biology in focus - chapter 26

CAMPBELL BIOLOGY IN FOCUS

© 2014 Pearson Education, Inc.

Urry • Cain • Wasserman • Minorsky • Jackson • Reece

Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge

26The Colonization of Land by Plants and Fungi


Overview: The Greening of Earth

For more than the first 2 billion years of Earth’s history, the terrestrial surface was lifeless

Cyanobacteria likely existed on land 1.2 billion years ago

Around 500 million years ago, small plants, fungi, and animals emerged on land

The first forests formed about 385 million years ago


Figure 26.1


Although not closely related, plants and fungi colonized the land as partners before animals arrived

Plants supply oxygen and are the ultimate source of most food eaten by land animals

Fungi break down organic material and recycle nutrients


Figure 26.2

Charophytealgae

Fungi

Animals

Plants


Concept 26.1: Fossils show that plants colonized land more than 470 million years ago

Green algae called charophytes are the closest relatives of land plants


Evidence of Algal Ancestry

Many characteristics of land plants also appear in some algae

However, land plants share certain distinctive traits with only charophytes, including Rings of cellulose-synthesizing complexes Structure of flagellated sperm


Figure 26.3

30 nm


Comparisons of both nuclear and chloroplast genes point to charophytes as the closest living relatives of land plants

Note that land plants are not descended from modern charophytes, but share a common ancestor with modern charophytes


Figure 26.4

40 m

Coleochaete orbicularis, adisk-shaped charophyte thatalso lives in ponds (LM)

Chara vulgaris, a pond organism


Figure 26.4a

Chara vulgaris, a pond organism


Figure 26.4b

40 mColeochaete orbicularis, a disk-shaped charophyte that also lives in ponds (LM)


Adaptations Enabling the Move to Land

In charophytes, a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out

Sporopollenin is also found in plant spore walls The movement onto land by charophyte ancestors

provided unfiltered sunlight, more plentiful CO2, and nutrient-rich soil

Land presented challenges: a scarcity of water and lack of structural support


The accumulation of traits that facilitated survival on land may have opened the way to its colonization by plants

Systematists are currently debating the boundaries of the plant kingdom

Until this debate is resolved, we define plants as embryophytes, plants with embryos

Animation: Moss Life Cycle

Animation: Fern Life Cycle


Figure 26.5

ANCESTRALALGA

Red algae

Chlorophytes

Plantae

Charophytes

Embryophytes

ViridiplantaeStreptophyta


Derived Traits of Plants

Key traits that appear in nearly all land plants but are absent in the charophytes include Alternation of generations Multicellular, dependent embryos Walled spores produced in sporangia Apical meristems


Alternation of generations The gametophyte is haploid and produces haploid

gametes by mitosis Fusion of the gametes gives rise to the diploid

sporophyte, which produces haploid spores by meiosis


Figure 26.6

FERTILIZATIONMEIOSIS

Key

Alternation of generations

Mitosis

Gametophyte(n)

Gamete fromanother plant

Wall ingrowths

Mitosis

Spore Gamete

Zygote

MitosisSporophyte(2n)

Placental transfercell (blue outline)

Multicellular, dependent embryos

EmbryoMaternal tissue

Haploid (n)Diploid (2n)

10 m2 m

2n

n

n

n

n


Figure 26.6a


Key


Mitosis

Gametophyte(n)

Gamete fromanother plant

Mitosis

Spore Gamete

Zygote

MitosisSporophyte(2n)

Haploid (n)Diploid (2n)

2n

n

n

n

n


Figure 26.6b

Wall ingrowthsPlacental transfercell (blue outline)

Multicellular, dependent embryos

EmbryoMaternal tissue

10 m2 m


Figure 26.6ba

EmbryoMaternaltissue

10 m


Figure 26.6bb

Wall ingrowthsPlacental transfercell (blue outline)

2 m


Multicellular, dependent embryos The multicellular, diploid embryo is retained within the

tissue of the female gametophyte Nutrients are transferred from parent to embryo

through placental transfer cells Land plants are called embryophytes because of the

dependency of the embryo on the parent


Walled spores produced in sporangia Sporangia are multicellular organs that produce

spores Spore walls contain sporopollenin, which makes them

resistant to harsh environments


Figure 26.7

Gametophyte

Sporophyte

SporangiumSpores

Longitudinal section ofSphagnum sporangium(LM)


Figure 26.7a

Gametophyte

Sporophyte

Sporangium


Figure 26.7b

SporangiumSpores

Longitudinal section ofSphagnum sporangium(LM)


Apical meristems Localized regions of cell division at the tips of roots

and shoots are called apical meristems Apical meristem cells can divide throughout the

plant’s life


Additional derived traits include Cuticle, a waxy covering of the epidermis that

functions in preventing water loss and microbial attack

Stomata, specialized pores that allow the exchange of CO2 and O2 between the outside air and the plant


Early Land Plants

Fossil evidence indicates that plants were on land at least 470 million years ago

Fossilized spores and tissues have been extracted from 450-million-year-old rocks


Figure 26.8

(a) Fossilizedspores

(b) Fossilizedsporophytetissue


Figure 26.8a

(a) Fossilizedspores


Figure 26.8b

(b) Fossilizedsporophytetissue


Large plant structures, such as the sporangium of Cooksonia, appeared in the fossil record 425 million years ago

By 400 million years ago, a diverse assemblage of plants lived on land

Unique traits in these early plants included specialized tissues for water transport, stomata, and branched sporophytes

Animation: Fungal Reproduction Nutrition


Figure 26.UN01

Cooksonia sporangium fossil (425 millionyears old)

0.3 mm


Figure 26.9

Sporangia

Rhizoids

25 m

2 cm


Figure 26.9a

25 m


Concept 26.2: Fungi played an essential role in the colonization of land

Fungi may have colonized land before plants Mycorrhizae are symbiotic associations between

fungi and land plants that may have helped plants without roots to obtain nutrients


Fungal Nutrition

Fungi are heterotrophs and absorb nutrients from outside of their body

Fungi use enzymes to break down a large variety of complex molecules into smaller organic compounds

Fungi can digest compounds from a wide range of sources, living or dead


Adaptations for Feeding by Absorption

Fungal cell walls contain chitin, a strong but flexible nitrogen-containing polysaccharide

The most common body structures are multicellular filaments and single cells (yeasts)

Some species grow as either filaments or yeasts; others grow as both


The morphology of multicellular fungi enhances their ability to absorb nutrients

Fungi consist of mycelia, networks of branched hyphae, filiments adapted for absorption

A mycelium’s structure maximizes its surface-area-to-volume ratio


Figure 26.10

Hyphae

Mycelium

60 m

Reproductivestructure

Spore-producingstructures


Figure 26.10a


Figure 26.10b

Mycelium


Figure 26.10c

60 m


Specialized Hyphae in Mycorrhizal Fungi

Some fungi have specialized hyphae called haustoria that allow them to extract or exchange nutrients with plant hosts


Figure 26.11

Fungal hypha

Haustorium

Plant cell

Plant cellplasmamembrane

Plantcellwall


Mycorrhizae are mutually beneficial relationships between fungi and plant roots

Ectomycorrhizal fungi form sheaths of hyphae over a root and also grow into the extracellular spaces of the root cortex

Arbuscular mycorrhizal fungi extend hyphae through the cell walls of root cells and into tubes formed by invagination of the root cell membrane


Sexual and Asexual Reproduction

Fungi propagate themselves by producing vast numbers of spores, either sexually or asexually

Fungi can produce spores from different types of life cycles


Figure 26.12-1

Key

GERMINATION

SporesASEXUALREPRODUCTION


Mycelium

Haploid (n)

Diploid (2n)Heterokaryotic


Figure 26.12-2

Zygote

PLASMOGAMY

Key

KARYOGAMY

GERMINATION

Spores SEXUALREPRODUCTION

ASEXUALREPRODUCTION

Heterokaryoticstage


Mycelium

Haploid (n)



Figure 26.12-3

Zygote

Spores

PLASMOGAMY

Key

KARYOGAMY

GERMINATION MEIOSISGERMINATION

Spores SEXUALREPRODUCTION

ASEXUALREPRODUCTION

Heterokaryoticstage


Mycelium

Haploid (n)



Plasmogamy is the union of cytoplasm from two haploid parent mycelia

Hours, days, or even centuries may pass before the occurrence of karyogamy, nuclear fusion

During karyogamy, the haploid nuclei fuse, producing diploid cells

The diploid phase is short-lived and undergoes meiosis, producing haploid spores


In addition to sexual reproduction, many fungi can reproduce asexually

Molds produce haploid spores by mitosis and form visible mycelia

Single-celled yeasts reproduce asexually through cell division


The Origin of Fungi

Fungi and animals are more closely related to each other than they are to plants or other eukaryotes


DNA evidence suggests that Fungi are most closely related to unicellular

protists called nucleariids Animals are most closely related to unicellular

choanoflagellates This suggests that multicellularity arose separately in

animals and fungi The oldest undisputed fossils of fungi are only about

460 million years old


Figure 26.13

50 m


The Move to Land

Fungi were among the earliest colonizers of land and probably formed mutualistic relationships with early land plants For example, 405-million-year-old fossils of

Aglaophyton contain evidence of fossil hyphae penetrating plant cells

Video: Phlyctochytrium Spores

Video: Allomyces Zoospores


Figure 26.14

100 nm

Zone of arbuscule-containing cells


Figure 26.14a

100 nm

Zone of arbuscule-containing cells


Figure 26.14b


Molecular evidence suggests that genes required for the establishment of mycorrhizal symbiosis were present in the common ancestor to land plants


Diversification of Fungi

Molecular analyses have helped clarify evolutionary relationships among fungal groups, although areas of uncertainty remain

There are about 100,000 known species of fungi, but there are estimated to be as many as 1.5 million species


Figure 26.15

2.5 m

Chytrids (1,000 species)

Zygomycetes (1,000 species)

Glomeromycetes (160 species)

Ascomycetes (65,000 species)

Basidiomycetes (30,000 species)

25 mHyphae


Chytrids (1,000 species) are found in freshwater and terrestrial habitats

Chytrids have flagellated spores and are thought to have diverged early in fungal evolution


Figure 26.15a

Chytrids (1,000 species)

25 m

Hyphae


Zygomycetes (1,000 species) include fast-growing molds, parasites, and commensal symbionts


Figure 26.15b

Zygomycetes (1,000 species)


Glomeromycetes (160 species) form arbuscular mycorrhizae with plant roots

About 80% of plant species have mutualistic relationships with glomeromycetes


Figure 26.15c

2.5 m

Glomeromycetes (160 species)


Ascomycetes (65,000 species) live in marine, freshwater, and terrestrial habitats

Ascomycetes produce fruiting bodies called ascocarps


Figure 26.15d

Ascomycetes (65,000 species)


Basidiomycetes (30,000 species) are important decomposers and ectomycorrhizal fungi

The fruiting bodies of basidiomycetes are commonly called mushrooms


Figure 26.15e

Basidiomycetes (30,000 species)


Concept 26.3: Early land plants radiated into a diverse set of lineages

Ancestral species gave rise to a vast diversity of modern plants


Figure 26.16

Origin of land plants

Origin of vascular plants

Origin of extant seedplants

ANCESTRALGREENALGA

Millions of years ago (mya)500

Angiosperms

450 400 350 300 50 0

3

2

1

Gymnosperms

Mosses

Hornworts

Lycophytes (clubmosses, spikemosses, quillworts)Monilophytes (ferns,horsetails, whisk ferns)

Liverworts

Land plantsVascular plantsSeed plantsSeedlessvascularplants

Nonvascular

plants(bryophytes)


Figure 26.16a

Origin of land plants

Origin of vascular plants

Origin of extant seedplants

ANCESTRALGREENALGA

Millions of years ago (mya)500

Angiosperms

450 400 350 300 50 0

3

2

1

Gymnosperms

Mosses

Hornworts

Lycophytes

Monilophytes

Liverworts


Figure 26.16b

Angiosperms

Gymnosperms

Mosses

Hornworts

Lycophytes (clubmosses, spikemosses, quillworts)Monilophytes (ferns,horsetails, whisk ferns)

Liverworts Land plantsVascular plantsSeed plantsSeedlessvascularplants

Nonvascular

plants(bryophytes)


Land plants can be informally grouped based on the presence or absence of vascular tissue

Most plants have vascular tissue for the transport of water and nutrients; these constitute the vascular plants

Nonvascular plants are commonly called bryophytes


Bryophytes: A Collection of Early Diverging Plant Lineages

Bryophytes are represented today by three clades of small herbaceous (nonwoody) plants Liverworts Mosses Hornworts

These three clades are thought to be the earliest lineages diverged from the common ancestor of land plants


Figure 26.UN03

AngiospermsGymnospermsSeedless vascular plantsNonvascular plants (bryophytes)


Figure 26.17

Sporophyte

Sporophyte(a sturdyplant thattakes monthsto grow)

Gametophyte

Gametophyte

Capsule

Seta

(b) Polytrichum commune, a moss

(c) Anthoceros sp., a hornwort

(a) Plagiochila deltoidea, aliverwort


Figure 26.17a

(a) Plagiochila deltoidea, aliverwort


Figure 26.17b

Sporophyte(a sturdyplant thattakes monthsto grow)

Gametophyte

Capsule

Seta

(b) Polytrichum commune, a moss


Figure 26.17c

Sporophyte

Gametophyte(c) Anthoceros sp., a hornwort


Bryophytes are anchored to the substrate by rhizoids

The flagellated sperm produced by bryophytes must swim through a film of water to reach and fertilize the egg

In bryophytes, the gametophytes are larger and longer-living than sporophytes

The height of gametophytes is constrained by lack of vascular tissues


Seedless Vascular Plants: The First Plants to Grow Tall Bryophytes were the prevalent vegetation during

the first 100 million years of plant evolution The earliest vascular plants date to 425–420 million

years ago Vascular tissue allowed these plants to grow tall Early vascular plants lacked seeds


Seedless vascular plants can be divided into clades

– Lycophytes (club mosses and their relatives)

– Monilophytes (ferns and their relatives)

Video: Plant time Lapse


Figure 26.UN04



Figure 26.18

(a) Diphasiastrum tristachyum, alycophyte

Strobili(conelikestructuresin whichspores areproduced)

(b) Athyrium filix-femina, amonilophyte

2.5 cm 2.5 cm


Figure 26.18a

(a) Diphasiastrum tristachyum, alycophyte

Strobili(conelikestructuresin whichspores areproduced)

2.5 cm


Figure 26.18b

(b) Athyrium filix-femina, amonilophyte

2.5 cm


Life Cycles with Dominant Sporophytes

In contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation, as in familiar ferns

The gametophytes are tiny plants that grow on or below the soil surface

Flagellated sperm must swim through a film of water to reach eggs

Animation: Pine Life Cycle


Figure 26.19

Sporophyte

Gametophyte

Example

PLANT GROUP

Mosses and othernonvascular plants

Ferns and otherseedless

vascular plants

Reduced, independent(photosynthetic andfree-living)

Reduced (usually microscopic), dependent onsurrounding sporophyte tissue for nutrition

Seed plants (gymnosperms and angiosperms)

Dominant

Dominant DominantReduced, dependenton gametophyte fornutrition

Gametophyte(n)

Gametophyte(n)

Sporophyte(2n)

Sporophyte(2n)

Sporophyte (2n) Sporophyte (2n)

Gymnosperm AngiospermMicroscopic femalegametophytes (n) insideovulate cone Microscopic female

gametophytes(n) inside these partsof flowers

Microscopicmalegametophytes(n) insidethese partsof flowersMicroscopic

malegametophytes (n)inside pollencone


Figure 26.19a

Sporophyte

Gametophyte

Example

Mosses and othernonvascular plants

Dominant

Reduced, dependent ongametophyte for nutrition

Gametophyte(n)

Sporophyte(2n)


Figure 26.19b

Sporophyte

Gametophyte

Example

Ferns and other seedlessvascular plants

Reduced, independent(photosynthetic and free-living)

Dominant

Gametophyte (n)

Sporophyte(2n)


Figure 26.19c

Sporophyte

Gametophyte

Example



Dominant

Sporophyte (2n)

GymnospermMicroscopic femalegametophytes (n)inside ovulatecone

Microscopic malegametophytes (n)inside pollen cone


Figure 26.19d

Sporophyte

Gametophyte

Example



Dominant

Sporophyte (2n)

Angiosperm

Microscopic female gametophytes(n) inside these partsof flowers

Microscopic malegametophytes (n) inside theseparts of flowers


Transport in Xylem and Phloem

Vascular plants have two types of vascular tissue: xylem and phloem

Xylem conducts most of the water and minerals and includes tube-shaped cells called tracheids

Water-conducting cells are strengthened by lignin and provide structural support

Phloem consists of cells arranged in tubes that distribute sugars, amino acids, and other organic products


Vascular tissue allowed for increased height, which provided an evolutionary advantage

Tall plants were better competitors for sunlight and could disperse spores much farther than short plants


Evolution of Roots and Leaves

Roots are organs that anchor vascular plants They enable vascular plants to absorb water and

nutrients from the soil


Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis

Leaves are categorized by two types Microphylls, small leaves with a single vein Megaphylls, larger, more productive leaves with a

highly branched vascular system


Seedless vascular plants were abundant in the Carboniferous period (359–299 million years ago)

Early seed plants rose to prominence at the end of the Carboniferous period


Concept 26.4: Seeds and pollen grains are key adaptations for life on land

Seed plants originated about 360 million years ago An adaptation called the seed allowed them to

expand into diverse terrestrial habitats A seed consists of an embryo and its food supply,

surrounded by a protective coat Mature seeds are dispersed by wind or other means


Extant seed plants are divided into two clades Gymnosperms have “naked” seeds that are not

enclosed in chambers Angiosperms have seeds that develop inside

chambers called ovaries


Figure 26.UN05



Terrestrial Adaptations in Seed Plants

In addition to seeds, the following are common to all seed plants: Reduced gametophytes Ovules Pollen


Reduced Gametophytes

The gametophytes of seed plants are microscopic Gametophytes develop within the walls of spores

that are retained within tissues of the parent sporophyte

The parent sporophyte protects and provides nutrients to the developing gametophyte


Ovules and Pollen

An ovule consists of an egg-producing female gametophyte surrounded by a protective layer of sporophyte tissue called the integument

Female gametophytes develop from large megaspores


Figure 26.20-1

Immatureovulate cone

Megaspore (n)

Integument (2n)

Spore wall

Megasporangium(2n)

Pollengrain (n)Micropyle

(a) Unfertilized ovule


Figure 26.20-2

Pollen tube

Femalegametophyte (n)

Egg nucleus(n)

Dischargedsperm nucleus(n)

Malegametophyte


Megaspore (n)

Integument (2n)

Spore wall

Megasporangium(2n)


(a) Unfertilized ovule (b) Fertilized ovule


Figure 26.20-3

Pollen tube

Femalegametophyte (n)

Seed coat

Sporewall

Foodsupply(n)

Embryo (2n)

Egg nucleus(n)

Dischargedsperm nucleus(n)

Malegametophyte


Megaspore (n)

Integument (2n)

Spore wall

Megasporangium(2n)


(a) Unfertilized ovule (b) Fertilized ovule (c) Gymnosperm seed


Male gametophytes develop from small microspores Microspores develop into pollen grains, which

consist of a male gametophyte enclosed within the protective pollen wall

Pollination is the transfer of pollen to the part of a seed plant containing the ovules

Pollen eliminates the need for a film of water and can be dispersed great distances by air or animals


The Evolutionary Advantage of Seeds

A seed develops from the whole ovule A seed is a sporophyte embryo, along with its food

supply, packaged in a protective coat


Seeds provide some evolutionary advantages over spores They may remain dormant from days to years, until

conditions are favorable for germination Seeds have a supply of stored food


Early Seed Plants and the Rise of Gymnosperms

Fossil evidence reveals that by the late Devonian period, some plants had begun to acquire features found in seed plants but did not bear seeds

Gymnosperms appeared in the fossil record about 305 million years ago

Gymnosperms largely replaced nonvascular plants as the climate became drier toward the end of the Carboniferous period


Gymnosperms were better suited than nonvascular plants to drier conditions due to adaptations including Seeds and pollen Thick cuticles Leaves with small surface area


Gymnosperms are an important part of Earth’s flora For example, vast regions in northern latitudes are

covered by forests of cone-bearing gymnosperms called conifers

Video: Flower Time Lapse


Figure 26.21

(b) Douglas fir (Pseudotsugamenziesii)

(a) Sago palm (Cycas revoluta)

(c) Creeping juniper (Juniperushorizontalis)


Figure 26.21a

(a) Sago palm (Cycas revoluta)


Figure 26.21b

(b) Douglas fir (Pseudotsuga menziesii)


Figure 26.21c

(c) Creeping juniper (Juniperus horizontalis)


The Origin and Diversification of Angiosperms

Angiosperms are seed plants with reproductive structures called flowers and fruits

They are the most widespread and diverse of all plants


Flowers and Fruits

The flower is an angiosperm structure specialized for sexual reproduction

Many species are pollinated by insects or animals, while some species are wind-pollinated


A flower is a specialized shoot with up to four types of modified leaves called floral organs Sepals, which enclose the flower Petals, which are brightly colored and attract

pollinators Stamens, which produce pollen Carpels, which produce ovules


Figure 26.22

Sepal

Ovule

Petal

Style

Ovary

Stigma CarpelStamen

Filament

Anther


A stamen consists of a stalk called a filament, with a sac called an anther where the pollen is produced

A carpel consists of an ovary at the base and a style leading up to a stigma, where pollen is received

The ovary contains one or more ovules


Seeds develop from ovules after fertilization The ovary wall thickens and matures to form a fruit Fruits protect seeds and aid in their dispersal


Various fruit adaptations help disperse seeds by wind, water, or animals

Fruits can function as Parachutes or propellers for wind dispersal Burrs that cling to animal fur or human clothing Food that is carried in the digestive system of animals

with seeds passing unharmed when the animal defecates


Angiosperm Evolution

Darwin called the origin of angiosperms an “abominable mystery”

Fossil evidence and phylogenetic analysis have led to progress in solving the mystery, but we still do not fully understand the evolution of angiosperms


Fossil evidence: Angiosperms originated at least 140 million years ago and dominated the landscape by the end of the Cretaceous period, 65 million years ago

Chinese fossils of 125-million-year-old angiosperms help us to infer traits of the angiosperm common ancestor

Archaefructus sinensis, for example, was herbaceous and may have been aquatic


Figure 26.23

Carpel

Stamen

(a) Archaefructus sinensis, a125-million-year-old fossil

(b) Artist’s reconstruction ofArchaefructus sinensis

5 cm


Figure 26.23a

(a) Archaefructus sinensis, a125-million-year-old fossil

5 cm


Angiosperm phylogeny: The ancestors of angiosperms and gymnosperms diverged about 305 million years ago

Angiosperms may be closely related to Bennettitales, extinct seed plants with flowerlike structures


Figure 26.24

Microsporangia(containmicrospores)

Ovules


Amborella and water lilies are likely descended from two of the most ancient angiosperm lineages


Figure 26.25

Most recent common ancestorof all living angiosperms

Magnoliids Monocots

Eudicots

Star anise

Water liliesAmborella

Amborella

Star aniseand relatives

Water lilies

Magnoliids

Monocots

Eudicots

Millions of years ago150 125 100 25 0


Figure 26.25a

Most recent common ancestorof all living angiosperms

Amborella

Star aniseand relatives

Water lilies

Magnoliids

Monocots

Eudicots

Millions of years ago150 125 100 25 0


Figure 26.25b

Amborella Star aniseWater lilies

Magnoliids Monocots

Eudicots


Amborella includes only one known species, a small shrub called Amborella trichopoda


Figure 26.25ba

Amborella


Water lilies are found in aquatic habitats throughout the world


Figure 26.25bb

Water lilies


Star anise naturally occur in southeast Asia and the southeastern United States

Extant species are likely descended from ancestral populations that were separated by continental drift


Figure 26.25bc

Star anise


Magnoliids include magnolias, laurels, avocado, cinnamon, and black pepper plants


Figure 26.25bd

Magnoliids


Monocots account for more than one-quarter of angiosperm species


Figure 26.25be

Monocots


Eudicots account for more than two-thirds of angiosperm species


Figure 26.25bf

Eudicots


Concept 26.5: Land plants and fungi fundamentally changed chemical cycling and biotic interactions

The colonization of land by plants and fungi altered the physical environment and the organisms that live there


Physical Environment and Chemical Cycling

A lichen is a symbiotic association between a photosynthetic microorganism and a fungus

Lichens are important pioneers on new rock and soil surfaces

They break down the surface, affecting the formation of soil and making it possible for plants to grow

Lichens may have helped the colonization of land by plants


Figure 26.26 A foliose (leaflike) lichen

Crustose(encrusting) lichens

(b) Anatomy of a lichen involving an ascomycete fungusand an alga

(a) Two common lichen growth forms

Fungal hyphae Algal cell

50

m


Figure 26.26a

A foliose (leaflike) lichen

Crustose(encrusting) lichens

(a) Two common lichen growth forms


Figure 26.26aa

Crustose (encrusting) lichens


Figure 26.26ab

A foliose (leaflike) lichen


Figure 26.26b

(b) Anatomy of a lichen involving an ascomycete fungusand an alga


50

m


Figure 26.26ba


50

m


Plants affect the formation of soil Roots hold the soil in place Leaf litter and other decaying plant parts add

nutrients to the soil

Plants have also altered Earth’s atmosphere by releasing oxygen to the air through photosynthesis


Plants and fungi affect the cycling of chemicals in ecosystems

Plants absorb nutrients, which are passed on to the animals that eat them

Decomposers, including fungi and bacteria, break down dead organisms and return nutrients to the physical environment


Plants play an important role in carbon recycling

Photosynthesis removes CO2 from the atmosphere

Increased growth and accelerated photosynthesis resulted from the formation of vascular tissue and may have contributed to global cooling at the end of the Carboniferous period


Figure 26.27

Lycophyte trees Horsetail FernLycophyte treereproductivestructures

Tree trunk coveredwith small leaves


Biotic Interactions

Biotic interactions can benefit both species involved (mutualisms) or be beneficial to one species while harming the other (as when a parasite feeds on its host)

Plants and fungi had large effects on biotic interactions because they increased the available energy and nutrients on land


Fungi as Mutualists and Pathogens

Mutualistic fungi absorb nutrients from a host organism and reciprocate with actions that benefit the host

Plants harbor harmless symbiotic endophytes, fungi that live inside leaves or other plant parts

Endophytes make toxins that deter herbivores and defend against pathogens


Figure 26.28

Endophyte not present; pathogen present (E−P) Both endophyte and pathogen present (EP)

E−P E−P EP EP

15

10

5

0

Leaf

mor

talit

y (%

)

Leaf

are

a da

mag

ed (%

)

30

20

10

0

Results


Parasitic fungi absorb nutrients from host cells, but provide no benefits in return

About 30% of known fungal species are parasites or pathogens, mostly on or in plants For example, Cryphonectria parasitica causes

chestnut blight


Figure 26.29

(a) Corn smut on corn

(c) Ergots on rye

(b) Tar spotfunguson mapleleaves


Figure 26.29a

(a) Corn smut on corn


Figure 26.29b

(b) Tar spot fungus on mapleleaves


Figure 26.29c

(c) Ergots on rye


Plant-Animal Interactions

Animals influence the evolution of plants, and vice versa For example, animal herbivory selects for plant

defenses For example, interactions between pollinators and

flowering plants select for mutually beneficial adaptations


Clades with bilaterally symmetrical flowers have more species than those with radially symmetrical flowers

This is likely because bilateral symmetry affects the movement of pollinators and reduces gene flow in diverging populations


Figure 26.UN06

Bilateral symmetry

Time since divergencefrom common ancestor

Radial symmetry

Commonancestor “Bilateral” clade

“Radial” clade

Comparenumbersof species


Angiosperms and other plant groups are being threatened by the exploding human population and its demand for space and resources

About 55,000 km2 of tropical rain forest are cleared each year

Deforestation leads to the extinction of plant, insect and other animal species

If current extinction rates continue, more than 50% of Earth’s species will be lost within the next few centuries


Figure 26.30

(b) By 2009, much moreof this same tropicalforest had been cutdown.

(a) A satellite image from2000 shows clear-cutareas in Brazil (brown)surrounded by densetropical forest (green).

4 km


Figure 26.30a

(a) A satellite image from2000 shows clear-cutareas in Brazil (brown)surrounded by densetropical forest (green).

4 km


Figure 26.30b

(b) By 2009, much moreof this same tropicalforest had been cutdown.

4 km


Figure 26.UN02a

No AM fungi

Thermal AM fungiNonthermal AM fungi

Soil treatment

Shoo

t dry

wei

ght (

g) 0.4

0.3

0.2

0.1

0.0


Figure 26.UN02b


Figure 26.UN02c

Root length

Soil temperature (C)

Roo

t len

gth

(cm

/g)

50

40

30

20

10Hyphal length

0

5

4

3

2

1

0

Hyp

hal l

engt

h (m

/g)

35 403020 250 45


Figure 26.UN07



MitosisGametophyte

Mitosis

Spore Gamete

Zygote

Mitosis

SporophyteHaploidDiploid

2n

n

n

n

n

SporesSporangium

Walled sporesin sporangia

21


Figure 26.UN08

Sepal

Ovule

Petal

Style

Ovary

Stigma Carpel (produces ovules)Stamen

(produces pollen)

Filament

Anther

Flower anatomy


Figure 26.UN09

Angiosperms

Gymnosperms

Mosses

Charophyte green algae

Ferns


Figure 26.UN10

biology in focus - chapter 26

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