categories: Animal-plant interactions; economic botany and plant uses; ecosystems.

Open land of a wide variety of types, including grasslands, shrublands, marshes, and meadows as well as some desertand alpine land, is known as rangeland.

Rangeland is a valuable and resilient ecosystemresource that supports considerable plant and

animal life. Rangeland generally refers to a kind ofland rather than a use of that land. The Society forRange Management defines rangelands as “land onwhich the native vegetation (climax or natural po-tential) is predominantly grasses, grass-like plants,forbs, or shrubs.” Rangeland “includes lands re-vegetated naturally or artificially” as well as “natu-ral grasslands, savannas, shrublands, most deserts,tundra, alpine communities, coastal marshes andwet meadows.”

Rangelands usually have some limitation for in-tensive agriculture, such as low and erratic precipi-tation, lack of soil fertility, shallow or rocky soil, orsteep slopes. In addition to livestock grazing, range-lands serve multiple-use functions such as provid-ing recreational opportunities, watersheds, mininglocations, and habitat for many animal species. Re-newable natural resources associated with range-lands are plants and animals (and, in some senses,water). Nonrenewable resources include mineralsand other extractable materials.

Location and CharacteristicsRangelands are extensive and extremely vari-

able. As defined by the Society for Range Manage-ment, they occupy more than 50 percent of theworld’s total land surface and about 1 billion acresin the United States alone. Rangelands are home tonomadic herders on nearly every continent. Theyvary from high-elevation alpine tundra and high-latitude Arctic tundra to tropical grasslands. Thetall-grass prairies in the United States (now mostlyplowed for intensive agriculture) and the richgrasslands of eastern Africa are among the mostproductive.

Rangelands grade into woodlands and forests aswoody species and trees become more abundant.Some forests are grazed by wild and domestic ani-mals, and the distinction between rangeland andforest is often not clear. The other difficult dis-tinction is between rangeland and pastureland.Pastureland is generally improved by seeding, fer-tilization, or irrigation, whereas rangelands sup-port native plants and have little intensive improve-ment.

Rangeland • 889

In the United States, rangeland improvementsduring the twenty years following World War II of-ten included brush control, grazing management,and seeding, but rangelands were not irrigated. Af-ter the 1970’s, when fuel costs increased and envi-ronmental concerns about pesticide use increased,brush control practices were reduced considerably.Today environmental concerns include rangelanddegradation from overgrazing, especially on ripar-ian vegetation along streams, and concern for en-dangered animal and plant species. These issueshave become controversial in the United States.

Rangelands as EcosystemsRangelands constitute natural ecosystems with

nonliving environmental factors such as soil andclimatic factors. Life-forms are primary producers(grasses, forbs, and shrubs), herbivores (livestock;

big game animals such as deer and bison; and manyrodents and insects), carnivores (such as coyotes,bears, and eagles), and decomposers (fungi and bac-teria) that break down organic matter into elementsthat can be utilized by plants. Plants convert carbondioxide and water into complex carbohydrates,fats, and proteins that nourish animals feeding onthe plants.

Individual chemical elements are circulatedthroughout the various components. Many of theseelements are present in the soil, including phospho-rus, magnesium, potassium, and sulfur. Nitrogen,on the other hand, is present in large amounts inthe atmosphere but must be converted (fixed) intoforms that can be utilized by plants before it can becycled. Energy is fixed through the process of pho-tosynthesis and transformed to forms useful for theplants, then the animals that feed on plants.

890 • Rangeland

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When chemicals are taken up by plant roots fromthe soil, they become available to a wide group ofherbivores, from small microbes to large ungulates.Eventually nutrients are passed on to organisms athigher trophic levels (omnivores and carnivores).Both plant and animal litter is eventually brokendown by decomposers—bacteria, fungi, and othersoil organisms—and returned to the soil or, in thecase of carbon or nitrogen, given off to the atmo-sphere.

However, energy is degraded at each step alongthe way; energy is transferred but not cycled. Graz-ing animals on rangelands influence plants by re-moving living tissue, by trampling, and by alteringcompetitive relations with other plants. Large graz-ing animals tend to compact the soil, reducing infil-tration and increasing surface runoff.

Rangeland DynamicsRangelands vary considerably with time. Scien-

tists are gaining a better understanding of some fac-tors related to rangeland change. Pollen recordsand, in the southwestern United States, packratmiddens have been used to reconstruct past climateand vegetational conditions. Some areas have be-come drier and others more mesic. The formationand retreat of glaciers influenced climatic patterns

and soil development. A recent general trend inmany rangelands is an increase in woody plants atthe expense of grasses. Many factors are probablyresponsible for these shifts, but fire control, over-grazing, climatic shifts, introduction of exotic spe-cies, and influence of native animals are likelycausal agents.

Rangelands are being threatened by encroach-ment from crop agriculture as worldwide develop-ment increases. Nomadic herders traditionally metperiodic drought conditions by having the flexibil-ity to move to areas not impacted by drought. Now,with area lost to livestock grazing and other politi-cal restrictions, herders are often forced to maintainhigher livestock numbers to support those directlydependent on livestock. Despite various kinds ofdisturbances and stresses on rangelands, these ar-eas have supported many large grazing animalsand people for centuries.

Rex D. Pieper

See also: Agriculture: history and overview; Agri-culture: modern problems; Forest and range policy;Forests; Grasses and bamboos; Grasslands; Graz-ing and overgrazing; Trophic levels and ecologicalniches; Tundra and high-altitude biomes.

Sources for Further StudyHeady, Harold F., and R. Dennis Child. Rangeland Ecology and Management. 2d ed. Boulder,

Colo.: Westview, 2000. General text on rangeland grazing, animal management, and veg-etation manipulation.

Holechek, Jerry L., Rex D. Pieper, and Carlton H. Herbel. Range Management: Principles andPractices. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2001. General text on rangelandsand their management.

Jacobs, Lynn. Waste of the West: Public Lands Ranching. Tucson, Ariz.: L. Jacobs, 1991. This crit-ical account of public lands ranching deals with historical and contemporary situationsand issues of environment, economics, politics, animal welfare, livestock production, andmore. Includes ideas for activism, bibliography, and index.

Longworth, John W., and Gregory J. Williamson. China’s Pastoral Region: Sheep and Wool, Mi-nority Nationalities, Rangeland Degradation, and Sustainable Development. Wallingford, En-gland: CAB International, 1993. Comprehensive picture of this increasingly importantpart of the world focuses on improving and preserving the livelihood of pastoral commu-nities in northern and northwestern China.

Sayre, Nathan F. The New Ranch Handbook: A Guide to Restoring Western Rangelands. Santa Fe,N.Mex.: Quivera Coalition, 2001. Describes practices of New Ranches in New Mexico, Ar-izona, and Colorado, where vegetation and soil have responded positively to innovativeways of managing livestock.

Rangeland • 891

RED ALGAE

Categories: Algae; economic botany and plant use; food; taxonomic groups; water-related life

Red algae, from the phylum Rhodophyta (from the Greek rhodos, meaning “red”), are named for their reddish color be-cause of their pigment phycoerythrin. Despite their name, not all rhodophytes are red. Some of them have very littlephycoerythrin and appear green or bluish. Most are multicellular and vary greatly in shape; platelike, coral-like,crustlike, leathery, and featherlike forms are known.

The bodies of some rhodophytes are relativelycomplex, characterized by a great deal of

branching of their leaflike structures as well as thepresence of a holdfast that resembles plant roots.These rhodophytes are commonly known as sea-weeds. Red algae are distinctive from other eukary-otic algae in that they lack flagella (or motile cells ofany kind) in their vegetative cells, spores, and ga-metes.

There are four thousand to six thousand speciesof red algae, and although some rhodophytes do in-habit fresh water (about fifty species), red algae aremost common in tropical marine environments.Many are found at great depths, living 210-260 me-ters below the surface of the ocean. The pigmentphycoerythrin allows red algae to live and photosyn-thesize at these depths. Phycoerythrin absorbs bluelight, which penetrates water to a greater depththan light of longer wavelengths normally used inphotosynthesis. Reds are the deepest-growingphotosynthetic eukaryotes.

HabitatsRhodophytes are important members of many

periphyton communities (which typically grow at-tached to substrata) from tropical to polar seas.Epiphytism is common among red algae. Epiphyticrhodophytes grow on the surface of larger, typicallybrown, algae. Although their holdfasts penetratethe tissue of their hosts, they do not obtain any nu-trients from host algae.

Nearly 15 percent of red algae are parasites, of-ten living on closely related species. Parasitic redstransfer nuclei into host cells and transform them.Host reproductive cells may then carry the para-site’s genes. Other Rhodophyta are extremophiles.For example, the red alga Cyanidium lives in acidic

(pH 2-4) hot springs at 55 degrees Celsius. Somecalcified rhodophytes are major contributors to theformation of tropical reefs. Reef-building red algaeare called coralline algae.

Structure and PropertiesA red algal cell is surrounded by a cell wall. In

many species, the main wall component is cellulose(similar to cell walls of various other algae andplants), but other reds have mannans (polymers ofmannose) and xylans (polymers of xylose). Otherpolymers associated with the cell walls include agarand carrageenan. The majority of coralline red algaecontain calcium carbonate, which forms limestonein the cell walls. Because of their ability to secretecalcium carbonate, red algae do not decay and havea better preserved fossil record than many other al-gae. Fossils of red algae have been found in rocks500 million years old. Production of calcium car-bonate is linked to photosynthetic carbon fixation.Apparently, carbon dioxide fixation results in a pHincrease (an increase in alkalinity), which facilitatescalcium carbonate precipitation.

An unusual feature of red algae compared toother algae is the occurrence of protein plugs (pitconnections) in cell walls between the cells, al-though some red algae lack them. All plugs consistof protein, and in some species protein polysaccha-rides are an additional component. Cells of theRhodophyta may contain several nuclei as a result ofeither the fusion of nongamete cells or mitosis with-out cytokinesis. Cell fusion is a very important fea-ture of parasitic reds. Red algae lack centrioles, butthe mitotic spindle radiates from the “nuclear-associated organelle,” which often appears as a pairof short, hollow cylinders. Some red algae havelarge vacuoles in the centers of their cells. Cells of

892 • Red algae

the Rhodophyta may produce mucilage, which playsan important role in the attachment of their repro-ductive cells. Mucilages are polymers of D-xylose,D-glucose, D-glucuronic acid, and galactose andare produced within Golgi apparatuses.

Pigments of red algae include chlorophyll aand two classes of accessory pigments: phycobilinsand carotenoids. Phycoerythrin, phycocyanin, andallophycocyanin are phycobilins. They attach toproteins known as phycobiliproteins, which oc-cur in highly organized structures called phyco-bilisomes. Phycoerythrin occurs in at least fiveforms in the Rhodophyta (B-phycoerythrin I and II,R-phycoerythrin I, II, and II). Carotenoids are alsofound in plants, and those in red algae are similar instructure and function.

Some parasitic forms of red algae lack photosyn-thetic pigments. In red algae that have pigments, allpigments are located in the chloroplasts. Red algaechloroplasts have a highly distinctive ultrastructure.Two membranes surround each chloroplast. Chlo-roplasts of red algae probably originated from cya-nobacteria that formed an ancient symbiotic rela-tionship with the reds. Both red algal chloroplastsand cyanobacteria share same phycobilin pig-ments. Inside the chloroplast are thylakoids, whichare not stacked. This is the same arrangement foundin cyanobacteria, but it is different from that ofother algae and from plants. On the thylakoid sur-face there are many phycobilisomes. Some red al-gae have chloroplasts that contain pyrenoids,which have no known function.

Photoautotrophy is the principal mode of nutri-tion in red algae; in other words, they are “self-feeders,” using light energy and photosyntheticapparatuses to produce their own food (organiccarbon) from carbon dioxide and water. A few Rho-dophyta are heterotrophic, and these organisms aregenerally obligate parasites (parasites that mustlive off a host) of other algae. Carbon and nitrogenmetabolism in red algae is similar to that in other al-gae. Various rhodophytes produce unusual carbo-hydrates, such as digeneaside, which is used to reg-ulate osmotic status of cells in response to droughtstress in shoreline environments. Some red algaeare covered by surface-protein cuticle, which is dif-ferent from that found in higher plants. The foodstorage of red algae is a unique polysaccharidefloridean starch. This starch differs from that syn-thesized by green algae and plants. Floridean starchgrains are formed in the cytoplasm. Red algae store

inorganic nitrogen in the form of phycobilin pig-ments.

ReproductionRed algae reproduce both asexually and sexu-

ally. Methods of asexual reproduction include dis-charging spores and fragmentation of the algalbodies. Sexual reproduction, as well as alteration ofgenerations, is widespread among the Rhodophyta,but two classes of red algae (floridean and bangean)have particular variations.

In contrast to the two phases in an alteration ofgenerations of other algae and plants (gametophyteand sporophyte, haploid and diploid stages, corre-spondingly), most species of red floridean algaehave three phases: free-living, haploid gameto-phytes, diploid carposporophytes, and diploid tet-rasporophytes. Male and female gametophytes areoften separate. The male gametophytes producemale nonflagellated gametes called spermatia. Fe-male gamethophytes produce a special branch, thecarpogonial branch, that produces a terminal car-pogonium (oogonium, an egg-bearing structure).

Contact between spermatia and carpogonia is fa-cilitated by water movements. The carposporophyteis a diploid stage that develops from the zygote (fer-tilized carpogonium) and produces carpospores.Diploid tetrasporophytes develop from carpo-spores. Tetrasporophytes form tetrasporangia,which produce four haploid tetraspores. When re-leased, tetraspores develop into new gameto-phytes. The gametophyte and tetrasporophyte mayappear nearly identical, and therefore can be said tobe isomorphic, as in the Polysiphonia. Alternatively,the tetrasporophyte and gametophyte may be verydifferent in size and appearance (heteromorphic),as in Phyllophora.

Diversity of Red AlgaeRed algae are divided into two subclasses or

classes: Florideophyceae (florideophyceans orfloridean) and Bangiophyceae (bangiophyceans orbangean). Floridean algae have numerous smallchloroplasts and a complex life cycle. Bangean al-gae have life cycles without carpogonia andcarposporophyte development and have a singlecentral chloroplast. Representative species ofFlorideophyceae are Batrachospermum, Chondrus,Corallina, Gelidium, and Polysiphonia. Representa-tives of Bangiophyceae include Porphyra, Bangia, andCyanidium.

Red algae • 893

UsesPeople have used red algae for thousands of

years. Most are collected along seashores for usein human food or for the extraction of gellingcompounds. A few red algae, such as Porphyra,Eucheuma, and Gracilaria, are cultivated. More than60,000 hectares of sea along Japanese coasts areoccupied by “red algal culture.” Thousands of peo-ple worldwide are engaged in cultivating red sea-weeds.

The most valuable of all algae is Porphyra. Theannual Porphyra harvest worldwide has been esti-mated to be worth 2.5 billion dollars. Porphyra (inJapanese, Nori; in Chinese, Zicai) is used as a wrap-per for sushi or may be eaten mixed with rice andfish and in salads. It is very rich in vitamins B and Cas well as minerals, including iodine. There areabout seventy species of Porphyra, but the mostwidely used species is Porphyra yezoensis. Two im-portant compounds derived from red algae areagar and carrageenan, both of which are polymersof galactose. Agar is used as a medium for culturingmicroorganisms, including algae; as a food gel (forjams and jelly); and in pharmaceutical capsules. In

the United States, agar is used in the canning indus-try as a protective agent against the unwanted ef-fects of metals. In addition, agar is the source ofagarose, which is widely used in recombinant DNA(deoxyribonucleic acid) technology for gel electro-phoresis. The first agar was produced in 1670 in Ja-pan, and Japan is still the largest producer of agar.

The red algae Gelidium, Gracelaria, and Ptero-cladia are harvested for extraction of agar. Carra-geenan is used in toothpaste, cosmetics, and food,such as ice cream and chocolate milk. Eucheuma,Kappaphycus, and Chondus (the so-called Irish moss)are the sources of carrageenan. The most importantproducer of carrageenan is Europe, followed by thePhilippines and Indonesia.

Sergei A. Markov

See also: Agriculture: marine; Algae; Brown algae;Cell wall; Charophyceae; Chlorophyceae; Chryso-phytes; Cryptomonads; Diatoms; Dinoflagellates;Electrophoresis; Euglenoids; Eutrophication;Green algae; Haptophytes; Marine plants; Photo-synthesis; Phytoplankton; Pigments in plants;Protista; Ulvophyceae.

Sources for Further StudyGraham, Linda E., and Lee W. Wilcox. Algae. Upper Saddle River, N.J.: Prentice Hall, 2000.

This textbook focuses on the major algal types and gives a comprehensive overview of thephylum Rhodophyta.

Van Den Hoek, C., et al. Algae: An Introduction to Phycology. Cambridge, England: CambridgeUniversity Press, 1995. Acomprehensive text on major algal groups, including red algae.

REFORESTATION

Categories: Forests and forestry; environmental issues

Reforestation is the growth of new trees in an area that has been cleared for human activities. It can occur naturally orbe initiated by people.

Many areas of the eastern United States, such asthe New England region, reforested natu-

rally in the nineteenth and early twentieth centu-ries after farmland that had been abandoned wasallowed to lie fallow for decades. After an area hasbeen logged, environmentalists, as well as the com-mercial logging industry, advocate planting treesrather than waiting for natural regrowth because

the process of natural regeneration can be bothslow and unpredictable. In natural regeneration,the mixture of trees in an area may differ signifi-cantly from the forest that preceded it. For exam-ple, when nineteenth century loggers clear-cut thewhite pine forests of the Great Lakes region, manylogged-over tracts grew back primarily in mixedhardwoods.

894 • Reforestation

Land that has been damaged by industrial pollu-tion or inefficient agricultural practices sometimesloses the ability to reforest naturally. In some re-gions of Africa, soils exposed by slash-and-burn agri-culture contain high levels of iron or aluminum ox-ide. Without a protective cover of vegetation, evenunder cultivation, soil may undergo a processknown as laterization. Laterite is a residual productof rock decay that makes soil rock-hard. Such aban-doned farmland is likely to remain barren of plantlife for many years. In polluted areas such as formermining districts, native trees may not be able to tol-erate the toxins in the soil; in these cases, more toler-ant species must be introduced.

Safeguarding Timber ResourcesReforestation differs from tree farming in that the

goal of reforestation is not always to provide wood-lands for future harvest. Although tree farming is atype of reforestation (trees are planted to replacethose that have been removed), generally only one

species of tree is planted, with explicit plans for itsfuture harvest. The trees are seen first as a crop andonly incidentally as wildlife habitat or a means oferosion control.

As foresters have become knowledgeable aboutthe complex interactions within forest ecosystems,however, tree farming methods have begun tochange. Rather than monocropping (planting onlyone variety of tree), the commercial forest industryhas begun planting mixed stands. Trees that pos-sessed no commercial value, once considered unde-sirable weed trees, are now recognized as nitrogenfixers necessary for the healthy growth of other spe-cies. In addition to providing woodlands for possi-ble use in commercial forestry, goals of reforesta-tion include wildlife habitat restoration and thereversal of environmental degradation.

Early EffortsReforestation to replace trees removed for com-

mercial purposes has been practiced in Western

Reforestation • 895

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Europe since the late Middle Ages. English mon-archs, including Queen Elizabeth I, realized thatforests were a vanishing resource and establishedplantations of oaks and other hardwoods to ensurea supply of ship timbers. Similarly, Sweden createda corps of royal foresters to plant trees and watchover existing woodlands. These early efforts at re-forestation were inspired by the reduction of avaluable natural resource. By the mid-nineteenthcentury it was widely understood that the removalof forest cover contributes to soil erosion, waterpollution, and the disappearance of many speciesof wildlife.

Ecological and Environmental AspectsWater falling on hillsides made barren by clear-

cutting timber washes away topsoil and causesrivers to choke with sediment, killing aquatic life.Without trees to slow the flow of water, rain canalso run off slopes too quickly, causing rivers toflood. For many years, soil conservationists advo-cated reforestation as a way to counteract the eco-logical damage caused by erosion.

In the mid-twentieth century, scientists estab-lished the vital role that trees, particularly those intropical rain forests, play in removing carbon diox-ide from the earth’s atmosphere through the pro-cess of photosynthesis. Carbon dioxide is a green-house gas: It helps trap heat in the atmosphere. Asforests disappear, the risk of global warming—caused in part by an increase in the amount of car-bon dioxide in the atmosphere—becomes greater.Since the 1980’s, scientists and environmental ac-tivists concerned about global warming havejoined foresters and soil conservationists in urgingthat for every tree removed anywhere, whether toclear land for development or to harvest timber, re-placement trees be planted. As the area covered bytropical rain forests shrinks in size, the threat of ir-

reversible damage to the global environment be-comes greater.

Reforestation ProgramsIn 1988 American Forests, an industry group, es-

tablished the Global ReLeaf program to encouragereforestation efforts in an attempt to combat globalwarming. In addition to supporting reforestationefforts by government agencies, corporations, andenvironmental organizations, Global ReLeaf andsimilar programs encourage people to practice re-forestation in their own neighborhoods. Trees serveas a natural climate control, helping to moderate ex-tremes in temperature and wind. Trees in a well-landscaped yard can reduce a homeowner’s en-ergy costs by providing shade in the summer andserving as a windbreak during the winter. GlobalReLeaf is one of many programs that support refor-estation efforts.

Arbor Day, an annual day devoted to plantingtrees for the beautification of towns or the foresta-tion of empty tracts of land, was established in theUnited States in 1872. The holiday originated in Ne-braska, a prairie state that seemed unnaturally bar-ren to homesteaders used to eastern woodlands.Initially emphasizing planting trees where none hadexisted before, Arbor Day is observed in U.S. publicschools to educate young people about the impor-tance of forest preservation. Organizations such asthe National Arbor Day Foundation provide sap-lings (young trees) to schools and other organiza-tions for planting in their own neighborhoods.

Nancy Farm Männikkö

See also: Deforestation; Erosion and erosion con-trol; Forest management; Logging and clear-cutting; Rain-forest biomes; Rain forests and theatmosphere; Slash-and-burn agriculture; Soil man-agement; Sustainable forestry; Timber industry.

Sources for Further StudyCherrington, Mark. Degradation of the Land. New York: Chelsea House, 1992. Addresses

large-scale ramifications of deforestation and reforestation.TreePeople, with Andy Lipkis and Katie Lipkis. The Simple Act of Planting a Tree: ACitizen For-

ester’s Guide to Healing Your Neighborhood, Your City, and Your World. Los Angeles: St. Mar-tin’s Press, 1990. Specific and local approaches to the ramifications of deforestation andreforestation.

Weiner, Michael. Plant a Tree: Choosing, Planting, and Maintaining This Precious Resource. NewYork: Wiley, 1992. Specific and local approaches to the ramifications of deforestation andreforestation.

896 • Reforestation

REPRODUCTION IN PLANTS

Categories: Genetics; physiology; reproduction and life cycles

Plants have evolved a remarkable number of ways to increase their numbers. These include not only a variety of forms ofsexual reproduction, in which two individuals produce specialized cells that fuse to become a new offspring, but alsomany ways of achieving asexual reproduction, in which a single plant produces offspring.

In unicellular organisms three steps result in cellu-lar reproduction. Among prokaryotic organisms

(made of cells that have no nuclei), the single loopof DNA (deoxyribonucleic acid, the molecule thatcarries genetic information) replicates; then onecopy is carried to each daughter cell as the originalcell elongates and then “pinches” in two, a processcalled fission.

In eukaryotic organisms (whose cells have nu-clei), the DNA is located within a nucleus in dis-crete chromosomes that must be precisely dividedbetween the two daughter cells. Nuclear division toproduce two identical daughter cells (asexual re-production) is called mitosis. Achromosome duringcell division consists of two halves, sister chromatids,each of which is identical to the other. During mito-sis, every chromosome in the nucleus splits in halfso that one chromatid will migrate to the firstdaughter cell, and the second chromatid migratesto the other. When cell division is complete, the re-sult is two genetically identical daughter cells.

In sexual reproduction, a nucleus must divide bymeiosis. In sexually reproducing organisms, at leastsome cells will have pairs of every type of chromo-some. Such cells are diploid, or 2n, where n is thenumber of different types of chromosomes. Duringmeiosis, the pairs separate so that each daughtercell has only one of each type of chromosomes andis haploid, or n. If two different haploid cells fuse, theresulting cell will have pairs of every type of chro-mosome but with one of each pair contributed byeach of the two parents. The offspring will thus bedifferent from either parent.

Sexual Reproduction: Alternation of GenerationsSexual reproduction provides an opportunity

for an organism to have different kinds of cells at

different stages of its life cycle. The most familiarexample is what is known from animals, includinghumans. The body cells of the adult are diploid, butin the reproductive organs, meiosis occurs to formhaploid cells, either eggs or sperm. If these haploidreproductive cells (gametes) fuse, a new diploid cellis formed, the zygote. The zygote divides mitoticallyto form an embryo and eventually a new adult con-sisting of diploid body cells similar to those of bothparents. This is a gametic life cycle, in which thegametes are the only haploid cells, and they areformed directly by meiosis. Fertilization occurs im-mediately after meiosis.

Some algae, particularly diatoms and some ofthe green algae, also have a gametic life cycle. Inmany plant species the gametes are large, immotileeggs and small, motile sperm, just as in animals.This condition is oogamy. However, many otherplants are not oogamous. In some cases the two ga-metes appear to be identical (isogamous), while inothers there may be two distinctive sizes of ga-metes, but their shape and motility are the same(anisogamous).

Some of the green algae have a life cycle exactlythe opposite of the gametic cycle described above.In these plants, the diploid zygote divides by meio-sis to produce haploid daughter cells, which multi-ply to form either a population of haploid unicellu-lar plants or a multicellular plant with a haploidbody. Eventually some of these haploid cells willdifferentiate into gametes, and two gametes willfuse to form a new zygote. In this type of zygotic lifecycle, the zygote is the only diploid cell in the plant’slife cycle, and fertilization is delayed after meiosisoccurs.

Even more interesting is the sporic life cycle, inwhich a plant will have both a haploid gametophyte

Reproduction in plants • 897

and diploid sporophyte body at different stages ofits life. This type of life cycle is often called alter-nation of generations. These bodies may look thesame (isomorphic), or they may look completelydifferent (heteromorphic). In some cases two differ-ent species have been described and later discov-ered to be simply the haploid and diploid formsof the same species. Most plants, including most al-gae and fungi, have some form of a sporic life cy-cle. In a sporic life cycle, the diploid adult sporo-phyte plant forms reproductive organs in whichmeiosis occurs to form spores. These spores ger-minate and undergo mitosis to form a multicellu-lar haploid gametophyte body. The gametophyteforms reproductive organs in which gametes areproduced. Following fertilization, the resulting zy-gote undergoes mitosis to form the new sporo-phyte.

Many red algae and some of the green andbrown algae are isomorphic; that is, the gameto-phyte and sporophyte bodies look identical. Bryo-phytes, some green algae, some brown algae, andmost fungi are heteromorphic, with the gameto-phyte being the dominant, more conspicuous body.For instance, in mosses the green leafy plant is thegametophyte. When it is mature, the gametophytewill produce two types of gametangia, archegoniaand antherida. Archegonia are flask-shaped struc-tures that produce an egg, and antheridia are saclikestructures that produce multiple sperm. After asperm cell fertilizes the egg, the zygote grows into asporophyte plant, still attached to and growing outof the gametophyte. The tip of the sporophyteswells to become a capsule, or sporangium, wheremeiosis occurs to form haploid spores. Each sporethat falls onto a suitable place will germinate andgrow into a new, leafy gametophyte plant.

Vascular plants and many brown algae areheteromorphic, with the sporophyte dominant. Forinstance, leafy ferns are the leaves of a sporophyteplant with an underground stem and roots.Sporangia typically develop on the underside ofthe fern, and meiosis occurs inside the sporangia toproduce haploid spores. If a spore falls onto a suit-able habitat it will germinate to form a small, incon-spicuous gametophyte plant. The gametophytetypically produces archegonia and antheridia. Fol-lowing fertilization, a new sporophyte plant growsout of the archegonium, but it is usually not seenuntil the first leaves enlarge enough to be visibleabove the soil surface.

Asexual ReproductionIn theory, any cell from a plant body should be ca-

pable of generating an entire new plant, because thenucleus of each cell has identical genetic informationto every other cell in that body. In fact, since the mid-1950’s it has been possible to clone many plants—produce entire new plants from single cells of a par-ent plant. The techniques of plant cell and tissueculture are used to propagate many commerciallyimportant species of ornamental plants. These tech-niques are also valuable tools in plant research. Thebasis for these tools is found in nature—the varietyof methods of asexual reproduction found in plants.

In nonvascular plants, particularly fungi andfilamentous algae, fragmentation can be an effec-tive way of increasing the number of individuals ofa plant. If the plant body is physically broken intopieces, each piece may continue to grow and de-velop as an independent plant. Most algae alsoform sporangia, asexual reproductive organs thatproduce motile zoospores. When the unicellularzoospores are released, they will swim for a periodand, if they settle in a suitable environment, willgerminate and grow to form a new plant. In somecases specialized multicellular asexual propagulesare formed. For instance, lichens may form isidia orsoridia, and liverworts may form gemmae. If thepropagule breaks off and lands in a suitable envi-ronment, it will grow into a new plant.

Asexual reproduction may also occur in vascu-lar plants. If a stem is laid horizontally on theground, it may produce adventitious roots, whichwill allow that portion of the stem to grow as a sepa-rate plant. Similarly, some plants have roots thatform buds and produce new stems at some distancefrom the original stem. The most dramatic exam-ples of this are aspen groves in which all the treesare clones of one another. These natural phenomenaare the basis of horticultural plant propagation bymeans of stem, leaf, or root cuttings.

Other plants form specialized structures forasexual propagation. For instance, strawberriesproduce stolons, specialized stems that grow outfrom a plant, then root and form new plantlets.Bulb-forming plants, such as gladiolus, frequentlyform new bulblets, and if a rhizome is split, such ason an iris, each half will continue to grow as a newplant. Some plants, such as kalenchoe, producecomplete plantlets on the edges of leaves, which falloff and disperse to propagate the plant.


898 • Reproduction in plants

See also: Algae; Angiosperm life cycle; Angio-sperm plant formation; Bacteria; Bryophytes; Chro-mosomes; Cloning of plants; DNA in plants; DNAreplication; Fungi; Germination and seedling de-

velopment; Hybridization; Lichens; Mitosis andmeiosis; Nonrandom mating; Nucleus; Plant bio-technology; Plant life spans; Pollination; Seeds;Shoots.

Sources for Further StudyBell, Peter Robert. Green Plants: Their Origin and Diversity. 2d ed. New York: Cambridge Uni-

versity Press, 2001. Includes detailed life cycles of green plants.Hartmann, Hudson T., et al. Plant Propagation: Principles and Practices. 5th ed. Englewood

Cliffs, N.J.: Prentice Hall, 1990. Ahandbook of techniques for reproducing plants with de-tails for most commercially important species.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Chapters on plant diversity cover basic life cycles.

REPRODUCTIVE ISOLATING MECHANISMS

Categories: Evolution; genetics; reproduction and life cycles

Reproductive isolating mechanisms are genetically influenced, as opposed to factors such as geographic isolation. Thesebuilt-in isolating mechanisms prevent interbreeding between species and thereby promote reproductive efficiency.

Reproductive isolating mechanisms prevent in-terbreeding between species. The term, which

was first used by Theodosius Dobzhansky in 1937in his landmark book Genetics and the Origin of Spe-cies, refers to mechanisms that are genetically influ-enced and built-in. Geographic isolation can pre-vent interbreeding among populations, but it is anexternal factor. The standard model for speciationrequires that populations be geographically iso-lated long enough to diverge genetically. Later, ifthe geographic barriers break down, built-in isolat-ing mechanisms maintain reproductive isolationbetween the divergent populations. As these mech-anisms continue to prevent hybridization, contin-ued divergence leads to new species.

Reproductive isolating mechanisms functiononly between sexually reproducing species. Theyhave no applicability to forms that reproduce onlyby asexual means. Hermaphrodites, organisms withboth male and female reproductive organs that re-produce only by self-fertilization (rare in animals,more common in plants), represent a distortion ofthe sexual process that produces essentially thesame results as asexual reproduction. Many loweranimals, many plants, and protists regularly em-

ploy both asexual and sexual means of reproduc-tion, and the significance of isolating mechanismsin such forms is essentially the same as in normalsexual species.

Prezygotic MechanismsReproductive isolating mechanisms are usually

classified into two main groups. Premating, orprezygotic, mechanisms operate prior to mating, orthe release of gametes, and therefore do not result ina waste of the reproductive potential of the individ-ual. By contrast, postmating, or postzygotic, mecha-nisms come into play after mating, or the release ofgametes, and could result in a loss of the geneticcontribution of the individual to the next genera-tion. This distinction is important in the theoreticalsense, in that natural selection should favor genesthat promote premating isolation. Genes that donot promote premating isolation presumablywould be lost more often through mating with anindividual from another species, which often leadsto no offspring or infertile offspring, in turn leadingto a reinforcement of premating isolation.

Ecological isolation (habitat isolation) often playsan important role in both animals and plants. Dif-

Reproductive isolating mechanisms • 899

ferent forms may be adapted to different habitats inthe same general area and may meet only infre-quently at the time of reproduction. Different plantspecies may occur on different soils, on differentdrainage profiles or exposures, or at different alti-tudes. This type of isolation, although frequent andwidespread, is often incomplete, as the differentforms may come together in transitional habitats.The importance of ecological isolation, however, isattested by the fact that when hybrid swarms (groupsof organisms that show signs of extensive hybrid-ization) are produced between forms that normallyremain distinct, they have often been found to re-sult from disruption of the environment, usually byhumans.

Mechanical isolation is a less important type ofpremating isolation in plants, though it does occurin some. Complex floral structures in certain plants(such as orchids) may favor one species of animalpollinator over others. Finally, temporal differencesoften contribute to premating isolation. The mostcommon type of temporal isolation is seasonal isola-tion: Species may reproduce at different times of theyear. One type of western pine normally sheds itspollen in February, while another does not shed itspollen until April. Differences can also involve thetime of day. In one species of desert plant, the flow-ers open in the early morning, while in another spe-cies of the same genus the flowers open in the lateafternoon. Such differences, as in the case of ecolog-ical isolation, are often incomplete but may be animportant component of premating isolation.

Postzygotic MechanismsIf premating mechanisms fail, postmating mech-

anisms can come into play. If gametes are released,there still may be a failure of fertilization (inter-sterility). In plants, the pollen may not germinate onthe foreign stigma or the pollen tube may fail todevelop. Fertilization failure is almost universalbetween remotely related species and occasionallyoccurs even between closely related species.

If fertilization does take place, other postmatingmechanisms may operate. The hybrid may beinviable (zygotic inviability). In other cases, develop-ment may be essentially normal, but the hybridmay be ill-adapted to survive in any available habi-tat (hybrid adaptive inferiority). Even if hybrids areproduced, they may be partially or totally sterile(hybrid sterility). Hybrids between closely related

forms are more likely to be fertile than those be-tween more distantly related species, but the corre-lation is an inexact one. The causes for hybrid steril-ity are complex and can involve genetic factors,differences in gene arrangements on the chromo-somes that disrupt normal chromosomal pairingand segregation at meiosis, and incompatibilitiesbetween cytoplasmic factors and the chromo-somes.

If the hybrids are fertile and interbreed orbackcross to one of the parental forms, a subtlerphenomenon known as hybrid breakdown some-times occurs. It takes the form of reduced fertility orreduced viability in the offspring. The basis for hy-brid breakdown is poorly understood but may re-sult from an imbalance of gene complexes contrib-uted by the two species.

A Fail-Safe SystemIn most cases of reproductive isolation that have

been carefully studied, more than one kind of iso-lating mechanism has been found. Even thoughone type is clearly of paramount importance, it isusually supplemented by others. Should the pre-dominant type fail, others may come into play. Inthis sense, reproductive isolation can be viewed as afail-safe system.

A striking difference in the overall pattern of re-productive isolation between animals and plants isthe much greater importance of premating isola-tion in animals and the emphasis on postmatingmechanisms in plants. Behavioral isolation, togetherwith other premating mechanisms, is highly effec-tive in animals, and postmating factors usuallyfunction only as a last resort. In contrast, behavioralfactors contribute little to premating isolation inplants. Pollen in many forms is widely distributedeither by the wind or by unselective animalpollinators, and postmating factors consequentlyare much more likely to come into play. This differ-ence is reflected in a much higher incidence of natu-ral hybridization in plants as compared with ani-mals.

John S. Mecham

See also: Gene flow; Genetic drift; Genetics: Men-delian; Genetics: mutations; Genetics: post-Mendelian; Hybrid zones; Hybridization; Non-random mating; Pollination; Population genetics;Species and speciation.

900 • Reproductive isolating mechanisms

Sources for Further StudyBaker, Jeffrey J. W., and Garland E. Allen. The Study of Biology. 4th ed. Reading, Mass.:

Addison-Wesley, 1982. Offers coverage of isolating mechanisms and speciation. Includeslist of suggested readings.

Dobzhansky, Theodosius. Genetics of the Evolutionary Process. New York: Columbia Univer-sity Press, 1970. Dobzhansky’s contributions over the years have had a major impact ongenetics and evolutionary biology. This book is a revision and expansion of his Geneticsand the Origin of Species, which first appeared in 1937. The book is directed at the advancedreader and presupposes a foundation in genetics. Much of chapter 10, however, which isdevoted to an excellent review of reproductive isolation, can be understood by the gen-eral reader. Includes extensive technical bibliography.

Futuyma, Douglas J. Evolutionary Biology. 3d ed. Sunderland, Mass.: Sinauer Associates,1998. This process-oriented textbook assumes some undergraduate training in biology,especially genetics, but most topics are prefaced with background information thatshould help the less prepared. Treatment of isolating mechanisms is brief, but the bookprovides one of the best introductions to current evolution theory available. Major refer-ences are given at the end of each chapter; includes extensive list of references.

Grant, Verne. Plant Speciation. 2d ed. New York: Columbia University Press, 1981. Acompre-hensive review of speciation and related topics in plants that should be required readingfor the advanced student with a special interest in this area. The approach is essentiallyclassical, however, and little attention is given to more recent contributions of molecularbiology. Two short chapters are devoted to isolating mechanisms. Includes technical bib-liography.

Stebbins, G. Ledyard, Jr. Variation and Evolution in Plants. New York: Columbia UniversityPress, 1950. When it appeared, this book represented a landmark synthesis of evolution asapplied to the plant kingdom. Although the book is now out of date, most of the materialstill rings true. Chapter 6 is largely devoted to a very complete treatment of isolatingmechanisms in plants, including numerous examples. Includes numerous technical ref-erences.

RESISTANCE TO PLANT DISEASES

Categories: Anatomy; diseases and conditions; physiology

Plants have a number of defense mechanisms by which they can resist biotic or abiotic stresses that might cause theirdeath or inhibit their growth. Such mechanisms are referred to as resistance.

Plants, the primary producers in all food chains,are besieged by a host of biological agents

throughout their life cycles. Each species of plant isattacked by at least one hundred different kinds ofmycoplasmas, viruses, bacteria, fungi, nematodes,and insects. In addition, regional weather patternschange, and plants are often faced with unfavor-able environmental conditions such as heat,drought, or cold. Plants must contend with the

chemicals used by people to deal with unwantedplants. These herbicides are designed to kill the plantor to restrict its growth.

When plants are exposed to biotic or abiotic stress,varying degrees of damage may occur, but manyplants manage to protect themselves. Collectively,their defense mechanisms make up what is referred toas plant resistance. Although all plants possesssome means of defense against stress, the term resis-

Resistance to plant diseases • 901

tant plant is usually reserved for varieties that havethe ability to produce a larger crop of good qualityfruit than would other varieties placed under thesame stress conditions.

Types of ResistanceResistance is often categorized according to its

intensity. An immune cultivar is one that will neverbe consumed or injured by a particular stress underany condition. Few, if any, cultivars are consideredimmune to biotic or abiotic factors known to inducestress in other cultivars of the same plant species.A high-resistance cultivar is one that exhibits onlyslight damage from a specific biotic or abiotic stress-ing agent under a given set of conditions. A low-resistance cultivar is one that demonstrates less dam-age from a stressing agent than the average for thespecies. Susceptible and highly susceptible cultivarsshow increasing damage from a biotic or abioticagent greater than the average for the species.

Resistance also varies according to environmen-tal conditions, genetic control, number of pests, and

plant age. Multiple resistance re-fers to cultivars that are resistantto multiple factors. Resistance un-der field conditions (called fieldresistance) may be considerablydifferent from the resistance ob-served in the laboratory or green-house.

Resistance may be controlledby a single gene (monogenic), bya few genes (oligogenic), or bymany genes (polygenic). Theterms “horizontal resistance” or“general resistance” are usedwhen describing resistance thatis expressed equally against allbiotypes of a pest species, and“vertical resistance,” or “specificresistance,” refers to resistanceexpressed against only some ofthe biotypes of a pest species.Resistance may be expressed atany stage of the life cycle fromseedling through maturity. Onoccasion, host plants may passthrough a particularly susceptiblestage of growth quickly, therebyavoiding infestation by a largenumber of pests.

Mechanisms of ResistanceThe mechanisms of plant resistance are gener-

ally grouped into three main categories: tolerance,nonpreference, and antibiosis. Tolerance is the sumof all plant responses that give the species the abil-ity to withstand a particular degree of stress. Theterm “tolerance” is particularly applicable to mech-anisms of resistance associated with environmen-tal stresses. Plants commonly develop tolerancesto stresses such as heat, cold, drought, or salt.Nonpreference is a phenomenon in which theplant is merely ignored by a particular pest. Theplant has no food or shelter to offer the pest andis not suitable for egg-laying; therefore, the plantis not a potential host. Antibiosis refers to a mecha-nism in which the plant exerts some deleteriousaction on the pest. For example, the plant may pro-duce a substance that inhibits some essential func-tion of the pest’s biology, such as reproductionor development, usually leading to death of thepest.

902 • Resistance to plant diseases

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Structural DefensesBoth structural and biochemical defenses can be

preexisting or induced by stress. Preexisting de-fense structures include the waxy surfaces of manyleaves, thickness of the cuticle that covers the epi-dermal cells, characteristics of openings into theplant, and thickness and toughness of the cell wallsof the plant cells. After a pest invades a plant, in-ducible changes in structure can provide some de-gree of defense. After invasion by pests such asfungi, bacteria, viruses, and nematodes, someplants will form layers of cork tissue that seal off theinvading organisms and prevent them from reach-ing the remainder of the plant.

Other structural defense strategies include theformation of structures called tyloses to seal off theinfected vascular tissue or the deposition of gumsaround lesions. Both tyloses and gum deposits pre-vent the spread of the agent. In some instances,plants will form abscission layers that seal off a sec-tion of leaf and cause it to die along with the pest.

Biochemical DefensesAlthough structural barriers provide some de-

gree of defense against invading organisms, chemi-cals produced by the plant during or after the in-duction of stress appear to be much more importantin conferring resistance. There are several preexist-ing biochemical defense systems. Although plantsdo not produce antibodies to specific invadingpests, some type of immunological response ap-pears to be operating. Plants that are resistant tospecific pathogens do not contain the antigens,chemicals that induce the resistance response, thatare found in the susceptible plants. Some cultivarsmaintain resistance by limiting the production ofcertain chemicals that are essential nutrients forinvading pathogens. Other preexisting defensemechanisms include the presence in the plants’cells of chemicals that inhibit the growth of an in-vading pest or the release into the environment ofchemicals that either inhibit or kill potential patho-gens.

When injured by a biotic agent, chemicals, or en-vironmental factors, plants respond with a series ofbiochemical reactions aimed at limiting the injuryand healing the wound. This response is muchmore pronounced in resistant plants than in sus-ceptible plants. The biochemical response to stressshows tremendous variation. Many resistant plants

respond to a pest invasion by releasing phenolics orother toxic compounds. Fungi produce a group oftoxic substances called phytoalexins in response toan invasion.

Many plants respond to stress by the inducedsynthesis of proteins and other enzymes that forman immune layer around the infected site. When re-sistant plants are confronted with the oxidativestress that usually accompanies environmentallyinduced stress, they increase the production of anti-oxidant enzymes. Enzymes produced by invadingorganisms are often responsible for the damage suf-fered by the host plant, but some resistant plantsproduce substances that either resist or inactivatethese enzymes. Some invading organisms producetoxins that damage the host plant, and plants resis-tant to these organisms generally produce chemi-cals that detoxify the toxins. Other plants developresistance by altering certain biochemical path-ways or initiating a hypersensitive response.

Genetically Engineered ResistanceWith the advent of recombinant DNA (deoxyri-

bonucleic acid) technology in the 1970’s, the devel-opment of new traits such as resistance was no lon-ger limited to mutation or natural selection from alimited pool of genes. Scientists first developedtransgenic animals and plants in the early 1980’s,and industry has made widespread use of geneti-cally modified organisms. Despite the beneficialapplications, potential risks and ethical issues as-sociated with the technology have led to contro-versy.

Genetic engineering has been used extensivelyin agriculture. Products of modified organisms areused to protect plants from frost and insects. In 1986the U.S. Environmental Protection Agency ap-proved the release of the first genetically modifiedcrop plant; by the end of the 1990’s, more than onethousand others had been field-tested. Plants havebeen designed to resist disease, drought, frost, in-sects, and herbicides as well as to improve the nu-tritional value or flavor of foods.

D. R. Gossett, updated by Bryan Ness

See also: Biopesticides; Biotechnology; Cloning ofplants; Diseases and disorders; DNA: recombinanttechnology; Genetically modified foods; Herbi-cides; Integrated pest management; Pesticides;Plant biotechnology; RNA.

Resistance to plant diseases • 903

Sources for Further StudyCampbell, Neil A., and Jane B. Reece. Biology. 6th ed. San Francisco: Benjamin Cummings,

2002. Introductory textbook provides an excellent discussion of the biology associatedwith plant reproduction. Includes superb graphics, lists of suggested readings, and glos-sary.

Chrispeels, Maarten J., and David E. Sadava. Plants, Genes, and Agriculture. Boston: Jonesand Bartlett, 1994. Outstanding treatise on the use of biotechnology in crop production.Contains sections related to the use of biotechnology to transfer resistance to susceptibleplants. Although written at an advanced level, it will interest the general reader. The well-illustrated text has a supplemental reading list.

Hale, M. G., and D. M. Orcutt. The Physiology of Plants Under Stress. New York: John Wiley &Sons, 1987. An excellent treatise on the way in which plants respond and develop resis-tance to environmental stress. Written for the advanced student, it contains much the gen-eral reader will understand. Sparsely illustrated; includes an excellent glossary.

Maxwell, Fowden G., and Peter R. Jennings, eds. Breeding Plants Resistant to Insects. NewYork: Wiley, 1980. Written for more advanced students, this book contains material of in-terest to the general reader. One section explains how plants respond and defend them-selves against an insect invasion. Sparsely illustrated. Has a list of general references anda glossary.

Smith, Michael C. Plant Resistance to Insects: A Fundamental Approach. New York: Wiley, 1989.An excellent treatise on the fundamental principles associated with insect resistance inplants. One section is devoted to the methods of studying resistance. The general readerwill not find the book overly technical. Well illustrated; includes supplemental readinglist.

RESPIRATION


All cells must have a source of energy in order to survive. Almost all cells utilize ATP as their energy currency. In otherwords, ATP is produced and stored up until it is needed to supply energy for metabolic activity. Respiration is the processby which cells oxidize a fuel, usually the simple sugar glucose, and use the energy released during this oxidation to pro-duce ATP.

The term “metabolism” refers to the sum total ofall the chemical activity that occurs within an

organism. Metabolism can be further divided intotwo large categories, anabolism and catabolism.Anabolism refers to those metabolic reactions asso-ciated with the synthesis of molecules, such as pro-teins or carbohydrates, while catabolism includesthose reactions involved in the degradation of mol-ecules. Respiration is a catabolic process.

Carbohydrates, especially the simple sugars glu-cose and fructose, serve as the initial substrates forthe respiratory process. In plants, these substratesare produced by photosynthesis in the chloroplasts.

Solar energy is used to convert carbon dioxide andwater into small sugar-phosphate molecules whichare then combined to form fructose. Energy is re-quired to form each carbon-to-carbon bond (calleda covalent bond), and a specified amount of energy isstored in each covalent bond. In other words, solarenergy is converted to chemical energy.

The fructose may directly enter glycolysis, thefirst series of reactions in cellular respiration, or itmay be converted to glucose. The glucose may alsoenter glycolysis directly, or it may be combinedwith a fructose molecule to form sucrose (tablesugar), which can be transported to other parts of

904 • Respiration

the plant. Once sucrose reaches the target cells, itcan be converted back to glucose and fructose,which can then be metabolized via glycolysis. Glu-cose can also be polymerized into large starch mol-ecules, which can be stored. When the plant experi-ences an energy deficit, starches can be brokendown to glucose molecules.

During respiration fuel molecules, such asglucose, undergo a series of reactions in which themolecule is oxidized into smaller molecules, and inaerobic respiration, the glucose will ultimately bedegraded to carbon dioxide and water. As the mol-ecules are degraded, the energy stored in the chem-ical bonds that held the glucose molecule togetheris released, and the cells trap this energy in the formof adenosine triphosphate (ATP) molecules. In asense, the oxidation of glucose as a fuel is similar tothe oxidation (burning) of any other organic fuel,such as gas, fuel oil, or coal, except the biologicaloxidation of glucose is a stepwise process and re-sults in the formation of ATP. Each step requires anenzyme, an organic catalyst. There are many en-zymes associated with respiration, but the threemost common types are kinases, decarboxylases,and oxioreductases. Kinases catalyze reactions asso-ciated with ATP formation or utilization; decarbox-ylases catalyze decarboxylation reactions which re-move chemical groups called carboxyls as carbondioxide; and oxioreductases catalyze oxidation/re-duction reactions. Oxidation is the removal of elec-trons from a molecule, and reduction is the additionof electrons. In general, oxidation involves the re-moval of two electrons and two hydrogen ions (H+)from the substrate molecule. The release of H+ intothe cytosol would result in acidification of the cell;therefore, oxioreductases require the presence ofcoenzymes which will accept the electrons and H+. Inother words, these coenzymes are reduced as glu-cose is oxidized. The two most important coen-zymes in respiration are nicotine adenine dinucleotide(NAD) and flavin adenine dinucleotide (FAD). The re-duced forms of these coenzymes are NADH + H+

and FADH2, respectively.

Glycolysis and FermentationGlycolysis, the first series of reactions in the re-

spiratory pathway, consists of nine or ten separatesteps, depending on whether the initial substrateis fructose or glucose. Because sugar moleculesare not very reactive, the first two or three stepsin the process use two ATP molecules to convert

the six-carbon glucose or fructose to a very reactivemolecule called fructose-1,6-bisphosphate. This six-carbon compound is then broken down into theequivalent of two molecules of a three-carbon com-pound called glyceraldehyde-3-phosphate (PGAL).Each molecule of PGAL undergoes a series of re-actions in which it is converted to pyruvic acid.During this conversion, one NADH + H+ is formed,and enough energy is released to produce twoATP molecules per PGAL. In summary, glycoly-sis is the conversion of glucose to two moleculesof pyruvic acid, two molecules of NADH + H+,and a net gain of two ATP molecules (four wereproduced, but two were used to initiate the pro-cess). Glycolysis occurs in the cytosol and is entirelyanaerobic, meaning that it occurs in the absence ofoxygen.

In anaerobic organisms, such as yeast, each mol-ecule of pyruvic acid is decarboxylated to producecarbon dioxide and a molecule called acetaldehyde.The NADH + H+ produced during glycolysis isthen used to convert the acetaldehyde to ethanol.This anaerobic process is called fermentation. Over-all, the process of fermentation results in the con-version of glucose to two molecules of carbon diox-ide, two molecules of ethanol, and a net gain of twoATP molecules.

The Krebs Cycle and Oxidative PhosphorylationIn aerobic plants, each molecule of pyruvic acid

is transported to the matrix of the mitochondriawhere it is oxidatively decarboxylated to produce acarbon dioxide molecule, a molecule of NADH +H+, and a compound called acetyl coenzyme A(acetyl CoA), which contains two carbon atomsfrom the initial glucose molecule. Acetyl CoA thenenters a second series of reactions called the Krebscycle, also known as the tricarboxylic acid cycle orthe citric acid cycle. The two carbons from the glu-cose molecule combine with a four-carbon com-pound called oxaloactic acid to form a six-carboncompound called citric acid. The citric acid isdecarboxylated twice, and the remaining four-carbon fragment is ultimately converted back tooxaloacetic acid. During this process, two mole-cules of carbon dioxide, three molecules of NADH+ H+, one FADH2 molecule, and one ATP moleculeare produced. During glycolysis, two molecules orpyruvic acid are produced per glucose; therefore,after two turns of the Krebs cycle, glucose is con-verted to six molecules of carbon dioxide, six (four

Respiration • 905

net) ATP molecules, ten molecules of NADH + H+,and two molecules of FADH2.

A third series of reactions, referred to as electrontransport, takes place within the mitochondrialmembranes. The electrons and H+ ions bound to theNADH + H+ and FADH2 are transferred to initialelectron receptors and then passed through a serieselectron transporters, each with a lower reductionpotential (the tendency to accept electrons). Severalof these electron transporters are iron-sulfur con-taining proteins called cytochromes. Hence, thiselectron transport system is sometimes referred toas the cytochrome system. The final electron accep-tor in this system is oxygen. When two electronsand H+ ions are transferred to oxygen, a molecule ofwater is formed. This provides the cells with a safemeans of removing excess H+ ions, but more impor-tant, additional ATP is produced. As electrons aretransported from NADH + H+ and FADH2 throughthe electron transport chain and ultimately to oxy-gen, energy is released. This energy can be usedto synthesize ATP from ADP (adenosine diphos-phate). Since the energy is stored in the phosphatebond (ADP to ATP) and occurs only in the presenceof oxygen, the production of ATP during aerobicrespiration is referred to as oxidative phosphoryla-tion. Each mole of NADH + H+ produced withinthe mitochondria results in the production of threemoles of ATP via the electron transport system.The NADH + H+ from glycolysis and FADH2 enterthe electron transport system downstream from

the NADH + H+ derived from inside the mitochon-dria. As a result, each mole of these moleculesproduces only two moles of ATP. The total ATPproduction per mole of glucose from electron trans-port and oxidative phosphorylation is thirty-twomoles.

3 × 8 NADH + H+ from within the mitochondria = 24;2 x 2 NADH + H+ from glycolysis = 4; 2 × 2 FADH2

from the Krebs cycle = 4: 24 + 4 + 4 = 32.

Overall, aerobic respiration uses glycolysis, theKrebs cycle, and electron transport and results inthe conversion of one mole of glucose to six molesof carbon dioxide, twelve moles of water and a netof thirty-six moles of ATP.

2 net ATPs from glycolysis + 2 ATPs from the Krebscycle + 32 ATPs from electron transport = 36 ATPs.

Each mole of ATP represents about 8,000 calories ofenergy; therefore the oxidation of one mole of glu-cose can produce a total of 288,000 calories of en-ergy available for cellular work.

D. R. Gossett

See also: ATP and other energetic molecules; Car-bohydrates; Energy flow in plants; Ethanol; Glycol-ysis and fermentation; Krebs cycle; Microbial nutri-tion and metabolism; Mitochondria; Oxidativephosphorylation; Photosynthesis.

Sources for Further StudyHopkins, William G. Introduction to Plant Physiology. 2d ed. New York: John Wiley and Sons,

1999. This general text for the beginning plant physiology student contains a very cleardiscussion of respiration in plants. Includes diagrams, illustrations, and index.

Nicholls, D. G., and S. J. Ferguson. Bioenergetics 2. London: Academic Press, 1992. An excel-lent source of information dealing with all aspects of respiration, for the more advancedstudent. Includes bibliographical references.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. A very good general botany text with a simplified discus-sion of respiration in plants. Includes diagrams, illustrations, and index.

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology. 4th ed. Belmont, Calif.: Wadsworth,1992. One of the standard texts for the more advanced plant physiology student, with anexcellent discussion on respiration in plants. Includes biographical references, diagrams,illustrations, and index.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology. 2d ed. Sunderland, Mass.: Sinauer Asso-ciates, 1998. A text written for the advanced plant physiology student, with an outstand-ing discussion of respiration in plants. Includes biographical references, diagrams, illus-trations, and index.

906 • Respiration

RHYNIOPHYTA


When first proposed by Harlan Banks in 1968, the Rhyniophyta were the first and oldest vascular land plants. Thetrimerophytes subsequently evolved from them.

In 1908, Octave Lignier developed a model ofwhat the sporophyte of the earliest vascular land

plants might look like. The sporophyte is a diploid(2n) plant that produces spores in a sporangium,while its counterpart, the gametophyte, is a haploid(n) plant that bears the male (antheridia) and fe-male (archegonia) sex organs. Lignier proposedthat the first vascular land plants would consist of aforked, photosynthetic stem lacking both roots andleaves. At the point of branching, the fork lookedlike a capital Y. Running along the ground or justbelow the soil surface was a horizontal stem (rhi-zome) bearing hairlike filaments (rhizoids) on itslower surface that functioned in anchorage and ab-sorption. Spores were produced either within theunmodified stem tips of some of the aerial branchesor in a specialized reproductive structure, a thick-walled, elongate sporangium, that terminatedsome branch tips.

In 1912, some unique plant fossils were found inthe Rhynie Chert (406 million to 401 million yearsold) in Scotland. The Rhynie Chert is a hot-springchert formed when silica-saturated water from ahot spring flooded a low-lying marsh, seeped intothe plants, and hardened within and about them.Because they are encased in chert, the Rhynie plantswere the first fossil plants to yield both structuraland anatomical data. Two of the plants, Rhynia ma-jor (now Aglaophyton) and R. gwynne-vaughanii, werereconstructed as near-perfect matches to Lignier’smodel of the earliest vascular land plant. With theirdiscovery, the science of paleobotany came of age.

As knowledge of the rhyniophytes increased, arevision of the group seemed in order. As originallydefined, the Rhyniophyta included both vascular(tracheophytes Rhynia and Cooksonia pertoni) andnonvascular plants that are intermediate betweenbryophytes and vascular plants (Aglaophyton andHorneophyton lignieri). These plants are placed in

two groups, the Rhyniophyta and Horneophyta, re-spectively.

Horneophyta or ProtracheophytesAglaophyton‘s rhizome was in contact with the

ground at intervals, and rhizoids were concen-trated at those positions rather than spread evenlyalong the axis. The tissue of the rhizome contained afungus, representing the earliest known example ofa plant-fungus symbiosis. The vast majority of landplants require this symbiotic relationship to live.The land plant provides the fungus with shelterwithin its cells and access to the products of photo-synthesis. The fungus absorbs water and mineralsfrom the soil, which it passes on to the land plant.

This partnership is so important to living plantsthat scientists were not surprised to find it veryearly in the fossil history of the land plants. Denseclusters of buds (possibly dormant stem tips) werefound on the rhizome at the base of the aerial stems.The aerial stem was naked and branched by forkinginto two daughter axes of equal size. Elongatesporangia were borne at the tips of some aerialbranches. The water-conducting cells of Aglao-phyton lack the internal wall thickenings character-istic of tracheids (thick-walled, dead cells found inthe xylem of vascular land plants) and resemble thethin-walled, water-conducting cells (hydroids) ofthe mosses. Because of the absence of tracheids,Aglaophyton is considered a nonvascular plant andmust be removed from the Rhyniophyta. A male re-productive structure (Lyonophyton rhyniensis) rep-resents the gametophyte of Aglaophyton.

Another nonvascularized sporophyte found inthe Rhynie Chert was Horneophyton. Instead of hav-ing obvious sporangia, the spores of Horneophytonwere produced within the stem tip and released bymeans of a terminal pore. Multicellular projectionswere found on the stem, which terminated in

Rhyniophyta • 907

stomates. The aerial stems did not grow from a rhi-zome. The aerial stems each terminated in a bul-bous, nonvascularized structure called a corm. Sev-eral corms were found attached to one another, butno vascular tissue connected them. As in Aglao-phyton, the water-conducting cells were thin-walledand lacked internal wall thickenings. Langiophy-ton mackiei represents the female gametophyte ofHorneophyton.

When gametophytes are known in the protra-cheophytes, they have an anatomical and structuralcomplexity similar to that of their correspondingsporophytes. In contrast, the gametophytes of vas-cular land plants are small and inconspicuous (of-ten subterranean). Scientists know more about thegametophytes of the plants of the Rhynie Chertthan they do about almost all other groups of fossilplants.

RhyniaRhynia gwynne-vaughanii was reconstructed as

a smaller version of Aglaophyton. The aerial stemof Rhynia was naked and branched by forking.The rhizome bore rhizoids. Thick-walled water-conducting cells (tracheids) were present. AlthoughRhynia was reconstructed bearing sporangia, nonewere originally found attached to the aerial stems.In addition to lacking sporangia, Rhynia had bumpsalong the aerial stem that were interpreted as fe-male reproductive structures (archegonia). As a re-sult, Rhynia was reinterpreted as the gametophyteof Aglaophyton. Subsequently, sporangia were foundattached to Rhynia, making the plant a sporophyteonce more. The sporangia were borne laterally alongthe stem on short side branches, and they were shedafter releasing their spores. The gametophyte ofRhynia is unknown.

The small, circular vascular strands and fleshystems associated with the rhyniophytes and horneo-phytes indicate that these plants did not grow verytall. Water pressing against the inside of the cellwalls in their stems (turgor pressure) held theseplants upright.

CooksoniaThe oldest vascular land plant (412 million to 406

million years old) and only cooksonioid with iden-tifiable vascular tissue is Cooksonia pertoni. The ae-rial stems of Cooksonia branched by forking andbore at their branch tips sporangia shaped like kid-ney beans. Plants whose sporangia resemble those

of C. pertoni are known from older sediments (414million to 412 million years in age), but no vasculartissue has been found in their stems. The oldest fos-sil plant from the Western Hemisphere that resem-bles Cooksonia comes from Bathurst Island in theCanadian Arctic (420 million to 414 million yearsin age). No roots or rhizomes are known for anyCooksonia. If the older specimens of Cooksonialacked vascular tissue, Cooksonia may not be a validgenus. These plants are best referred to collectivelyas cooksonioids, and plants showing these traitscould be bryophytes, protracheophytes, or rhynio-phytes.

Fossil SporesFossils spores are known from sediments that

are about 40 million years older than those fromwhich cooksonioids are recovered. The most abun-dant spores were not produced by the vascular landplants and are called cryptospores. Banded tubulesand sheets bearing cell outlines (collectively callednematoclasts) are recovered with the cryptospores.Similar structures are produced by the sporangialepidermis of modern mosses (cellular sheets) andliverworts (banded tubes). The cryptospores had aworldwide distribution and showed limited diver-sity. These spores were probably produced by earlybryophytes.

To identify the type of plant that produced a spe-cific spore, researchers must find the spore within asporangium. Although free spores are common, nospores are known from sporangia for the first 65million years of land plant evolution. The presenceof cryptospores and the occasional spore that mighthave been produced by a vascular land plant indi-cates that land plants first appeared about 500 mil-lion years ago. Most of the 65-million-year periodfrom which sporangia are unknown was charac-terized by high sea levels, and little continental de-position occurred. The missing continental depos-its are where the necessary sporangia would havebeen found. The oldest spores from continentaldeposits are about 435 million years old. Crypto-spores are found in sporangia from England thatare between 414 million and 412 million years old.Although they predominated early, cryptosporesform a very minor component of the spore floraabout by 414 million years ago. The decrease inthe number of cryptospores reflects the growingimportance (diversification) of the vascular landplants.

908 • Rhyniophyta

Ancestors of the RhyniophytesThe first land plants, bryophytes, protracheo-

phytes, and tracheophytes, evolved from green al-gal (charophycean) ancestors that lived in freshwa-ter habitats whose appearance was unpredictable(not seasonal) and of short duration. Their algal an-cestors were preadapted to life on land. They wereresistant to microbial attack due to the structure oftheir cell walls. They had a genetic (heritable) basisfor lowering or suspending metabolic activity dur-ing drought (dry conditions that could last for daysor months) and desiccation (dry conditions thatcould last for decades). Their reproductive bodies

did not dry out during air transport from one waterbody to another. The first to colonize the land suc-cessfully were the bryophytes. The protracheo-phytes and tracheophytes appeared after the bryo-phytes were well established on land.

Gary E. Dolph

See also: Adaptive radiation; Algae; Charophyceae;Evolution of plants; Fossil plants; Hornworts; Liv-erworts; Mosses; Paleobotany; Plant tissues; Seed-less vascular plants; Shoots; Species and speciation;Stems; Trimerophytophyta; Zosterophyllophyta.

Sources for Further StudyGensel, Patricia G., and Henry N. Andrews. Plant Life in the Devonian. New York: Praeger,

1984. Good descriptions of the rhyniophytes with valuable illustrations. Illustrations,bibliographical references, index.

Gensel, Patricia G., and Dianne Edwards. Plants Invade the Land: Evolutionary and Environ-mental Perspectives. New York: Columbia University Press, 2001. Provides the current in-formation on the rhyniophytes in the context of the first land plants. Illustrations, biblio-graphical references, index.

Kenrick, Paul, and Peter R. Crane. The Origin and Early Diversification of Land Plants: ACladistic Study. Washington, D.C.: Smithsonian Institution Press, 1997. Detailedcladistical treatment of the rhyniophytes. Excellent line drawings, bibliographical refer-ences, index.

RIBOSOMES


Ribosomes are complex cellular structures found in all cells and are responsible for making proteins. They are composedof ribosomal RNA (rRNA) and protein and are most abundant in the cytoplasm, although some functional ribosomescan be found in the nuclei of eukaryotic cells. Chloroplasts and mitochondria also have their own ribosomes.

Ribosomes are responsible for synthesizing theproteins in all cells by a process called transla-

tion. It is called translation because ribosomes usemessenger ribonucleic acids (mRNAs) as theirguide and must “translate” the message containedin the nucleotides of mRNAs. The general structureof ribosomes is the same in all cells, but ribosomesof prokaryotes are smaller than ribosomes in thecytoplasm of eukaryotes. The ribosomes in chloro-plasts and mitochondria are more similar to thesmaller ribosomes of prokaryotes but are oftensmaller yet.

Ribosome StructureBecause ribosomes, and rRNAs, are so large,

they are not described on the basis of their molecu-lar weight but rather in Svedbergs, or Svedbergunits. Svedbergs (denoted S) are a measure of howquickly a large molecule or aggregation of mole-cules sediment, or sink to the bottom of a centrifugetube while being spun around. Higher S valuesmean faster sedimentation and thus greater mass.For example, prokaryotic ribosomes sediment at70’s, whereas eukaryotic ribosomes, which arelarger, sediment at 80’s. Svedbergs cannot be added

Ribosomes • 909

together, because they are not directly related tomass but to a combination of mass and overall mo-lecular size and shape. Consequently, if two mole-cules that are both 30’s are joined together, theircombined sedimentation rate would be about 50’sor a little more but not 60’s.

Prokaryotic ribosomes constitute three rRNAsand fifty-two different proteins. Like all ribosomes,they are composed of two subunits, referred to sim-ply as large and small subunits. Large subunitshave a sedimentation rate of 50’s and are composedof a 23S rRNA, a 5S rRNA, and thirty-one proteins.Small subunits have a sedimentation rate of 30’sand are composed of a single 16S rRNA andtwenty-one different proteins. To function, a smalland large subunit must come together.

Eukaryotic ribosomes are more complex, withfour rRNAs and more than eighty proteins. Largesubunits have a sedimentation rate of 60’s and arecomposed of a 28S rRNA, a 5.8S rRNA, a 5S rRNA,and about forty-nine proteins. Small subunits havea sedimentation rate of 40’s and are composed of asingle 18S rRNA and about thirty-three proteins.Assembly of the subunits takes place in a region ofthe nucleus called the nucleolus. When completed,the subunits are transported through nuclear poresto the cytoplasm. Although translation of mRNAstakes place primarily in the cytoplasm, a smallamount occurs in the nucleus.

Translation in ProkaryotesBefore translation can take place, transcription

of a gene must occur. Transcription converts the de-oxyribonucleic acid (DNA) code of a gene into acomplimentary RNAcode, in the form of an mRNA.Even while RNA polymerase catalyzes the joiningof nucleotides for the mRNA, translation can begin.Translation includes three steps: initiation, elonga-tion, and termination.

In initiation, special proteins called initiation fac-tors (IFs) enable the small subunit of a ribosome tobind to an mRNA and form the initiation complex.Next, a special tRNA called fMet-tRNA (N-formyl-methionyl-tRNA), which carries a specially modi-fied amino acid derivative of methionine, bindsto the start codon of the mRNA (AUG). This modi-fied methionine is the first amino acid in all pro-karyotic proteins, but many proteins are modifiedafter transcription, often by removing the first fewamino acids. Lastly, the large ribosomal subunitbinds.

Elongation is a complex process in which theribosome adds amino acids, one at a time, to thegrowing protein. Ribosomes have three sites whereevents in translation takes place. The A site(aminoacyl-tRNA site) is where new tRNAs, withtheir attached amino acids, bind to the ribosomeand mRNA. The P site (peptidyl-tRNAsite) is wherethe tRNAwith the growing protein (polypeptide) isattached to the ribosome and the mRNA. The E site(exit site), only recently recognized, is where tRNAsleave the ribosome after they have completed theirwork.

The sequence of events in elongation can be diffi-cult to visualize, but it is a cyclical process that re-peats until elongation is done. The steps are as fol-lows. First, while the tRNA with the growingprotein (called a peptidyl-tRNA) rests in the P site, atRNA with an amino acid (called an aminoacyl-tRNA) enters the A site and binds to the codon onthe mRNA. Second, the peptidyl-tRNA attachesthe growing protein to the amino acid on theaminoacyl-tRNA. Now the tRNA in the P site hasno amino acids attached to it. Third, the tRNAin theP site now leaves the ribosome by passing throughthe E site. Fourth, as the tRNA leaves the P site,the tRNA at the A site (now a peptidyl-tRNA) istranslocated (moved) to the P site by moving themRNA over one codon, leaving the A site open forthe next aminoacyl-tRNA to attach. These stepsrepeat until the stop codon, near the end of themRNA, is reached. Many of the events of elonga-tion were long believed to be catalyzed by proteinsin the ribosome. It is now known that some of the ri-bosome’s catalytic properties are due to the rRNAs,which, as a result, are sometimes called ribozymes.

The stop codon is not recognized by any of thetRNAs. Termination occurs when some special pro-teins called releasing factors bind to the ribosome.When they bind, the protein that is attached to thetRNA in the P site is released from the tRNA, thetRNA leaves through the E site, and the two sub-units come apart.

Translation in EukaryotesThe process of translation in eukaryotes varies

only in minor details from translation in prokary-otes, and these differences are due to the greatercomplexity of eukaryotic mRNA and ribosomes.Eukaryotic mRNA must go through extensive pro-cessing after being transcribed, including intron ex-cision/exon splicing and addition of a special 5′ cap

910 • Ribosomes

and a 3′ poly-adenosine tail. Eukaryotic mRNAcannot be translated until these modifications arecompleted. Although some mRNA is translated inthe nucleus, most must be transported to the cyto-plasm first. Consequently, transcription and trans-lation are completely separate processes, unliketheir coupling in prokaryotes.

Initiation in eukaryotes involves a larger num-ber of initiation factors. Once the small ribosomalsubunit binds, it then starts at the 5′ end of themRNA and searches for the start codon (AUG).When the start codon is found, a methionyl-tRNAbinds to the ribosome and the start codon of themRNA. In eukaryotes, like in prokaryotes, the firstamino acid in all proteins is methionine, but in eu-karyotes it is not a N-formyl-methionine. Elonga-tion and termination are essentially the same inboth eukaryotes and prokaryotes. As in prokary-otes, completed proteins are typically modified invarious ways before being used by the cell.

Many ribosomes will typically translate anmRNA simultaneously. An mRNA with several ri-bosomes lined up along it, all of them translating atonce, is often called a polyribosome. Polyribosomesare observed in both eukaryotes and prokaryotes.

Distribution in Eukaryotic CellsAfter being synthesized in the nucleolus, a few

ribosomes will function for an unknown length oftime in the nucleus. Most, though, are transportedto the cytoplasm as separate subunits. Once in the

cytoplasm, ribosomes will either bind to endoplas-mic reticulum (ER), in regions called rough ERbecause of the fuzzy appearance of these areas inelectron micrographs, or they will remain free inthe cytoplasm. Ribosomes bound to the rough ERspecialize in making proteins that will either beembedded in membranes or transported in vesiclesto the Golgi complex. The different fates of proteinsmade at the rough ER are determined by special sig-nal sequences, sequences of amino acids at the begin-ning of proteins which are typically removed later.

Ribosomes that are free in the cytoplasm makeproteins of various kinds that are needed for pro-cesses occurring in the cytoplasm. Regardless of lo-cation, ribosomes are usually found disassembled,and the two subunits come together only whenneeded for translation. Cells have anywhere fromseveral thousand ribosomes to as many a few mil-lion in cells that have particularly high rates of pro-tein synthesis.

Bryan Ness

See also: Angiosperm cells and tissues; Cell cycle;Cell theory; Chloroplasts and other plastids; Cy-toplasm; Cytoskeleton; Cytosol; DNA: historicaloverview; DNA in plants; DNA replication; Endo-membrane system and Golgi complex; Endoplasmicreticulum; Eukaryotic cells; Membrane structure;Microbodies; Nucleus; Oil bodies; Peroxisomes;Photosynthesis; Plant cells: molecular level; Plasmamembranes; Respiration; RNA.

Sources for Further StudyGarrett, Roger A., Stephen R. Douthwaite, and Anders Liljas, eds. The Ribosome: Structure,

Function, Antibiotics, and Cellular Interaction. Washington, D.C.: ASM Press, 2000. Acollec-tion of papers based on presentations given on a ribosome conference. Detailed and up todate information on ribosomes, best for college-level and more advanced readers.

Gunning, Brian E. S., and Martin W. Steer. Plant Cell Biology, Structure, and Function. Sudbury,Mass.: Jones & Bartlett, 1996. An integrated textbook copiously illustrated with electronmicrographs and containing detailed information on plant cell and molecular biology.

Smith, H., ed. The Molecular Biology of Plant Cells: Botanical Monographs. Berkeley: Universityof California Press, 1978. Somewhat out of date but provides a good overview for all as-pects of plant cell biology, including ribosomes and translation.

Spedding, Gary. Ribosomes and Protein Synthesis: A Practical Approach. Oxford, England: Ox-ford University Press, 1990. A practical guide to the methods of studying the structureand function of ribosomes in light of the ability or rRNA to act as a catalyst.

Spirin, Alexander S. Ribosomes. New York: Plenum, 2000. The author is considered the worldauthority on ribosomes, and this book covers every aspect of ribosome structure andfunction. Written for college-level and more advanced readers.

Ribosomes • 911

RICE

Categories: Agriculture; economic botany and plant uses; food

Rice, the starchy seeds of an annual cereal grass, is the most commonly consumed food grain for a majority of the world’spopulation.

The rice plant, Oryza sativa, is a member of thegrass family, classified into indica and japonica

varieties. World production of rice exceeds 500 mil-

lion tons. Most countries—particularly in Asia—cultivate rice for domestic consumption, so lessthan 5 percent enters the export market. The United

States generates only about 2 percentof world rice production, but almosthalf of U.S. production is exported. Ricecultivation almost certainly began inIndia, where it dates back to about3000 b.c.e. During medieval times itspread westward to southern Europe.

CultivationMonsoon tropics are ideal for indica

rice, which is commonly cultivated inChina and Southeast Asia. The plantscan adapt to uncertain conditions. Thejaponica type of rice requires precisewater control as well as weed and in-sect control. It is cultivated in temper-ate zones such as the United States,Australia, Japan, Korea, and certainparts of China.

Rice is self-pollinated, and the grainis enclosed in the palea, or hull. Har-vested but unmilled rice is called paddyor rough rice. Milling of rough rice byany of several processes yields the pol-ished grain that is ready for consump-tion. Rough rice contains approxi-mately 10 percent protein, 65 percentstarch, 2 percent lipids, 5 percent min-erals, and 18 percent hull/bran. Theunhulled whole rice kernel also con-tains thiamine, niacin, and riboflavin.Parboiled rice can be stored for longperiods.

The International Rice Research In-stitute in the Philippines has contrib-uted significantly to the development

912 • RiceD

igit

al

Sto

ck

Rice paddies in Indonesia.

of high-yielding types of rice, beginning in the mid-1960’s. The development of these plants is consid-ered a significant part of the 1960’s Green Revolutionin agriculture. Some of these varieties demandcomplete irrigation systems year-round that helpkeep the soil submerged under about 6 inches (15centimeters) of water. Next to corn, rice providesthe farmer with the greatest yield when plants arecultivated with the necessary care. The crop growswell in irrigated and flooded areas.

Cooked rice is mostly consumed in its wholegrain form. Puffed rice and flaked rice are commonbreakfast cereals, and rice flour is used in bakeryproducts. Laundry starch is made from rice starch.Rice hull is used in cattle feed as well as fertilizers.The rice plant produces oil for food and industryand thatching material for roofs and mats. The Jap-anese alcoholic beverage sake is made from a pro-cess that involves the fermentation of rice.

Wild RiceThe plant commonly known as wild rice, Zizania

aquatica, is actually a separate genus found in NorthAmerica. Like rice, wild rice is an annual grass, andit grows mostly in lakes and streams. Lakes in Min-nesota, Wisconsin, and southern Canada provide agood harvest of wild rice. Wild rice, once a staple ofthe diet of American Indians in those regions, hasbecome a popular side dish.

Mysore Narayanan

See also: African agriculture; Agriculture: worldfood supplies; Alternative grains; Aquatic plants;Asian agriculture; Australian agriculture; Carib-bean agriculture; Central American agriculture;Green Revolution; Grains; High-yield crops; Hy-bridization; North American agriculture; PacificIsland agriculture; Plant biotechnology; Plant do-mestication and breeding; South American agri-culture.

Sources for Further StudyCarney, Judith Ann. Black Rice: The African Origins of Rice Cultivation in the Americas. Cam-

bridge, Mass.: Harvard University Press, 2001. Establishes the independent domestica-tion of rice in West Africa and its vital significance there well before Europeans arrived,discrediting the notion that Europeans introduced rice to West Africa and then brought

Rice • 913

Leading Rice-Producing Countries, 1994

50 100 150 200Millions of Metric Tons

Bangladesh

Brazil

Burma

China

India

Indonesia

Japan

Philippines

Thailand

United States

27.5

10.6

19.1

178.3

118.4

46.2

15.0

10.2

18.4

9.0

Note: World total for 1994 was approximately 535 million metric tons.Source: U.S. Department of Commerce, Statistical Abstract of the United States, 1996, 1996.

the knowledge of its cultivation to the New World. Carney asserts that Africans and Afri-can American slaves transferred rice seeds and cultivation skills to the Americas.

Piper, Jacqueline M. Rice in South-East Asia: Cultures and Landscapes. New York: Oxford Uni-versity Press, 1993. Shows how rice, as a staple food and as a source of livelihood, deter-mines the yearly rhythm as its major ceremonials mark the changing seasons for much ofthe population of Southeast Asia.

Shimamoto, Ko. Molecular Biology of Rice. New York: Springer, 1999. Describes how ad-vances in genome analysis and transformation have made rice a model monocot for mo-lecular biologists. Discusses these technological impacts on rice breeding.

RNA


Ribonucleic acid (RNA), a molecule that plays many roles in the effective usage of genetic information, exists in severalforms, each with its own unique function. RNA functions in the process of protein synthesis, during which informationfrom DNA is used to direct the construction of a protein, and possesses enzymatic and regulatory capabilities.

Ribonucleic acid (RNA) is a complex biologicalmolecule that is classified along with deoxyri-

bonucleic acid (DNA) as a nucleic acid. Chemically,RNA is a polymer (long chain) consisting of sub-units called ribonucleotides linked together byphosphodiester bonds. Each ribonucleotide con-sists of three parts: the sugar ribose (a five-carbonsimple sugar), a negatively charged phosphategroup, and a nitrogen-containing base. There arefour types of ribonucleotides, and the differencesbetween them lie solely in which of four possiblebases they contain. The four bases are adenine (A),guanine (G), cytosine (C), and uracil (U).

The structures of DNAand RNAare very similar,but there are three important differences. The sugarfound in the nucleotide subunits of DNA is deoxy-ribose, which is related to but differs slightly fromthe ribose found in the ribonucleotides of RNA. Inaddition, while DNA nucleotides also contain fourpossible bases, there is no uracil in DNA; instead,DNA nucleotides may contain a different base,called thymine (T). Finally, while DNA exists as adouble-stranded helix in nature, RNA is almost al-ways single-stranded.

Folding of RNA MoleculesThe significance of many types of RNA lies in

the order of their nucleotides, which represents in-formation transcribed from DNA. This nucleotide

order is called the primary structure of the mole-cule. An important aspect of many biological mole-cules, however, is the way their primary struc-tures fold to create a three-dimensional shape. Asingle strand of DNA, for example, associates withanother strand in a particular way to form the fa-mous double helix, which represents its actual three-dimensional shape in nature. Similarly, protein mol-ecules, especially enzymes, must be folded into avery specific three-dimensional shape if they areto perform their functions; loss of this shape willcause their inactivation.

Since RNA is single-stranded, it was recognizedshortly after the discovery of some of its major rolesthat its capacity for folding is great and that thisfolding might play an important part in the func-tioning of the molecule. The nucleotides in an RNAmolecule can form hydrogen-bonded base pairs, ac-cording to the same rules that govern DNA basepairing. Cytosine binds to guanine, and uracilbinds to adenine. What this means is that in a par-ticular single-stranded RNA molecule, comple-mentary portions of the molecule are able to foldback and form base pairs with one another. Theseare often local interactions, and a common struc-tural element that is formed is called a “hairpinloop” or “stem loop.” A hairpin loop is formedwhen two complementary regions are separated bya short stretch of bases so that when they fold back

914 • RNA

and pair, some bases are left unpaired, forming theloop. The net sum of these local interactions is re-ferred to as the RNA’s secondary structure and isusually important to an understanding of how theRNA works. All transfer RNAs (tRNAs), for exam-ple, are folded into a secondary structure that con-tains three stem loops and a fourth stem arrangedonto a “cloverleaf” shape.

Finally, local structural elements may interactwith other elements in long-range interactions,causing more complicated folding of the moleculein space. The cloverleaf arrangement of a tRNA un-dergoes further folding so that the entire moleculetakes on a roughly L-shaped appearance. An un-derstanding of the three-dimensional shape of anRNA molecule is crucial to understanding its func-tion. By the late 1990’s, the three-dimensional struc-tures of many tRNAs had been worked out, but ithad proven difficult to do X-ray diffraction analy-ses on most other RNAs because of technical prob-lems. More advanced computer programs and al-ternate structure-determining techniques havenow enabled research in this field to proceed.

Three Classes of RNAWhile all RNAs are produced by transcription,

several classes of RNA are created, and each hasa particular function. By the late 1960’s, three ma-jor classes of RNAs had been identified, and theirrespective roles in the process of protein synthe-sis had been elucidated. In general, protein synthe-sis refers to the assembly of a protein using infor-mation encoded in DNA, with RNA acting as anintermediary to carry information and assist inprotein building. In 1956, Francis Crick, one of thescientists who had discovered the double-helicalstructure of DNA, referred to this information flowas the “central dogma,” a term that continues to beused.

Messenger RNA (mRNA) is the molecule that car-ries a copy of the DNA instructions for building aparticular protein. It usually represents the infor-mation provided by a single gene and carries thisinformation to the ribosome, the site of protein syn-thesis. This information must be decoded so that itwill specify the order of amino acids in a protein.Nucleotides are read in groups of three (codons). Inaddition to the information required to orderamino acids, the mRNA contains signals that tellthe protein-building machinery where to start andstop reading the genetic information.

Ribosomal RNA (rRNA) exists in three distinctsizes and is part of the structure of the ribosome.The three ribosomal RNAs interact with many pro-teins to complete the ribosome, the cell structurethat directs the events of protein synthesis. One ofthe functions of the rRNA is to interact with mRNAat a particular location and orient it properly so thatreading of its genetic code can begin at the correctlocation. Another rRNA acts to facilitate the trans-fer of the growing polypeptide chain from onetRNA to another (peptidyl transferase activity).

Transfer RNA (tRNA) serves the vital role of de-coding the genetic information. There are at leasttwenty and usually fifty to sixty different tRNAs ina given cell. On one side, they contain an “anti-codon” loop, which can base-pair to the mRNAcodon according to its sequence and the base-pairing rules. On the other side, they contain anamino acid binding site, to which is attached theappropriate amino acid for its anticodon. In thisway, tRNA allows the recognition of any particularmRNA codon and matches it with the appropriateamino acid. The process continues until an entirenew protein molecule has been constructed.

Split Genes and mRNA ProcessingIn bacterial genes, there is a colinearity between

the segment of a DNA molecule that is transcribedand the resulting mRNA. In other words, themRNA sequence is complementary to its templateand is the same length, as would be expected. In thelate 1970’s, several groups of scientists made aseemingly bizarre discovery regarding mRNAs ineukaryotes (organisms whose cells contain a nu-cleus, including all living things that are not bacte-ria or archaebacteria): The sequences of mRNAsisolated from eukaryotes were not collinear withthe DNA from which they were transcribed. Thecoding regions of the corresponding DNA were in-terrupted by seemingly random sequences thatserved no immediately obvious function. Theseintrons, as they came to be known, were apparentlytranscribed along with the coding regions (exons)but were somehow removed before the mRNA wastranslated. This completely unexpected observa-tion led to further investigations that revealed thatmRNA is extensively processed, or modified, afterits transcription in eukaryotes.

After a eukaryotic mRNA is transcribed, it con-tains all of the intervening sequences and is re-ferred to as immature, or a “pre-mRNA.” Before it

RNA • 915

can become mature and functional, three majorprocessing events must occur: splicing, the addi-tion of a “cap,” and the addition of a “tail.”

The process of splicing is a complex one that oc-curs in the nucleus with the aid of the “splice-osome,” a large complex of RNAs and proteins thatidentify intervening sequences and cut them out ofthe pre-mRNA. In addition, the spliceosome mustrejoin the sequences from which the intron-encodednucleotides were removed so that a complete,functional mRNA results. The process must be ex-tremely specific, since a mistake that caused theremoval of only one extra nucleotide could changethe protein product of translation so radically that itmight fail to function.

While splicing is occurring, two other vital eventsare being performed to make the immature mRNAready for action. Aso-called cap, which consists of amodified guanine, is added to one end of the pre-mRNA by an unconventional linkage. The cap ap-pears to function by interacting with the ribosome,helping to orient the mature mRNA so that trans-lation begins at the proper location. A tail, whichconsists of many adenines (often two hundred ormore), is also attached to the other end of the pre-mRNA. This so-called poly-A tail, which virtuallyall eukaryotic mRNAs contain, seems to be involvedin determining the relative stability of an mRNA.These important steps must be performed aftertranscription in eukaryotes to enable the creation ofa mature, functional messenger RNA molecule thatis now ready to be translated. Most mRNAs mustbe transported out of the nucleus before they areused to make proteins, but about 10 to 15 percent ofthe mRNAs produced remain in the nucleus, wherethey are used to make proteins.

Other Specialized FunctionsThe traditional roles of RNA in protein synthesis

were originally considered the only roles RNA wascapable of performing. RNA in general, while con-sidered an important molecule, was thought of as a“helper” in translation. This all began to change in1982, when the molecular biologists Thomas Cechand Sidney Altman, working independently andwith different systems, reported the existence ofRNAmolecules that had catalytic or enzymelike ac-tivity, meaning that RNA molecules can function asenzymes. Until this time, it was believed that all en-zymes were protein molecules. The importance ofthese findings cannot be overstated, and Cech and

Altman ultimately shared the 1989 Nobel Prize inChemistry for the discovery of these RNA en-zymes, or ribozymes. Both of these initial ribozymescatalyzed reactions that involved the cleavage ofother RNA molecules—that is, they acted as nu-cleases. Subsequently, many ribozymes have beenfound in various organisms, from bacteria to hu-mans. Some of them are able to catalyze differenttypes of reactions, and there are new ones reportedevery year. Thus ribozymes are not a mere curiositybut play an integral role in the molecular machin-ery of many organisms. Their discovery also gaverise to the idea that at one point in evolutionary his-tory, molecular systems composed solely of RNAperforming many roles existed in an “RNA world.”

At around the same time as these momentousdiscoveries, still other classes of RNAs were beingdiscovered, each with its own specialized func-tions. In 1981, Jun-Ichi Tomizawa discovered RNAI,the first example of what would become anothermajor class of RNAs, the antisense RNAs. The RNAsin this group are complementary to a target mole-cule (usually an mRNA) and exert their function bybinding to that target via complementary base pair-ing. These antisense RNAs usually play a regula-tory role, often acting to prevent translation of therelevant mRNA to modulate the expression of theprotein for which it codes.

Another major class of RNAs, the small nuclearRNAs (snRNAs), was also discovered in the early1980’s. Molecular biologist Joan Steitz was workingon the autoimmune disease systemic lupus whenshe began to characterize the snRNAs. There are sixdifferent snRNAs, now called U1-U6 RNAs. TheseRNAs exist in the nuclei of eukaryotic cells and playa vital role in mRNA splicing. They associate withproteins in the spliceosome, forming so-calledribonucleoprotein complexes (snRNPs), and play aprominent role in detecting proper splice sites anddirecting the protein enzymes to cut and paste atthe proper locations.

It has been known since the late 1950’s that manyviruses contain RNA, and not DNA, as their geneticmaterial. This is another fascinating role for RNAinthe world of biology. The viruses that cause influ-enza, polio, and a host of other diseases are RNAvi-ruses. Of particular note are a class of RNA virusesknown as retroviruses because they have a par-ticularly interesting life cycle. Retroviruses, whichinclude human immunodeficiency human immu-nodeficiency virus (HIV), the virus that causes ac-

916 • RNA

quired immunodeficiency syndrome (AIDS) inhumans, use a special enzyme called reverse tran-scriptase to make a DNA copy of their RNA instruc-tions when they enter a cell. That DNA copy is in-serted into the DNA of the host cell, where it isreferred to as a provirus, and never leaves. Clearly,understanding the structures and functions of theRNAs associated with these viruses will be impor-tant in attempting to create effective treatments forthe diseases associated with them.

An additional role of RNA was noted during theelucidation of the mechanism of DNAreplication. It

was found that a small piece of RNA, called aprimer, must be laid down by the enzyme primasebefore DNA polymerase adds DNA nucleotides tothis initial RNA sequence, which is subsequentlyremoved.

Matthew M. Schmidt

See also: Chloroplast DNA; DNA: historical over-view; DNA in plants; DNA replication; Gene regu-lation; Genetic code; Genetics: mutations; Mito-chondrial DNA; Nucleic acids; Proteins and aminoacids.

Sources for Further StudyEckstein, Fritz, and David M. J. Lilly. Catalytic RNANew York: Spring, 1996. Offers a compre-

hensive overview of ribozyme diversity and function. Illustrated. Bibliography, indexes.Erickson, Robert P., and Jonathan G. Izant, eds. Gene Regulation: Biology of Antisense RNA and

DNA. New York: Raven Press, 1992. Provides a comprehensive overview of naturalantisense RNA function and prospects for its uses in gene therapy. Illustrations, biblio-graphical references, index.

Murray, James A. H., ed. Antisense RNA and DNA. New York: Wiley-Liss, 1992. Coversantisense RNA. Illustrations, bibliography, index.

Simons, Robert W., and Marianne Grunberg-Manago, eds. RNA Structure and Function. ColdSpring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 1998. An advanced and heftytext (more than seven hundred pages) that, like Murray’s book, takes a detailed look atthe various structures of RNA, their relationships to function, and the techniques for de-termining RNA structure. Illustrations, bibliography, index.

Watson, James D., et al. Molecular Biology of the Gene. 4th ed. 2 vols. Menlo Park, Calif.:Benjamin/Cummings, 1987. The codiscoverer of DNA’s double-helix structure providesan insightful discussion of the various structures of RNA and their relationships to func-tion. Bibliographies, indexes, illustrations.

ROOT UPTAKE SYSTEMS

Categories: Anatomy; physiology; transport mechanisms; soil

Root uptake systems are processes by which root cells transport water and nutrients from the soil, across the root surface,and to the tissues that will move the water and nutrients throughout the plant.

Fertile soil is a complex mixture of a variety ofminerals, many different types of organic mat-

ter in different stages of decay, and a host of livingmicroorganisms. This complex medium holds alarge quantity of water, which it supplies to plants.In addition to water, the soil supplies the plantswith the thirteen mineral nutrients required for

normal growth and development. These nutrients(and the ionic forms taken up by the root) are nitro-gen, phosphorus, potassium, sulfur, calcium, mag-nesium, iron, manganese, boron, chlorine, zinc,copper, and molybdenum. Varying amounts ofthese mineral nutrients exist both as constituents ofthe soil particles and as dissolved ions in the soil

Root uptake systems • 917

water. Root uptake systems are responsible for tak-ing these nutrient ions and water from the soil andmoving them into the root tissues.

Root StructureMost plant roots extend below the soil surface as

either a fibrous root system or taproot system. In a fi-brous root system, the major root branches numer-ous times, and each branch divides again andagain, until a meshlike network is formed withinthe soil. This system does not penetrate very deepinto the soil, but it does cover considerable areaclose to the surface.

In a taproot system, only small secondary lateralroots branch off the main root (the taproot) as itgrows downward into the soil. The taproot mayextend several meters in depth. Along the outerperiphery of either of these root systems, largenumbers of small filamentous root fibers and roothairs can be found. The root hairs are filamentlikeprojections of the epidermal (outer layer) cells.These root hairs, in conjunction with the cells ofthe filamentous root fibers, are responsible for the

vast majority of water and nutrient uptake.Each root cell, as is the case in all plant cells, is

surrounded by a porous cell wall. Immediately in-side the cell wall is a semipermeable membrane,which regulates the movement of ions and mole-cules into the cytosol of the cell. This semipermeablemembrane is a fluid mosaic structure composedprimarily of lipid and protein. A double layer oflipid provides the basic stable structure of the mem-brane. The protein is then interspersed periodicallythroughout the lipid bilayer. Some of these pro-teins, called peripheral proteins, penetrate only oneof the layers of lipid, while integral proteins extendthrough both lipid layers to interface with the envi-ronment both inside and outside the cell. The rateand extent of water and ion movement throughmembranes are largely determined by this struc-tural configuration.

Ion Uptake MechanismsUptake of ions across membranes is accom-

plished by three mechanisms: simple diffusion, facili-tated diffusion, and active transport. The first two are

918 • Root uptake systemsP

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Most plant roots extend below the soil surface.

passive processes, with no direct input of cellularenergy required. The latter, as the name indicates,is an active process requiring the cell to expend en-ergy.

In its simplest analysis, diffusion is the net move-ment of suspended particles down a concentrationgradient. Thus, certain ions dissolved in the soil so-lution will move into the root cell cytosol as long asthe external concentration is higher than the con-centration inside the cell.

Because of the lipid nature of the membrane, therate of this movement will be determined by thelipid solubility of the particle. Those particles withhigh lipid solubility will diffuse across the mem-brane much faster than those with low lipid solu-bility.

Many nutrients exhibit low lipid solubility, yetstill diffuse across the membrane. This is accom-plished by facilitated diffusion. The ion with lowlipid solubility combines with a membrane protein,which then facilitates its uptake across the mem-brane.

In both free and facilitated diffusion, the parti-cles move down a concentration gradient. In nu-merous instances, however, the ion concentrationof the root cell cytosol is greater than the concentra-tion of the ion in the soil solution. The ion will nev-ertheless continue to accumulate within the root tis-sues.

Diffusion of any sort cannot account for this “up-hill” movement against a concentration gradient.In this instance, the nutrient ion will combine with amembrane protein referred to as a carrier. This pro-tein carrier will transport the particle across themembrane. Uptake mediated by this protein carriersystem is called active transport and requires theinput of energy supplied by the hydrolysis ofadenosine triphosphate (ATP), the cell’s primaryenergy currency.

Osmotic PotentialRegardless of the mechanism utilized to trans-

port ions into the root cytosol, the final result isthe establishment of an osmotic potential across themembrane. Osmotic potential is a measure of thetendency of water to move across a semipermeablemembrane in response to a difference in solute con-centration. The addition of a solute to water lowersits osmotic potential, and water will always movefrom the side of the membrane containing the solu-tion with the lower solute concentration (higher

osmotic potential) to the side of the membrane con-taining the solution with the higher solute concen-tration (lower osmotic potential). Hence, the up-take of the mineral ions by the plant root cellsestablishes a lower osmotic potential within thecytosol than exists in the soil solution, and waterflows across the membrane into the cell.

In order for the water and ions to move acrossthe root from the epidermal layer to the internaltransport vessels called xylem, several layers of cor-tex cells and the endodermis must be crossed. Thereare two pathways water and ions can take. One isthrough the cytosol of the epidermal cells, corticalcells, and endodermal cells by means of the plas-modesmata, which are cytoplasmic strands that passbetween plant cells, thereby connecting the cells asmicroscopic “bridges.” These plasmodesmata pro-vide a means by which the cytosol from all cells canexist as a continuous mass referred to as thesymplast, and transport through this system iscalled symplastic transport.

A second pathway, referred to as apoplastic trans-port, occurs through the apoplast (the region of con-tinuous cell walls among cells). Because thisapoplast is freely permeable to water and ions, thecells of the epidermis, cortex, and endodermis arein intimate contact with the soil water. Apoplastictransport, however, cannot occur across the en-dodermis because of the presence of an imperme-able waxy layer in the cell walls called the Casparianstrip. Thus, water and ions can travel through theapoplast until reaching the endodermis. There,movement into the endodermal cytosol by one ofthe three mechanisms mentioned above is required.

After passing the Casparian strip, the water andions can move back into the apoplast. Althoughthere is some disagreement among plant scientistsas to which pathway is most important, it is highlyprobable that both pathways are involved. Regard-less of whether the transport across the root isapoplastic or symplastic, the water and ions reachthe xylem tubes within the interior of the root,where subsequent transport throughout the plantcan take place.

D. R. Gossett

See also: Active transport; Angiosperm cells andtissues; Cells and diffusion; Liquid transport sys-tems; Osmosis, simple diffusion, and facilitated dif-fusion; Plant tissues; Roots; Vesicle-mediated trans-port; Water and solute movement in plants.

Root uptake systems • 919

Sources for Further StudyBegon, Michael, and Alastair Fitter, eds. Advances in Ecological Research. Vol. 27. San Diego:

Academic Press, 1997. Includes chapters on ecology of root life span and age-related de-cline in forest productivity.

Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. San Francisco: Benjamin Cummings,2002. An introductory college-level textbook for science students. The chapter “TrafficAcross Membranes” provides a clear, concise, detailed description of the membranetransport process. The well-written text, combined with superb graphics, furnishes thereader with a very clear understanding of membrane structure and transport. List of sug-gested readings at the end of the chapter. Includes glossary.

Mader, Sylvia S. Inquiry into Life. 10th ed. Dubuque, Iowa: McGraw-Hill, 2002. An introduc-tory-level textbook for the nonscience student. The chapter “Plant Form and Function”provides a simple overview of the root uptake and transport process. Clear, concise dia-grams with a very readable text establish a general picture of the process. Key terms andsuggested reading list at the end of each chapter. Includes glossary.

Nissen, P. “Uptake Mechanisms: Inorganic and Organic.” Annual Review of Plant Physiology25 (1974): 53-79. Detailed review article outlining much of the pertinent information re-lated to the control of root uptake. Ahistorical perspective as well as a discussion of the re-cent advances is presented by the reviewer. A very informative article with a detailed listof complex literature citations.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. An introductory college-level textbook for science students.The chapters “The Root: Primary Structure and Development” and “The Movement ofWater and Solutes in Plants” provide a discussion of root anatomy and transport acrossthe root. This profusely illustrated text furnishes a good pictorial study of the root uptakeprocess. Includes glossary.

Steward, F. C. Plant Physiology: A Treatise. 15 vols. New York: Academic Press, 1959-1991. Atechnical treatise for the more advanced student. An excellent review of plants in their re-lation to water and solutes. Ahistorical perspective, an overview, and in-depth treatmentare provided for each topic. Includes extensive bibliography.

ROOTS

Categories: Anatomy; physiology

Roots are those underground portions of a plant that store food, absorb water and minerals from the soil, and anchor theplant in the earth.

Roots account for more than 80 percent of plants’biomass in ecosystems such as tundra and

shortgrass prairies. In many plants, roots are longerand spread wider than the shoots. The extensiveroot systems of plants are effective collectors ofwater and minerals necessary for the life of theplant.

Root Cap and Quiescent CenterThe root’s structure facilitates each of its func-

tions. Tips of roots are covered by a thimble-shapedroot cap. At the base of the root cap is a meristem thatproduces cells that form the cap. The meristempushes cells forward into the cap, which protectsthe tip of the growing root as it forces its way

920 • Roots

through the soil. As these cells move through thecap, they differentiate into rows of columella cells.Columella cells contain plastids that sediment, in re-sponse to gravity, to the lower side of the cell. Thissedimentation is how roots perceive gravity.

Surrounding the outside of the root cap are pe-ripheral cells. Peripheral cells produce and secretelarge amounts of a slimy, water-soluble substancecalled mucigel. Mucigel has several important func-tions. It protects roots from desiccation and containscompounds that diffuse into the soil and inhibit

growth of other roots. Mucigelalso lubricates roots as they forcetheir way between soil particles.Soil particles cling to mucigel,thus increasing the root’s contactwith the soil, which helps rootsabsorb water.

Just behind the root cap is thequiescent center, which is made upof five hundred to one thousandseemingly inactive cells. Cells ofthe quiescent center divide onlyabout once every twenty days,while those of the adjacent meri-stem divide as often as twice perday. Cells of the quiescent centerbecome active when the tip of theroot is damaged. When this oc-curs, the quiescent cells dividerapidly to form cells to repair thedamaged root tip. The quiescentcenter also organizes the patternsof primary growth in roots.

Subapical RegionThe subapical region of roots

consists of three zones: the zoneof cellular division, the zone of cel-lular elongation, and the zone ofcellular maturation. These regionsof the root intergrade and arenot sharply defined. The zone ofcellular division surrounds thequiescent center and is a dome-shaped meristem 0.5 to 1.5 milli-meters behind the root tip. Thus,the meristem of a root is subter-minal and is made of small, multi-sided cells. Meristematic cells di-vide between one and two times

per day. In some plants, the meristem produces al-most twenty thousand new cells per day. The rate ofthese divisions is influenced by hormones such asethylene.

The zone of cellular elongation is 4 to 15 millime-ters behind the root tip. Cells in this zone elongaterapidly by filling their vacuoles with water. As a re-sult, the elongating zone is easily distinguishedfrom the root cap and zone of cellular division by itslong, vacuolate cells. Cellular elongation in theelongating zone pushes the root cap and apical

Roots • 921

Root cap

Mucigel sheath

Root hairs

Emerginglateral root

Lateral root

CortexLateral root

Pericycle

Vascular tissue(xylem and phloem)

Parts of a Root

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meristem through the soil as fast as 2 to 4 centime-ters per day. Cellular elongation is typically inhib-ited by the hormone auxin and stimulated by lowconcentrations of ethylene.

Cells behind the elongating zone do not elon-gate. Elongation begins the process of cellular dif-ferentiation, or specialization. Differentiation iscompleted in the zone of cellular maturation, whichis 1 to 5 centimeters behind the root tip. The matura-tion zone is distinguished by the presence of nu-merous root hairs—as many as forty thousand persquare centimeter. Root hairs increase the surfacearea of the root by a factor of several thousands andare usually less than 1 millimeter long. They liveonly a few days and form only in the mature,nonelongating region of the root. Because root hairsare fragile extensions of epidermal cells, they usu-ally break off when plants are transplanted.

All mature tissues of roots form behind the zoneof cellular maturation. The root is surrounded by anepidermis, which is usually only one cell thick. Epi-dermal cells usually lack a cuticle. The epidermiscovers all of the root except the root cap and typi-cally has no openings.

CortexImmediately inside the epidermis is the cortex.

The cortex occupies most of a root’s volume andconsists of three concentric layers: the hypodermis,storage parenchyma cells, and the endodermis. Thehypodermis is a waxy, protective layer that slowsoutward movement of water. Thus, the hypoder-mis helps roots retain water and nutrients that havebeen absorbed.

Most of the cortex consists of thin-walled paren-chyma cells that store carbohydrates. These cellsare separated by large intercellular spaces occupy-ing as much as 30 percent of the root’s volume. Theinnermost layer of the cortex is the endodermis.Unlike other cortical cells, endodermal cells arepacked tightly together and lack intercellularspaces. Their radial and transverse walls, further-more, are impregnated with a Casparian strip oflignin and suberin. If endodermal cells are com-pared to bricks in a brick wall, then the Casparianstrip is analogous to the mortar surrounding eachbrick. The Casparian strip prevents inward move-ment of water and nutrients through the cell walland intercellular spaces. The endodermis functionssomewhat like a valve that regulates movement ofnutrients.

Collectively, the tissues inside the cortex arecalled the stele, which consists of the pericycle, vascu-lar tissues, and sometimes a pith. The pericycle is theoutermost layer of the stele and is a meristematiclayer of cells one to several cells thick; it producessecondary, or lateral, roots.

Inside the pericycle is the root’s vascular tissue,which consists of xylem and phloem. Vascular tissuestransport water, minerals, and sugars throughoutthe plant and differentiate in response to auxin, aplant hormone, coming from the shoot. Roots ofmost dicotyledons (dicots) and gymnosperms havea lobed, solid core of primary xylem in the center ofthe root. Roots of monocots and a few dicots have aring of vascular tissue that surrounds a pith. Bundlesof primary phloem differentiate between lobes of xy-lem. In dicots and gymnosperms, a vascular cambiumlater forms between the xylem and phloem andproduces secondary growth that thickens the root.

Types of Root SystemsDifferent kinds of plants often have different

kinds of root systems. Most gymnosperms and di-cots have a taproot system consisting of a large pri-mary root and smaller branch roots. In plants suchas the carrot, fleshy taproots store large amounts ofcarbohydrates. Not all taproots store food. Longtaproots of plants such as poison ivy and mesquiteare modified for reaching water deep in the groundrather than storing food. Many plants have verylong taproots. Engineers digging a mine in thesoutheastern United States uncovered the taprootof a mesquite tree more than 50 meters down.

Most monocots such as corn and other grasseshave a fibrous root system that consists of an exten-sive mass of similarly sized roots. Most of theseroots are adventitious roots, which form on organsother than roots themselves. Fibrous roots of someplants are edible—for example, sweet potatoes arefleshy parts of fibrous root systems of ipomoea(morning glory) plants. Plants with fibrous rootsystems reduce erosion because their root systemsare extensive and cling tightly to soil particles.

Adventitious roots are common in ferns, clubmosses, and horsetails. In plants such as tree ferns,adventitious roots form in stems, grow downthrough the cortex, and finally emerge at the base ofthe stem. Adventitious roots are a primary meansof asexual reproduction in many plants. For example,prairie grasses and forests of quaking aspen treesare often derived from a single individual propa-

922 • Roots

gated by adventitious roots. Humans use adventi-tious roots to propagate plants such as raspberries,apples, and brussels sprouts. Formation of adventi-tious roots is controlled by hormones such as auxin,which is often an ingredient in “rooting” com-pounds sold commercially.

FunctionsThe structure of a root relates directly to its four

primary functions: absorption, anchorage, conduction,and storage. For example, most water and nutrientsare absorbed by root hairs in the zone of maturation

of the root. The water thereafter moves through theroot either inside cells or in spaces between cells.Water seeping between cells finally encounters theendodermis, which is the primary barrier to ab-sorption. The Casparian strip in the endodermis en-sures that water and nutrients enter the stele via theplasmodesmata (narrow strands of cytoplasm thatconnect the cytoplasms of adjacent cells). Most nu-trients are absorbed and accumulate in the apical0.3 to 0.5 meter of the root.

Few nutrients are absorbed past a few centime-ters beyond the root tip because these parts of the

Roots • 923

Taproot

Taproot systemFibrous root system

Two Root Systems

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root lack root hairs and have a waxy endodermis.These nonabsorptive regions of roots anchor plantsand may later produce branch roots. Water and dis-solved nutrients absorbed by roots move to theshoot in xylem. Roots receive nutrients from theshoot via the phloem. These nutrients either areused for growth or are stored in cortical cells for fu-ture use.

Roots of many plants are modified for specialfunctions. For example, roots of plants such asbeets, radishes, dandelions, and cassava store largeamounts of starch. Sweet potato roots store carbo-hydrates as sugars. Roots of other plants are usedfor asexual reproduction.

Roots of cherry, apple, and teak possess adventi-tious buds that form shoots called suckers. Whenseparated from the parent plant, suckers becomenew individuals. Adventitious buds are a commonmeans of propagating many other plants. For ex-ample, most groups of creosote bushes are clonesderived from a single plant. Some of these clonesare more than twelve thousand years old—mean-ing that the first seed germinated approximatelyfour thousand years before humans began writing.

The roots of many plants minimize competitionfor water and nutrients by growing in differentparts of the soil. For example, mesquite trees grow-ing in deserts often have taproots more than 20 me-ters long that obtain water from the undergroundwater table. Nearby cacti, however, survive in thesame environment by producing a shallow rootsystem that spreads as far as 30 meters. Cactus rootsdo not reach the water table; rather, they quickly ab-sorb water after the infrequent and often heavyrains that occur in the desert.

Roots can protect the plant from other organ-isms. Most root defenses against soil pathogens arechemical rather than structural. Roots often secretenoxious chemicals that inhibit the growth of patho-gens and other organisms, including other plants,in some cases.

Prop roots are aerial roots that grow into the soil.They are common in plants such as corn and ban-yan trees. Banyan trees produce thousands of proproots that grow down from horizontally orientedstems and form pillarlike supports.

Environmental InfluencesThe growth and distribution of roots are con-

trolled by several environmental factors. For ex-ample, the short days and cooler temperatures of

winter typically cause roots to become inactive.Microbes living in the soil also affect how rootsgrow. Most microbes in soil live within 10 microm-eters of the root and secrete compounds that sig-nificantly affect growth and distribution of roots.These compounds can affect the anatomy, mor-phology, number of root hairs, and branching pat-terns of root systems.

Microbes also affect how plants absorb andtransport minerals from the soil. Plants growing insterile soil absorb fewer minerals than those grow-ing in soil containing microbes. Beneficial fungicalled mycorrhizae live in and on roots of almost allplants in a form of mutualism, meaning that boththe plant and the fungus benefit from the associa-tion. The mycorrhizae absorb nutrients from the en-vironment, while the host plant provides the fun-gus with carbohydrates, amino acids, vitamins,and other organic substances. Plants with mycor-rhizae tolerate drought and other types of stressbetter than uninfected plants.

Roots of legumes such as beans are often infectedwith Rhizobium (from rhiza, the Greek word for“root”), a genus of the nitrogen-fixing bacteria.Swellings in response to these infections are callednodules. Bacteria receive carbohydrates and othersubstances from the host, while the host plants re-ceive large amounts of usable nitrogen from thebacteria.

Many organisms compete with roots to collectnutrients and water in the soil. For example, 1 gramof fertile soil contains approximately 109 bacteria,106 actinomycetes (a group of fungilike bacteria),105 fungi, and 103 algae. Plants have evolved sev-eral different strategies for competing with theseorganisms. One is to produce an extensive root sys-tem consisting of many roots that permeate the soil.Most roots grow in the upper 3 meters of soil, how-ever, where nutrients are most abundant.

The narrow zone of soil surrounding a root andsubject to its influence is called the rhizosphere.Roots modify the rhizosphere by secreting organicmatter, compressing the soil, and absorbing nutri-ents. As a result of these effects, the rhizosphere issignificantly different from bulk soil. It usually con-tains large amounts of energy-rich molecules.These molecules are eaten by fungi and bacteria.

Roots tend to grow best in moist, loosely packedsoil. Roots of many plants grow two to four timesfaster in loose, sandy soil than in tightly packedclay. The slow growth of roots in poorly aerated soil

924 • Roots

is probably caused by the accumulation of ethyl-ene, a plant hormone that slows root growth.

Roots of plants that grow in wet areas are usuallysmall and modified for gas exchange. They possesssmall amounts of xylem, lack root hairs, and con-tain large intercellular spaces, thereby improvingventilation. Roots of aquatic plants are also modi-fied for gas exchange. For example, the black man-grove produces specialized roots called pneumato-phores, which grow up into the air, where theyfunction like snorkels through which oxygen dif-fuses to submerged roots.

Epiphytes, including some bromeliads and or-chids, are plants that grow on other plants but arenot parasites. Adaptations among these “air plants”include a thickened root epidermis which protectsthe cortex and retards water loss. The “flower potplant” (Dischidia rafflesiana) grows a “pot” that col-lects water and debris; the plant’s aerial roots grow

into the pot, where they can absorb the mineralscollected there.

Roots of plants that grow in dry areas are oftenextensive and modified for rapid transport ofwater. They contain large amounts of xylem, whichallows them to move water rapidly to the shoot af-ter rainfall. Plants such as witchweed and broom-rape use their roots to parasitize other plants.Witchweed is a red-flowered plant that infectsgrains such as corn and sorghum; it is the secondleading cause of cereal famine in Africa.

Randy Moore, updated by Elizabeth Slocum

See also: Angiosperm cells and tissues; Bulbs andrhizomes; Gas exchange in plants; Germinationand seedling development; Growth and growthcontrol; Liquid transport systems; Nitrogen fixa-tion; Plant tissues; Root uptake systems; Water andsolute movement in plants.

Sources for Further StudyBohm, W. Methods of Studying Root Systems. New York: Springer-Verlag, 1979. Excellent ac-

count of the various ways of studying root systems. Includes numerous diagrams andphotographs.

Epstein, Emanuel. “Roots.” Scientific American 228 (May, 1973): 48-58. This article describesthe structure and function of roots, including roots modified for unusual functions. Dia-grams of root structure are included to show the integration of structure and function, es-pecially as related to ion uptake.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. The chapter “The Root: Primary Structure and Function” isa well-illustrated description of the primary structure of roots. Numerous micrographsand diagrams help explain the relationship of root structure to root function.

Waisel, Yoav, Amram Eshel, and U. Kafkafi, eds. Plant Roots: The Hidden Half. 3d rev. ed.New York: Marcel Dekker, 2002. This overview of root systems addressees structure anddevelopment, methods of study, growth and metabolism, ion and water relations, therhizosphere, root pests, and more.

RUBBER

Category: Economic botany and plant uses

Rubber is a product of rubber tree bark or a macromolecule or polymer of repeated chains of carbon and hydrogen atoms.Rubber’s properties of extensibility, stretchability, toughness, and resilience make it a useful commodity in applicationsranging from tires to clothing.

The name “rubber” originates from the mate-rial’s ability to erase pencil marks; its chemical

designation is polyisoprene with several isomers.About 60 to 65 percent of the rubber produced to-

Rubber • 925

day is synthetic. When explorer Christopher Co-lumbus arrived in Haiti in 1492 he found Indiansplaying a game with a ball made from the latex ofrubber. American Indians were also known to haveused latex for making footwear, bottles, and cloaks.By 1735 latex had been described as caoutchouc by aFrench geographical expedition in South America.

The role that rubber could play in clothing andfootwear attracted the attention of chemists and in-ventors throughout the world in the late eighteenthand mid-nineteenth centuries. Charles Macintoshand Thomas Hancock, working as colleagues, dis-covered two separate means of using rubber in fab-rics and footwear. Macintosh found that placingrubber between layers of fabric resulted in a fabricwith no sticky and brittle surfaces. Hancock devel-oped the rubber masticator, which welded rubberscraps to be used for further manufacturing.

The dramatic increase in the use of rubber thatoccurred in the twentieth century is attributable

largely to the development of the automobile in-dustry and advances in industrial technology. Al-though rubber’s percentage of use compared withother elastomers decreased from the end of WorldWar II to the late 1970’s, the development of radialautomobile tires in Europe in the late 1940’s andtheir popularization in the United States in the late1960’s resulted in increased use of natural rubber.

Origin of RubberThe early use of rubber involved all-natural rub-

ber formed from a number of different plant speciesbelonging to the Euphobiaceae family, of which therubber tree (Hevea brasiliensis), native to Brazil, hasbecome the exclusive commercial source. As a co-agulated milk substance, rubber is obtained from afluid in latex vessels located in the bark of the tree.A number of other tropical and subtropical plantspecies also contain such latex vessels, includingManihot, Castilla, the Russian dandelion, guayule

926 • Rubber

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(Parthenium argentatum), and Funtumia elastica.Both the Russian dandelion and guayule were

widely used during World War II. Research hascontinued on guayule, a plant native to the south-western United States and northern Mexico. Simi-larly, Funtumia elastica, native to West and CentralAfrica, has received some research attention.Guayule, used by American Indians, is still consid-ered a possible alternative rubber source to syn-thetic rubber in North America, particularly thesouthwestern United States.

Today the production of natural rubber is basedon Hevea brasiliensis, which is grown mostly in trop-ical and subtropical environments. While produc-tion is concentrated in developing countries, con-sumption occurs mostly in industrialized countries.Between 1955 and 1988 production of rubber morethan doubled, with Malaysia the leading worldproducer and the United States the world’s largestconsumer.

Growing Rubber PlantsTrees for commercial rubber plantations are

vegetatively propagated by means of bud grafting.The bud from a high-yielding tree is cut and in-serted under the bark of a rootstock. Upon a suc-cessful take, the bud grows, and the rootstock istopped or removed at the point of growth. It is thentransplanted from the nursery to the field. The treeis ready for tapping in five to seven years, when treegirth reaches 50 centimeters at 1.60 meters fromground level. Crown budding may also be done be-fore budded stumps are transferred to the field.This type of budding is used to provide a crownthat is tolerant of or resistant to disease or winddamage. Stand density in rubber plantationsranges from 250 trees to 400 trees per hectare at anaverage spacing of about 6 meters by 6 meters.

Rubber grows best in deep, well-drained soil butcan be grown on a wide range of soils. Rainfallshould exceed 2,000 millimeters yearly, evenly dis-tributed without any marked dry season. Tempera-ture should average 25 to 28 degrees Celsius, withhigh (80 percent) humidity and bright sunshine ofabout six hours per day year-round. These condi-tions exist in the major rubber-producing countriesof the world.

In Hevea, latex is obtained from latex vesselscalled secondary laticifers. The quantity of latici-ferous tissue in the tree is determined by a numberof anatomical factors, such as vessel rings, size of

laticifers, girth of trees, and the distribution of latexand latex vessel rows. The flow of latex and, subse-quently, the yield of a rubber tree is dependent onthese anatomical features.

Latex ProcessingUntil about 1913 Brazil was the major producer

of natural rubber, obtained mostly from wild rub-ber trees growing in the jungles of the Amazon ba-sin. In the early twentieth century, plantation pro-duction of rubber began, based on the work ofHenry N. Riley in Singapore around 1890. Riley de-veloped the “tapping” method for extracting latexfrom Hevea. Later improvements to this method in-cluded the mechanization of the tapping knife.

During tapping, a slice of bark is systematicallyremoved from one side (panel) of the tree, startingfrom an upper left corner and shaving to a lowerright corner, with care being taken not to damagethe cambium. The cut usually has an angle of 25 to30 degrees. Once the cut is made, latex flows into acollecting cup through a spout inserted on the tree.Generally, tapping is done from just before sunriseto about 10:00 a.m., to take advantage of maximumturgor pressure within the tree in the early morninghours. Stoppage of latex flow is attributable to a co-agulum that plugs latex vessels.

About four to five hours after tapping, the latexis collected from the trees. Field latex or cuplumpsand “tree-lace” latex (strips or sheets of latex coagu-lated on a tapping cut) are collected and taken toa factory, laboratory, or small-holder processingcenter. At the processing center, latex is sieved toremove foreign objects, such as stones, branches,and leaves, and is then blended by the addition ofwater or dilute acetic or formic acid. About 10 per-cent of the latex is shipped as latex concentrate, fol-lowing blending. Concentrates of natural rubber la-tex are obtained by the process of centrifugationand creaming. Meanwhile, the remainder of the la-tex and field coagulum are processed, either intoconventional types of rubber or into technicallyspecified rubber (TSR).

A number of fundamental weaknesses associ-ated with manufactured rubber were resolved in1839 with the development of vulcanization byCharles Goodyear, an American inventor. Vulcani-zation is the process of treating natural rubber withsulfur and lead and subjecting the compounds tointense heat, resulting in what Goodyear first called“fire proof gum” but later called vulcanized rubber.

Rubber • 927

Current vulcanization technology is a modificationof Goodyear’s invention. New forms of vulcaniza-tion are available based on diurethanes, which arestable at processing temperatures as high as 200 de-grees Celsius or more. Vulcanized rubber can thenbe processed into a wide range of applications, in-cluding tires, fabrics, bridge constructions, andother latex products such as adhesives and foot-wear.

Future Uses of Natural RubberThere is continuing interest and effort on the part

of research scientists and natural rubber producersto find new uses for natural rubber. Thus, projec-tions for new uses range from snowplow blades toearthquake-resistant building construction materi-als. Although many proposed uses are engineeringapplications, there are other applications in the areaof wood products that may eventually make thelarge acreages of rubber plantations importantsources for environmental restoration, given the in-creasing deforestation that is taking place in thenatural rubber-producing areas of the world. Inrubber plantations that are more than forty yearsold, the regeneration of secondary forests with as-sociated wildlife species occurs frequently. Thus,natural rubber is both an important industrial cropspecies and a major renewable resource.

Synthetic RubberMuch of what people typically consider rubber

today is actually synthetic rubber. Synthetic rubberis a polymer of several hydrocarbons; its basis ismonomers such as butadiene, isoprene, and sty-rene. Almost all monomers for synthetic rubber arederived from petroleum and petrochemicals. Theemulsion polymerization process occurs at veryhigh temperatures. There are different types of syn-thetic rubbers, three of which are dominant in therubber industry. These are styrene-butadiene rub-ber (SBR), polyisoprene rubber (IR), and poly-butadiene rubber (BR). Unlike natural rubber, syn-thetic rubber is produced mainly in industrializedcountries. The United States is the world’s leadingproducer.

Theoretically, synthetic rubber production datesback to 1826, when scientist Michael Faraday indi-cated that the empirical formula for synthetic rub-ber was (C5H8)x. The technology for synthetic rub-ber production was not developed until 1860,however, when Charles Williams found that natu-ral rubber was made of isoprene monomers. Greatinterest in using synthetic rubber as a substitute fornatural rubber began during World War II, whenthe Germans were looking for alternatives to natu-ral rubber. The severe shortages of natural rubberduring and immediately after the war stimulated

928 • Rubber

Latex products 8%

Footwear 5%

Non-automotive engineering 3%

Belting and hose 2%

Other miscellaneous uses 12%

Tires andrelated

products70%

End Uses of Natural Rubber

Source: Adapted from R. E. Brice and K. P. Jones, in Natural Rubber Science and Technology, edited by A. D. Roberts, 1988.

significant research in synthetic rubber and its tech-nology. Today, synthetic rubber is used in a widerange of applications, and it constitutes 60 to 65 per-cent of the total rubber produced and consumed.

Oghenekome U. Onokpise

See also: Asian agriculture; Asian flora; CentralAmerican flora; Forest management; Metabolites:primary vs. secondary; Plants with potential; SouthAmerican flora; Vacuoles.

Sources for Further StudyCosner, Sharon. Rubber. New York: Walker, 1986. Concise, factual history describes the earli-

est use of rubber and traces it to the present.Johnson, Peter S. Rubber Technology: An Introduction. Cincinnati, Ohio: Hanser Gardner, 2001.

Overview deals with all aspects of rubber processing. For newcomers to the industry,suppliers, processors, or end-product users.

Korman, Richard. The Goodyear Story: An Inventor’s Obsession with Rubber at the Dawn of the In-dustrial Revolution. San Francisco: Encounter Books, 2002. Biography describes howCharles Goodyear, in his quest to find the recipe for rubber, mired his family in povertyand provoked powerful enemies, ultimately becoming an American industrial legend.

Stokes, Charles E., Jr. The Amazon Bubble: World Rubber Monopoly. Fort McKavett, Tex.: C. E.Stokes, Jr., 2000. Constructs the background and history of the Amazon rubber trade,from the perspective of all the South American countries involved.

RUSTS

Categories: Diseases and conditions; fungi; pests and pest control; poisonous, toxic, and invasive plants;taxonomic groups

Rust fungi belong to the taxonomic class Teliomycetes within the basidiosporic phylum and include some seven thou-sand known species, all plant parasites, with many being extremely important plant pathogens.

Rust diseases have caused serious problems forcenturies and continue to cost billions of dol-

lars annually worldwide. Important examples in-clude: leaf, stem, and crown rusts of wheat, barley,and oats; the blister, fusiform, and gall rusts ofpines; coffee rust; many field crop rusts, such assoybean, peanut, sunflower, and flax rusts; andrusts of many horticultural crops and ornamentalplants, such as cedar-apple rust, bean rust, and roserust.

Symptoms and SignsSymptoms, the visible effects on a host caused by

rust diseases, are often less noticeable than are thesigns of infection, that is, the visible presence of apathogen, such as reproductive spores. Symptomssuch as bushy overgrowths, witches’ brooms (denseclusters of shoots), and large galls are more com-mon in some tree hosts than in herbaceous hosts. In

most herbaceous rusts and the foliar rusts of sometrees, such as coffee and poplar, little, if any, distor-tion of the host occurs. Most noticeable are therust’s reproductive spores borne in various types ofpustules (or sori) that rupture leaf and stem epider-mis. Loss of water vapor through ruptured epi-dermis can cause wilt symptoms during hot, dryperiods. Several kinds of spores and pustules maydevelop during different stages of the rust’s life cy-cle. Combined results of parasitism, water loss, celldeath, and other effects on the host can result in sig-nificant reductions in yield.

Life Cycle and Related Spore StagesRust fungi have a complex life cycle that may

contain up to five different spore types, or stages,each performing specific functions in the cycle.Some rust species lack one or more spore stage orstages. Autoecious forms of rusts, such as bean rust,

Rusts • 929

complete their life cycle on one host species. Heter-oecious rusts require two unrelated hosts on whichto complete their life cycle. Puccinia graminis, thewheat stem rust pathogen, which produces all fivespore stages on two unrelated hosts, is a good ex-ample of a rust with the heteroecious life cycle.

Distribution and DisseminationInitial distribution of rusts between continents,

especially if separated by oceans, is often the resultof human activity, such as the inadvertent introduc-tion of the pathogen on seeds, cuttings, or otherplant parts. Once established in a region, rusts are

most often spread locally and long distance bywind-blown urediniospores. In North America,south-to-north dispersal of urediniospores ofwheat stem and leaf rust fungi occurs annuallyfrom Mexico and south Texas through the GreatPlains into Canada. Coffee rust, long absent in theWestern Hemisphere, rapidly spread across Southand Central America by wind dispersal followingits 1970 introduction into Brazil.

Obligate ParasitismAlthough a few rusts can be grown on chemi-

cally defined, nonliving nutrients, most rusts re-quire a living host on which to growand reproduce. Consequently, rustsare termed obligate parasites or bio-trophs, in contrast to the majority ofplant pathogens, which are faculta-tive parasites or heterotrophs. Thosecan grow and reproduce on manynutritional media in culture, in deadplant tissue, or as parasites of livingplants. Within infected host tissue,rust fungi grow primarily as fila-mentous hyphae in intercellularspaces between host cells. Rusts ob-tain nutrition through specializedbranches, haustoria, that extendinto and absorb nutrients from liv-ing host cells. Those absorbed nutri-ents are then transported throughintercellular hyphae to expandingor spore-producing portions of therust colony.

Disease ControlTypically, total control of rust dis-

eases is impossible, both biologicallyand economically; hence, the usualgoal is management of rust diseases,not control. Several different man-agement methods are commonlypursued, depending on the host,the type of rust involved, its majormeans of dissemination, and eco-nomics or aesthetic value of the hostplant or crop. The ideal and mostcost-effective management tool ishost resistance, either naturally pres-ent in the host population or result-ing from plant breeding.

930 • Rusts

Image Not Available

Breeding for rust resistance has been especiallyimportant in rusts of cereals and other major agro-nomic plants. Unfortunately, many resistant varie-ties, or cultivars, may eventually succumb to new,more virulent or aggressive strains or races of thepathogen that arise by natural selection from popu-lations of the pathogen in nature. Foliar fungicidesprays can be cost-effective management tools forrusts of some cereal crops, beans, coffee, and roses.

In some instances, removing or eradicating the al-ternate host, such as common barberry, in heteroe-cious rusts has controlled such diseases but hasusually proven ineffective economically.

Larry J. Littlefield

See also: Basidiosporic fungi; Diseases and disor-ders; Fungi; Parasitic plants; Resistance to plantdiseases.

Sources for Further StudyAgrios, G. Plant Pathology. 4th ed. San Diego: Academic Press, 1997. University-level plant

pathology text; in-depth discussion of all types of plant pathogens and plant diseases.Alexopoulos, C. J., C. W. Mims, and M. Blackwell. Introductory Mycology. 4th ed. New York:

John Wiley & Sons, 1996. Comprehensive university-level mycology text; covers allgroups of fungi, including pathogens, nonpathogens, beneficial fungi, and taxonomy,morphology, and ecology.

Littlefield, L. J. Biology of the Plant Rusts: An Introduction. Ames: Iowa State University Press,1981. A broad, introductory overview of rust fungi and rust diseases, including taxon-omy, morphology, aerobiology, life cycles, and historical and economic importance.

_______. “Smut and Rust Diseases.” In Plant Pathology: Concepts and Laboratory Exercises, ed-ited by R. N. Trigiano et al. Boca Raton, Fla.: CRC Press, 2002. College-level text provides abroad, introductory presentation of all aspects of plant pathology. The smut and rust dis-eases chapter covers those pathogens and diseases. Includes CD of illustrations.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Outstanding basic botany text includes a brief, well illus-trated introduction to rust fungi. With glossary, index.

Schumann, G. L. Plant Diseases: Their Biology and Social Impact. St. Paul, Minn.: APS Press,1991. An overview of a wide range of plant diseases and pathogens, including rusts, withemphasis on both basic biology and economic, social, and historical impacts of plant dis-eases.

Rusts • 931

Wheat Stem Rust: Five StagesStage Spore-Bearing Structure Spores Spores’ Nuclear Condition

0 Pycnia, bearing1 Pycniosporesand receptive hyphae Haploid (n)I Aecia, bearing1 Aeciospores Dikaryotic (n + n)II Uredinia, bearing2 Urediniospores Dikaryotic (n + n)III Telia, bearing2 Teliospores Dikaryotic, later diploid (2n)IV Basidia, bearing Basidiospores3 Haploid (n)

1. Produced on the barberry host.2. Produced on the wheat or barley host.3. Produced upon germination of teliospores, on crop debris or on soil.

SAVANNAS AND DECIDUOUS TROPICAL FORESTS

Categories: Biomes; ecosystems; forests and forestry

Savannas are areas of continuous grass or sedge cover beneath trees that range from scattered, twisted, and gnarled indi-viduals to open woodlands. Deciduous tropical forests have continuous to open forest cover and undergo a leafless periodduring a seasonally lengthy dry season.

Where the annual rainfall in tropical regions isless than 2,000 millimeters and three to six

months out of the year are dry, savannas and decid-uous forests are common. Both savannas and de-ciduous tropic forests often occur where the annualrainfall is less than that of savannas. Together, thetwo biomes are referred to here as the dry tropicalbiome.

A pronounced pattern of seasonally wet and dryperiods is the most important factor affecting thedistribution of these types of plant cover. Highersoil fertility favors forest over grasses and savannasuch as in the cerrado of Brazil, which occurs onlyon certain geological formations and low-nutrientsoils. Fire has been a dominant feature of thesebiomes, and human influences—fires, agriculture,and grazing of animals—have interacted with cli-mate to produce a varied landscape.

The dry tropical biome is most geographicallywidespread on the continents of Africa, SouthAmerica, and Australia, with smaller enclaves inAsia. The world’s largest expanses of dry forest—the Brachystegia woodland across Central Africa,the cerrado (savanna) and caatinga (dry forest) of theAmazon basin, and much of interior Australia—arenotable examples. “Elephant grass savanna,” withtall grasses up to 4 meters tall and scattered trees,occurs exclusively in Africa. In the West Indies, dryforest occurs in rain-shadow zones on the leewardsides of islands affected by the tradewinds.

Plant Adaptations and Diversity of Life-FormsAs the rainfall decreases below 2,000 millime-

ters, and especially below 1,000 millimeters, theheight of the forest decreases and the proportion oftrees that are deciduous increases. In the dry trop-ics, leaf fall occurs in response to drought, andtherefore the lengthy dry season becomes a selec-

tive pressure to which plants have adapted. Treeleaves tend to be compound, with small leafletsthat help plants exchange heat with their surround-ings better than large, simple leaves; rates of leafrespiration and transpiration are thereby reduced.Sclerophyllous leaves are common, aiding in mois-ture retention, and the drier, more open woodlandsmay have cacti or other succulents.

The dry forest is far less species-rich than the rainforest, but the diversity of life-forms and the pro-portion of endemics are greater. For example, dryforests may contain xerophytic (dry-adapted) ever-greens, either obligatively or facultatively decidu-ous trees, trees with photosynthetic bark, plantsthat use the crassulacean acid metabolism (CAM)photosynthesis as well as C3 and C4 dicots, grasses,bromeliads, lianas, epiphytes, and hemiparasites.Trees from Fabaceae (the legume family) are themost well-represented family among trees.

Dry forests contain a higher proportion of wind-dispersed species than wetter forests, and manytrees will have their flowering and fruiting con-trolled by the duration and intensity of the dryseason. Synchronous flowering within and amongspecies is common, and many produce seed duringthe dry season. Flowers are often conspicuous andvisited by specialized pollinators such as hawk-moths, bats, and bees.

It is incorrect to generalize about savannas anddry tropical forests because, although they both oc-cur in the drier tropics, the two vegetation types oc-cur in different habitats and are adapted differentlyto their respective environments.

Trees of the cerrado in northeast Brazil aredeeply rooted, tap groundwater, and have highrates of transpiration. Drought here is atmospheric,as water is always available below 2 meters of soildepth. The deciduous caatinga of central Brazil,

932

however, receives only 500 millimeters of rainyearly, and transpiration of trees is low. Here, treessuffer significant water deficits during the long, dryseason, are truly xerophytic, and exhibit classic ad-aptations to drought.

Trees of the cerrado have a number of adapta-tions that confer resistance to fire. These include athick, corky bark, the ability to form adventitiousroots from buds on roots following the burning ofthe stem, and the cryptophyte or hemicryptophytelife-form (cryptophytes produce buds under-ground). Many herbaceous species are induced toflower by fire.

Human Impacts and ConservationFires have occurred in the Brazilian cerrado for

thousands of years based on carbon 14 dating ofcharcoal fragments. Fire is thus an environmentalfactor to which the vegetation has become adapted.Yet, the human influence has been to increase theincidence of fire. The cerrado has changed as a re-

sult to a more open form of plant cover with fewertrees and shrubs. In addition, timber extraction,charcoal production, and ranching have altered thesavanna landscape. The ability of belowground or-gans to survive such types of disturbance has in-creased the ability of the cerrado to persist. Yet it isestimated that 50 percent of the cerrado has beendestroyed, much of this the result of clearing for ag-riculture since the 1960’s.

Because of better soils and fewer pests, humansin tropical areas of Central America have mostlychosen the dry and moist life zones as places to liverather than the wetter rain-forest zones. As a result,dry forest ecosystems have been subject to massivedisturbance. Today, only a small fraction of the orig-inal dry forest remains. Fire has been used as ameans of clearing the forest for farming, but, unlikethe savanna, the dry forest is not adapted to fire. AtGuanacaste, Costa Rica, a well-known tropical con-servation project, restoration of dry forest, is depen-dent on controlling annual fires set by farmers and

Savannas and deciduous tropical forests • 933C

orbis

Savannas are landscapes of dense grass and scattered trees, such as these yellow fever trees growing in Nakura NationalPark in Kenya. Common on the continent of Africa, savannas are also found in India, Australia, and the northern part ofSouth America.

ranchers and supporting the return of forest vege-tation to dry areas. In Africa, large areas of dry for-est are burned annually by farmers, and areas ofdense, dry forest have been converted to more openforest or even savanna. Sustainable land-use sys-tems are urgently needed for dry tropical regions.

Allan P. Drew

See also: African flora; Australian flora; Biomes:types; Biomes: definitions and determinants; Bi-omes: types; Central American flora; Community-ecosystem interactions; Drought; Slash-and-burnagriculture; South American flora; Sustainable ag-riculture.

Sources for Further StudyAllen, W. Green Phoenix: Restoring the Tropical Forests of Guanacaste, Costa Rica. New York: Ox-

ford University Press, 2001. Discusses Daniel Janzen’s work with ecological restoration ofdry forest to form the Guanacaste Conservation Area. Photographs, bibliographical refer-ences, index.

Bullock, S. H., H. A. Mooney, and E. Medina. Seasonally Dry Tropical Forests. New York: Cam-bridge University Press, 1995. Geographical distribution of dry forests, plant and life-form diversity, drought responses, plant reproduction, plant-herbivore interactions, nu-trient cycling, conversion to pasture and agriculture, and ethnobotany. Tables, figures,photographs, maps, references, index.

Rizzini, C. T., A. F. C. Filho, and A. Houaiss. Brazilian Ecosystems. Rio de Janeiro: ENGE-RIO,Index Editora, 1988. Major ecosytems are presented through excellent photographs andknowledgeable commentary. Map, references, side notes providing scientific names.

SEEDLESS VASCULAR PLANTS


Seedless vascular plants possess vascular tissues (xylem and phloem) for transport of materials through the body but donot produce seeds bearing dormant embryos as part of the reproductive process. They are among the oldest of land plants.

Modern seedless vascular plants include spe-cies from several different phyla, including

the club mosses, spike mosses, and quillworts ofthe phylum Lycophyta, the horsetails of the phy-lum Sphenophyta, the whiskferns of the phylum Psi-lotophyta, and the great diversity of ferns in thephylum Pterophyta. Lycophytes, sphenophytes, andpsilotophytes are generally referred to as fern allies.Carolus Linnaeus used the word Cryptogamia (fromthe Greek kryptos, meaning “hidden,” and gamos,meaning “marriage,” or reproduction) as an inclu-sive taxonomic category for a wide range of organ-isms such as bryophytes, ferns, fern allies, algae,and fungi whose sexual reproductive parts wereconcealed from observation. The term cryptogam inmodern usage refers to plants that do not produceseeds.

Origin and RelationshipsTheories on the origin of vascular plants suggest

that they probably arose from an ancestor in thegroup of nonvascular plants known as the bryo-phytes. Fossil spores from the Ordovician period(about 500 million years ago) are linked to the earli-est land plants, which were likely an early liver-wort (phylum Hepatophyta) or hornwort (phylumAnthocerophyta). The most derived bryophytes, themosses (phylum Bryophyta), share an ancestor withthe line that gave rise to vascular plants. A line thatgave rise to the phylum Lycophyta appears to havediverged early in vascular plant development, soonafter the advent of phloem and the rise of the domi-nance of the branching sporophyte generation. Theremaining line diverged into a branch that gave riseto modern sphenophytes (horsetails) and ptero-

934 • Seedless vascular plants

phytes (ferns) and a branch that gave rise to mod-ern seed plants.

Seedless vascular plants are well represented inthe fossil record. Cooksonia, a representative of theextinct phylum Rhyniophyta, is thought to be oneof the earliest vascular plants, dating to the mid-Silurian period (around 430 million years ago). Nu-merous other examples dating to the Devonianperiod (around 400 million years ago) are well doc-umented.

Life Cycle and Gametophyte AnatomySeedless vascular plants express the typical life

cycle pattern called alternation of generations foundin many algae and members of the kingdomPlantae. As in all vascular plants, the diploidsporophyte generation, which produces haploidspores for the asexual reproductive phase, is domi-nant. Haploid spores are produced by meiosis inspecial structures called sporangia (singular,sporangium). In psilotophytes, some lycophytes,the sphenophytes, and the pterophytes, only onekind of spore is produced in a phenomenon calledhomospory. Sexually reproducing haploid gameto-phytes that have both male and female sex organsarise from these spores. In some lycophytes, twokinds of spores are produced in a phenomenoncalled heterospory. Large spores called megasporesare produced in megasporangia. Megaspores de-velop into female gametophytes. Small sporescalled microspores are produced in microspor-angia. Microspores develop into male gameto-phytes.

The haploid gametophytes of seedless vascularplants, which produce gametes (sex cells) for thesexual reproductive phase, are independent andsmaller than the sporophytes. Gametophytes range

in size from a few millimeters to a few centimetersin length or diameter, while sporophytes can rangein size from a few millimeters to several meters inheight. Since there is a distinct difference betweenthe appearance of the sporophyte and the gameto-phyte, these organisms are said to express hetero-morphy. In psilotophytes and some lycophytes,gametophytes are nonphotosynthetic and under-ground, while in most of the other groups gameto-phytes grow on the soil surface and are photosyn-thetic. Underground gametophytes must rely onmutualistic fungi in mycorrhizal relationships tosupply them with energy and carbon sources forgrowth and development.

Gametes are produced by mitotic cell division.Male gametes are produced in organs calledantheridia (singular, antheridium). Sperm cells aremultiflagellated and require liquid water for trans-mission to the female organs of a nearby plant. Eggsare produced in organs called archegonia (singular,archegonium). Sperm cells swim through the neck ofthe archegonium and into a swollen region at thebase called the venter, which contains the egg. Fer-tilization results in the production of a diploid zy-gote, which develops into an embryo that thengrows into a sporophyte.

Sporophyte AnatomyAccording to the fossil record, the earliest vascu-

lar plants were quite similar in structure to themodern genus Psilotum, of the phylum Psilotophyta:the whiskferns. The Psilotum sporophyte lacks trueleaves and roots (that is, the organs lack vasculartissue). Small, nonvascularized flaps of tissuecalled prophylls are found on the stem of Psilotum,which some theorize either are reduced leaves ormay be suggestive of the evolutionary origin of

Seedless vascular plants • 935

Seedless Vascular Plants: Phyla and CharacteristicsPhylum Status Anatomical Characteristics Reproduction Exhibits

Lycophyta Living Stems, roots, leaves, some branching Homospory, heterosporyPsilotophyta Living Stems, no leaves or roots, branching HomosporyPterophyta Living Stems, roots, leaves, no branching Homospory, minimal heterosporyRhyniophyta Extinct Stems, no leaves or roots, often branching HomosporySphenophyta Living Stems, roots, leaves, no branching Homospory, some (extinct) heterosporyTrimerophytophyta Extinct Stems, no leaves or roots, branching rare HomosporyZosterophyllophyta Extinct Stems, no leaves or roots, often branching Homospory, some heterospory

Source: Some data adapted from Peter H. Raven et al., Biology of Plants, 6th ed. (New York: W. H. Freeman/Worth, 1999).

leaves—that is, that true leaves are the result ofthe evolutionary vascularization of prophylls. Theother living psilotophyte genus, Tmesipteris, pos-sesses true leaves, each with a single vein, referredto as microphylls. Other groups produce either theone-veined microphylls or megaphylls that havenumerous, often branched veins. The most dra-matic leaf expression in seedless vascular plants isfound in the delicate, pinnately compound, mega-phyllous fronds of many of the ferns in phylumPterophyta. In lycophytes, sporangia are producedon specialized leaves called sporophylls. Some in-terpret the spore-bearing sporangiophores thatmake up the cones of sphenophytes to be special-ized leaves, as well.

The aerial stem of Psilotum exhibits a primitivepattern of isotomous dichotomous branching, withtwo equal branches arising from one branch point,or node. Other seedless vascular plants may showdifferent patterns: For example, lycophytes oftenshow an unequal dichotomous branching pattern,or anisotomy.

In most cryptogams, aerial stems arise fromunderground stems called rhizomes. Rhizomes ofpsilotophytes have many rhizoids, rootlike epider-mal extensions, to aid in absorption of water andmineral nutrients. The sporophytes of all othergroups of seedless vascular plants possess vascu-larized roots for absorption and anchorage.

The vascular tissues of all members of this broadgroup include the primitive type of xylem calledtracheids and the primitive type of phloem calledsieve cells. The vascular cylinder, or stele, ofPsilotum is composed of a solid core of xylem sur-rounded by a layer of phloem. This arrangement iscalled a protostele. Variations on the protostele pat-tern are common in the roots, rhizomes, and shootsof members of the Lycophyta. Protosteles are themost common stele types found in vascular plantroots, in general.

The shoot steles of sphenophytes and ptero-phytes are composed of a ring of xylem surround-ing a pith made of parenchyma tissue. A ring ofphloem surrounds the xylem ring. This arrange-ment is called a siphonostele. Variations on thesiphonostele pattern are common in the shoots ofall vascular plants from the sphenophytes throughthe seed plants.

The endodermis is a layer of cells that surroundsthe vascular cylinder of roots and rhizomes andsome shoots in certain psilotophytes; it is is usually

localized in the roots and rhizomes of lycophytes,sphenophytes, and pterophytes. The waterproof-ing suberin deposited in the Casparian strip of theendodermis regulates water flow to the vasculartissue.

LycophytaFossil evidence suggests that the phylum Lyco-

phyta, the lycophytes, arose during the Devonianperiod. There are between ten and fifteen genera,consisting of about one thousand species, dividedamong three broad groups: the club mosses ofthe order Lycopodiales, the spike mosses of theorder Selaginellales, and the quillworts of the or-der Isoetales. All possess microphylls; however, thehomosporous club mosses differ significantly fromthe heterosporous spike mosses and quillworts.Some extinct lycophytes were woody and grew totree size. Lycophytes were among the dominantspecies during the Carboniferous period (around350 million years ago), during which the plantsthat eventually became coal deposits were pro-duced.

SphenophytaPhylum Sphenophyta arose during the Devonian

period and reached its peak during the late Devo-nian and Carboniferous periods. Among the largestsphenophytes were members of the genus Cala-mites, treelike plants that exceeded 18 meters (60feet) in height and nearly 0.5 meter (18 inches) inbasal stem diameter. There are fifteen living herba-ceous species in the only remaining genus, Equi-setum, the horsetails.

PsilotophytaThe fossil record is incomplete regarding the an-

cestry of modern psilotophytes, so it is difficult toassign a date of origin. Although once consideredto possess characteristics that allied them to themost primitive vascular plants, molecular studieshave suggested that psilotophytes are possiblyclosely related to pterophytes (ferns), perhaps rep-resenting a reduction from an early fern ancestor.The phylum includes only two living genera, con-sisting of a total of six to eight species.

PterophytaPterophytes, or ferns, arose during the Devonian

period and reached the height of abundance and di-versity during the Carboniferous. Today, there are

936 • Seedless vascular plants

about eleven thousand species of ferns ranging insize from a few millimeters across (such as Azolla)to several meters in height (such as Dicksonia). Ex-cluding flowering plants, ferns are the most abun-dant and diverse plants on earth, exhibiting a vari-ety of growth habits, from floating aquatics totropical epiphytes to temperate terrestrial plants.They are important both ecologically and economi-cally, serving as habitat and food sources for many

organisms. Humans use ferns as food, fuel, anddecoration.

Darrell L. Ray

See also: Bryophytes; Evolution of plants; Ferns;Hornworts; Horsetails; Liverworts; Lycophytes;Mosses; Mycorrhizae; Plant tissues; Psilotophytes;Reproduction in plants; Rhyniophyta; Tracheobionta;Trimerophytophyta.

Sources for Further StudyBell, Peter R., and Alan R. Hemsley. Green Plants: Their Origin and Diversity. 2d ed. New York:

Cambridge University Press, 2000. Reviews relationships among major groups of seed-less vascular plants. Photographs, diagrams.

Ennos, Roland, and Elizabeth Sheffield. Plant Life. Malden, Mass.: Blackwell Science, 2000.Introduces major groups of seedless vascular plants. Photographs, diagrams.

Gifford, Ernest M., and Adriance S. Forster. Morphology and Evolution of Vascular Plants.3d ed. New York: W. H. Freeman, 1989. Astandard among plant morphology texts. Exten-sive discussions of origins and relationships of seedless vascular plants. Photographs,diagrams.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman, 1999. A standard text for college botany. Introduces history and relation-ships among major groups of seedless vascular plants. Photographs, drawings, dia-grams.

SEEDS

Categories: Anatomy; reproduction and life cycles

A seed is a mature, fertilized plant ovum containing an embryo, a food supply, and a protective covering called the testa,or seed coat.

A mature seed typically consists of a matureplant ovum containing a minute, partially de-

veloped young plant, the embryo, surrounded by anabundant supply of food and enclosed by a protec-tive seed coat. Seed plants are divided into two maingroups: the gymnosperms, primarily cone-bearingplants such as pine, spruce, and fir trees, and the an-giosperms, the flowering plants. The gymnospermshave naked ovules which, at the time of pollination,are exposed directly to the pollen grains. Their foodsupply in the seed is composed of a female gameto-phyte, rather than the endosperm found in angio-sperms.

In angiosperms, seeds develop from ovules thatare enclosed in a protective ovary. The ovary is the

basal portion of the carpel, typically a vase-shapedstructure located at the center of a flower. The top ofthe carpel, the stigma, is sticky, and when a pollengrain lands upon it, the grain is firmly held. Thegerminating pollen grain produces a pollen tube thatgrows down through the stigma and style into theovary and pierces the ovule.

Two male sperm nuclei are released from thepollen grain and travel down the pollen tube intothe ovule. One of the sperm nuclei fuses with an eggcell inside the ovule. This fertilized egg dividesmany times and develops into the embryo. The sec-ond male nucleus unites with other parts of theovule and develops into the endosperm, a starchy orfatty tissue that is used by the embryo as a source of

Seeds • 937

food during germination. Angiosperm seeds re-main protected at maturity. While the seed devel-ops, the enclosing ovary also develops into a hardshell, called a seed coat or testa, often enclosed in afibrous or fleshy fruit.

StructureAlthough the characteristics of different plant

seeds vary greatly, some structural features arecommon to all seeds. Each seed contains an embryowith one, two, or several cotyledons. In angiospermseeds, the embryo may have either one or two coty-ledons. Angiosperms with one cotyledon are plantsare called monocots; those with two cotyledons arecalled eudicots (formerly dicots). A typical example

of a monocot is corn, or maize (Zea mays), whereasthe bean (Phaseolus vulgaris) is a typical eudicot. Ingymnosperms, the embryo may have between twoand sixteen cotyledons; for example, the embryo ofScots pine (Pinus sylvestris) possesses eight cotyle-dons.

Immediately below the cotyledons is the hypo-cotyl, at the tip of which lies the growing point of theroot. Above the cotyledons lies the epicotyl, whichconsists of a miniature shoot tip and leaves. Upongermination of the seed, the epicotyl develops intothe stem and leaves of the new young plant. Almostall seeds carry with them a supply of food, which inangiosperms is the endosperm. Although the em-bryo is usually surrounded by the endosperm, insome seeds (such as maize) embryos and endo-sperm lie side by side. In the seeds of the pea family(Leguminosae), the food reserves of the endospermare absorbed by the embryo, resulting in enlargedcotyledons. Gymnosperm seeds differ from thoseof angiosperms in the origin of their stored food. Ingymnosperms the stored food is provided by a fe-male gametophyte housed with the embryo insidethe seed, whereas in angiosperms the food reserveis the endosperm.

All seeds are surrounded by a seed coat, thetesta. Variability in the appearance of the testa isconsiderable, and these variations are used by tax-onomists as an aid in distinguishing among differ-ent genera and species. The testa is of great im-portance to the seed; it is often the only barrierprotecting the embryo from the external environ-ment. The seed coats of some plants swell and pro-duce a jellylike layer in response to contact withwater. The gel retains water needed by the seed forgermination. Cotton fibers are formed as exten-sions from some of the outermost cells of the seedcoat in cotton plants (Gossypium). The seed coats ofnutmeg contain aromatic substances.

Size and ChemistryThe range of seed size is extreme—more than

nine orders of magnitude. The largest known seedis that of the double coconut (Lodoicea maldivica); theseed and fruit together weigh as much as 27 kilo-grams. At the other end of the scale, the dustlikeseeds of some orchids, begonias, and rushes weighonly about 5 milligrams per seed. It is thought thatthe size of seed displayed by each species repre-sents a compromise between the requirements fordispersal (which would favor smaller seeds that

938 • Seeds

Colerorhiza

Radicle

Plumule

Coleoptile

Scutellum

Endosperm

Pericarp

Maize seed (monocot)

Cotyledons

Hypocotyl-root axis

Epicotyl

Foliage leaves

Seed coat

Bean (eudicot)

Seeds: Eudicots vs. Monocots

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In angiosperm seeds, the embryo may have either one ortwo cotyledons. Angiosperms with one cotyledon areplants called monocots; those with two cotyledons arecalled eudicots (formerly dicots). A typical example of aeudicot is the bean (Phaseolus vulgaris), whereas a typ-ical monocot is corn, or maize (Zea mays).

can be borne on wind or picked up by animals) andthe requirements for establishment of the seedling(which would favor larger seeds that can adhere toa growth medium).

The chemical composition of seeds varies widelyamong species. In addition to the normal com-pounds found in all plant tissues, seeds containunique food reserves that are used to support earlyseedling growth. About 90 percent of plant speciesuse lipids (fats and oils) as their main seed reserves.The cotyledons of soybeans and peanuts are rich inoil, whereas in other legumes such as peas andbeans, starch is the reserve material. Sixty-four per-cent of the weight of a castor bean is derivedfrom the oil stored in the endosperm. Inseeds of cereal crops, the endosperm storesmuch starch; in corn it can be up to 80 per-cent of the weight of the seed. All seeds, par-ticularly legumes, also store protein as a re-serve substance.

DispersalA seed can be regarded as a vessel in

which lies a partially developed young plantin a condition of arrested growth, waitingfor the correct conditions for growth to re-sume. Successful reproduction depends onseed dispersal to places appropriate for ger-mination to occur. During the evolutionaryhistory of plants, seeds and fruits have de-veloped a great variety of specialized struc-tures that enhance seed dispersal.

Wind is one major means of seed distribu-tion. Very small seeds, such as the dustlikeseeds of orchids, heathers, and some rushesand grasses, are dispersed by wind. Suchseeds have been recovered from the atmo-sphere by airplanes at elevations up to 1,000meters. Heavier seeds have evolved a vari-ety of structures to ensure wind dispersal.For example, some members of the daisyfamily, such as dandelions, bear numerousone-seeded fruits to which are attached afeathery, tuftlike structure that acts as a para-chute. Similar structures aid in the dispersalof the seeds of many other plants, such ascattails and milkweeds. Heavier seeds, suchas those of ash, maple, and pine trees, havedeveloped large, flat wings that allow theseeds to fly in a propeller-like manner for aconsiderable distance from the parent plant.

Often adaptations for wind dispersal of seedscan be seen not only in the seed’s structure but alsoin structures of the parent plant. Many plants offertheir seeds to the wind by bearing them on longflower stalks that tower above the surroundingvegetation. Tumbleweed bushes are small and al-most spherical. When mature, they develop aweakness of the main stem at soil level. Wind canbreak the stem of the bush from which, as it rollsover the ground, the seeds are shaken loose and arescattered.

The seeds or fruits of many plants are dispersedby sticking to the outsides of birds or land animals.

Seeds • 939D

igital

Stock

The size of seed displayed by each species is thought to representa compromise between the requirements for dispersal (whichwould favor smaller seeds that can be borne on wind, as are thesedandelion seeds, or picked up by animals) and the requirementsfor establishment of the seedling (which would favor larger seedsthat can adhere to a growth medium).

Seeds are transported in mud that sticks to the feetof animals. The large number of species whoseseeds show no obvious special dispersal adaptationare probably spread in this manner. Many seeds,however, can attach themselves to a passerby bymeans of adhesive substances, hooks (such as thefruit of bedstraw), or burrs (such as the seeds of theburdock plant).

Some plant species have seeds that are adaptedfor dispersal by animals and birds that transportthem internally. The attractive and tasty fleshyfruits and berries of plants can be considered an ad-aptation to aid in seed dispersal. The seeds of mostfruits eaten by animals and birds have a digestion-resistant coat. The animal deposits excrement con-taining seeds at a location at some distance fromthe parent plant, where the seeds grow into newplants. In some species, germination will not evenoccur unless the seed has passed through an ani-mal’s digestive tract. The presence of seeds in birddroppings is responsible for the appearance ofsome plants on remote, barren, volcanic islands.Various animal behaviors, including the collectingbehavior of ants and the seed-burying activitiesof mice, squirrels, and jays, also aid in seed dis-persal.

Several plants have evolved mechanisms thatexpel seeds explosively away from the parentplant. The pods of Impatiens, for example, developstrongly unbalanced tension forces as they ripen.When the pod is fully mature, the tension is so in-

tense that the slightest disturbance causes the podto split open, violently expelling the seeds.

DormancySeeds are adapted for conditions other than geo-

graphic dispersal. When a seed arrives at its desti-nation after dispersal, conditions may not be suit-able for establishment of the plant. The delayingmechanism which prevents germination under ad-verse conditions is called dormancy. Seeds can re-main in a condition of dormancy for varying lengthsof time, depending on the species, until the correctbalance of oxygen, moisture, and temperature trig-gers germination.

Viability varies greatly from species to speciesand may last only a few weeks or many years. Seedsof the cocoa plant are viable for only ten weeks.Some seeds, however, remain viable for decades oreven hundreds of years. Seeds of the Indian lotushave been shown to remain viable for almost onethousand years. No claims for long-term viabilityhave surpassed those made for the Arctic lupine,however: Seeds of this species have been success-fully germinated after having been buried in theArctic tundra for ten thousand years.

Iain Miller and Randy Moore

See also: Angiosperm life cycle; Dormancy; Flowerstructure; Fruit: structure and types; Germinationand seedling development; Gymnosperms; Plantlife spans; Pollination; Reproduction in plants.

Sources for Further StudyBewley, J. Derek, and Michael Black. Seeds: Physiology of Development and Germination. 2d ed.

New York: Plenum Press, 1994. Intermediate-level college text presenting a general yetrigorous treatment of the essential aspects of seed physiology and biochemistry. Assumessome knowledge of plant physiology, although fundamentals are explained where neces-sary. Well illustrated.

Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. San Francisco: Benjamin Cummings,2002. A well-written textbook suitable for high school and college students as well as thelayperson. The chapter “Plant Reproduction” covers the reproductive process in angio-sperms from pollen and ovule development through fruit development and seed dis-persal. The chapter “Plants and the Colonization of the Land” compares and contrastssexual reproduction in gymnosperms and angiosperms. Easy to read; good illustrations.

Fenner, Michael. Seed Ecology. New York: Chapman and Hall, 1993. Lower-level college textthat stresses an ecological approach to the study of seeds. Highlights the simplicity ofmany ecology experiments and field observations and thus is useful to both the studentand interested amateurs wishing to make some experimental investigations of their own.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Lower-level college text presenting a detailed and compre-hensive look at plant biology. The chapter “Seed Plants” describes the life cycles of both

940 • Seeds

gymnosperms and angiosperms in some detail. The chapter “Early Development of thePlant Body” addresses embryo development and seed germination. Well-drawn dia-grams and sharp color photographs enhance understanding of the complex processes in-volved in plant reproduction. Glossary, index, references.

Stidworthy, John, ed. Plants and Seeds: Through the Microscope. New York: Gloucester Press,1990. The main strength of this book is its striking collection of photographs of enlargedmicroscopic images.

SELECTION

Categories: Evolution; genetics

Selection refers to any process by which some individuals are allowed to reproduce at the expense of others, leading to ashift in the composition of a population over generational time.

In the context of animal and plant breeding by hu-mans, the process of selection is referred to as arti-

ficial selection. In the wild, it is referred to as naturalselection and is viewed by scientists as the principalmeans by which adaptations (traits that promote thesurvival and reproduction of organisms) arise andnew species evolve over geological time frames.

Natural SelectionIn On the Origin of Species by Means of Natural Se-

lection, published in 1859, Charles Darwin pre-sented arguments for two related theories. The firstargument documents evidence for evolution on thebasis of an extensive study of biogeographical,embryological, and fossil data. Within ten years ofits publication, nearly all individuals who todaywould be considered scientists were convinced thatevolution had occurred.

Darwin’s second argument develops the case forwhat he identified as the chief mechanism of evolu-tion: his theory of natural selection. This argumentelegantly draws attention to the probabilistic con-sequences of three conditions in nature. First, or-ganisms vary from one another in ways that affecttheir ability to survive and reproduce. Second, atleast some of this variation is heritable. Third, thereis competition, owing to the fact that organisms pro-duce more offspring than can possibly survive.Darwin’s genius was to recognize that as a conse-quence of these conditions, members of a popula-tion possessing favored variations would be morelikely to leave offspring than those that did not, and

as such, the composition of the population as awhole would change over generational time in thedirection of the favored form.

EvidenceIn addition to evidence for the existence of the

above conditions in nature, Darwin also gave indi-rect evidence in support of the long-term conse-quences of these conditions he had deduced. In ad-dition to thought experiments, he developed ananalogy between artificial and natural selection. Hepointed out that during the relatively short timethat pigeon and dog species had been domesti-cated, breeders had been able, by picking which in-dividuals were allowed to reproduce, to create newvarieties as distinct in appearance as true species innature. Darwin then drew attention to the fact thatconditions in nature (coupled with the occasionalintroduction of new variants by mutation) operat-ing over geological time periods could have a simi-larly dramatic effect on populations in the wild.

Although the vast time and spatial frames in-volved make it difficult to observe directly the ori-gin of new adaptations and species, many field ex-periments in the years since Darwin first wrotehave confirmed both the power and ubiquity ofnatural selection in nature. A particularly impor-tant example in the context of botany was providedin the late 1960’s by Janis Antonovics and others,who demonstrated that the evolution of heavymetal tolerance in many plant species was the re-sult of natural selection. Other contemporary areas

Selection • 941

of research on selection include the study of devel-opmental plasticity, or the ability of an organism torespond to environmental conditions during itsdevelopment, as occurs when a growing saplingforms leaves to maximize its light exposure in thepresence of partial shading by other trees in a for-est. This ability is itself the object of natural se-lection.

Genotypes vs. PhenotypesAlthough evolution is often discussed in terms

of a change in the frequency of genes in a popu-lation, in fact, natural selection acts directly onthe phenotypes (observable characteristics of organ-isms) and only indirectly on the genotypes (the spe-cific forms of the gene, or alleles, an individual hasinherited from its parents) responsible for them.While often portrayed as a process that removesgenetic variability from populations, natural selec-tion can promote variability, as occurs when thepopulation exists in an environment where oneform of a trait is advantageous in some areas butanother form is advantageous in other areas. Suchprocesses may lead to clines, or gradations, in thefrequencies of genes over the range of the popu-lation.

EcosystemsThe foregoing has discussed selection with refer-

ence to isolated populations. In ecosystems com-posed of multiple populations of distinct species,natural selection can promote the evolution ofecotypes, or populations having a distinct set ofcharacteristics unique to the region they inhabitand the mode of life they pursue. Natural selectioncan favor the coevolution of populations of distinct

species with one another. This occurs in the evolu-tion of predator and prey species, in which, for ex-ample, the origin of an adaptation that allows apredator to consume a grass species that was previ-ously toxic to it changes the selective environmentof the prey species, leading to the evolution of en-tirely new plant defenses.

Convergent vs. Divergent EvolutionThe presence of common selective conditions in

distinct locations may lead to the independent evo-lution of similar characteristics in separate species;that is, convergent evolution. A good example is pro-vided by cacti in North and South American desertsand euphorbs in African deserts: Both havethornless leaves with a similar structure that hasevolved to maximize water retention. Natural se-lection can also lead to a partitioning of resourcespace, a phenomenon known as divergent evolution,in which distinct populations of a species evolvedivergent modes of life. This may reflect the impo-sition of a barrier that prevents interbreeding orcompetition among cohabiting populations lead-ing to a partitioning of niche space. Agood exampleis the common mistletoe, Viscum album, a parasitichigher plant having three distinct races that special-ize on deciduous trees, firs, and pines.

David W. Rudge

See also: Adaptations; Clines; Coevolution; Com-petition; Evolution: convergent and divergent; Evo-lution: historical perspective; Evolution of plants;Genetic drift; Genetics: Mendelian; Genetics: muta-tions; Genetics: post-Mendelian; Plant domestica-tion and breeding; Population genetics; Species andspeciation.

Sources for Further StudyBell, Graham. The Basics of Selection. New York: Chapman & Hall, 1997. Acollege-level intro-

duction to different types of selection and how they are studied.Bowler, Peter J. Evolution: The History of an Idea. Berkeley, Calif.: University of California

Press, 1984. An overview of the history of the concept of natural selection in connectionwith debates on evolution.

Cragg, J. B. Advances in Ecological Research. London: Academic Press, 1971. Contains JanisAntonovics et al.’s classic paper reviewing research on the evolution of heavy metal toler-ance in plants.

Endler, John. Natural Selection in the Wild. Princeton, N.J.: Princeton University Press, 1986. Asurvey of many studies documenting the presence and ubiquity of natural selection.

Jones, Steve. Darwin’s Ghost: The Origin of Species Updated. New York: Random House, 2000.An assessable retelling of Charles Darwin’s argument, augmented in light of contempo-rary research.

942 • Selection

SERPENTINE ENDEMISM

Category: Soil

Serpentine endemics are plants that grow only in serpentine soils. These uncommon soils present serious challenges toplants and are often sparsely covered with dwarfed vegetation.

Serpentine rock is one form of ultramafic rock, anuncommon rock found in mountain-building

zones. Soils derived from ultramafic rock are calledserpentine soils. The most important chemicalcharacteristics of ultramafic rock and of the serpen-tine soils formed from it are high magnesium con-centrations; low calcium concentrations; low cal-cium/magnesium ratios; low concentrations ofother macronutrients (such as nitrogen, phospho-rus and potassium); high concentrations of toxicheavy metals (such as nickel, chromium, and co-balt); and low micronutrient (molybdenum, boron)concentrations. All these factors are detrimental toplant growth.

Physical factors also tend to be harsh in serpen-tine soils. They are rocky and low in humus (the or-ganic matter formed from decomposing plants),and temperatures tend to fluctuate widely. In part,these physical factors are caused by the chemicalfactors discussed above and their restriction ofplant growth. Sparse plant cover results in lessplant material to decompose, thus less humus; anda greater plant cover would mitigate temperaturechanges. Whatever their causes, these physical fac-tors further restrict plant growth. Not all serpentinesoils have all these characteristics, but those presentcombine in various ways to severely restrict plantgrowth on serpentine soils.

Plants of Serpentine SoilsMany plant species cannot grow in serpentine

soils. Others can grow in serpentine soils or in othersoils, but the serpentine varieties are often dwarfedand require special adaptations. Still others, the ser-

pentine endemics, grow only in serpentine soils.Two examples are Quercus durata, a shrubby oakthat grows in some western serpentines, and Asterdepauperatus, a small aster of some Appalachianserpentine soils.

One common adaptation of serpentine plants isa high tolerance for nickel, the most troublesome ofthe serpentine heavy metals. A second is the abilityto use calcium efficiently in the presence of excessmagnesium. Magnesium is an essential nutrient,but in high concentrations it interferes with theplant’s use of calcium, another essential nutrient.These two adaptations are widespread among ser-pentine endemics, suggesting that they are two ofthe adaptations important to serpentine endemics.Additional adaptations are probably necessary be-cause a plant growing on serpentine must over-come all of the soil’s troublesome characteristics.

Competition may explain why these plants areunable to grow in more favorable soils. Many ser-pentine endemics cannot compete with otherplants outside the serpentine environment and arethus excluded from nonserpentine soils. Becausemost plants cannot grow in serpentine soil, serpen-tine endemics are freed from competition and cangrow successfully in those soils.

Because they only occur on these uncommonsoils, serpentine endemics are especially suscepti-ble to extinction. Preservation efforts have focusedon some of these areas and their plant life.

Carl W. Hoagstrom

See also: Adaptations; Agronomy; Halophytes;Nutrient cycling; Selection; Soil.

Sources for Further StudyBrooks, Robert R. Serpentine and Its Vegetation: A Multidisciplinary Approach. Portland, Oreg.:

Dioscorides Press, 1987. This world survey offers an understandable explanation of ser-pentine endemics.

Serpentine endemism • 943

Dann, Kevin T. Traces on the Appalachians: A Natural History of Serpentine in Eastern NorthAmerica. New Brunswick, N.J.: Rutgers University Press, 1988. Nontechnical explanationof the history of serpentine ecosystems.

Roberts, Bruce A., and John Proctor, eds. The Ecology of Areas with Serpentinized Rocks: AWorldView. Boston: Kluwer Academic, 1992. This understandable coverage of serpentine ecol-ogy includes extensive examples.

SHOOTS

Category: Anatomy

“Shoot” is a term used to refer collectively to the stem and associated leaves of a vascular plant.

The stem of a vascular plant is typically a cylin-drical, aboveground axis that grows upright,

away from the pull of gravity. Young stems aregreen and photosynthetic, and in some plants, suchas horsetails and most cacti, the stems are the pri-mary site of photosynthesis. In most plants, how-ever, the primary function of the stem is to supportleaves in a position where they are well exposed tolight. The part of the stem where a leaf attaches isa node. The section of stem between nodes is aninternode. Internode elongation spaces the leavesand is responsible for most of the increase in lengthof the stem every growing season.

The cylindrical shape of the stem is a structuraladaptation that provides maximum strength pervolume of tissue. The vascular tissues, which containtough, thick-walled fiber cells, contribute to thestrength of the stem. In non-seed plants, such asferns, these tough cells form a core in the center ofthe stem with softer tissues around them. In seedplants, such as conifers and flowering plants, thevascular tissues tend to lie in a subperipheral ringbelow the surface, surrounding a softer core of tis-sue. This is analogous to the arrangement of steelreinforcing bars in a column of reinforced concrete.In gymnosperms and dicots, the vascular tissuesare formed in a single ring. In monocots, strands ofvascular tissue form throughout the stem but evenso tend to be more numerous and more tightlypacked around the periphery of the stem.

Leaves are lateral appendages of the stem that aretypically flattened to expose a maximum amount ofsurface area to sunlight. They are the primary site ofphotosynthesis in most vascular plants. The flat-

tened portion of the leaf is the blade, or lamina. Insome plants, such as grasses, the lamina appears toattach directly to the stem. In most plants, however,there is a stemlike stalk, the petiole, that extends thelamina away from the stem. In many cases the cellsof the petiole enable the blade to be repositioned tomaximize exposure to light. The leaf base is the at-tachment zone where the leaf connects to the stem.In some plants the cells of the leaf base enlarge toform stipules on each side of the petiole. The leafbase merges with the node. One or more axillarybuds arise in the angle formed by this merger.

Shoot DevelopmentAt the tip of every shoot is a terminal bud. If the

leaves of this bud are carefully peeled away, a tiny,rounded dome of tissue is exposed—the shoot apicalmeristem. A meristem is a region of the plant wherecell division is concentrated. Cell division and dif-ferentiation in the shoot apical meristem producethe tissues that form the stem and leaves of thatshoot.

The structure of the shoot apical meristem var-ies, depending on the plant being examined. Seed-less vascular plants, such as ferns, have a large,pyramidally shaped apical cell that ultimatelygives rise to all the cells and tissues of the shoot. Theseed plants have a multicellular apical meristemdifferentiated into several zones. Each zone is asso-ciated with one of three primary meristems thatform the tissues of the shoot system. Asurface layerof cells covers the dome and differentiates intoprotoderm, the primary meristem that forms epider-mal cells. The epidermis is a continuous protective

944 • Shoots

layer that covers the stem and leaves of the shootsystem. Within the apical meristem, certain cells be-gin to elongate and form the procambium, the pri-mary meristem that forms vascular tissues. The re-mainder of the internal cells of the apical meristembecome ground meristem, the primary meristemthat forms ground tissues, such as cortex and pith.

As the apical meristem grows and new cells areformed, the dome of tissue enlarges. This growth isnot symmetric; rather, surface or subsurface cellsof a localized region will proliferate more rapidlythan the rest to form a small bulge of tissue. Thisbulge is the first sign of initiation of a new leaf, a leafprimordium. As the leaf primordium enlarges, theprotoderm also proliferates to maintain a continu-

ous covering over the developing leaf that is contig-uous with the developing stem. This continuity ismaintained as cells differentiate into mature epi-dermal cells.

After initiation of a leaf primordium, the shootapical meristem continues to grow, and procam-bium begins to differentiate within the developingnode. These procambial cells connect to alreadyformed procambium in older portions of the stemto form the template for the vascular system of thestem. Simultaneously, as the leaf primordium en-larges, procambial strands differentiate in the leafbase region. Further development is bidirectional.Cells from the basal end of each strand differentiateinto the node and connect with the already-formed

Shoots • 945

Stem

Petiole

Leaf base

Node

Internode

Node

Internode

Node

Axillary bud

Shoot apical meristem

Youngleaf

Leaves

Parts of the Shoot

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procambium there. Cells from the upper end ofeach strand continue to develop into the enlargingleaf primordium. In this way the vascular tissues ofthe leaf and stem are integrated into a continuoussystem. Similarly, the ground tissues of leaf andstem are continuous as the leaf primordia are initi-ated and develop.

Specialized ShootsSpecialized shoots are usually associated with

storage or vegetative reproduction. Bulbs, such asonions, have a shortened stem with enlarged, over-

lapping, fleshy leaves. Tubers, such as potatoes, aresubterranean storage shoots with thick, starch-filled stem cells and undeveloped leaves subtend-ing axillary buds (the “eyes”). Rhizomes are hori-zontal shoots running on, or below, the soil surface.Their leaves are typically much reduced.


See also: Angiosperm cells and tissues; Bulbs andrhizomes; Cloning of plants; Growth habits; Leafanatomy; Leaf arrangements; Stems; Plant tissues;Thigmomorphogenesis; Tropisms; Tracheobionta.

Sources for Further StudyBell, Adrian D. Plant Form: An Illustrated Guide to Flowering Plant Morphology. New York: Ox-

ford University Press, 1991. Photos and line art illustrate the common and specializedstructure of flowering plants. Includes bibliographical references, index.

Harris, James G., and Melinda Woolf Harris. Plant Identification Terminology: An IllustratedGlossary. Spring Lake, Utah: Spring Lake, 1994. Clear line drawings illustrate specific ter-minology used to identify and describe plants.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. Basic botany text covers all aspects of shoot structure, func-tion, and development.

Rudall, Paula. Anatomy of Flowering Plants: An Introduction to Structure and Development. 2ded. New York: Cambridge University Press, 1993. Basic introduction to angiosperm struc-ture and development.

SLASH-AND-BURN AGRICULTURE

Categories: Agriculture; economic botany and plant uses; environmental issues

Slash-and-burn agriculture, also called swidden agriculture, is a practice in which forestland is cleared and burned foruse in crop and livestock production. While yields are high during the first few years, they rapidly decline in subsequentyears, leading to further clearing of nearby forestland.

Slash-and-burn agriculture has been practicedfor many centuries among people living in trop-

ical rain forests. Initially, this farming system in-volved small populations. Therefore, land could beallowed to lie fallow (unplanted) for many years,leading to the full regeneration of the secondary for-ests and hence a restoration of the ecosystems. Dur-ing the second half of the twentieth century, how-ever, several factors led to drastically reduced fallowperiods. In some places such fallow systems are nolonger in existence, resulting in the transformationof forests into shrub and grasslands, negative effects

on agricultural productivity for small farmers, anddisastrous consequences to the environment.

Among the factors that have been responsiblefor reduced or nonexistent fallow periods are in-creased population in the tropics, increased de-mand for wood-based energy, and, perhaps mostimportant, the increased worldwide demand fortropical commodities during the 1980’s and 1990’s,especially for products such as palm oil and naturalrubber. These last two factors have helped industri-alize slash-and-burn agriculture, which was prac-ticed for centuries mainly by small farmers. Ordi-

946 • Slash-and-burn agriculture

narily, small farmers are able to control their fires sothat they are similar to a small forest fire triggeredby lightning in the northwestern or southeasternUnited States. However, the continued reduction infallow periods, coupled with increased burning bysubsistence farmers and large agribusiness, espe-cially in Asia and Latin America, is resulting in in-creased environmental concern.

While slash-and-burn agriculture seldom takesplace in temperate regions, some agricultural burn-ing occurs in the Pacific Northwest of the UnitedStates, where it is estimated that three thousand tofive thousand agricultural fires are set each yearin Washington State alone. These fires also createproblems for human health and the environment.

Habitat FragmentationOne of the most easily recognized results of

slash-and-burn agriculture is habitat fragmentation,which leads to a significant loss of the vegetationneeded for the maintenance of effective gaseous ex-change in tropical regions and throughout theworld. For every acre of land lost to slash-and-burn

agriculture, 10 to 15 acres (4 to 6 hectares) of landare fragmented, resulting in the loss of habitat forwildlife, plant species, and innumerable macro-and microorganisms yet to be identified. This alsocreates problems for management and wildlife con-servation efforts in parts of the world with little orno resources to feed their large populations. Frag-mentation has also led to intensive discussions onglobal warming. While slash-and-burn agricultureby itself is not completely responsible for globalwarming, the industrialization of the process couldmake it a significant component of the problem, asmore and more vegetation is fragmented.

Human HealthThe impact of slash-and-burn agriculture on hu-

man health and the environment is best exempli-fied by the 1997 Asian fires that resulted from suchpractices. Monsoon rains normally extinguish thefires set by farmers, but a strong El Niño weatherphenomenon delayed the expected rains, and thefires burned out of control for months. Thick smokecaused severe health problems. It is estimated that

Slash-and-burn agriculture • 947

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more than 20 million people in Indonesia alonewere treated for asthma, bronchitis, emphysema,and eye, skin, and cardiovascular problems as a re-sult of the fires. Similar problems have been re-ported for smaller agricultural fires.

Three major problems are associated with air pol-lution: particulate matter, pollutant gases, and volatileorganic compounds. Particulate compounds of 10 mi-crons or smaller that are inhaled become attached tothe alveoli and other blood cells, resulting in severeillness. Studies by the U.S. Environmental ProtectionAgency (EPA) and the University of Washington in-dicate that death rates associated with respiratoryillnesses increase when fine particulate air pollutionincreases. Meanwhile, pollutant gases such as car-bon monoxide, nitric oxide, nitrogen dioxide, andsulfur dioxide become respiratory irritants whenthey combine with vapor to form acid rain or fog.Until the Asian fires, air pollutants stemming fromthe small fires of slash-and-burn agriculture that

occur every planting season often went unnoticed.Thus, millions of people in the tropics experienceenvironmental health problems because of slash-and-burn agriculture that are never reported.

Soil and Water QualityThe loss of vegetation that follows slash-and-

burn agriculture causes an increased level of soilerosion. The soils of the humid tropics create a hardpan underneath a thick layer of organic matter.Therefore, upon the removal of vegetation cover,huge areas of land become exposed to the torrentialrainfalls that occur in these regions. The result is se-vere soil erosion. As evidenced by the impact ofHurricane Mitch on Honduras during 1998, theseexposed lands can give rise to large mudslides thatcan lead to significant loss of life. While slash-and-burn agriculture may not be the ultimate cause forsudden mud slides, it does predispose these landsto erosional problems.

948 • Slash-and-burn agriculture

0

10

20

30

40

50

60

70

80

90

100

Rel

ativ

eyi

eld

(per

cent

age)

1 2 3 4 5 1 21 21 21 2 31 21 2 3Corn

(S. Sudan)Corn

(Belize)Cassava(Zaire)

Rice(Malaysia)

Corn(Guatemala)

Rice(Zaire)

Peanuts(Zaire)

Years after clearing

Declining Crop Yields with Successive Harvests on Unfertilized Tropical Soils

Source: Data adapted from John Terborgh, Diversity and the Tropical Rain Forest. New York: W. H. Freeman, 1992.

Associated with erosion is the impact of slash-and-burn agriculture on water quality. As erosioncontinues, sedimentation of streams increases. Thissedimentation affects stream flow and freshwaterdischarge for catchment-area populations. Mixedwith the sediment are minerals such as phosphorusand nitrogen-related compounds that enhance al-gal growth in streams and estuaries, which depletesthe supply of oxygen that aquatic organisms re-quire to survive. Although fertility is initially in-creased on noneroded soils, nutrient depositionand migration into drinking water supplies contin-ues to increase.

Controlling Slash-and-Burn AgricultureGiven the fact that slash-and-burn agriculture

has significant effects on the environment not onlyin regions where it is the mainstay of the agricul-tural systems but also in other regions of the world,it has become necessary to explore different ap-proaches to controlling this form of agriculture.However, slash-and-burn agriculture has evolvedinto a sociocultural livelihood; therefore, recom-mendations must be consistent with the way of life

of a people who have minimal resources for exten-sive agricultural systems.

Among the alternatives are new agroecosystemssuch as agroforestry systems and sustainable agricul-tural systems that do not rely so much on the slash-ing and burning of forestlands. These systems al-low for the cultivation of agronomic crops andlivestock within forest ecosystems. This protectssoils from being eroded. Another possibility is theeducation of small rural farmers, absentee land-lords, and big agribusiness concerns in developingcountries to understand the environmental impactof slash-and-burn agriculture. While small ruralfarmers may not have the resources for renovatingutilized forestlands, big business can organize eco-systems restoration, as has been done in many de-veloped nations of the world.

Oghenekome U. Onokpise

See also: Agriculture: modern problems; Air pollu-tion; Deforestation; Erosion and erosion control;Forest fires; Rain-forest biomes; Rain forests andthe atmosphere; Soil degradation; Sustainable agri-culture.

Sources for Further StudyJordan, C. F. An Amazonian Rain Forest: The Structure and Function of a Nutrient Stressed Ecosys-

tem and the Impact of Slash and Burn Agriculture. Boca Raton, Fla.: CRC Press, 1990. Acollec-tion of studies on the effects of slash-and-burn agriculture in the Amazon.

Simons, L. M., and M. Yamashita. “Indonesia’s Plague of Fire.” National Geographic 194 (Au-gust, 1998): 100-120. Details the extensive effects of the 1997 Asian fires.

Terborgh, J. Diversity and the Tropical Rain Forest. New York: Scientific American Library,1992. Contains a good overview of tropical rain forests and the problems associated withslash-and-burn agriculture.

Youth, Howard. “Green Awakening in a Poor Country.” World Watch 11, no. 5 (September/October, 1998): 28-37. Describes attempts to counter the effects of slash-and-burn agricul-ture in Honduras.

SOIL

Categories: Nutrients and nutrition; soil

Soil is a product of the physical and chemical breakdown of the earth’s surface into small fragments, including sand, silt,and clay. Soil contains the products of organic matter decomposition—the composting of dead plant and animal debris.

Soils are classified on the basis of soil profile andsoil formation. They can be grouped according to

a number of characteristics, including agronomicuse, color, organic matter content, texture, and

Soil • 949

moisture condition. Typical soil is about 45 percentminerals and about 5 percent organic matter. Theother 50 percent of soil consists of pores that holdeither water or air. The liquid portion of soil con-tains dissolved minerals and organic compounds,produced by plants and microorganisms. The gasesfound in soil often are the same as those found inthe air above it. Soil can support plant life if climateand moisture are suitable. It is a changing and dy-namic body, adjusting to conditions of climate, to-pography, and vegetation. In turn, soil influencesplant and root growth, available moisture, and thenutrients available to plants. While “the soil” is acollective term for all soils, “a soil” means one indi-vidual soil body with a particular length, depth,and breadth.

Soil ProfileIn a typical soil, the top layer is

usually dark with decomposingorganic matter; the layers beloware sand, silt, clay, or some com-bination of the three. Soil scien-tists classify soils on the basis ofsoil profile and soil formation.

Typically the top soil layer iscalled the O horizon, or organicmatter horizon. It has rotten logs,leaf litter, and other recognizablebits of plants and animals. Un-derneath the O horizon is the Ahorizon. It is characterized bythoroughly decomposed organicmatter. Water passing through theA horizon carries clay particlesand organic acids through it intothe B horizon. Clay or organic sub-stances passing into the B horizonglue soil particles together, form-ing soil aggregates. Soil aggre-gates—granular, columnar, andso on—are indicators of a mature,healthy soil. The lowest level ofthe soil profile is the C horizon. Itcontains bedrock or soil parentmaterial that shows little or noevidence of plant growth or soilformation.

Soil FormationSoil formation takes hundreds,

even thousands, of years. Parentmaterial, climate, organisms, topography, and timeall contribute. Sources of parent material includeigneous, sedimentary, and metamorphic rocks(fragments of which may be deposited by water,wind, and ice), and plant and animal deposits.

Soil formation is the result of the physical, chem-ical, and biochemical breakdown of parent mate-rial. It also reflects the processes of weathering andchange within the soil mass. Many substances areadded to soil—rain, water from irrigation, nitrogenfrom bacteria, sediment, salts, organic residues, anda variety of substances created by humans. How-ever, many substances are also removed from thesoil—water-soluble minerals, clay, plants, carbon di-oxide, and nitrogen. Other transformations also areoccurring: Organic matter is decomposing, and min-

950 • Soil

Soil Horizons

O = organic debris

A = topsoil(minerals)

B = subsoil(clay, iron oxide,

carbonate calcium)

C = regolith(soil base)

Bedrock

erals are solubilizing and changing chemical form.Clays and soluble salts that move along with the soilwater cause color and chemical changes in the soil.

Parent material is a primary determinant of soiltype or soil classification. All soils at the lowest cat-egory of soil classification are distinct if the parentmaterial differs. The differences in parent materi-als—weathering rates, the plant nutrient content,and soil texture resulting from parent materialbreakdown—contribute to the formation of distinc-tive soils. For example, sandstone yields sandy soilwith low fertility.

Chemical WeatheringSoils slowly change color and density as a result

of wetting and drying, warming and cooling, andfreezing and thawing. During weathering—the rub-bing, grinding, and moving of rocks by water,wind, and gravity—rocks are split into smaller andsmaller fragments. Soil is composed of fragments 2millimeters or less in diameter.

The expansion force of water as it freezes issufficient to split minerals. However, water alsois involved in chemical weathering—solution, hy-drolysis, carbonation, reduction, oxidation, and hy-dration. A simple example of solution, the dissolv-ing of minerals in liquid, is the dissolving of saltin water. The salts then move along with the liquid.In hot arid climates, salts can move to the surfaceas water evaporates, creating salt flats. In wetter cli-mates, salts can move through the soil, depletingit of necessary plant nutrients and contaminatinggroundwater.

Hydrolysis is the splitting of a water moleculeto form hydroxides and soluble hydroxide com-pounds, such as sodium hydroxide. Hydration isthe addition of water to minerals in rock. When amineral such as hematite (an oxide of iron) hy-drates, it expands, softens, and changes color. Car-bonation is the reaction of a compound with car-bonic acid, a weak acid produced when carbondioxide dissolves in water. Water often containscarbonic acid and other organic acids produced byorganic matter decomposition. These acids increasethe power of the water to disintegrate rock. Oxida-tion is the addition of oxygen to a mineral, and re-duction is the removal of oxygen from a mineral.

Biological WeatheringBiological weathering is a combination of physi-

cal and chemical disintegration of rocks to produce

soil. The roots of plants can crack rocks and breakthem apart. Plant roots also produce carbon diox-ide, which combines with water to produce car-bonic acid. Carbonic acid dissolves certain miner-als, speeding the breakdown of parent material andchemically changing the soil.

Plants and animals also add humus (organicmatter) to soil, increasing its fertility and water-holding capacity and speeding rock weathering.Animals such as earthworms, ants, prairie dogs,gophers, and moles also contribute to soil aerationand fertility by mixing the soil. In areas where ani-mal populations are large, they can influence boththe formation and destruction of soil.

Climate and TopographyClimate also influences soil formation indirectly

through its action on vegetation. Soils in arid cli-mates have sparse vegetation, less organic matter,and little soil profile development. Wet soil, how-ever, usually has thick vegetation and high organicmatter, and therefore a deep soil profile.

The shape of the land is referred to as its topogra-phy. Each landform—valleys, plains, hills, andmountains—is covered with a crazy quilt of differ-ent soil types. For example, the steep sides of theSandia Mountains near Albuquerque, New Mex-ico, which are severely eroded by wind and sum-mer rains, contain a variety of soil types—forestsoils, sandy soils, and rocky soils. Sand, silt, andclay eroded from the mountains and nearby extinctvolcanoes combine in the moist and fertile RioGrande Valley. The valley has deep sandy soils, lay-ered sand and clay soils, and soils eroded by flashfloods.

Soils located in similar climates that developfrom similar parent material on steep hillsides usu-ally have thin A and B horizons because less watermoves through the soil. Similar materials on shal-lower slopes allow more water to pass throughthem. Topography and climate work together ei-ther to allow or to prohibit plant growth and or-ganic matter deposition. Without moisture, plantscannot grow to impede soil erosion, and soil devel-opment is slow. With moisture, plants can grow,hold the soil in place, add organic matter to the soil,and speed soil development.

The age of a soil may be reckoned in tens,hundreds, or thousands of years. Under ideal con-ditions, a soil profile may develop in two hun-dred years; however, under less favorable condi-

Soil • 951

tions soil development may take several thousandyears.

Soil ClassificationScientists have identified and classified soils for

hundreds of years. Soils can be grouped accordingto agronomic use, color, organic matter content, tex-ture, moisture condition, and other characteristics.Each of these groupings serves a particular pur-pose. U.S. soil scientists adopted a system of soil

classification on January 1, 1965, that was based onthe knowledge they had about soil genesis, mor-phology, and classification. The U.S. system is di-vided into six categories: order, suborder, greatgroup, subgroup, family, and series. Soil taxonomyis patterned after the worldwide system of plantand animal taxonomy, which contains phylum,class, order, family, genus, and species.

Changes to the system have proceeded througha number of major revisions or approximations.

952 • Soil

The Twelve Soil Orders in the U.S. Classification SystemSoil Order Features

Alfisols Soils in humid and subhumid climates with precipitation from 500 to 1,300 millimeters (20 to 50inches), frequently under forest vegetation. Clay accumulation in the B horizon and availablewater most of the growing season. Slightly to moderately acid soils.

Andisols Soils with greater than 60 percent volcanic ash, cinders, pumice, and basalt. They have a dark Ahorizon as well as high absorption and immobilization of phosphorus and very high cationexchange capacity.

Aridisols Aridisols exist in dry climates. Some have horizons of lime or gypsum accumulations, saltylayers, and A and slight B horizon development.

Entisols Soils with no profile development except a shallow A horizon. Many recent river floodplains,volcanic ash deposits, severely eroded areas, and sand are entisols.

Gelisols Soils that commonly have a dark organic surface layer and mineral layers underlain bypermafrost, which forms a barrier to downward movement of soil solution. Common in tundraregions of Alaska. Alternate thawing and freezing of ice layers results in special features in thesoil; slow decomposition of the organic matter due to cold temperatures results in a peat layer atthe surface in many gelisols.

Histosols Organic soils of variable depths of accumulated plant remains in bogs, marshes, and swamps.

Inceptisols Soils found in humid climates that have weak to moderate horizon development. Horizondevelopment may have been delayed because of cold climate or waterlogging.

Mollisols Mostly grassland soils, but with some broadleaf forest-covered soils with relatively deep, dark Ahorizons, a possible B horizon, and lime accumulation.

Oxisols Excessively weathered soils. Oxisols are over 3 meters (10 feet) deep, have low fertility, havedominantly iron and aluminum oxide clays, and are acid. Oxisols are found in tropical andsubtropical climates.

Spodosols Sandy leached soils of the cool coniferous forests, usually with an organic or O horizon and astrongly acidic profile. The distinguishing feature of spodosols is a B horizon with accumulatedorganic matter plus iron and aluminum oxides.

Ultisols Strongly acid and severely weathered soils of tropical and subtropical climates. They have clayaccumulation in the B horizon.

Vertisols Soils with a high clay content that swell when wet and crack when dry. Vertisols exist intemperate and tropical climates with distinct dry and wet seasons. Usually vertisols have only adeep self-mixing A horizon. When the topsoil is dry, it falls into the cracks, mixing the soil to thedepth of the cracks.

The system can be used to classify soils anywherein the world, especially with the addition of a newsoil order, the andisols. The new soil classificationcontinues to be tested, and minor modificationsmay be anticipated. The approximation being usedas of 1997 treated soil as a collection of three-dimensional entities that can be grouped based onsimilar physical, chemical, and mineralogical prop-erties. The minimum volume of soil that scientistsconsider when they classify a soil is the pedon,which can range from 1 to 10 meters square and isas deep as roots extend into a soil.

By 1999, the U.S. soil classification system recog-nized twelve soil orders. The differences among or-ders reflect the dominant soil-forming processesand the degree of soil formation. Each order is iden-tified by a word ending in “-sol.” Each order is di-vided into suborders, primarily on the basis ofproperties that influence soil genesis, are importantto plant growth, and reflect the most importantvariables within the orders. The last syllable in thename of a suborder indicates the order. An exampleis “aquent,” meaning water, plus “-ent,” from“entisol.”

Suborders are distinctive to each order and arenot interchangeable between orders. Each suborderis divided into great groups on the basis of addi-tional soil properties and horizons resulting fromdifferences in soil moisture and soil temperature.

Great groups are denoted by a prefix that indi-cates a property of the soil. An example is “psam-maquents” (“psamm” referring to sandy textureand “aquent” being the suborder of the entisols thathas an aquic moisture regime). Soil scientists haveidentified more than three hundred great groups inthe United States. Great groups are distinguishedon the basis of differing horizons and soil features.The differing soil horizons include those with accu-mulated clay, iron, or organic matter and thosehardened or cemented by soil cultivation or otherhuman activities. The differentiating soil featuresinclude self-mixing of soil due to clay content, soiltemperature, and differences in content of calcium,magnesium, sodium, potassium, gypsum, andother salts.

There are more than twenty-four hundred sub-groups, and each great group is divided into threekinds of subgroups: a typic subgroup, an intergradesubgroup, and an extragrade subgroup. The typicsubgroup represents the central spectrum of a soilgroup. The intergrade subgroup represents soils

with properties like those of other orders, sub-orders, or great groups. The extragrade subgrouprepresents soils with some properties that are notrepresentative of the great group but do not indi-cate transitions to any other known kind of soil.Each subgroup is identified by one or more adjec-tives preceding the name of the great group.

Families are established within a subgroup on thebasis of physical and chemical properties and othercharacteristics that are important to plant growth orthat are related to the behavior of soils that are im-portant for engineering concerns. Among the prop-erties and characteristics considered are particlesize, mineral content, temperature regime, depth ofthe root zone, moisture, slope, and permanentcracks. Afamily name consists of the name of a sub-group preceded by terms that indicate soil proper-ties. Several thousand families have been identifiedin the United States.

Finally, the series is the lowest soil category, withmore than nineteen thousand recognized in theUnited States as of 1999. Aseries might share one ormore properties with those of an entire family, butfor at least one property only a narrower range ispermitted.

Texture, Structure, and ConsistencySoil texture is determined by the percent of sand,

silt, and clay in a soil sample. Most fertile or pro-ductive soils have a loam texture, or about equalamounts of sand, silt, and clay, and a high organicmatter content (about 5 to 10 percent). Soil tex-ture determines the water-holding and nutrient-holding capacity of a soil. Thus, clay soils have ahigh nutrient-holding capacity, but they waterlogeasily. Sandy soils have a lower nutrient-holdingcapacity but dry out easily. Farmers base their plansof how to fertilize and irrigate their crops partly onthe texture of the soil.

Soil structure refers to how soil particles are gluedtogether to form aggregates. During soil formation,soil particles are glued together with clay, dead mi-croorganisms, earthworm slime, and plant roots,and they form air and water channels. Plants needthese channels so they can absorb nutrients, water,and air. Soil structure may be destroyed when farm-ers cultivate wet or waterlogged soils with heavyfarm machinery. Destroying soil structure makes asoil unsuitable for plant growth.

Soil consistency is the “feel” of a soil and the easewith which a lump can be crushed in one’s fingers.

Soil • 953

Common soil consistencies are loose, friable, firm,plastic, sticky, hard, and soft. Clay soils, for exam-ple, are sticky or plastic when they are wet, but theybecome hard or harsh when they are dry. The besttime to work a clay soil is when it is soft or friable.Sandy soils, on the other hand, do not become plas-tic or sticky when they are wet or hard or harshwhen they are dry. They have a tendency to stayloose, which makes them easier to work. Loam andsilt loam soils are intermediate in behavior. Whenfarmers are trying to determine whether to workthe soil or wait for better soil moisture conditions,they usually check the soil consistency.

Aeration and MoistureSoil aeration relates to the exchange of soil air

with atmospheric air. Growing roots need oxygenand are constantly expiring carbon dioxide. Unlessthere is a continuous flow of oxygen into soil andcarbon dioxide out of the soil, oxygen becomes de-pleted. When their oxygen supply is cut off, theroots will die.

Soil moisture refers to water held in soil pores. Aplant draws water from soil the same way a childdraws water from a cup with a straw. When thecup is full, it is easy for the child to draw up thewater, but as the cup empties, the child must workharder to get water. Similarly, plants draw waterfrom soil easily when the soil has plenty of water.As the soil dries, however, plants must work harderto pull water out of the soil until they reach wiltingpoint.

FertilityPlants absorb many of the nutrients they need

from soil, including phosphorus, potassium, cal-cium, magnesium, sulfur, boron, chlorine, cobalt,copper, iron, manganese, molybdenum, and zinc.They may obtain carbon, hydrogen, and nitrogenfrom the air and water.

Soil testing services give farmers specific fertil-izer and lime recommendations based on soil tex-ture and chemical analysis. Farmers use soil tests todetermine if their soil has enough essential nutri-ents for a crop to grow. The absence of one essentialnutrient can limit overall crop growth. Nitrogen,phosphorus, and potassium are commonly appliedto the soil as commercial fertilizer and manure. Cal-cium and magnesium are applied as lime, which isalso used to reduce the acidity of soil and to in-crease the solubility of some minerals. Manure andother organic matter added to soils increase water-holding and nutrient-holding capacity and there-fore boost crop yields.

Agricultural extension services offer guidelinesfor the maximum amounts of manure, sewagesludge, fertilizer, and other chemicals that farmersshould apply to soils. Farmers are encouraged toapply nitrogen fertilizer in small applications attimes when plants are growing rapidly. This soilmanagement practice decreases deep percolationlosses that could pollute groundwater.

With an understanding of soil characteristics,farmers and gardeners can learn to manage a widevariety of soils. Some soils are naturally fertile andneed few amendments to promote high crop yields.Other soils, whether because of their parent mate-rial or climate, are naturally infertile and might bestbe used for purposes other than agriculture. Likethe water and the air, the soil is a crucial natural re-source. From an airplane, all soils look about thesame, but from an ant’s view, soils are all different.Differences in soil type make huge differences toplants, animals, humans, and the environment.

J. Bradshaw-Rouse, updated by Christina J. Moose

See also: Agronomy; Composting; Hydroponics;Nutrients; Soil conservation; Soil contamination;Soil degradation; Soil management; Soil saliniza-tion.

Sources for Further StudyDonahue, Roy L., Roy Hunter Follett, and Rodney W. Tulloch. Our Soils and Their Manage-

ment: Increasing Production Through Environmental Soil and Water Conservation and FertilityManagement. 6th ed. Danville, Ill.: Interstate Publishers, 1990. Covers soil management;emphasis is on agricultural applications. Bibliography, indexes.

Harpstead, Milo I., Thomas J. Sauer, and William F. Bennett. Soil Science Simplified. 3d ed.Ames: Iowa State University Press, 1997. A concise introduction. Illustrations, maps,index.

Miller, Raymond, and Duane T. Gardiner. Soils in Our Environment. 9th ed. Upper SaddleRiver, N.J.: Prentice Hall, 1997. This is the authoritative textbook on soil formation. This

954 • Soil

edition includes a CD-ROM as well as sixty full-color photographs and expanded andupdated coverage touching on the latest technologies and environmental issues. Glos-sary, index.

SOIL CONSERVATION

Categories: Environmental issues; soil

Soil conservation is the effort by farmers and other landowners to prevent the buildup of salts and fertilizer acids in thesoil, as well as the loss of topsoil from wind erosion, water erosion, desertification, and chemical deterioration. This can beachieved by implementing management practices to maintain soil quality and reduce pollution.

According to the United Nations EnvironmentProgramme, approximately 17 percent of the

earth’s vegetated land is degraded, which poses athreat to agricultural production around the world.The introduction of minerals, metals, nutrients, fer-tilizers, pesticides, bacteria, and pathogens sus-pended in topsoil runoff into waterways is a signifi-cant source of water pollution and is a threat tofisheries, wildlife habitat, and drinking water sup-plies. The introduction of soil particles into the airthrough wind erosion is a significant source of airpollution.

Threats and ResponsesThe Industrial Revolution of the nineteenth cen-

tury and the population explosion of the twentiethcentury encouraged people to till new land, cutdown forests, and disturb soil for the expansion oftowns and cities. The newly exposed topsoilquickly succumbed to erosion from rainfall, floods,wind, ice, and snow. The Dust Bowl, which oc-curred in the Great Plains in the United States dur-ing the 1930’s, is one example of the devastatingeffects of wind erosion.

Hugh Hammond Bennett, the so-called father ofsoil conservation, lobbied for congressional estab-lishment of the United States Soil Conservation Ser-vice (approved in 1937) and the establishment ofvoluntary Soil Conservation Districts in each state.Bennett was named the first chief of the U.S. SoilConservation Service in 1937. On August 4, 1937,the Brown Creek Conservation District in Bennett’shome county, Anson County, North Carolina, be-came the first Soil Conservation District in theUnited States. Local landowners voted to establish

the district by three hundred to one, proving thatfarmers were concerned about soil conservation.A reporter for the Charlotte Observer newspapersought out the one negative voter; after having theprogram explained to him, he changed his opinion.By 1948, more than twenty-one hundred districtshad been established nationwide. They were even-tually renamed Soil and Water Conservation Dis-tricts. There are more than three thousand such dis-tricts in the United States.

The U.S. Food Security Act of 1985 authorizedthe Conservation Reserve Program to take landhighly susceptible to erosion out of production andrequired farmers to develop soil conservation plansfor the remaining susceptible land. The Natural Re-source Conservation Service estimates that the lossof topsoil was nearly cut in half, reduced from 1.6billion tons per year to 0.9 billion tons. The Euro-pean Community and Australia also adopted soilconservation measures during the 1990’s.

PracticesSoil conservation practices include covering the

soil with vegetative cover, reducing soil exposure ontilled land, creating wind and water barriers, and in-stalling buffers. Vegetative cover slows the wind atground level, slows water runoff, protects soil par-ticles from being detached, and traps blowing orfloating soil particles, chemicals, and nutrients. Be-cause the greatest wind and water erosion damageoften occurs during seasons in which no crops aregrowing or natural vegetation is dormant, soil con-servation often depends on permitting the deadresidues and standing stubble of the previous cropto remain in place until the next planting time. An-

Soil conservation • 955

nual tree-foliage loss serves as a natural groundmulch in forested areas. Planting grass or legumecover crops until the next planting season, or as partof a crop rotation cycle or no-till planting system, alsoreduces erosion.

Modern no-till and mulch-till planting systems re-duce soil exposure to wind and rain, while plowingthe land brings new soil to the surface and buriesthe ground cover. No-till systems leave the soilcover undisturbed before planting and insert cropseeds into the ground through a narrow slot in thesoil. Mulch-till planting keeps a high percentageof the dead residues of previous crops on the sur-face when the new crop is planted. Row crops areplanted at right angles to the prevailing winds andto the slope of the land in order to absorb wind andrainwater runoff energy and trap moving soil parti-cles. Crops are planted in small fields to preventavalanching caused by an increase in the amount ofsoil particles transported by wind or water as thedistance across bare soil increases. As the amount ofsoil moved by wind or water increases, the erosive

effects of the wind and water also increase. Smallerfields reduce the length and width of unprotectedareas of soil.

Wind and water barriers include tree plantingsand crosswind strips of perennial shrubs and 1-meter-high (3-foot-high) grasses, which act as windbreaks to slow wind speeds at the surface of the soil.The protected area extends for ten times the heightof the barrier. In alley cropping, which is used in ar-eas of sustained high wind, crops are planted be-tween rows of larger mature trees. Contour stripfarming on slopes, planting grass waterways inareas where rainwater runoff concentrates, andplanting 3-meter-wide (10-foot-wide) grass fieldborders on all edges of cultivated or disturbed soilare additional methods for reducing wind speedand rainwater runoff and trapping soil particles,chemicals, and nutrients.

Buffers filter runoff to remove sediments andchemicals. Riparian buffers are waterside plantingsof trees, shrubs, and grasses, usually 6 meters (20feet) in width. Riparian buffers planted only in

956 • Soil conservationP

ho

toD

isc

Soil conservation practices include protecting soil particles from becoming detached and trapping blowing or floatingsoil particles, chemicals, and nutrients.

grass are called filter strips. Grassed waterways,field borders, water containment ponds, and con-tour grass strips are other types of soil conservationbuffers.

Gordon Neal Diem

See also: Agriculture: modern problems; Agron-omy; Composting; Erosion and erosion control; Hy-droponics; Nutrients; Slash-and-burn agriculture;Soil; Soil contamination; Soil degradation; Soil man-agement; Soil salinization; Sustainable agriculture.

Sources for Further StudyGershuny, Grace, and Joseph Smillie. The Soul of Soil: A Guide to Ecological Soil Management.

3d ed. Davis, Calif.: agAccess, 1995. Soil conservation and related issues are discussedfrom an ecological perspective. Illustrations, bibliography, index.

Morgan, R. P. C. Soil Erosion and Conservation. New York: Wiley, 1995. This nearly two-hundred-page discussion is supplemented by illustrations, maps, bibliography, andindex.

Richter, Daniel D., and Daniel Markowitz. Understanding Soil Change: Soil Sustainability overMillennia, Centuries, and Decades. New York: Cambridge University Press, 2001. Aimed atfarmers and agricultural professionals, this book discusses the forces of erosion and pro-cedures to sustain soil fertility.

Sanders, David W., Paul C. Huszar, and Samran Sombatpanit, eds. Incentives in Soil Conser-vation: From Theory to Practice. Enfield, N.H.: Science Publishers, 1999. A series of studieson the results of incentives to soil conservation in worldwide soil management programs.

Troeh, Frederick R., et al. Soil and Water Conservation: Productivity and Environmental Protec-tion. Upper Saddle River, N.J.: Prentice Hall, 1998. Considers various hazards—erosion,sedimentation, pollution—with reference to various world soils. Takes an agronomic ap-proach. Appendices present conversion factors and common and scientific plant names.

Yudelman, Montague, John C. Greenfield, and William B. Magrath. New Vegetative Ap-proaches to Soil and Moisture Conservation. Washington, D.C.: World Wildlife Fund and theConservation Foundation, 1990. Examines soil and water conservation in developingcountries. Bibliographical references, illustrations.

SOIL CONTAMINATION

Categories: Environmental issues; pollution; soil

Soils contaminated with high concentrations of hazardous substances pose potential risks to human health and theearth’s thin layer of productive soil.

Productive soil depends on bacteria, fungi, andother soil microbes to break down wastes and

release and cycle nutrients that are essential toplants. Healthy soil is essential for growing enoughfood for the world’s increasing population. Soilalso serves as both a filter and a buffer between hu-man activities and natural water resources, whichultimately serve as the primary source of drinkingwater. Soil that is contaminated may serve as asource of water pollution through leaching of con-

taminants into groundwater and through runoffinto surface waters such as lakes, rivers, andstreams.

The U.S. government has tried to address theproblem of soil contamination by passing two land-mark legislative acts. The Resource Conservationand Recovery Act (RCRA) of 1976 regulates haz-ardous and toxic wastes from the point of genera-tion to disposal. The Comprehensive Environmen-tal Response, Compensation, and Liability Act

Soil contamination • 957

(CERCLA) of 1980, also known as Superfund, iden-tifies past contaminated sites and implements re-medial action.

Sources of ContaminationSoils can become contaminated by many human

activities, including fertilizer or pesticide applica-tion, direct discharge of pollutants at the soil sur-face, leaking of underground storage tanks or pipes,leaching from landfills, and atmospheric deposition.Additionally, soil contamination may be of naturalorigin. For example, soils with high concentrationsof heavy metals can occur naturally because of theirclose proximity to metal ore deposits. Common con-taminants include inorganic compounds such as ni-trate and heavy metals (for example, lead, mercury,cadmium, arsenic, and chromium); volatile hydro-carbons found in fuels, such as benzene, toluene,ethylene, and xylene BTEX compounds; and chlori-nated organic compounds such as polychlorinatedbiphenyls (PCBs) and pentachlorophenol (PCP).

Contaminants may also include substances thatoccur naturally but whose concentrations are ele-vated above normal levels. For example, nitrogen-and phosphorus-containing compounds are oftenadded to agricultural lands as fertilizers. Since nitro-gen and phosphorus are typically the limiting nu-trients for plant and microbial growth, accumula-tion in the soil is usually not a concern. The realconcern is the leaching and runoff of the nutrientsinto nearby water sources, which may lead to oxy-gen depletion of lakes as a result of the eutrophica-tion encouraged by those nutrients. Furthermore,nitrate is a concern in drinking water because itposes a direct risk to human infants (it is associatedwith blue-baby syndrome).

Contaminants may reside in the solid, liquid,and gaseous phases of the soil. Most will occupy allthree phases but will favor one phase over the oth-ers. The physical and chemical properties of thecontaminant and the soil will determine whichphase the contaminant favors. The substance maypreferentially adsorb to the solid phase, either theinorganic minerals or the organic matter. The at-traction to the solid phase may be weak or strong.The contaminant may also volatize into the gaseousphase of the soil. If the contaminant is soluble inwater, it will dwell mainly in the liquid-filled poresof the soil.

Contaminants may remain in the soil for yearsor wind up in the atmosphere or nearby water

sources. Additionally, the compounds may be bro-ken down or taken up by the biological componentof the soil. This may include plants, bacteria, fungi,and other soil-dwelling microbes. The volatile com-pounds may slowly move from the gaseous phaseof the soil into the atmosphere. The contaminantsthat are bound to the solid phase may remain intactor be carried off in runoff attached to soil particlesand flow into surface waters. Compounds that fa-vor the liquid phase, such as nitrate, will eitherwind up in surface waters or leach down into thegroundwater.

Metals display a range of behaviors. Some bindstrongly to the solid phase of the soil, while otherseasily dissolve and wind up in surface or ground-water. PCBs and similar compounds bind stronglyto the solid surface and remain in the soil for years.These compounds can still pose a threat to water-ways because, over long periods of time, they slowlydissolve from the solid phase into the water at tracequantities. Fuel components favor the gaseousphase but will bind to the solid phase and dissolveat trace quantities into the water. However, eventrace quantities of some compounds can pose a seri-ous ecological or health risk. When a contaminantcauses a harmful effect, it is classified as a pollutant.

TreatmentsThere are two general approaches to cleaning up

a contaminated soil site: treatment of the soil inplace (in situ) or removal of the contaminated soilfollowed by treatment (non-in situ). In situ meth-ods, which have the advantage of minimizing ex-posure pathways, include biodegradation, volatil-ization, leaching, vitrification (glassification), andisolation or containment. Non-in situ methods gen-erate additional concerns about exposure duringthe process of transporting contaminated soil. Non-in situ options include thermal treatment (incin-eration), land treatment, chemical extraction, solid-ification or stabilization, excavation, and asphaltincorporation. The choice of methodology will de-pend on the quantity and type of contaminants, andthe nature of the soil. Some of these treatment tech-nologies are still in the experimental phase.

John P. DiVincenzo

See also: Agronomy; Composting; Environmentalbiotechnology; Hydroponics; Nutrients; Soil; Soilconservation; Soil degradation; Soil management;Soil salinization.

958 • Soil contamination

Sources for Further StudyPierzynski, Gary M., J. Thomas Sims, and George F. Vance. Soils and Environmental Quality.

Boca Raton, Fla.: Lewis, 1994. A good overview of the environmental health of soils.Sparks, Donald L. “Soil Decontamination.” In Handbook of Hazardous Materials, edited by

Morton Corn. San Diego, Calif.: Academic Press, 1993. Provides a good introduction tothe sources of soil contamination and then gives a brief overview of the options for decon-tamination.

Testa, Stephen M. The Reuse and Recycling of Contaminated Soil. Boca Raton, Fla.: Lewis, 1997.Includes a good introduction that provides an overview of the problem and the regula-tory aspects of soil contamination. The remainder of the book is geared for the more ad-vanced reader and discusses technical aspects of decontaminating soil.

SOIL DEGRADATION

Categories: Environmental issues; soil

A decline in soil quality, productivity, and usefulness due to natural causes, human activities, or both, is known as soildegradation. It is often caused by unfavorable alterations in one or all of a soil’s physical, chemical, and biological attrib-utes.

In 1992, for the first global study of soil degrada-tion, the World Resources Institute in Washing-

ton, D.C., reported that 3 billion acres of landworldwide had been seriously degraded sinceWorld War II. They also stated that 22 million acresof once usable land could no longer support crops.

Natural Processes and Human ActivitiesOf the total acreage lost to soil degradation, al-

most two-thirds is in Asia and Africa; most of theloss is attributable to water and wind erosion result-ing from agricultural activities, overgrazing, defores-tation, and firewood collection. There are also seri-ously degraded soils in Central America, wheredegradation is caused primarily by deforestationand overgrazing. In Europe, industrial and urbanwastes, pesticides, and other substances have poi-soned soils in much of Poland, Germany, Hungary,and southern Sweden. In the United States, the U.S.Department of Agriculture estimates that a quarterof the nation’s croplands have been depletedthrough deep plowing, removal of crop residue,conversion to permanent pasture, and other con-ventional agricultural practices. Although unwisemanagement practices contribute significantly tosoil degradation, soil degradation also involves

three natural soil processes: physical, chemical, andbiological degradation.

Physical DegradationPhysical soil degradation involves deterioration

in soil structure, leading to compaction, crusting,accelerated erosion, reduced water-holding capac-ity, and decreased aeration. Soil compaction is thecompression of soil particles into a smaller volume.Excessively compacted soil suffers from poor aera-tion and reduced gas exchange, which can restrictthe depth of root penetration. Soil compaction alsocauses accelerated runoff and erosion of soils.

Crusting is the formation of a hard layer a fewmillimeters or a few tens of millimeters thick atthe soil surface. Crusts affect drainage, leading towaterlogging at the soil surface and to salinity oralkalinity problems. Once crusts called duricrustsform, soil moisture recharge declines, and vegeta-tion cannot root. Sheet and gully erosion increasesas the land fails to absorb precipitation. Hard layerscan also form below the cultivation depth and arecalled hard pans (other names are plow soles, trafficpans, and plow pans). These compacted layers canrestrict root growth, making crops and trees vulner-able to drought and lodging (falling over).

Soil degradation • 959

Chemical DegradationChemical degradation comprises changes in

soil’s chemical properties that regulate nutrientavailability. Nutrient depletion is the major factor inchemical soil degradation. Soil nutrient depletionmay be caused or exacerbated by many factors, in-cluding monocropping, leaching of nutrients, andsalt buildup.

A historic example of nutrient depletion is thedepletion of soils in the southeastern United Statesby the growing of cotton. As late as 1950, “King Cot-ton” was the most valuable farm commodity pro-duced in Alabama, Arkansas, Georgia, Louisiana,Mississippi, South Carolina, Tennessee, and Texas.In the eighteenth and nineteenth centuries, thegrowing of cotton ruined soil fertility as it spreadwestward from the Atlantic to the Texas panhandle.Cotton growth without regard to topography inhilly regions contributed to soil erosion. Topsoilwas eventually removed from many fields, whichfurther depleted nutrients. One reason that peanuts

became a major crop in the South is that they arenitrogen-fixing plants that can grow in soils de-pleted of nitrogen by cotton.

Nutrient leaching is another problem. Continu-ous irrigation can leach nutrients and cause saltbuildup in soils where drainage is poor. Leachingcan move essential but soluble nutrients past theroot zone deeper into the soil and into groundwa-ter. In addition, the water used to irrigate soil oftencontains salts that can accumulate to toxic levelsand inhibit plant growth where evaporation occursreadily. Thick crusts of salt on farmland in Pakistan,Australia, Ethiopia, Sudan, and Egypt have madesoil unfit for crops.

Laterization refers to the product and process ofwetting and drying that leads to the irreversibleconsolidation and hardening of aluminum and iron-rich clays into hard pans, sometimes of great thick-ness, called plinthitic materials (Greek plinthosmeans “brick”). Laterization is particularly com-mon in the humid and subhumid tropics.

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Of the total acreage lost to soil degradation, most of the loss is attributable to water and wind erosion resulting from agri-cultural activities, overgrazing, deforestation, and firewood collection. Soil compaction also causes accelerated runoffand erosion of soils.

Biological DegradationThe loss of organic matter and soil nutrients

needed by plants can occur in any environment, butit is most dramatic in hot, dry regions. Organic mat-ter is important in maintaining soil structure, sup-porting microorganisms, and retaining plant nutri-ents. Because organic matter is near the soil surface,it is generally the first soil component to be lost.Organic matter may be lost through brush fires,stubble-burning, overgrazing, or the removal ofcrops, fodder, wood, and dung. Loss of organicmatter can be accelerated when soil moisture isreduced, when soil aeration is increased, or both.

For example, peat soils that aredrained decompose rapidly andsubside. In drier climates, the lossof organic matter reduces thesoil’s moisture-holding capacityand lowers soil fertility, whichleads to lower crop yields andthus to less organic matter beingreturned to the soil.

Tropical rain forests such asthose of the Amazon basin inSouth America seem lush, sopeople widely assume tropicalsoils to be fertile and high in or-ganic matter. Although tropicalforests do produce considerableorganic matter, the amount thatstays in the soil is surprisinglysmall, and the soils actually havelow nutrient levels. Soil microor-ganisms in the rain forest breakdown the organic matter and re-lease nutrients that are absorbedby growing plants. However,warm temperatures and highrainfall cause accelerated nutri-

ent loss if plants are absent. Nutrients that wouldbuffer the pH of the soil are lost. Consequently,the clearing of rain forests exposes the soil to ero-sion, leaching, acidification, and rapid nutrient de-pletion.

J. Bradshaw-Rouse

See also: Agronomy; Composting; Deforestation;Erosion and erosion control; Fertilizers; Grazingand overgrazing; Herbicides; Hydroponics; Mono-culture; Nutrients; Pesticides; Slash-and-burn agri-culture; Soil; Soil conservation; Soil contamination;Soil management; Soil salinization.

Sources for Further StudyBridges, E. M., ed. Responses to Land Degradation. Enfield, N.H.: Science Publishers, 2001. As-

sesses legislative responses to soil degradation.Lindert, Peter. Shifting Ground. Cambridge, Mass.: MIT University Press, 2000. Quantitative

historical study of soil fertility in China and Indonesia suggests that modern agriculturalpractices actually remedy soil degradation.

Murnaghan, Niamh, and Michael A. Stocking, eds. A Handbook for the Assessment of LandDegradation. London: Earthscan Publications, 2001. A guidebook for farmers, intended tohelp assess soil degradation and choose appropriate options for ameliorating the prob-lems.

Soil degradation • 961

Permafrost 6%

Soil tooshallow

22%

Soil too wet10%

Soiltoo dry

28%

Chemicalproblems

23%

Soil that can be farmed withoutbeing irrigated, drained, orotherwise improved 11%

Soil Limits to Agriculture, by Percentage ofTotal World Land Area

Source: United Nations Food and Agriculture Organization (FAOSTAT Database, 2000).

SOIL MANAGEMENT

Categories: Agriculture; disciplines; environmental issues; soil

Tillage, conservation, and cropping practices are soil management techniques that are used to preserve soil resourceswhile optimizing soil use.

Soils are managed differently depending on theirintended use. Soil management groups are soil

types with similar adaptations or management re-quirements for specific purposes, such as use withcrops or cropping rotations, drainage, fertilization,

forestry, highway engineering, and construction.In managing soil for agriculture, soil managementincludes all tillage and planting operations, crop-ping practices, fertilization, liming, irrigation, her-bicide and insecticide application, and other treat-

ments conducted on or applied to the soilsurface for the production of plants.

TillageThe most basic aspect of soil management

is the way in which it is cultivated or tilledfor crop growth. Tillage is the mechanicalmanipulation of the soil profile to modifysoil conditions, manage crop residues orweeds, or incorporate chemicals for cropproduction. Tillage can be exhaustive orminimal. Conventional tillage uses multipletillage operations to bury existing crop resi-due and prepare a uniform, weed-free seedbed for planting. This method breaks up soilaggregates in the process and destroys soilstructure. Consequently, it can result in ex-cessive wind and water erosion.

Conservation tillage, or minimum tillage,involves soil management practices thatleave much more crop residue on the soilsurface and cause much less soil disruption.As a result, the soil is less susceptible to ero-sion, and the plant residue acts as a mulch toprotect the soil surface from the destructiveimpact of rainfall as well as to reduce evapo-ration.

No-tillage, or chemical tillage, is a soilmanagement practice adapted to slopingsoils in which herbicides rather than tillageare used to control weeds, while the disrup-tion of soil structure is limited to a narrowslit in the soil surface in which the seeds areplanted.

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Soil management extends to the way in which soils are manipu-lated. Terraces, for example, are raised horizontal strips of earthconstructed along the contour of a hill to slow the movement ofdownward-flowing water.

Hillsides and WetlandsSoil management extends to the way in which

soils are manipulated. Terraces, for example, areraised horizontal strips of earth constructed alongthe contour of a hill to slow the movement of down-ward-flowing water. Tile drains are perforated ce-ramic or plastic pipes buried in poorly drained soilsthat act as underground channels to carry wateraway, lower the water table, and allow a soil todrain faster after rainfall. The benefit of managingpotentially erodible soils on hill slopes as perma-nent pastures is being recognized as another way ofmanaging hillside soils.

Likewise, the value of retaining wet soils aswetlands has been acknowledged. Wetlands pro-vide wildlife habitat, assist in flood control, and actas buffers to protect surface waterways from nutri-ent and soil runoff from cultivated fields.

Chemical ManagementSoil management also involves the addition of

chemicals to soil: lime to make acid soils more neu-

tral, fertilizers to increase the nutrient level, herbi-cides and insecticides to control weed and insectpests, soil conditioners to improve soil aggregation,structure, and permeability. A growing technologyis the use of mobile global positioning system (GPS)units attached to the equipment that applies thesechemicals to soil. Called site-specific management, ituses computer technology to regulate chemical ad-dition based on the exact position in a field and pre-vious yield or fertility maps that indicate whetherthe soil needs to be amended. The goal of site-specific management is to optimize chemical useand profit while minimizing potential chemicalloss to other environments by only applying thechemicals to areas where they are needed.

J. Bradshaw-Rouse

See also: Agronomy; Composting; Fertilizers; Hy-droponics; Nutrients; Soil; Soil conservation; Soilcontamination; Soil degradation; Soil salinization;Sustainable agriculture.

Sources for Further StudyDonahue, Roy L., Roy Hunter Follett, and Rodney W. Tulloch. Our Soils and Their Manage-

ment: Increasing Production Through Environmental Soil and Water Conservation and FertilityManagement. 6th ed. Danville, Ill.: Interstate Publishers, 1990. Covers soil management;emphasis is on agricultural applications. Bibliography, indexes.

Fisher, Richard, and Dan Brinkley. Ecology and Management of Forest Soils. 3d ed. New York:John Wiley & Sons, 2000. Provides basics of soil management for forestry professionals.

Prasad, Rajendra, and James F. Power. Soil Fertility Management for Sustainable Agriculture.Boca Raton, Fla.: Lewis, 1997. Provides a broad picture of soil management practices ap-propriate to different environmental conditions.

Richter, Daniel D., and Daniel Markowitz. Understanding Soil Change: Soil Sustainability overMillennia, Centuries, and Decades. New York: Cambridge University Press, 2001. Aimed atfarmers and agricultural professionals, this book discusses the forces of erosion and pro-cedures to sustain soil fertility.

SOIL SALINIZATION

Categories: Agriculture; environmental issues; pollution

Soil salinization is a process in which water-soluble salts build up in the root zones of plants, blocking the movement ofwater and nutrients into plant tissues.

Soil salinization rarely occurs naturally. It be-comes an environmental problem when it oc-

curs as a result of human activity, denuding once-vegetated areas of all plant life. Rainwater is virtu-

Soil salinization • 963

ally free of dissolved solids, but surface waters andunderground waters (groundwater) contain signifi-cant quantities of dissolved solids, ultimately pro-duced by the weathering of rocks. Evaporation ofwater at the land surface results in an increase indissolved solids that may adversely affect the abil-ity of plant roots to absorb water and nutrients.

Arid ClimatesIn arid regions, evaporation of soil water poten-

tially exceeds rainfall. Shallow wetting of the soilfollowed by surface evaporation lifts the availabledissolved solids to near the surface of the soil. Thenear-surface soil therefore becomes richer in solu-ble salts. In natural arid areas, soluble salts in thesubsurface are limited in quantity because rockweathering is an extremely slow process. Degreesof soil salinization detrimental to plants are un-common.

Irrigating arid climate soils with surface orgroundwater provides a constant new supply ofsoluble salt. As the irrigation water evaporates andmoves through plants to the atmosphere, the dis-solved solid content of the soil water increases.Eventually, the increase in soil salt will inhibit orstop plant growth. It is therefore necessary to ap-ply much more water to the fields than requiredfor plant growth to flush salts away from the plant

root zone. If the excess water drains easily to thegroundwater zone, the groundwater becomes en-riched in dissolved solids, which may be detri-mental.

If the groundwater table is near the surface, or ifthere are impermeable soil zones close to the sur-face, overirrigation will not alleviate the problem ofsoil salinization. This condition requires the instal-lation of subsurface drains that carry the excess soilwater and salts to a surface outlet. The problemwith this method is that disposing of the salty drainwater is difficult. If the drain water is released intosurface streams, it degrades the quality of thestream water, adversely affecting downstream us-ers. If the water is discharged into evaporationponds, it has the potential to seep into the ground-water zone or produce a dangerously contami-nated body of surface water, as occurred at theKesterson Wildlife Refuge in California, where con-centrations of the trace element selenium rose tolevels that interfered with the reproduction of resi-dent birds.

Robert E. Carver

See also: Agronomy; Composting; Deserts; Halo-phytes; Hydroponics; Nutrients; Soil; Soil conser-vation; Soil contamination; Soil degradation; Soilmanagement.

Sources for Further StudyAbrol, I. P., J. S. P. Yadev, and F. I. Massoud. Salt-Affected Soils and Their Management. Rome:

Food and Agriculture Organization, 1988. AUnited Nations-sponsored study of soil sali-nization and its remedies.

Frenkel, H., and A. Mieri, eds. Soil Salinity: Two Decades of Research in Irrigated Agriculture.New York: Van Nostrand Reinhold, 1985. A collection of studies on soil salinization inIsrael.

Ghassemi, F., A. J. Jakeman, and H. A. Nix. Salinization of Land and Water Resources: HumanCauses, Extent, Management, and Case Studies. Wallingford, England: CAB International,1995. Practical assessment of soil salinization.

McClug, Sue. “Salts: Old Dilemma, New Solutions.” Western Water (September/October,1995): 4-13. Approaches being taken to address soil salinization in the San Joaquin andImperial Valleys in California.

964 • Soil salinization

SOUTH AMERICAN AGRICULTURE


Increasing urbanization in South America raises questions regarding use of both land that is farmed and land that isabandoned.

In South America as elsewhere, landforms, soil,water, climate, and culture interact to produce an

arrangement of agricultural production specific tothe region. South America can be divided into sixgeneral landform regions: the Andes Mountains,the plateaus of the interior of the continent, theriver lowlands, the coastal lowlands, the tierra tem-plada, and the tierra fria.

Andes MountainsThe Andes Cordillera reaches from Venezuela in

the north to Tierra del Fuego at the south tip of thecontinent. Some Andean peaks exceed 20,000 feet(6,100 meters) in height. The mountain soils arerocky and steeply graded. This makes farming dif-ficult but not impossible. Farmers use terrace sys-tems, building up steplike fields carved into theside of a mountain. Because of the limitations of thesoils and the difficulty in farming them, the regionsupports only subsistence agricultural settlements.People in the highlands grow small plots of corn,barley, and especially potatoes on the high-altitudesoils.

Highland Growing RegionsIn the central eastern region of Brazil are the Bra-

zilian Highlands. To the north of the Amazon basinlies another plateau region called the Guiana High-lands. Both plateaus, which are not much higherthan 9,000 feet (2,743 meters), are old geologic struc-tures with relatively rough surfaces to farm. TheBrazilian Highlands constitute the world’s primarycoffee-growing region. More than one-third of theworld’s coffee is grown there, along with soybeansand oranges. The Guiana Highlands, which stretchthrough southern Venezuela and Guyana, are cov-ered in savannas (grasslands with trees and shrubs).People there use slash-and-burn agriculture to growcorn and rice.

River LowlandsSome of the most remote regions in South Amer-

ica are its fastest-developing areas. The northern in-terior of Brazil in the Amazon basin is covered withthe world’s largest tropical rain forest, the size ofthe forty-eight contiguous United States. Withinsome areas of this rich habitat, gold and huge ironore deposits have been discovered. Developmentprograms have encouraged mining, hydroelectricprojects, ranching, and farming there. This has pro-duced a need for clearing the forests and caused alarge migration of people into this otherwise re-mote, isolated region. It has been estimated that30,000 square miles (78,000 square kilometers) oftropical rain forest were destroyed annually in the1990’s.

The rain forest is not a highly productive grow-ing region for crops. Soils there are thin and havefew nutrients. To grow crops and grasses for cattle,slash-and-burn agriculture is practiced. Trees andbrush on the land are cut down and burned, and theashes enrich the soils for a time. This allows for lim-ited production of crops such as corn and rice, butafter about three years the nutrients are washed outof the soil by the heavy tropical rains. Farmers andranchers move on and find new forest area to cut,and the cycle goes on. Rain forests cover only about5 percent of the total land area of the earth but con-tain more than half the different types of plants andanimals on earth. These forests provide lumberproducts, medicines, and food.

The Orinoco River drains the Guiana Highlandsand the llanos region of Venezuela, flowing into theAtlantic Ocean. The llanos form a large, expansiveplain of grassland between the Andes in Venezuelaand the Guiana Highlands. Soils are flooded in thisarea during the rainy seasons, providing for richgrass development and the support of large cattleranches.

South American agriculture • 965

Belem

Cali

Medellin

Rio de JaneiroSao Paulo˜

Porto Alegre

Arica

CoffeeCoffee

Coffee

YamsCassava

Root Crops

Coffee

Soybeans

Coffee

CattleSugarcane

Sugarcane

Sugarcane

Rice

Rice

BananasYams

Manioc

BananasOranges

Cotton

Bananas

Cacao

Cacao

Potatoes

Alpacas

Llamas

Grapes

Apples

Peaches

Plums

Fish

Wheat

Sheep

Sheep

BeefCattle

Taunin

Tobacco

Timber

Barley

Bananas

Cotton

La PazBrasilia

Santiago

Bogota

Quito

MontevideoBuenos Aires

Asuncion`

Lima

'

Am a zo n B a s i n

ARGENTINA

BOLIVIA

COLOMBIA

VENEZUELA

PERUBRAZIL

FRENCHGUIANA

SURINAMEGUYANA

CHILE

ECUADOR

PARAGUAY

URUGUAY

AN

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SM

OU

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AI

NS

Falkland Islands

GalapagosIslands

'

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O c e a n

S o u t h

A t l a n t i c

O c e a n

S o u t h

P a c i f i c

O c e a n

Selected Agricultural Products of South America

The Amazon River drains the north and westernregion of Brazil. Although more water movesthrough the Amazon River than any other river inthe world, its location in dense tropical forests un-der a humid tropical climate makes agriculturethere a challenge. Amerindians of the region prac-tice subsistence agriculture, growing yams and ba-nanas and raising some small animals. Cassava, ormanioc (a root crop from which tapioca is made)and sugarcane are also grown there in slash-and-burn fashion.

The Paraná, Paraguay, and Uruguay Rivers,south of the Amazon lowlands, dissect a great grass-land region known as the Pampas and a forested re-gion known as the Gran Chaco, which extends tonorthern Argentina. The Pampas is not unlike theMidwest of North America. It is a hugegrassy plain nearly 400 miles (640 kilome-ters) long in the central part of Argentina.Agriculture is the primary industry in Ar-gentina and encompasses 60 percent ofthe country’s land. Large estancia (cattleranches) are common in the region. Thesoils are well suited to wheat and othergrains as well as alfalfa, a grass grown forcattle and horse feed. The level characterof the land makes it easy to work, but it isdifficult to drain and is prone to flooding.

Paraguay’s agricultural zones are di-vided by the Paraguay River. Tobacco,rice, and sugarcane grow to the east of theriver in the more humid climates. To thewest, in the drier climates and the Chacoregion, is the unique growing area of theQuebracho Forest. Quebracho is a hard-wood that grows only in the Chaco. Thewood contains tannin, which is used toproduce tannic acid, a chemical used fortanning leather. This area is suited for cat-tle, but because of its rocky and steep to-pography, the population of goats andsheep rises as one moves farther west to-ward the Andes. Irrigated with mountainstreams from the eastern slopes of the An-des, grapes also grow well and are madeinto wine.

Coastal LowlandsThe coastal lowlands, up to an altitude

of about 2,500 feet (750 meters), encom-pass an area known as the hot land (tierra

calienta). Temperatures there average between 75and 80 degrees Fahrenheit (22 and 24 degrees Cel-sius), and plantation agriculture abounds. Planta-tions are huge commercial farming operations thatgrow large quantities of crops that are usually soldfor export. Because of their easy access to port facili-ties, coastal lowlands historically have been linkedwith the markets of Europe and North America.

The banana is one of the best-known examples ofa plantation crop. It grows well in the wet, hot cli-mate of this zone and has been cultivated there forU.S. and European markets since 1866. In the1990’s, more bananas were traded on the interna-tional fruit market than any other commodity. InSouth America, Ecuador and Brazil are the leadingbanana producers, with Colombia third. Cacao, the

South American agriculture • 967P

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Farmers sell local produce at a market in Ecuador.

bean pods from which cocoa and chocolate aremade, is also grown on plantations in this zone. Thelargest producing area for cacao is Ghana in WestAfrica, but Brazil and Ecuador are fifth and sixth inworld yearly production.

Although sugarcane is grown in almost everycountry in South America, it does well as a planta-tion crop in the lowlands of eastern Brazil, theworld’s largest exporter. There the crop is not usedjust to produce sugar but also to produce ethanolfor gasohol, an alcohol-based gasoline that fuelsmore than half the automobiles in Brazil. This typeof commercial agriculture has made agriculturalbusiness the fastest-growing part of the Brazilianeconomy. Yams, cassava, and other root crops usedas staple foods also grow well in this humid, hot cli-mate.

Tierra TempladaJust above the calienta zone lies a zone of cooler

temperatures, the tierra templada, that extends toabout 6,000 feet (1,850 meters). Temperatures thererange from 65 to 75 degrees (17 to 22 degrees Cel-sius), and the commercial crop that dominates thelandscape is coffee. It is grown on large plantationscalled fazendas. Brazil, the largest South Americancoffee producer, exports about one-quarter of theworld’s coffee, producing nearly forty million bagsof about 132 pounds each (60 kilograms) annually.Colombia is South America’s second-largest coffeeproducer. Coffee was once the leading export fromColombia, but as a result of a coffee-worm infesta-tion and lower world prices, other products are tak-ing the lead.

Other commercial crops from this zone includefresh fruit. In the central valley of Chile, grape vine-yards and apple orchards have emerged. Produce

from this region enters stores in the United Statesand elsewhere as the growing seasons of the do-mestic products are finishing up. Chilean grapes,apples, peaches, and plums are now sold world-wide. Corn and wheat are also grown in this zone.These staple foods are produced for local consump-tion and sold only at local markets.

Tierra FriaA cooler climate, the tierra fria, extends from

6,000 feet (1,850 meters) to about 15,000 feet (4,570meters). Average temperatures there range from 55to 64 degrees Fahrenheit (12 to 17 degrees Celsius).This zone extends throughout the Andes and canmaintain only that plant life that can withstand thelimited soils and the cold conditions. In this regionof subsistence-type farming, crops and animals aregrown and raised mostly for family use.

In the lower reaches of this zone, barley growswell because it requires a short growing season. Inthe cooler portions of this zone, the potato began. Atuber, the potato can flourish in cold conditionswith moderate moisture. The loose soils of this zoneare perfect for its production, but it requires a con-siderable amount of cultivation. Although the po-tato is used throughout South America, potato pro-duction for the continent constitutes only about 5percent of the world total production. Alpacas, atype of goat, and llamas are also raised there.

M. Mustoe

See also: Central American agriculture; Corn; Etha-nol; Grains; Grasslands; Rain-forest biomes; Rainforests and the atmosphere; Rangeland; Rice; Sa-vannas and deciduous tropical forests; SouthAmerican flora; Wheat.

Sources for Further StudyCaufield, Catherine. In the Rainforest: Report from a Strange, Beautiful, Imperiled World. Chi-

cago: University of Chicago Press, 1991. A comprehensive study of the world’s rain for-ests looks at these threatened resources from historical, political, economical, and biologi-cal viewpoints. The afterword includes addresses of organizations working to save rainforests.

Grigg, David B. An Introduction to Agricultural Geography. 2d rev. ed. London: Routledge,1995. Provides a comprehensive introduction to agriculture in developing and developednations, describing both human and environmental issues. Covers the physical environ-ment, economic behavior and demands, institutional, social and cultural influences, andthe impact of farming upon the environment.

Stanfield, Michael Edward. Red Rubber, Bleeding Trees: Violence, Slavery, and Empire in North-west Amazonia, 1850-1933. Albuquerque: University of New Mexico, 1998. Explores the

968 • South American agriculture

complex transformation of northwestern Amazonia by the rubber boom from 1850 to1933. Examines the historical myths and realities of northwest Amazonia before its incor-poration and then shows how the Indians and environment were radically altered by therubber boom and international trade.

SOUTH AMERICAN FLORA


South America is the most diverse continent in terms of flora, primarily because of its location and geography.

South America’s floristic diversity is increased byits high mountains, especially the Andes Moun-

tains, which extend from north to south along thewestern part of the continent for much of its length.South America has such diverse biomes as tropicalrain forests, tropical savannas, extremely dry deserts,temperate forests, and alpine tundra. The largest ofthese biomes are deserts, savanna, and tropical for-est. With the rapid rate of deforestation in placeslike the Amazon basin, some plants may becomeextinct before being cataloged, let alone studied.

The subtropical desert biome is the driest biomein South America and is considered the driestdesert in the world, with an average annual precipi-tation of less than 0.25 inch (4 millimeters). Thedesert biome is restricted primarily to the westcoast of South America from less than 10 degreessouth of the equator to approximately 30 degreessouth. Dry conditions prevail from the coast to rela-tively high elevations in the Andes. The AtacamaDesert, in northern Chile, and the Patagoniandesert, in central Chile, are the most notable SouthAmerican deserts. Smaller desert regions also occurin the rain shadow portions of the Andes.

Next on the moisture scale are the savannabiomes. Savanna occurs in two distinctly differentareas of South America. The largest savanna regionincludes three distinctive regions: the cerrado; thePantanal; and farther south, in southern Brazil,Uruguay, and northern Argentina, the grasslandcalled the Pampas. The other savanna region, thellanos, is found in lower-elevation areas of Vene-zuela and Colombia.

Although a few of the forests in South Americaare dry, most are rain forests, receiving annual pre-cipitation from 79 inches to 118 inches (2,000-3,000

millimeters). The Amazon rain forest, the world’slargest, accounts for more than three-fourths of therain-forest area in South America. One of the mostspecies-rich areas of the world, it is being rapidlydestroyed by logging, ranching, and other humanactivities. Smaller rain forests are located along thesoutheastern coast of Brazil and in the northernpart of Venezuela.

Covering much smaller areas are a small mediter-ranean region in central Chile characterized by cool,wet winters and warm, dry summers. In the farsouth of Chile and Argentina is a small area of tem-perate forest, becoming alpine tundra in the farsouth. Temperatures are relatively cool and mildyear-round (except in the far south, where it can beextremely cold in the winter.

Plants of the Subtropical DesertIn the Atacama Desert, one of the world’s driest,

some moisture is available, but it is limited to cer-tain zones. Coastal regions below 3,280 feet (1,000meters) receive regular fog (called camanchacas).Rainfall is so low in the Atacama Desert that evencacti (which normally store water) can hardly ac-quire enough water from rainfall alone, so manyplants, including bromeliads, receive a portion oftheir water from the fog. At midelevation areasthere is no regular fog; thus, there is almost no plantcover. At higher elevations, the rising air has cooledsufficiently to produce moderate amounts of rain-fall, although the vegetation is still desertlike.Shrubs typically grow near streambeds, wheretheir roots can reach a permanent source of water.

The Atacama Desert often appears barren, butwhen a good dose of moisture becomes availableephemerals change its appearance, seemingly over-

South American flora • 969

night. Ephemerals are typically annuals that re-main dormant in the dry soil as seeds. When mois-ture increases, they quickly germinate, grow,flower, and set seed before dry conditions prevailagain. In the days and weeks following a good rain,many grasses appear and provide a backdrop forendless varieties of showy flowers, many endemicto (found only in the region of) the Atacama Desert.Among the showier flowers are species of Alstroe-meria (commonly called irises, although they are ac-tually in the lily family) and Nolana (called pansies,although they are members of a family found onlyin Chile and Peru).

Conditions in the Patagonian desertare less harsh. The vegetation rangesfrom tussock grasslands near the An-des to more of a shrub-steppe commu-nity farther east. Needlegrass is espe-cially abundant throughout Patagonia,and cacti are a common sight. In theshrub-steppe community in the easternPatagonian desert, the shrubs quilembaiand the cushionlike colapiche are com-mon. Where the soil is salty, saltbushand other salt-tolerant shrubs grow.

Plants of the Tropical Savanna BiomeThe cerrado region of east central

Brazil and southward is not only thelargest savanna biome of South Amer-ica but also one of the most romanti-cized of the world’s savannas. As in theOld West of North America, the grass-lands of Brazil have cowboys who tra-ditionally have used the cerrado forfarming and cattle ranching. With ever-increasing pressure from agriculture,the cerrado is now under attack in vari-ous ways. Extensive fertilization, asso-ciated with modern agriculture, plant-ing of trees for timber production, andthe introduction of foreign species, es-pecially African grasses, have all begunto change the cerrado. Frequent firesalso have taken their toll. The cerradocontains more than ten thousand spe-cies of plants, with 44 percent of themendemic. As much as 75 percent of thecerrado has been lost since 1965, andwhat remains is fragmented. Anumberof conservation groups are trying to

save as much as possible of what remains.Two other savanna regions farther south are the

Pantanal and the Pampas. Although the Pantanal isa savanna, during the rainy season it becomes a wet-land and is a haven for aquatic plants. Later, thePantanal dries out and grasslands appear in placeof the water. This unique area is under attack by avariety of human activities, including navigationand artificial drainage projects, mining, agricul-ture, and urban waste.

The Pampas, like the great prairies that once cov-ered central North America, is composed almostsolely of grass. Trees and shrubs grow near bodies

970 • South American floraP

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An aerial view of the rain forest in Guyana. In addition to rain forests,South America is home to flora of such diverse biomes as tropical sa-vannas, extremely dry deserts, temperate forests, and alpine tundra.

of water, but everywhere else grass predominates.Cattle ranching and wheat and corn farming are theprimary occupations of the area and are thus theprimary threat. Because the area is farther souththan the Pantanal, it has a more temperate climate.Pampas grass from this area has been exported asan ornamental plant.

The last major savanna region is the llanos, lo-cated at lower elevations in the drainage area of theOrinoco River in Venezuela and Colombia. Thisarea has pronounced wet and dry seasons. At thelowest elevations, treeless grasslands persist afterthe water from the rainy season subsides. On thehigher plains is a scattering of smaller trees. Themauritia palm can also be found here in poorlydrained areas.

Plants of the Tropical Forest BiomeThe Amazon rain forest is the largest contiguous

rain forest in the world. It is so large and so lushwith tree growth that its transpiration is actually re-sponsible, in part, for the wet climate of the region.Plant diversity is so great here that no comprehen-sive plant guide currently exists for many parts ofthe Amazon rain forest. Of tens of thousands ofplant species, a large number have never been de-scribed.

This one-of-a-kind botanical treasure is beingdestroyed at a rapid pace of between 5,000 and10,000 square miles (13,000-26,000 square kilome-ters) per year. The causes for this destruction areprimarily logging, agriculture, and cattle ranching.A common practice for preparing an area for cattleranching or farming has been to simply burn theforest, not even necessarily logging it first, and thento allow the grass and other vegetation or cropsgrow in its place. The soils of the rain forest are sopoor, however, that this practice usually depletesthe soil within a few years, and the land becomes auseless wasteland. Mining and oil drilling havealso taken their toll.

The Amazon rain forest is an extremely complexbiome. The main plant biomass is composed oftrees, which form a closed canopy that preventsmuch of the sunlight from reaching the forest floor.Consequently, the forest floor has little herbaceousgrowth, and most smaller plants tend to grow asepiphytes on the branches and trunks of trees. Com-mon epiphytes in the Amazon rain forest includeorchids, bromeliads, and even some cacti. There is alarge diversity of bromeliads, ranging from small,

inconspicuous species to larger species, such astank bromeliads that can collect significant amountsof water in their central whorl of leaves. The waterin these plants can contain a whole miniature eco-system, complete with mosquito larvae, aquaticinsects, and frogs. Ferns are another significantmember of the epiphyte community. Some largerspecies of ferns, often called tree ferns, also grow inthe understory. Lianas, or vines, are a prominentcomponent as well.

The trees that form the canopy are stratified intothree fairly discrete levels. The lowest two levels arethe most crowded, and the highest level comprisesextremely tall trees, often referred to as emergenttrees because they stand out randomly above thefairly continuous lower two layers. Many of thetallest trees are buttressed at the base, an adapta-tion that seems to give them greater stability. Be-neath the canopy, there are some smaller palms,shrubs, and ferns, but they only occur densely wherethere is a break in the canopy that allows in greaterlight.

Some rain-forest species of trees are well known,primarily because of their economic value. A favor-ite tree for use in furniture is the mahogany. Be-cause its wood is highly prized, many species ofmahogany are becoming rare or extinct. The SouthAmerican rain forests are also the original source ofrubber. Brazil had a monopoly on rubber untilseeds were smuggled out and planted in Malaysia.Synthetic rubber has now replaced natural rubberfor many applications. Another popular tree is theBrazil nut tree, an abundant food source that hasbeen exploited by suppliers of mixed nuts. The na-tive cacao tree produces fruits from which cacaobeans are extracted, then processed to make choco-late.

Every year during the rainy season, the lowestelevation areas of the Amazon rain forest areflooded with several feet of water, which recedes af-ter a few months. The trees flourish in this floodingcycle. A few even have unique adaptations, such asproducing fruits that are eaten by fish, thus assur-ing the spread of their seeds. The flooding can be soextensive in some areas that the water reaches thelower parts of the canopy.

Coastal tropical rain forests also occur in north-western and southeastern South America. Thereare a high number of endemic species in each ofthese forests. Some tree species are so rare that theymay be found in an area of only a few square miles

South American flora • 971

and nowhere else. Where the tropical rain forestmeets the ocean, mangrove trees have becomeadapted to the tidal environment. Mangroves haveprop roots, which make the trees look like they aregrowing on stilts. They also frequently have specialroot structures that extend above the water at hightide and allow the roots to breathe. Mangrove treesare also extremely salt-tolerant.

Plants of the Mediterranean and TemperateForest Biomes

One of the world’s five mediterranean climateregions is found in central Chile. This climate ischaracterized by warm, dry summers and cool, wetwinters. The vegetation, called matorral, is com-posed primarily of leathery-leaved, evergreenshrubs that are well adapted to the long summerdrought. The matorral is the only mediterraneanarea that has bromeliads. At lower elevation areas,somewhat inland, many of the shrubs are drought-deciduous; that is, they drop their leaves in thesummer. In more inland parts of this biome, theespino tree is common.

Because South America extends so far south, itactually has a small region containing temperateforests. These forests range from temperate rain for-est to drier temperate forest, and in all cases are typ-ically dominated by southern beeches. The under-growth is dominated by small evergreen trees andshrubs. Fuchsias, which are valued the world overfor their showy flowers, are common in the under-growth. Although not rich in species, the temperaterain forests of southern South America can be lush.In the far south, before the extreme climate restrictsthe vegetation to alpine tundra, a region of elfinwoodlands predominates. These woodlands can benearly impenetrable, with the densest growth oftenassociated with patches of tall bamboo.

Bryan Ness

See also: Deforestation; Deserts; Grasslands; Log-ging and clear-cutting; Mediterranean scrub; Rain-forest biomes; Rain forests and the atmosphere;Rangeland; Rice; Savannas and deciduous tropicalforests; South American agriculture; Sustainableforestry.

Sources for Further StudyGentry, Alwyn H., ed. Four Neotropical Rainforests. New Haven, Conn.: Yale University Press,

1993. A comparative study of four neotropical rain forests. Six sections describe the rain-forest sites and forest dynamics.

Gentry, Alwyn H., and Adrian G. Foryth. A Field Guide to the Families and Genera of WoodyPlants of Northwest South America: Colombia, Ecuador, Peru. Washington, D.C.: Conserva-tion International, 1996. Unlike many field guides, which rely for their identifications onflowers and fruits that are present only during certain seasons, this book focuses on char-acteristics such as bark, leaves, and odor that are present year-round. The guide is filledwith clear illustrations, step-by-step keys to identification, and a wealth of previously un-published data.

Kircher, John. A Neotropical Companion. 2d ed. Princeton, N.J.: Princeton University Press,1997. An introduction to the American tropics, the lands of Central and South America,their rain forests and other ecosystems, and the creatures that live there. Revised and ex-panded to incorporate the abundance of new scientific information that has been pro-duced since the book was first published in 1989.

Maxwell, Nicole. Witch-Doctor’s Apprentice: Hunting for Medicinal Plants in the Amazon. NewYork: Citadel Press, 1990. A memoir of Maxwell’s travels in the Amazon with the triplegoal of conserving the jungle, improving the lot of humanity, and having a great deal offun.

Plotkin, Mark J. Tales of a Shaman’s Apprentice: An Ethnobotanist Searches for New Medicines inthe Amazon Rain Forest. New York: Penguin Books, 1994. Plotkin’s account of wanderingthrough the Amazonian jungles focuses on local knowledge about plants, whose usesrange from the mundane to the magical. The rain forests of the world, Plotkin notes, are agreat natural resource and intercultural pharmacy of cures and therapies both knownand yet unvisited.

Rice, Richard E., Raymond E. Gullison, and John W. Reid. “Can Sustainable Management

972 • South American flora

Save Tropical Forests?” Scientific American 276, no. 4 (April, 1997): 44-49. In a quest to rec-oncile conservation with the production of tropical timber, a study finds that sustainabil-ity of the forests is problematic.

Young, Kenneth R., and Blanca Leon. “Nature on the Rebound.” Americas 51, no. 1 (January/February, 1999): 32-34. Botanical exploration in Rio Abiseo National Park is both difficultand rewarding; the beauties of nature in the park are discussed.

Zimmerer, Karl S. “The Ecogeography of Andean Potatoes.” Bioscience 48, no. 6 (June, 1998):445-454. Zimmerer discusses how ecogeography offers key insights about the viability ofagricultural biodiversity by analyzing the occurrence and adaptive nature of diversetypes of primary crops. The diversity and farming of Andean potatoes is studied as an ex-ample of food-plant ecogeography.

SPECIES AND SPECIATION

Categories: Classification and systematics; ecology; evolution

A species is any group of organisms recognized as distinct, with members able to interbreed with one another and pro-duce fertile offspring. Speciation is the evolutionary process whereby a species comes into existence.

Species are distinct kinds of organisms, in thatthe organism can be recognized as different

from other kinds of similar organisms by a combi-nation of characteristic shapes, sizes, behaviors,physiology, or other attributes. For instance, whiteoaks can be recognized as different from other oakspecies by growth form and habitat, along with acombination of leaf, fruit, and bark characteristics.To be useful as a diagnostic feature, a characteristicmust lend itself to measurement and remain rela-tively constant generation to generation. Such ahereditary pattern implies that members of a spe-cies share a common pool of genetic information.

Although most field guides and keys for speciesidentification are based largely on measurable mor-phological traits, the biological species concept, attrib-uted to Ernst Mayr in the 1940’s, defines speciesas groups of interbreeding populations reproduc-tively isolated from other such groups. Sometimesa greater range of variation can be observed inlarge, dispersed populations than is found betweensimilar but reproductively isolated species. What,then, determines when a species is formed?

Unfortunately there is no clear-cut answer. In-vestigators may use different criteria or assign vari-able levels of importance to characteristics; there-fore, a population of plants may be assigned speciesstatus by one expert and varietal status by another.

While this is confusing to those searching for aname, the problem is indicative of the dynamic andchanging nature of life. Because evolution is an on-going process, some groups of plants are expectedto be in transition.

The processes that contribute to variation inlarge, sexually reproducing populations also are re-sponsible for the origin of species. Isolation and se-lection of genetically based variation are the onlyadditional requirements. Speciation generally isconceived to involve the separation, isolation, anddivergence of a genetic pool of information. Plantsthat share a common pool of genetic informationare split into two pools that remain isolated untilidentifiable genetic differences accumulate. Clas-sifying a segment of this pool as a species requiresrecognition of significant genetic differences andthe relative isolation of the population.

Allopatric SpeciationThe physical separation of a gene pool into pop-

ulations that are geographically or spatially iso-lated is termed allopatric speciation. The physicalseparation could be the result of continental drift,uplift or subsidence of landmasses, glaciers, flood-ing, or radical dispersal of population members.Dispersal of a few fertile population members to anisolated island is a good example. The resulting

Species and speciation • 973

gene pool, although small, is immediately isolated.For example, if the fruiting inflorescence of a com-mon grass were transported on the struts of anairplane to a small, isolated island, any plants re-sulting from germination of those seeds would rep-resent a new population, now isolated from theparent population on the distant landmass wherethe seeds originated. A drastic change in gene fre-quency could, and likely would, result: A genepresent in one out of one thousand in the origi-nal population might, in the new, isolated locationand within the smaller gene pool, increase in fre-quency from 0.05 percent to 25 percent if presentin one of two plants forming the invading popula-tion. This sudden change in frequency is referredto as the founder’s effect. When such a population isintroduced into a new environment, it may rapidlychange and give rise to a number of additional newspecies. The latter process is known as adaptive radi-ation and is credited for the assemblage of uniquespecies often found in isolated areas.

Sympatric SpeciationIt is possible for segments of a parent gene pool

to diverge without spatial separation, ultimatelybecoming reproductively isolated. This process istermed sympatric speciation. Examples of situationsthat may lead to sympatric speciation include dis-ruption of pollination that could result from differ-ent flowering times among members of a popula-tion. Such differences could be caused by variationin soil, moisture, or exposure. One example of thisprocess is associated with the genus Achillea, ormilfoil. In the late 1940’s, investigators separatedclumps of two species of milfoil to produce geneti-cally identical clones that were subsequently trans-planted to different elevations in the Sierra Nevada.Plants grown during this process exhibited a widerange of morphological variation. Not only didthey look different; they flowered at different times.Thus, even genetically identical individuals of onespecies can be reproductively isolated if the timingof flowering precludes visitation by a commonpollinator.

PolyploidyGenetic variation can occur rapidly through an

increase in chromosome number. Although recom-bination of genes brought about by sexual repro-duction provides the greatest amount of the ob-servable variation in a population, abrupt and

large-scale change is also associated with a processcalled polyploidy. Through polyploidy, a plant canhave its entire set of chromosomes multiplied. If theincrease in chromosome number is brought aboutwhen chromosomes fail to separate after they du-plicate during meiosis, an individual’s number ofchromosomes may double. That process is knownas autopolyploidy. An increase in chromosome num-ber associated with hybridization resulting fromcombination of two separate sets of chromosomesis termed allopolyploidy. Although typically sterile,an allopolyploid may duplicate its combined set ofchromosomes, resulting in a fertile autoallopoly-ploid.

Although polyploidy occurs naturally in manyplants, the results so frequently are associated withdesirable changes in the bloom of ornamentals thatpolyploidy is often deliberately induced by chemi-cal treatment. It is estimated that one-half or moreof all flowering plant species may have arisenthrough some form of polyploidy.

HybridizationOffspring produced from the interbreeding of

related species, hybridization, may be either sterileor fertile. If the offspring are sterile, the geneticpools of parental species are not altered, becausegenes are not able to flow between the two. If theoffspring are fertile, as are hybrids of crosses be-tween North American and European species ofsycamores, interbreeding among fertile hybridsand parent populations can provide a bridge for themerging of genetic information. With time, the flowof such genetic material can lead to an increase invariation, a decrease in interspecific differences,and an interesting taxonomic problem as to whenthe two parent species should be reclassified as one.Currently geographic isolation maintains the ge-netic integrity of these parental species.

Recombination SpeciationAlthough the original source of variation is per-

manent change in DNA(mutation), recombination ofgenetic material through sexual reproduction ac-counts for the vast majority of variation betweengenerations. The result of this variation, acted uponby natural selection, leads to changes in the fre-quencies of genes within a population. The latter isoften stated as a definition of evolution. Rapid se-lection of recombinants has been proposed as themethod of speciation by which the anomalous sun-

974 • Species and speciation

flower, Helianthus anomalus, was produced fromhybrids of two other species of sunflower, Helian-thus annulus and Helianthus petiolaris. Studies, infact, have duplicated the proposed process. Afterseveral generations of natural selective pressure,experimentally produced hybrids of Helianthus an-nulus and Helianthus petiolaris were demonstratedto be the genetic equivalents of naturally occurringHelianthus anomalus.

Sterile HybridsPlant hybrids often are sterile because newly

combined hereditary material is so different thatchromosomes will not pair during meiosis. Viablegametes, therefore, are not produced. This appar-ent genetic blind alley does not always translateinto a lack of evolutionary success, however. Manysterile hybrids demonstrate an ability to reproduceapomictically, meaning they reproduce vegetativelyrather than sexually. Species of blackberry, aspen,and many grasses reproduce by apomictic meth-ods. Some, like dandelion, may have populationsthat are fertile and populations of infertile hybridsthat produce seed with embryos that were pro-duced asexually. Apomixis obviously does not pro-mote variation, but it may permit expansion ofpopulations where there is little chance for cross-pollination, as in areas of high disturbance or envi-ronmental stress.

Reproductive IsolationOnce a gene pool has separated and diverged

into populations identifiable as separate species bygenetic or morphological traits, the resulting seg-ments of the pool must remain separated. As longas they are separate and distinct, populations canbe labeled as separate species.

Barriers that prevent the flow of hereditary ma-terial may be classed as prezygotic or postzygotic.Prezygotic mechanisms prevent successful fertiliza-tion and include geographic isolation, temporalisolation (flowering at different times), mechanicalisolation (flowers structurally different), and in-compatible gametes. Postzygotic mechanisms pre-vent production of fertile adults and include hybridinviability (offspring that do not live to sexual ma-turity), hybrid sterility, and offspring of hybridsthat are weak.

John F. Logue

See also: Adaptations; Angiosperm evolution; Co-evolution; Evolution: convergent and divergent;Evolution: gradualism vs. punctuated equilibrium;Evolution of plants; Gene flow; Genetic drift; Ge-netics: mutations; Hybrid zones; Hybridization;Nonrandom mating; Population genetics; Repro-ductive isolating mechanisms; Selection; Systemat-ics and taxonomy; Systematics: overview.

Sources for Further StudyMoore, Randy, et al. Botany. 2d ed. New York: WCB/McGraw Hill, 1998. This basic textbook

devotes an entire chapter to speciation. The preceding chapter on evolution providesgood background. It is well organized and contains references to Internet sites, suggestedreadings, ample photographs, diagrams, and color drawings.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. The chapter on evolution and speciation contains examples,diagrams, photographs, and clear illustrations.

Stearns, Stephen C., and Rolf F. Hoekstra. Evolution: An Introduction. New York: Oxford Uni-versity Press, 2000. Provides a separate chapter on speciation plus many cross-referencedsections in the index. Easy to understand for most introductory students, with examplesand illustrations as well as an excellent bibliographical reference section.

Townsend, Colin R., et al. Essentials of Ecology. Malden, Mass.: Blackwell Science, 2000. Ageneral ecology textbook that includes good sections on speciation and evolution withinspecies. Well written text does not require a technical background. Plant and animal ex-amples are used.

Species and speciation • 975

SPERMATOPHYTA

Categories: Plantae; taxonomic groups

The Spermatophyta are those members of the Tracheobionta that produce seeds. At one time this category was consid-ered to be natural, composed only of closely related groups with homologous seeds. It is now known that seeds evolvedmany different times during evolutionary history and that different groups of extant seed plants may or may not beclosely related to other groups of extinct or extant spermatophytes.

Scientists once thought that the seed plants weremonophyletic, derived from a single common

ancestor. It is now known they are polyphyletic, de-rived from more than one ancestral group. The dis-tinctions can be seen in the variations among seedstructures and functions.

Seed StructureA seed is a complex reproductive structure com-

posed of multiple generations of tissue that de-velop from an ovule. The outer layer of the seed isusually composed of thick-walled cells of the pa-rental sporophyte that form a seed coat. Internal tothe seed coat is typically a layer of stored food,which may consist of megagametophyte tissue or of aunique tissue of a new generation called endosperm.At the center of the seed is a new embryonic plantthat emerges to become the seedling when the seedgerminates. Scientists recognize two basic typesof seed plants: gymnosperms and angiosperms.Gymnosperms are plants with “naked seeds.” Thatis, the ovule is exposed to the environment and isnot covered by a protective layer. In angiosperms,by contrast, the ovule is contained within a special-ized structure, the carpel.

The outer layer of a seed, the seed coat, developsfrom the outer layers of the ovary, the integument.The integument is composed of sporophyte tissueof the parent plant. As the integument develops, itforms an enlarging sheath around the central ovuletissue, the nucellus. The integument does not com-pletely enclose the nucellus but leaves a small pore,the micropyle, through which pollen grains or thepollen tube will move prior to fertilization. Amegaspore mother cell within the nucellus undergoesmeiosis to produce four megaspores. In gymno-sperms, only one megaspore is functional, and it

develops into a female gametophyte containingarchegonia. In angiosperms, one, two, or all four ofthe megaspore nuclei may contribute to the femalegametophyte.

In gymnosperms, a single egg forms within eacharchegonium. Pollen grains are drawn into themicropyle, and the pollen germinates to form a pol-len tube. In most gymnosperms, the pollen tubegrows through the nucellus until it reaches thearchegonium, where the sperm are released to fer-tilize the egg. The fertilized egg, or zygote, growsinto an embryo, a new sporophyte generation.Meanwhile, the integument enlarges to close themicropyle and hardens to form the seed coat. Theremaining tissue of the female gametophyte func-tions as a store of food to nourish the embryo untilthe seed germinates. Because multiple archegoniaare formed in the female gametophyte, more thanone egg may be fertilized within a single ovule, butusually only a single embryo grows to full size.

In angiosperms the entire female gametophyteusually consists of a seven-celled embryo sac. Threecells, the egg and two synergids, lie near themicropyle. Three cells, the antipodals, form on theend opposite the micropyle. The remaining centralcell contains two nuclei, the polar nuclei. Pollen ger-minates on the outside of the carpel in a specializedregion called the stigma. The pollen tube growsdown through the carpel tissue toward the ovulesand eventually grows into the micropyle andthrough the nucellus of one ovule until it reaches asynergid. The sperm nuclei are discharged into thesynergid cell, which subsequently degenerates.One sperm goes on to fertilize the egg, while thesecond fertilizes both polar nuclei. The fertilizedegg develops into a diploid embryo in a mannersimilar to that of gymnosperms. The product of the

976 • Spermatophyta

other sperm and the two polar nuclei is a triploidcell, the primary endosperm. The endosperm cellalso undergoes mitosis to form a multicellulartriploid endosperm tissue, which functions directlyas a stored food reserve in monocots. In dicots, theendosperm typically degenerates, and the nutri-ents are absorbed by the cotyledons of the embryo,which then serve as a food reserve.

Seed FunctionReproduction by means of seeds confers a num-

ber of advantages to seed plants for colonizingterrestrial habitats. First, the seed coat providesphysical protection from predators and from dehy-dration. In many cases seeds, protected by the seedcoat, pass unharmed through the digestive tracts ofanimals who have eaten them. In fact, the physicalabrasion of chewing and stomach acid action asso-ciated with passage through an animal’s digestivetract may be necessary to soften the coat and permitseed germination. The seed coat may also promotedormancy through chemical inhibition or by limit-ing water uptake. In this way the seed coat helps theseed survive periods of environmental stress, suchas dry or cold seasons during which plant growth

cannot occur. During the period of dormancy,seeds can be dispersed to new areas by a number ofmeans, such as lofting through the air, floating onwater, or being transported in an animal’s digestivetract or on its body.

The seed will germinate only when conditions aresuitable for growth. Stored food provides a nutrientsupply to support early growth of the seedling.Most seeds germinate belowground, where the en-vironment is moister and the temperature moreuniform than on the soil surface. Thus early growthoccurs before the seedling is exposed to light and isable to photosynthesize. Finally, the typical embryoin the seed has already formed root and shootprimordia, which are ready to function as soon asthey absorb water and begin to grow.


See also: Angiosperm evolution; Angiosperm lifecycle; Angiosperm plant formation; Angiosperms;Conifers; Cycads and palms; Dormancy; Flowerstructure; Flower types; Germination and seedlingdevelopment; Ginkgos; Gnetophytes; Gymno-sperms; Plant life spans; Pollination; Reproductionin plants; Roots; Seeds; Stems; Tracheobionta.

Sources for Further StudyBell, Peter, and Alan Hemsley. Green Plants: Their Origin and Diversity. 2d ed. New York:

Cambridge University Press, 2000. Includes a current interpretation of the phylogeneticrelationships of all the green plants, including the vascular plants.

Foster, Adriance S., and Ernest M Gifford, Jr. Morphology and Evolution of Vascular Plants.New York: W. H. Freeman, 1996. This remains the classic text on the comparative struc-ture of vascular plants.

SPICES

Categories: Economic botany and plant uses; food

Spices are a group of plant products used to impart flavor to foods. Unlike herbs, spices are generally derived not from theleafy and other green portions of the plant, but rather from the seeds, fruits, flower parts, bark, or rhizomes. Spices arealso generally distinguished from herbs by the greater strength, intensity, or pungency of their flavors.

For thousands of years, humans have addedspices and herbs to their foods in order to im-

prove taste. Before modern refrigeration and can-ning, the first signs of spoiling could be masked byadding parts of aromatic plants—spices and herbs.

In modern times, spices and herbs are added onlyto enhance the flavor of foods.

Spices are distinguished from herbs based uponwhat part of a plant is used and the way it is pre-pared. Generally spices are flavorings from dried

Spices • 977

seeds, fruits, or flower parts. A few spices are fromdried bark and rhizomes. Spices can be used as in-tact dried fruits or seeds, but most often they areground into a powder used in food preparation.Herbs, by contrast, are typically flavorings fromleaves or other green parts of plants. Spices areused in a wide variety of dishes, from sweet to sa-vory: desserts, breads, pickles, fruits, meats, andvegetables. Different spices are characteristic of dif-ferent ethnic and regional cuisines.

In the late twentieth century, with the rise inglobal communications and travel as well as massmovement of migrant populations, the “typical”spices of North American cuisine increased and be-came more diverse, particularly in regions wheremany different cultures came together.

Allspice, from the myrtle family, is preparedfrom the dried fruit of a West Indian tree, Pimentadioica. Whole allspice is used in preparing picklesand relishes and to flavor meats such as pot roast;the powder is used in cakes, cookies, pies, andmincemeat.

Anise seeds, from the celery family, come fromthe herbaceous plant anise, Pimpinella anisum, fromthe Mediterranean region. Whole anise seeds areused as flavorings in stews, pot roasts, and someChinese meat dishes. Crushed or powdered aniseseeds are used as flavorings in Italian cookies,cakes, and liqueurs, as well as German breads.

Cardamom, from the ginger family, is preparedfrom the seed pods of large herbaceous cardamomplants, Elettaria cardamonum, native to India. Thedried pods are crushed and used in many Indianand African meat and vegetable dishes, in ricedishes, and in Scandinavian breads.

Cinnamon, from the laurel family, is made fromthe dried bark of several species of small tropicaltrees in the genus Cinnamomum. The bark is cut into3- or 4-inch lengths and is permitted to curl. Thesedried “cinnamon sticks” often flavor hot cider andother beverages as well as meat stocks. Ground cin-namon is widely used to flavor cookies, pies, cakes,sweet rolls, and candies and is added to Mexicanmeat dishes.

Cloves, from the myrtle family, are the dried, un-opened flower buds of a tropical tree, Eugeniacaryophyllata, possibly native to the Moluccas Is-lands. The dried buds are ground for use in cookies,gingerbread, pies, and cakes. They are used wholein pickle brines or to flavor ham and other meats.

Coriander, from the celery family, comes from

the dried seeds of the herbaceous plant Coriandrumsativum, a Mediterranean native. The seeds areused either whole in pickles and preserves orground in cookies and puddings.

Cumin, from the celery family, comes from thedried seeds of the herbaceous plant Cuminumcynimum, also native to the Mediterranean region.Powdered cumin is widely used to flavor stews,fish and meat dishes, curries, and Mexican dishes,and it is one of the main ingredients of commercialchili powder.

Ginger, from the ginger family, is prepared fromthe thick, underground rhizomes (often called gin-ger root) of a large perennial, herbaceous plant,Zingiber officinale, native to islands of the Pacific.Powdered ginger, from the dried rhizome, iswidely used in cookies, gingerbread, and otherbaked goods. Fresh ginger rhizome is used in manyAsian dishes, and sweetened crystallized ginger isused in pickles or is eaten as candy.

Mace and nutmeg, from the Myristicaceae family,are two spices that come from the seeds of the nut-meg tree, Myristica fragrans, native to the MoluccasIslands. The hard seed has a covering of strips ofreddish material; dried and ground, that material iscalled mace. The rest of the seed becomes nutmeg.Both mace and nutmeg are used in cakes, cookies,pies, and puddings.

Mustard, from the mustard family, comes fromthe dried seeds of herbaceous mustard plants. Bras-sica sinapis is the most widely used species. Mustardhas been used in cooking for a very long time; its na-tive origins are unknown. Ground to a powder,mustard is used in sauces and meat dishes, andwhole mustard seeds are used in pickles.

Paprika, from the nightshade family, is a spiceprepared from the dried fruits of a sweet pepper,Capsicum annuum, a native of Central America. Pa-prika is used to flavor a variety of meat, vegetable,egg, and cheese dishes.

Black and white pepper, from the pepper treefamily, are both produced from the dried fruits(peppercorns) of Piper nigrum shrubs, native to In-dia. Black pepper is made by picking and dryingthe unripe fruits. White pepper is prepared fromriper fruits that are soaked in water. After the outerfruit wall is rubbed off, a smooth white seed re-mains. The dried peppercorns are either ground orshipped whole to be ground at the time of use. Pep-per is used in all sorts of meat, cheese, vegetable,and salad dishes.

978 • Spices

Red pepper, from the nightshade family, is aspice made by grinding the dried fruits of any ofseveral species of hot peppers or chilies of the genusCapsicum, native to tropical regions of the Ameri-cas. Capsicum frutescens produces cayenne pepper.Ground or crushed, the peppers are used to flavormeat and vegetable dishes characteristic of Mexi-can, Caribbean, Indian, Chinese, Thai, and manyother cuisines.

Saffron, from the iris family, is the most expen-sive spice, made from the stigmas of the flowers of acrocus plant, Crocus saturis, native to southern Eu-rope and Asia. On top of the female part of theflower, the stigmas consist of three slender fila-

ments; it takes about seventy-five thousand driedstigmas to make one pound of saffron, and eachmust be picked by hand. Used intact or ground, saf-fron adds a deep yellow color and a slightly bittertaste to breads, sauces, soups, meat, and rice dishes.Turmeric (Ginger family) is prepared from thedried rhizome of a perennial herbaceous plant,Curcuma longa, native to the East Indies. Turmericgives color and a slightly sweet flavor to pickles,cakes, cookies, salad dressings, sauces and meatdishes.

John P. Shontz

See also: Herbs.

Sources for Further StudyMiloradovich, M. The Art of Cooking with Herbs and Spices: A Handbook of Flavors and Savors.

Garden City, N.Y.: Doubleday, 1950. Includes a history of spices and herbs as well asrecipes.

Spices • 979P

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Red pepper, from the nightshade family, is made into a spice by grinding the dried fruits of any of several species of thegenus Capsicum, native to tropical regions of the Americas. Peppers are used to flavor meat and vegetable dishes char-acteristic of Mexican, Caribbean, Indian, Chinese, Thai, and many other cuisines.

Morris, S., and L. Mackley. The Spice Book. Singapore: Southwater, an Imprint of Anness Pub-lishing, 2000. Descriptions and uses of a wide variety of spices.

Tighe, E. Woman’s Day Encyclopedia of Cookery. 12 vols. New York: Fawcett Publications, 1965.Each of the common spices is described, and several recipes using that spice are includedwith the description.

Walker, H., ed. Spicing up the Palate: Studies of Flavourings, Ancient and Modern. Proceedings ofthe Oxford Symposium of Food and Cookery 1992. Totnes, Devon, England: ProspectBooks, 1993. A series of essays about the history of spices in cooking, treating where,when, how, and why spices are grown.

STEMS

Category: Anatomy

The stem is the part of a vascular plant that supports the leaves and reproductive structures, often at some distance abovethe ground.

Stems have two chief functions: to support vari-ous plant structures and to transport water and

nutrients. Most stems support the leaves and repro-ductive organs at some distance above the ground.Such elevation is important to both leaves, whosefunction is to trap light energy for photosynthesis,and flowers, because being positioned above theground helps attract pollinators. Fruits also benefitfrom being held aloft because height improves thechances of long-distance seed dispersal.

While being off the ground has advantages forshoot-borne organs, it also has one big disadvan-tage. These organs are far from their source ofwater, the soil, and therefore run the risk of dryingout unless adequate water can be supplied. Thereinlies the second function of stems: to conduct a sup-ply of water from the roots to the organs. Stems alsoconduct nutrients in both directions between theroots and organs of the plant.

StructureUnlike roots, which branch at irregular intervals

and do not bear organs such as leaves or reproduc-tive structures, stems have a definite pattern of or-ganization that consists of alternating nodes andinternodes. Each node is a point on a stem at whichan organ is attached. Adjacent nodes are separatedby an internode, a zone that lacks any attached or-

gans. The part of the stem closest to the ground iscalled the proximal end, while the part farthest awayis called the distal end. In many plants, internodesat the proximal end are the longest. Toward the dis-tal end, the internodes tend to be progressivelyshorter.

An actively growing stem has an apical meristemat its extreme distal end. In this region, new stemtissue is produced. In the apical meristem, cells ac-tively divide and elongate, causing the stem to be-come longer. That elongation process is called pri-mary growth. Immature leaves are also produced atthe apical meristem. Immediately after the newcells enlarge, they undergo differentiation to formprimary tissue.

Newly formed stems tend to be soft, because thewalls of the young cells are thin. Moreover, thestems are typically green because of the presence ofphotosynthesizing cells on the periphery. Such soft,green stems are termed herbaceous. These herbaceousstems often turn hard and woody through time.

At each node, the angle that is formed when aleaf attaches to the stem is called an axil. At thatjuncture is found an embryonic shoot, called anaxillary bud. Normally, axillary buds remain dor-mant for some time. Eventually, however, manybuds break dormancy and elongate to form a lateralshoot, called a branch.

980 • Stems

Different species of plants have different pat-terns of leaf arrangement. Most plants display analternate arrangement, in which only one leaf isinserted at a node. Plants with alternate leaves in-clude oak trees, goldenrod, geraniums, and lilies.Plants with two leaves at a node, in what is calledan opposite arrangement, include maples, lilacs,phlox, and mints. Finally, many plants, such astrillium, catalpa, and bedstraw, have three or moreleaves at a node, in what is called a whorled ar-rangement.

Some plants exhibit very little internodal devel-opment. The result is that the plant consists of acluster of leaves right at the soil surface. These arecalled rosette plants and are represented by dande-lion, hawkweed, and plantain.

Woody PlantsSome plants produce an herbaceous stem that

will develop a secondary growth of hard, woodytissue. These woody plants can be categorized astrees, which have only a single main stem emerg-ing from the ground, or as shrubs, which have sev-eral stems coming from the ground. The devel-opment of woody tissue from herbaceous tissueresults from a process called lignification, in whichthe walls of cells inside the stem become thick andhard because of the presence of a material calledlignin.

The lignified inner cells of the stem are collec-tively called wood. Lignification causes the stem tobecome more rigid, allowing it to support moreweight. In temperate climates with pronouncedcold winters, an herbaceous stem becomes lignifiedduring its first winter. In more moderate climates,stems become lignified gradually. In either case, theoutside of the stem becomes covered with water-impermeable bark.

Woody plants that grow in temperate areas varyin their rates of growth from one season to another.Stems grow most rapidly during the spring andsummer. The rate of growth declines markedly atthe end of summer and during fall. Little, if any,growth occurs during winter.

Woody plants, especially deciduous ones, have afew features that are lacking in herbaceous plants,which have unprotected stems. First, when a leafdrops off a woody plant at the end of the grow-ing season, it leaves a scar on the stem. The shapesof leaf scars vary from one species to another;some are circular, others are triangular, still oth-

ers look like small lines. A second feature found ona woody plant is a terminal bud. That structureusually forms when the apical meristem stops ac-tively growing at the end of the summer. The ter-minal bud is typically dormant during the winterand is often enclosed by one or more hard, modi-fied leaves called terminal bud scales. Those scalesprotect the bud against injury by cold and preda-tors.

When warm weather or abundant rainfall re-turns, the bud breaks its dormancy, and a new stemis produced. As the young shoots begin to elongate,the bud scales fall off, resulting in scars that encirclethe stem. Thus, the bud scales mark the juncture be-tween the growth that occurs in two successiveyears. Because a new set of bud scale scars is pro-duced at the beginning of every growing season,one can tell the age of a stem by counting the num-ber of sets of bud-scale scars from the base to theextreme distal end.

Finally, the bark that encircles woody stems doesnot allow air to pass between the inside of the stemand the outside. That poses a problem for the cellsimmediately under the bark, which need oxygento survive. To help overcome this problem, manywoody stems contain raised corky dots calledlenticels scattered over the surface. The cells of thelenticels are spongy and allow air to diffuse to theliving cells beneath.

Modified StemsMany plants have stems that are horizontal in-

stead of erect. Stolons, also called runners, are hori-zontal stems that lie above the soil surface. Stolonsare found on strawberry plants, for example. Rhi-zomes are horizontal stems that lie below the soilsurface. Ferns, irises, milkweeds, and goldenrodsproduce rhizomes. Both stolons and rhizomes oftenproduce adventitious roots, which are roots that formalong a stem or anywhere else roots typically do notgrow. Tubers are thickened rhizomes that storestarch. The potato is probably the best-known tu-ber. Interestingly, plants such as kohlrabi havethickened tuberlike stems that are borne aboveground.

A bulb is a budlike modified stem that has sets ofthick leaves closely overlapping one another (as anonion does). A corm is similar to a bulb except thatthe leaves are thin and papery, and they surround astem that is short and fleshy. Gladiolus and crocusboth produce corms.

Stems • 981

Some plants, especially those that grow in desertregions, have thick, succulent stems that serve tostore and conserve water. The stem is either padlike(such as that of a prickly-pear cactus) or barrel-shaped (such as that of a saguaro cactus) and is typ-ically covered by a thick layer of green photosyn-thesizing cells and sharp spines. Asparagus and afew other plants produce stems called cladophylls,which are flattened, highly branched, and capableof photosynthesis.

Some branch stems, such as those of hawthorn,are modified to form sharp structures called thorns.In contrast, the sharp structures on the surfaces ofrose and blackberry stems, commonly referred to asthorns, are actually prickles, while sharpened leavessuch as those of holly are called spines. Finally, somewoody plants, such as birches, produce stubby sidebranches called spur shoots that grow only a fewmillimeters each year.

GrowthThree types of primary tissue form the stem’s in-

terior: dermal tissue, vascular tissue, and ground tis-sue. Dermal tissue forms a thin layer surroundingthe herbaceous stem and protects it from dryingout. Vascular tissue is composed of xylem andphloem and serves to conduct water, minerals, andorganic substances throughout the plant. In mostplants, it forms a ring within the stem. Woodyplants have a ring of cells called the vascular cam-bium located within the vascular tissue. The vascu-lar cambium actively divides during the growingseason, producing wood to the inside and bark tothe outside, and causes the stem to increase inwidth. Finally, ground tissue has two main func-tions: photosynthesis and storage. It is found bothat the extreme core of the stem and in a ring imme-diately inside the dermal tissue but outside the vas-cular tissue.

Many plants vary from these basic patterns. Forexample, in plants such as lilies and grasses, thecells of the vascular tissue occur in small clustersthat are interspersed with the ground tissue. In

woody plants, the vascular tissue makes up thebulk of the stem.

Many internal (hormonal) and external (envi-ronmental) factors affect stem growth. At least twoclasses of hormones appear to increase the growthof stems when applied to the plant: auxins and gib-berellins. Auxins cause individual cells to becomelonger, whereas gibberellins stimulate both cell di-vision and elongation. Gibberellins appear to beimportant in the life cycles of biennials, which areplants that typically spend their first year as a ro-sette and then flower in their second year. Beforeflowering, the second-year plant must produce anerect flowering stem, a process called bolting. Whena biennial bolts, the new internodes are long ratherthan short, and gibberellins appear to cause thiselongation.

Many environmental factors can influence stemgrowth, but light and physical damage to the apicalmeristem are perhaps the most dramatic. Plantsthat grow in light that comes from one side (insteadof from the top) tend to bend toward the light, a pro-cess called phototropism. Plants that grow in com-plete darkness tend to have stems that are abnor-mally long and thin and are yellow in color. Suchstems are said to be etiolated.

Finally, when the apical meristem is intact, lat-eral buds typically remain dormant. When, how-ever, the meristem is removed by herbivory or clip-ping, at least one of the lateral buds is released fromdormancy, producing a branch that then becomesthe new meristem. The process by which an intactapical meristem inhibits the growth of the laterals istermed apical dominance. Research has shown thatphototropism, etiolation, and apical dominance aremediated through the hormone auxin.

Kenneth M. Klemow

See also: Bulbs and rhizomes; Cacti and succulents;Dormancy; Flowering regulation; Garden plants:shrubs; Growth and growth control; Growth habits;Hormones; Leaf anatomy; Leaf arrangements;Plant tissues; Roots; Shoots; Wood.

Sources for Further StudyBrowse, Philip McMillan. Plant Propagation. New York: Simon & Schuster, 1988. This spiral-

bound book guides the nonspecialist through the various techniques of plant propaga-tion with a detailed, step-by-step approach supported by clear illustrations. Numeroustechniques for layerage, grafting, and making cuttings are clearly explained.

Gartner, Barbara L., ed. Plant Stems: Physiology and Functional Morphology. San Diego: Aca-demic Press, 1995. Chapter topics include opportunities and constraints in the placement

982 • Stems

of flowers and fruits; stems and fires; and chemical defense against herbivores. Mostlyblack-and-white photographs, with two pages of color plates.

Kaufman, Peter B. Plants: Their Biology and Importance. New York: Harper & Row, 1989. Thisbook was written to introduce nonbiology students to the basic biology and economic as-pects of plants. Chapters 3-5 describe the basic structures of stems and their modifica-tions. Chapter 26 discusses the environmental and hormonal factors that regulate the de-velopment of stems and other plant organs.

Raven, Peter, Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York:W. H. Freeman/Worth, 1999. An excellent introduction to all phases of modern botanicalscience; includes sections on cell biology, energetics, genetics, diversity, structure, evolu-tion, and ecology. The external and internal structure of the shoot is detailed. Also con-tains an authoritative review of the roles that various external factors and hormones playin stem growth.

Stern, Kingsley R. Introductory Plant Biology. 8th ed. Boston: McGraw-Hill, 2000. A fine gen-eral text covering all phases of plant science, including stems, with special attention paidto woody stems and their commercial use as lumber and fiber. The process of stem growthand how it is influenced by the environment is also covered.

STRIP FARMING


Strip farming is the growing of crops in narrow, systematic strips or bands to reduce soil erosion from wind and waterand otherwise improve agricultural production.

The origins of strip farming can be traced to theenclosure movement of postmedieval Great Brit-

ain. Landlords consolidated the small, fragmentedstrips of land farmed by tenant peasants into largeblock fields in an effort to increase agricultural pro-duction. Peasant plots were typically 1 acre in size:220 yards, or one furlong in length (the distance ateam of oxen can plow before resting) and 22 yardsin width (the amount one team of oxen can plow inone day). After enclosure, fields were 100 or moreacres in size. Larger fields were more productivebut were also more exposed to wind and water ero-sion and nutritional exhaustion.

As agricultural production gradually shifted tonew lands in the Americas and colonial Africa,farmers continued to use large-field farming tech-niques and developed large-field plantations. Bythe early twentieth century, all readily tilled landshad been opened by the plow and were sufferingthe effects of water and wind erosion. Strip farm-ing, also known as strip cropping, was developed

as a soil conservation measure during the 1930’s.During the 1960’s strip farming became an impor-tant tool to prevent water and air pollution and im-prove wildlife habitat.

Agricultural HazardsWind erosion begins when wind velocity at 1

foot (0.3 meter) above soil level increases beyond13 miles (21 kilometers) per hour. Saltation and sur-face creep also allow soil to move. In saltation, smallparticles are lifted off the surface. These articlestravel ten to fifteen times the height to which theyare lifted, then spin downward with sufficient forceto dislodge other soil particles and break earthclods into smaller particles. Surface creep occurswhen particles too small to be lifted move along thesurface in a rolling motion. The wider the field, thegreater the cumulative effect of saltation and sur-face creep. These factors can lead to an avalanche ofsoil particles across the widest fields even duringmoderate wind gusts.

Strip farming • 983

Water erosion begins when raindrops or flowingwater suspends soil particles above the surface andtransports them downslope by splash or runoff. Icecrystals expand, then contract when melted, dis-lodging soil particles and making them availablefor both water and wind erosion. Water also leachesnutrients and chemicals from the soil, causing thesoil to experience both nutrient loss and an increasein salts and acids.

The U.S. Department of Agriculture computesannual soil loss from agricultural and developedland using the formula A = RKLSCP. In this for-mula, A equals annual soil loss, R equals theamount of rainfall on the plot, K equals the erosionfactor for the type of soil on the plot, L equals thelength of the slope on which the plot is located, Sequals the angle of the slope, C equals the type ofcrop or soil cover on the plot, and P equals the pres-ence of management conservation practices such asbuffers, terraces, and strip farming. Soil loss toler-ances are calculated for each plot. The tolerance isthe amount of soil that can be lost without reducingproductivity. Loss tolerances range from 1 to 5 tonsper acre per year. Farmers and developers reducesoil losses to tolerance levels by reducing soil expo-sure to wind and rain and by utilizing conservationpractices, such as strip farming.

Alleviating HazardsStrip farming reduces field width, thus reducing

erosion. Large fields are subdivided into narrow,cultivated strips. Planting crops along the contourlines around hills is called contour strip cropping.Planting crops in strips across the top of predomi-nant slopes is called field stripping. Crops are ar-ranged so that a strip of hay or sod (such as grass,clover, or alfalfa) or a strip of close-growing smallgrain (such as wheat or oats) is alternated with a

strip of cultivated row crop (such as tobacco, cot-ton, or corn). Rainwater runoff or blown dust fromthe row-crop strip is trapped in this way as it passesthrough the subsequent strip of hay or grain, thusreducing soil erosion and pollution of waterways.Contour or field strip cropping can reduce soil ero-sion by 65 to 75 percent on a 3 to 8 percent slope.

Cropping in each strip is usually rotated eachyear. In a typical four-strip field, each strip will becultivated with a cover crop for one or two years,grain for one year, and row-crop planting for oneyear. Each strip benefits from one or two years of ni-trogen replenishment from nitrogen-fixing covercrops, such as alfalfa, and each strip benefits fromone year of absorbing nutrient and fertilizer runofffrom the adjacent row-crop strip.

Strip widths are determined by the slope of theland: the greater the slope, the narrower the strips.In areas of high wind, the greater the average windvelocity, the narrower the strips. The number ofgrass or small-grain strips must be equal to orgreater than the number of row-cropped strips.

Terraces are often constructed to reduce the slopeof agricultural land. At least one-half of the land be-tween each terrace wall is cultivated with grass or aclose-growing crop. Diversion ditches are oftenused to redirect water from its downhill courseacross agricultural land. These ditches usually runthrough permanently grassed strips, throughdownhill grass waterways constructed across thewidth of the strips, and through grassed field bor-ders surrounding each field.

Gordon Neal Diem

See also: Agriculture: modern problems; Erosionand erosion control; Slash-and-burn agriculture;Soil conservation; Soil management; Soil saliniza-tion; Sustainable agriculture.

Sources for Further StudyFrancis, Charles, et al. “Strip Cropping Corn and Grain Legumes: AReview.” American Jour-

nal of Alternative Agriculture 1, no. 4 (1986): 159-164. Results of experimental studies onstrip cropping yields versus conventional techniques.

Hart, John Fraser. The Rural Landscape. Baltimore, Md.: Johns Hopkins University Press,1998. A description of historic land division and cropping systems.

984 • Strip farming

STROMATOLITES

Categories: Algae; bacteria; evolution; paleobotany

Stromatolites are laminated, sedimentary fossils formed from layers of blue-green algae (also known as blue-green bacte-ria or cyanobacteria). Located throughout the world, these ancient remnants of early life have revealed much about the“age of algae.”

Stromatolites are the most common megascopicfossils, contained within ancient rocks dating to

3.5 billion years in age. In both the living and fossilform, they are created by the trapping and bindingof sediment particles and the precipitation of cal-cium carbonate to the sticky surface of matlike fila-ments grown on a daily cycle by blue-green algae(also known as cyanobacteria). Modern stromat-olites are found throughout the world; they are ofparticular use in the creation of hydrocarbon reser-voirs, in geologic mapping, and as indicators ofpaleoenvironments.

Distribution of StromatolitesIn the geologic record, stromatolites are the most

abundant of fossils found in rocks dating to the Pre-cambrian era, that period of time from the origin ofEarth (approximately 4.6 billion years ago) up to544 million years ago. The oldest fossil stromat-olites are contained in the 3.3-billion- to 3.5-billion-year-old Warrawoona group of rocks in Australia.Close in age are the 3.4-billion-year-old stro-matolites of the Swaziland group of South Africa.Somewhat more removed are those associated withthe 2.5-billion- to 2.8-billion-year-old BulawayanLimestone, also in Africa.

All these examples originated in the Archeaneon, the first recorded period of geologic history,extending from approximately 4 billion to 2.5 bil-lion years ago. During the Proterozoic eon, imme-diately following the Archean eon and extending to544 million years ago, stromatolites became pro-lific. This Proterozoic expansion is probably reflec-tive of the initial development of continental land-masses and associated warm, photic continentalshelf regions, as plate tectonics became a control-ling process in the early development of the earth’scrust.

Throughout the Archean and Proterozoic eons,blue-green algae underwent a steady and progres-sive state of biologic evolution, recognized today asthe singular, common megascopic fossil of the Pre-cambrian time period. For this reason, the Precam-brian is often referred to as the “age of algae,” or,more specifically, the “age of blue-green algae.”Throughout the Phanerozoic eon, defined as 544million years ago to the present, blue-green algaeunderwent minimal evolution, probably becausetheir evolutionary state had become adapted to avariety of environments, reducing the need for fur-ther diversification.

Stromatolitic-building algae maintained theirdominance of the aquatic world during the EarlyPhanerozoic eon (544 million to about 460 millionyears ago). With the rather abrupt appearance,however, of shelled, grazing, and cropping inverte-brates in the early Phanerozoic eon, blue-green al-gae began to decline in significance. Today, as com-pared to their Precambrian domination, they have,on a relative scale, become endangered.

Geographically, fossil stromatolites are ubiqui-tous on every continent, especially within sedi-mentary carbonate rock sequences older than 460million years. On southeastern Newfoundland,stromatolites built by blue-green algae of the ge-nus Girvanella are found in conglomerate and lime-stone strata of the Bonavista Formation (approxi-mately 550 million years in age). In the Transvaalregion of South Africa, delicately banded stromat-olitic structures compose one of the most wide-spread of early Proterozoic shallow-water carbon-ate deposits in the world, extending over an areaexceeding 100,000 square kilometers.

In nearby Zambia, algal stromatolites are closelyassociated with rock sequences containing eco-nomic levels of copper and cobalt. Upper Permian

Stromatolites • 985

(250 million years ago) stromatolite horizons can betraced over an area of northern Poland exceeding15,000 square kilometers. Miocene age (15 millionyears old) algae of the species Halimeda composethe limestone-forming rocks of the island of Saipanin the Mariana Islands of the Pacific Ocean. InNorth America, fossil algal-bearing rocks includethe 2-billion-year-old Gunflint (Iron) Formation ofOntario, Canada, and the well-developed stromat-olitic horizons of Early Paleozoic era (450 millionyears ago) composing the Ellenberger Formation ofOklahoma and Texas.

Classification and IdentificationThe classification and identification of stromat-

olites are often concluded on the basis of overallmorphology, particularly the size, shape, and inter-nal construction of the specimen. The relevant liter-ature makes use of a variety of morphological

terms, including the adjectives “frondose” (leaf-like), “encrusting,” “massive,” “undulatory” (wave-like), “columnar,” “laminar,” “domed,” “elliptical,”and “digitigrade” (divided into fingerlike parts).Through the study of modern blue-green algae, it issuggested that three environmental criteria are ofimportance in stromatolite geometry development.These are direction and intensity of sunlight, direc-tion and magnitude of water current, and directionof sediment transport. As an example, the extantelliptical stromatolites of Shark Bay, western Aus-tralia, are oriented at right angles to the shorelineas the result of strong current-driven wave andscour action. Under certain environmental condi-tions, cyanobacteria growth surfaces are not pre-served, producing fossil algal structures character-ized by a lack of laminae (leaf blades). Thesestructures are termed thrombolites, in contrast tolaminar-constructed stromatolites.

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Stromatolites are formed by a slow buildup of microbial mats trapping sediment particles and the precipitation of cal-cium carbonate. Modern stromatolites are found throughout the world.

While stromatolites are generally describedas megascopic in size, discussion of specific di-mensions relates both to laminae thickness and tooverall size. Stromatolite laminae of the Precam-brian-aged Pethei Formation, an outcropping alongthe shores of Great Slave Lake in the NorthwestTerritories of Canada, are both fine and coarse indimension. The coarse-grained layers, formed oflime-mud pellets and calcium and magnesium car-bonate rhombs, are principally less than 5.0 milli-meters in thickness. The fine-grained laminae,composed of calcium carbonate clay and silt-sizedparticles, are, on average, only 0.5 millimeter thick.

In size, individual stromatolites can range fromcentimeters up to several meters. Fossils of the

common Precambrian genus Conophyton occur in arange of sizes, from pencil-sized shapes to columnsup to 10 meters in diameter. Subspherical varietiesof stromatolite-like structures, formed by the accre-tion of successive gelatinous mats of blue-greenalgae and generally less than 10 centimeters indiameter, are termed oncolites. Stromatolitic com-plexes in the Great Slave Lake district measure 80meters long by 45 meters wide by 20 meters inthickness and can be continuously traced for dis-tances exceeding 160 kilometers.

Albert B. Dickas

See also: Algae; Cyanobacteria; Evolution of cells;Fossil plants; Paleoecology.

Sources for Further StudyDott, R. H., Jr., and D. R. Prothero. Evolution of the Earth. 6th ed. Dubuque, Iowa: McGraw-

Hill, 2002. A well-illustrated textbook on the geologic history of Earth. Contains severalsections on stromatolites, their significance to Precambrian history, and their role in theunderstanding of very early forms of life.

Gebelein, C. D. “Biologic Control of Stromatolite Microstructure: Implications for Precam-brian Time Stratigraphy.” American Journal of Science 274 (1974): 575-598. Lengthy paperdiscusses the various environmental factors, including water and wind currents, that af-fect the detailed structure of modern stromatolitic organisms. Such knowledge is usefulin worldwide mapping of stromatolite horizons. Written for undergraduate college stu-dents.

Levin, H. L. The Earth Through Time. 6th ed. Fort Worth, Tex.: Saunders College Publishing,1999. This college-level textbook discusses fossil stromatolites of the Proterozoic eon andtheir importance as heliotrophic rock structures. Places these fossils within the context ofthe evolution of prokaryotic organisms. An easy-to-read source complete with numerousphotographs.

McMenamin, Mark. The Garden of the Ediacara: Discovering the First Complex Life. New York:Columbia University Press, 1998. Entertaining study of the earliest complex life-forms onthe planet details the author’s work on these organisms. Written for the student but un-derstandable by the general reader.

Schopf, J. William, ed. Major Events in the History of Life. Boston: Jones and Bartlett, 1992. Ex-cellent overview of the origin of life, the oldest fossils, and the early development ofplants and animals. Written by specialists in each field but at a level suitable for highschool students and undergraduates. Although technical language is used, most termsare defined in the glossary.

Stewart, W. N. Paleobotany and the Evolution of Plants. 2d ed. New York: Cambridge Univer-sity Press, 1993. Discusses the preservation, preparation, and age determination of fossilplants, the geologic fossil record, and the Precambrian eon, when stromatolites were com-mon throughout the aquatic world. Suitable for college-level readers.

Walter, M. R. “Understanding Stromatolites.” American Scientist, September/October, 1977,563-571. An in-depth paper on stromatolites, thrombolites, and oncolites. Discussesmeans of development of differing morphologies, modern analogues, and uses in strati-graphic mapping of stromotolitic-bearing rocks. Contains a complete reference section.Written for the high school science reader.

_______, ed. Stromatolites. New York: Elsevier, 1976. Contains papers on zonation, paleo-

Stromatolites • 987

environments, environmental diversity, morphologies, origin, geologic significance, anddistribution of fossil stromatolites. Several papers discuss modern algal stromatolitesand the organisms that build them. A valuable reference for the serious student.

SUCCESSION

Categories: Ecology; ecosystems; genetics; reproduction and life cycles

Succession is the progressive and orderly replacement of one biological community by another until a relatively stable,self-maintaining community is achieved.

Succession is an important ecological phenome-non because it allows the maximum variety and

number of species to occupy a given area throughtime and leads to the establishment of an ecologi-cally stable climax community that represents themost complex and diverse biological system possi-ble, given existing environmental conditions andavailable energy input. As succession proceeds,significant changes occur in species composition,nutrient cycling, energy flow, productivity, andstratification. Changes also occur within the climaxcommunity; however, these changes act to main-tain the climax, not alter it.

Immature communities tend to have high popu-lations of a few species that are relatively small andsimple. Biomass (weight of living material) is low,and nutrient conservation and retention are poor.Food chains are short, and available energy is sharedby few species. Community structure is simple andeasily disrupted by external forces. As communi-ties mature, larger and more complex organismsappear, and there is a higher species diversity (num-ber of different species). Biomass increases, and nu-trients are retained and cycled within the commu-nity. The greater number of species results in morespecies interactions and the development of com-plex food webs. Community productivity (conver-sion of solar energy to chemical energy), initiallyhigh in immature communities, becomes balancedby community respiration as more energy is ex-pended in maintenance activities.

Stages of SuccessionThe entire sequence of communities is called a

sere, and each step or community in the sequence is

a seral stage. The climax community is in balance, orequilibrium, with the environment and displaysgreater stability, more efficient nutrient and energyrecycling, a greater number of species, and a morecomplex community structure than that of eachpreceding seral stage.

Each seral stage is characterized by its own dis-tinctive forms of plant and animal life, which areadapted to a unique set of chemical, physical, and bi-ological conditions. Excepting the climax commu-nity, change is the one constant shared by all seralstages. Changes can be induced by abiotic factors,such as erosion or deposition, and by biotic factors,modification of the environment caused by the ac-tivities of living organisms within the community.

These self-induced factors bring about environ-mental changes detrimental to the existing commu-nity but conducive to invasion and replacement bymore suitably adapted species. For example, li-chens are one of the first colonizers of barren rockoutcrops. Their presence acts to trap and holdwindblown and water-carried debris, therebybuilding up a thin soil. As soil depth increases, soilmoisture and nutrient content become optimal forsupporting mosses, herbs, and grasses, which re-place the lichens. These species continue the pro-cess of soil-building and create an environmentsuitable for woody shrubs and trees.

In time, the trees overtop the shrubs and estab-lish a young forest. These first trees are usuallyshade-intolerant species. Beneath them, the seedsof the shade-tolerant trees germinate and grow up,eventually replacing the shade-tolerant species.Finally, a climax forest community develops onwhat once was bare rock, and succession ends.

988 • Succession

Primary and Secondary SuccessionThe sere just described—from barren rock to cli-

max forest—is an example of primary succession. Inprimary succession, the initial seral stage, or pio-neer community, begins on a substrate devoid oflife or unaltered by living organisms. Successionthat starts in areas where an established commu-nity has been disturbed or destroyed by naturalforces or by human activities (such as floods, wind-storms, fire, logging, and farming) is called second-ary succession.

An example of secondary succession occurs onabandoned cropland. This is referred to as old-fieldsuccession and begins with the invasion of theabandoned field by annual herbs such as ragweedand crabgrass. These are replaced after one or twoyears by a mixture of biennial and perennial herbs,and by the third year the perennials dominate.Woody shrubs and trees normally replace the

perennials within ten years. After another ten ortwenty years have passed, a forest is established,and ultimately, after one or two additional seralstages in which one tree community replaces an-other, a climax forest emerges.

Both primary and secondary succession begin onsites typically low in nutrients and exposed to ex-tremes in moisture, light intensity, temperature, andother environmental factors. Plants colonizing suchsites are tolerant of harsh conditions, are character-istically low-growing and relatively small, and haveshort life cycles. By moderating the environmentalconditions, these species make the area less favor-able for themselves and more favorable for plantsthat are better adapted to the new environment.Such plants are normally long-lived and relativelylarge. Secondary succession usually proceeds at afaster rate than primary succession, because a well-developed soil and some life are already present.

Succession • 989

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Aquatic EnvironmentsSuccession can also take place in aquatic envi-

ronments, such as a newly formed pond. The pio-neer community consists of microscopic organismsthat live in the open water. Upon death, their re-mains settle on the bottom and join with sedimentand organic matter washed into the pond. An accu-mulation of sediment provides anchorage and nu-trients for rooted, submerged aquatic plants such aspondweeds and waterweeds. These add to thebuildup of sediment, and as water depth decreases,rooted, floating-leaved species such as water liliesprevent light from reaching the submerged aquat-ics and eliminate them.

At the water’s edge, emergent plants rooted inthe bottom and extending their stems and leavesabove water (cattails, rushes, and sedges) trap sedi-ment, add organic matter, and continue the filling-in process. The shallow margins fill first, and even-tually the open water disappears and a marsh orbog forms. A soil rich in partially decomposed or-ganic matter and saturated with water accumu-lates. As drainage improves and the soil becomesraised above the water level, trees and shrubs toler-ant of wet soils invade the marsh. These act to lowerthe water table and improve soil aeration. Treessuited to drier conditions move in, and once again aclimax community characteristic of the surround-ing area develops.

Influence of ClimateThe American ecologist Frederic E. Clements

(1874-1945) believed that the characteristics of a cli-max community were determined solely by re-gional climate. According to Clements, all commu-nities within a given climatic region, despite initialdifferences, eventually develop into the same cli-max community. Some seral stages might be abbre-viated or skipped entirely, while others could belengthened or otherwise modified; however, theend result would always be a single climax commu-nity suited to the regional climate. This phenome-

non is called convergence, and Clements’s single-climax concept is known as the monoclimax theory.

Some ecologists have found the monoclimaxtheory to be simplistic and have offered other theo-ries. One of these, the polyclimax theory, holds that,within a given climatic region, there could be manyclimaxes. It was noted that in any single climatic re-gion, there were often many indefinitely main-tained communities that could be considered sepa-rate and distinct climaxes. These developed as aresult of differences caused by soil type, soil mois-ture, nutrients, slope, fire, animal activity (grazingand browsing), and other factors. Clements coun-tered that these would eventually reach true climaxstatus if given enough time and proposed termssuch as subclimax (a long-lasting seral stage preced-ing the climax) and disclimax (a nonclimax main-tained by continual disturbance) to describe suchsituations.

A third theory, the climax pattern concept, viewsthe climax as a single large community composedof a mosaic or pattern of climax vegetation insteadof many separate climaxes or subclimaxes. Numer-ous habitat and environmental differences accountfor the patterns of populations within the climax;no single factor such as climate is responsible.

While there is little doubt about the reality ofsuccession, it is apparently not a universal phenom-enon. For example, disturbed areas within tropicalrain forests do not undergo a series of seral stagesleading to reestablishment of the climax commu-nity. Instead, the climax is established directly bythe existing species. Nevertheless, in most regionssuccession is the mechanism by which highly orga-nized, self-maintained, and ecologically efficientcommunities are established.

Steven D. Carey

See also: Community-ecosystem interactions;Ecology: concept; Ecosystems: overview; Geneticdrift; Reforestation; Reproductive isolating mecha-nisms; Selection; Species and speciation.

Sources for Further StudyBazzaz, F. A. Plants in Changing Environments: Linking Physiological, Population, and Commu-

nity Ecology. New York: Cambridge University Press, 1996. Discusses how disturbancechanges the environment, how species function, coexist, and compete for resources, andhow species replace one another over time.

Brewer, Richard. The Science of Ecology. Fort Worth, Tex.: Saunders College Publishers, 1994.A clearly written text suitable for high school reference and college use. Includes a chap-ter on community change and succession. Contains biographical sketches of ecology pio-

990 • Succession

neers Frederic E. Clements and Henry Chandler Cowles. Discusses ecosystem develop-ment through the earth’s history. Illustrated with black-and-white photographs and linedrawings. Includes glossary, bibliography.

Perry, David A. Forest Ecosystems. Baltimore: Johns Hopkins University Press, 1994. Text-book for students, land managers, scientists, and policymakers. Topics range from theworld’s forest types and subdisciplines of ecology to more complex discussions of suchthings as productivity, succession, nutrient cycling, and stability.

Smith, Robert L. Ecology and Field Biology. 6th ed. San Francisco: Benjamin Cummings, 2001.The chapter “Succession” is a clear and informative treatment of the subject. Good discus-sion of land use and effects on succession as well as succession of human communities.The latter part of the chapter discusses ecosystem development through time with em-phasis on continental drift. Glossary and extensive bibliography. Illustrated with black-and-white photographs and line drawings.

SUGARS

Categories: Economic botany and plant uses; food; nutrients and nutrition; photosynthesis andrespiration

Refined from sugarcane, sugar beets, and corn, sugars are a major and vital part of nutrition and provide the basic molec-ular structure for most living matter.

Sugars, through the process called cellular respi-ration, are the primary power sources used to

produce adenosine triphosphate (ATP), the energyexchange molecule that sustains all life. Energy isstored in organisms as starch or fat but is burned assugar. Sugars include some of the simplest carbo-hydrates, and they are building blocks of morecomplicated molecules. Common sugars includeglucose, fructose, sucrose, maltose, and lactose. Al-cohol (ethanol) derived from sugar fermentation isan important product to the food and beverage,fuel, and drug manufacturing industries. Theprimary sugar derived from plants is sucrose, adisaccharide composed of two simpler sugars, glu-cose and fructose, joined together.

History of Use in FoodDemand for sugar and other food flavorings was

particularly strong before canning and refrigera-tion. People often had to eat partially spoiled food,and they eagerly sought ways to improve its flavor.Items with high sugar levels, such as berries andgrapes, were especially prized.

Sugarcane (Saccharum officinarum y officinarum)has a stem that can be squeezed to deliver a sucrose-

rich syrup, and the leftover woody material (ba-gasse) can be dried and burned to boil off water. Mo-lasses syrup or solid sugar can be made in this way.Arab traders brought sugarcane to Europe in the1100’s. By the 1500’s European colonization in Bra-zil and the Caribbean provided rich growing fields.So important and valuable were sugar crops that in1800 the new countries of Haiti and the UnitedStates had similar gross national products.

However, the cravings and rivalries that createdsugar empires caused their decline. England and itsallies fought a series of wars with France in the late1700’s and early 1800’s, and British blockades keptmolasses out of Europe. France’s Napoleon re-sponded by offering a prize for a process to producesugar from a European-grown plant. A sugar beetprocess won, and cane sugar was never again ascentrally important.

Yearly, sucrose production from sugarcane andsugar beets is more than 100 million metric tons.Both crops are excellent soil conditioners. Sugar-cane is a tall, periodically harvested grass, so it lim-its soil erosion that would occur in bare ground.Meanwhile, cane roots steadily grow and increasehumus in the soil. Sugar beets require plowing and

Sugars • 991

are dug from the ground during harvesting, butthey, too, leave extensive roots.

When sucrose prices rose in the 1950’s, high-fructose sweeteners were developed as alterna-tives. Enzymes are used to break starches into fruc-tose. Corn syrup, one fructose sweetener, is cheaperthan sucrose and has displaced much U.S. sucroseuse. Elsewhere, fructose syrups are made fromwheat, rice, tapioca, and cassava.

Sugar and AlcoholThe fermentation of alcohol has been a major

aspect of sugar use since antiquity. Fungi calledyeasts convert sugars into ethyl alcohol (ethanol);the yeasts can produce only mild alcohol levels. Al-cohol, although medically classified as a depres-sant, can provide a short-term energy boost. It alsoacts as a mild poison that desensitizes the centralnervous system, allowing drinkers to feel relaxed.Ancient Egyptians and Mesopotamians fermentedgrains into beer, while Greeks and Phoenicianstraded wine from grapes. In the Middle Ages, al-

chemists experimented with distillation, a processin which a substance boiled out of one substance iscooled back into liquid elsewhere. Distillation trans-formed beers and wines into whiskeys and bran-dies, alcoholic drinks several times more potent.

Caribbean sugar and alcohol formed one leg ofthe “triangle trade” from the 1600’s through theearly 1800’s. New Englanders sold fish for Carib-bean molasses, fermented it, and distilled it intorum. They traded rum in Africa for slaves, soldlargely in the Caribbean.

Fuel and Other UsesEthyl alcohol can burn more efficiently than gas-

oline in the internal combustion engines used in au-tomobiles. However, because gasoline has histori-cally been cheaper, research and developmentwork on ethanol as a fuel was minimal until the1970’s. The energy crisis of 1973 involved oil short-ages and soaring prices, and it stimulated many ex-perimental ethanol programs. Most experimentswere abandoned when oil prices dropped in the

992 • Sugars

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mid-1980’s, but Brazil persevered in a national pro-gram of sugar-cane alcohol fuel.

Even Brazilian ethanol is only barely economi-cally competitive with fossil fuels. The major prob-lem is that energy expenditures in the manufactureof ethanol include fuel for tending the fields andgathering cane, energy lost to yeasts (about half,although the yeasts do yield high-protein by-product), and another half of the remainder ex-pended for distilling the material to 95 percentalcohol. Suggested improvements include devel-oping more efficient yeasts and performing the dis-tillation process under partial vacuum, whichwould allow continuous processing rather thanbatch processing.

Theoretically, the most efficient way to use sugarenergy would be the development of electrical fuelcells that would take energy from sugar, just as liv-ing organisms do. Losses from yeast digestion anddistillation would be eliminated, and a fuel cell

might achieve 50 percent efficiency, rather than the25 percent efficiency of internal combustion en-gines, yielding eight times more energy. This ap-proach could create an energy revolution if thetechnical problems could be overcome. (Energy-efficient processes must be developed for sacchar-ification, or hydrolysis, of cellulose from wood andgarbage.)

Sugars can also be nutrients for generating otherproducts. Specialized groups of other cells, suchas juice-producing cells from oranges or fiber-producing cells from cotton, can be cultured withsugar nutrients, for example, and genetic engineer-ing has developed yeasts that produce specialtychemicals such as catalysts.

Roger V. Carlson

See also: ATP and other energetic molecules; Car-bohydrates; Ethanol; Grasses and bamboos; Micro-bial nutrition and metabolism; Respiration; Yeasts.

Sources for Further StudyDaviss, Bennett. “Power Lunch.” Discover 16, no. 3 (March, 1995): 58-63. Describes develop-

ment of an electrochemical battery powered by bacteria; these sugar-eating “living fuelcells” could burn waste products to light cities.

Peach, W. N., and James A. Constantin. Zimmermann’s World Resources and Industries. 3d ed.New York: Harper and Row, 1972. Summarizes the history and economic context of sugarproduction.

Ross, James R. Fuel Alcohol: How to Make It, How to Use It. New York: St. Martin’s Press, 1981.An ethanol fuel how-to book.

SUSTAINABLE AGRICULTURE

Categories: Agriculture; disciplines; economic botany and plant uses; environmental issues

Sustainable agriculture is the practice of growing and harvesting crops in a manner that will allow the land to return toits precultivation state. It seeks to maintain the quality of air, soil, and water resources for future human use and for wild-life.

Most twentieth century agricultural practiceswere based upon continued economic growth.

This practice demonstrated dramatic increases inproduction but had negative impacts on the envi-ronment through the losses of plant and animalhabitats, depletion of soil nutrients, and increases inpollution of water supplies. The concept of sustain-able development is based on using renewable re-

sources and working in harmony with ecosystems.The World Commission on Environment and De-velopment described the concept of sustainable de-velopment as being able “to meet the needs of thepresent without compromising the ability of futuregenerations to meet their own needs.” Sustainableagriculture strives to manage agricultural activitiesin such a way as to protect air, soil, and water qual-

Sustainable agriculture • 993

ity as well as conserve wildlife habitats and biodi-versity.

Problems Caused by AgricultureWater pollution is one of the most damaging and

widespread effects of modern agriculture. Runofffrom farms accounts for more than 50 percent ofsediment damage to natural waterways. Cleanupof the chemicals and nutrients associated with thisrunoff in the United States costs an estimated $2 bil-lion to $16 billion per year. Heavy application of ni-trogen fertilizers and pesticides has raised the po-tential for groundwater contamination. Feedlotsthat concentrate manure production lead to furthergroundwater contamination. Several of the mostcommonly used pesticides have been detected inthe groundwater of at least one-half of the states inthe United States. In addition, growing highly spe-cialized monoculture crops, which requires a heavyreliance on agricultural chemicals, has depleted thenatural organic nutrients that were formerly rich inNorth American topsoils.

Research has shown that many of the farm-based chemical agents, pesticides, fertilizers, plant-growth regulators, and antibiotics are now found inthe food supply. These chemicals can be harmful tohumans at moderate doses, and chronic effects candevelop with prolonged exposure at lower doses.Further, widespread pesticide use has been shownto severely stress animals other than the target pest,including bee populations. Pesticides have oftenled to resurgences of pests after treatment, occur-rences of secondary pest outbreaks, and resistanceto pesticides in the target pest.

Because of these growing problems, manyAmerican farmers are turning to sustainable agri-culture. The U.S. federal government has offeredguidance for this transition through the SustainableAgriculture Farm Bill, passed by Congress in 1990.The bill provides that sustainable agriculture,through an integrated system of plant and animalproduction practices, can, over the long run, meethuman food and fiber needs, enhance environmen-tal quality and natural resources, make the most ef-ficient use of nonrenewable resources, maintaineconomic viability of farm operations, and enhancethe quality of life for farmers and consumers.

Water ConservationWater is one of the most important resources for

agriculture and for society as a whole. In the west-

ern United States, it allows arid lands to producecrops through irrigation. In California, limitedsurface water supplies have caused overdraft ofgroundwater and the consequent intrusion of saltwater, which causes a permanent collapse of aqui-fers. In order to counteract these negative effects,sustainable farmers in California are improvingwater conservation and storage methods, selectingdrought-resistant crop species, using reduced-volume irrigation systems, and managing crops toreduce water loss. Drip and trickle irrigation canalso be used to dramatically reduce water usageand water loss while helping to avoid such prob-lems as soil salinization.

Salinization and contamination of groundwaterby pesticides, nitrates, and selenium can be tempo-rarily managed by using tile drainage to removewater and salt. However, this often has adverseaffects on the environment. Long-term solutionsinclude conversion of row crops to production ofdrought-tolerant forages and the restoration ofwildlife habitats.

Contour Plowing and TerracingOne of the most important aspects of sustainable

agriculture is soil conservation. Water runoff from afield having a 5 percent slope has three times thewater volume and eight times the soil erosion rateas a field with a 1 percent slope. In order to preventexcessive erosion, farmers can leave grass strips inthe waterways to capture soil that begins to erode.Contour plowing, which involves plowing across thehill rather than up and down the hill, helps captureoverland flow and reduce water runoff. Contourplowing is often combined with strip farming,where different kinds of crops are planted in alter-nating strips along the contours of the land. As onecrop is harvested, another is still growing and helpsrecapture the soil and prevent water from runningstraight down the hill. In areas of heavy rainfall,tiered ridges are constructed to trap water and pre-vent runoff. This involves a series of ridges con-structed at right angles to one another. Such con-struction blocks direct runoff and allows water tosoak into the soil.

Another method of soil conservation is terracing,in which the land is shaped into level shelves ofearth to hold in the water and soil. To provide fur-ther stability to soil, soil-anchoring plants aregrown on the edges of the terraces. Terracing, al-though costly, can make it possible to farm on steep

994 • Sustainable agriculture

hillsides. Some soils that are fairly unstable on slop-ing sites or waterways can require that perennialspecies of grasses be planted to protect the fragilesoil from cultivation every year.

Green ManureFarmers also use green manure—crops that are

raised specifically to be plowed under—to intro-duce organic matter and nutrients into the soil.Green manure crops help protect against erosion,cycle nutrients from lower levels of the soil into theupper layers, suppress weeds, and keep nutrientsin the soil rather than allowing them to leach out.Legumes such as sweet clover, ladino clover, and al-falfa are excellent green-manure crops. They areable to extract nitrogen from the air into the soil andleave a supply of nitrogen for the next crop that isgrown. Some crops, such as beans and corn, cancause high soil erosion rates because they leave theground bare most of the year. In sustainable farm-ing, crop residues are left on the ground after har-vest. Residues help reduce soil evaporation andeven excessive soil temperatures in hot climates.Many farmers choose to use cover crops rather thanresidue crops. Which cover crop touse depends on which geographicalarea farmers live in and if they wishto control erosion, capture nitrogenin the fall, release nitrogen to thecrop, or improve soil structure andsuppress weeds.

Cover CropsWhen planting crops with high

nitrogen requirements, such as to-matoes or sweet corn, farmers canuse cover crops such as hairy vetchor clover. Both these cover crops de-compose and release nutrients intothe soil within one month. To fighterosion, a farmer might choose arapid-growing cover crop, such asrye. Rye provides abundant groundcover and an extensive root systembelow the soil to stop erosion andcapture nutrients. Alfalfa, rye, orclover can be planted after harvestto protect the soil and add nutrientsand can then be plowed under atplanting time to provide a greenmanure for the crop. Cover crops

can also be flattened with rollers, and seeds can beplanted in their residue. This gives the new plants aprotective cover and discourages weeds from over-taking the young plants. Use of natural nitrogenalso reduces the risk of water contamination by ag-ricultural chemicals.

Reduced TillageSustainable agriculture emphasizes the use of

reduced tillage systems. There are three reducedtilling systems that sustainable farmers use to dis-turb the soil as little as possible. Minimum till in-volves using the disc of a chisel plow to make atrench in the soil where seeds are planted. Plant de-bris is left on the surface of the ground between therows, which helps further prevent erosion. Severalsustainable planting techniques help prevent soilerosion. Conser-till farming uses a coulter to open aslot just wide enough to insert seeds without dis-turbing the soil. No-till planting involves drillingseeds into the ground directly through the groundcover or mulch. When mulch is still in place, a nar-row slit can be cut through the cover or crop resi-dues in order to plant the new crops.

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Crop Rotation vs. MonoculturePlanting the same crop every year on the same

field can result in depleted soils. In order to keepthe soil fertile, nitrogen-depleting crops (such assweet corn, tomatoes, and cotton) should be ro-tated every year with legumes, which add nitro-gen to the soil. Planting a winter cover crop, such asrye grass, protects the land from erosion. Suchcover crops will, when plowed under, provide anutrient-rich soil for the planting of a cash crop.Crop rotations improve the physical condition ofthe soil because of variations in root depth and cul-tivation differences.

In nature, plants grow in mixed meadows,which allow for them to avoid insect infestations.Agricultural practices that use monoculture place agreat quantity of the food of choice in easy proxim-ity of the insect predator. Insects can multiply out ofproportion when the same crop is grown in the fieldyear after year. Since most insects are instinctivelydrawn to the same home area every year, they willnot be able to proliferate and thrive if crops are ro-tated and their crop of choice in not in the same fieldthe second year.

Crop rotation not only helps farmers use fewerpesticides but also helps to control weeds naturally.Some crops and cultivation methods inadvertentlyallow certain weeds to thrive. Crop rotations canincorporate a successor crop that eradicates theweeds. Some crops, such as potatoes and wintersquash, work as cleaning crops because of the dif-ferent style of cultivation that is used on them.Pumpkins planted between rows of corn will helpkeep weeds at bay.

Integrated Pest ManagementMost sustainable farmers use integrated pest man-

agement (IPM) to control insect pests. Using IPMtechniques, each crop and its pest is evaluated as anecological system. A plan is developed for usingcultivation, biological methods, and chemicalmethods at different timed intervals. Although ef-fective, profitable, and safe, the IPM techniqueshave been widely adopted only for a few crops,such as tomatoes, citrus, and apples.

The goal of IPM is to keep pest populations be-low the size where they can cause damage to crops.Fields are monitored to gauge the level of pest dam-age. If farmers begin to see crop damage, they putcultivation and biological methods into effect tocontrol the pests. Techniques such as vacuumingbugs off crops are used in IPM. IPM encouragesgrowth and diversity of beneficial organisms thatenhance plant defenses and vigor. Small amountsof pesticides are used only if all other methods failto control pests. It has been found that integratedpest management, when done properly, can reduceinputs of fertilizer, lower the use of irrigation water,and reduce preharvest crop losses by 50 percent.Reduced pesticide use can cut pest-control costs by50 to 90 percent and increases crop yield without in-creasing production costs.

Toby R. Stewart and Dion Stewart

See also: Agriculture: modern problems; Biofer-tilizers; Biopesticides; Composting; Erosion anderosion control; Hydroponics; Integrated pest man-agement; Monoculture; Organic gardening andfarming; Soil conservation; Soil management; Stripfarming; Sustainable forestry.

Sources for Further StudyLanger, Richard W. Grow It: The Beginner’s Complete, in-Harmony-with-Nature Small Farm

Guide, from Vegetable and Grain Growing to Livestock Care. New York: Saturday ReviewPress, 1972. An easy-to-read, hands-on book explaining how to use and apply sustainablefarming methods to both farm and garden.

National Research Council Board on Agriculture. Alternative Agriculture. Washington, D.C.:National Academy Press, 1989. Provides pages of detailed information on sustainable ag-ricultural methods.

President’s Council on Sustainable Development. Sustainable Agriculture Task Force. Sus-tainable Agriculture: Task Force Report. Washington, D.C.: Author, 1994. Lists the goals andstrategies for initiating sustainable agriculture in the United States.

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