new david sams tennesseemadisoncountymg.org/resources/master_gardener_handbook... · 2017. 8....

Basic Botany, Plant Physiology, and Plant Classification3

DAVID SAMSProfessor EmeritusUniversity of Tennessee

Taxonomic section additions by Carol Reese.

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3 - 3Basic Botany, Plant Physiology, and Plant Classification

CONTENTS

INTRODUCTION 3-5

PLANTPARTSANDFUNCTIONSINRELATIONTOGARDENING 3-5Basic Concepts and Vocabulary ............................................................................................................3-5

Root Versus Shoot .......................................................................................................................3-6Reproductive Versus Vegetative Parts ....................................................................................3-6Node Versus Internode ..............................................................................................................3-7Axillary Versus Apical Bud .........................................................................................................3-7Applications to Gardening .........................................................................................................3-7

What a Master Gardener Should Know About Stems .....................................................................3-8Characteristics of Typical Mature Dicot Stems .....................................................................3-8Characteristics of Typical Monocot Stems .............................................................................3-8Stem Growth Characteristics ....................................................................................................3-9Functions of Vascular Tissues ...................................................................................................3-9Internode Length .........................................................................................................................3-9Applications to Gardening .........................................................................................................3-9

What a Master Gardener Should Know About Roots ..................................................................3-12Development of the Root ........................................................................................................3-12Internal Characteristics of a Typical Mature Root ..............................................................3-13Types of Root Systems .............................................................................................................3-13Applications to Gardening .......................................................................................................3-14

What a Master Gardener Should Know about Leaves ..................................................................3-15The Leaf Blade ............................................................................................................................3-15Functions of Leaf Parts .............................................................................................................3-16Physical Variation in Leaves ...................................................................................................3-17Applications to Gardening .......................................................................................................3-18

What a Master Gardener Should Know About Flowers ...............................................................3-21The Four Main Parts of a Complete Flower .........................................................................3-21Pollination and Fertilization.....................................................................................................3-21Self- Versus Cross-pollinated Plants ......................................................................................3-22Monoecious Plants ....................................................................................................................3-22Dioecious Plants ........................................................................................................................3-23Applications to Gardening .......................................................................................................3-23

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Basic Botany, Plant Physiology, and Plant Classification3 - 4

PLANT PHYSIOLOGICAL AND ENVIRONMENTAL INTERACTIONS INRELATIONTOGARDENING 3-24

Plant Growth Requirements ................................................................................................................3-24Plant Hardiness ......................................................................................................................................3-24

Factors Affecting Plant Hardiness ..........................................................................................3-24Hardiness Variability Among Plant Parts ..............................................................................3-25Loss of Hardiness .......................................................................................................................3-25

Respiration .............................................................................................................................................3-25Flower Initiation .....................................................................................................................................3-25

Long-day Versus Short-day Plants .........................................................................................3-25Effect of Temperature and Moisture on Flowering ............................................................3-26

PLANTCLASSIFICATIONANDTAXONOMYRELATEDTOGARDENING 3-26Classification Based on Lifespan ........................................................................................................3-26Modern Plant Classification ................................................................................................................3-28Taxonomic Keys ......................................................................................................................................3-28The Genuine Usefulness and Joys of Plant Taxonomy ..................................................................3-28

GLOSSARY 3-33

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INTRODUCTIONIt is common to assume that botany, physiology, and taxonomy are academic rather than practical topics. While it is true that these subject areas can be complex and their study can lead to advanced degrees, it is also true that a basic introduction to these areas can help one be a better gardener. A basic understanding of plant function also prepares a Master Gardener to help others become better growers and stewards, which is the central goal of the Tennessee Extension Master Gardener (TEMG) program. Therefore, this chapter concentrates on basic botany and physiology topics that are of practical use in plant management and points out how we can apply them. The first two chapters of this manual presented an overview of the TEMG program and key geographic and plant features of our beautiful state of Tennessee. This chapter and the three that follow provide the foundation for an understanding of plant function and plant relationships with soil and water. The

remaining chapters build on those concepts, presenting sound gardening practices and useful information about ornamental and edible plants.

PLANT PARTS AND FUNCTIONS IN RELATION TO GARDENING

Basic Concepts and VocabularyA good starting point is to learn some of the basic terms and gain a uniform initial understanding of plants. The glossary at the end of this chapter contains definitions of all words in boldface type. Plants vary in size from those that are single celled (such as some algae) to those that are hundreds of feet tall and weigh many tons. They also reproduce in a range of ways that can include vegetative or asexual methods (such as many kinds of bananas), from spores (such as ferns), or from seed.

A young pepper seedling in greenhouse production. It is an example of a newer cultivar with edible fruit and ornamental value.

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Higher plants are those that gardeners deal with most commonly. They can be recognized by a well-developed system of veins. They are broadly called vascular plants. (See Figure 1.) Plants may be found high in trees (including mistletoe, some orchids, and bromeliads), floating in water, or most commonly with their roots in the ground. Most of this discussion focuses on vascular plants that grow from seed planted in soil. Figure 1 illustrates some of the important parts of a typical vascular plant.

ROOT VERSUS SHOOTRoots usually develop below the soil surface, but there are exceptions. Roots provide support for the plant and absorb the majority of the water and nutrients the plant uses in its various processes (metabolism). The parts of the plant above the soil surface are generally referred to as the shoot. In plant management, the most attention is focused on the above-ground shoot, but the root function and relationship with soil plays a vital role in plant survival and

productivity. That relationship is an important element introduced here and referenced in several following chapters.

REPRODUCTIVE VERSUS VEGETATIVE PARTSFlower buds, flowers, fruit, and seed are considered the reproductive parts of the plant. Roots, stems, leaves, and leaf buds are examples of vegetative parts. There is some overlap in the classification of structures, though, because many vegetative plant parts are used in asexual or vegetative reproduction (cuttings, grafting, layering, or division, for example). Vegetative reproduction produces plants that are genetically identical and that are therefore clones of each other. Asexual or vegetative reproduction is especially important when plants do not produce seed or do not come true to type when grown from seed. Currently, cloning is used to maintain many superior cultivars and has been used by people and in nature since before the beginning of agriculture.

Principal parts of a vascular plant

Lateral Root

Root CapRoot Hairs

Primary Root

Roo

t

Apical Bud

Auxillary Bud

Leaf BladeNodeInternode

Petiole

Leaf Axil

Flower

Shoo

t

Vascular System

FIGURE 1 Key parts of a

vascular plant.

Flower

Leaf Axil

Petiole

Apical Bud

Auxillary Bud

Leaf BladeNodeInternode

Vascular System

Primary Root

Root HairsLateral Root

Root Cap

SHO

OT

ROO

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In studying plants, it is important to understand that there are two broad types of structures and processes. Vegetative structures are focused on capturing light energy and turning it into sugars, which fuel growth. Reproductive structures are focused on producing flowers and other features needed to produce propagules to carry on the species. Obviously, both of these processes are critical to the long-term success of plants. However, both require energy and can compete for energy and building materials. Plant managers focus many of their efforts on this balance between vegetative and reproductive structures and processes. However, as will be described, there is overlap between these processes and structures.

NODE VERSUS INTERNODEAn overview of stem features is helpful in understanding the anatomy of plant shoots in greater detail. Points along the stem where leaves are attached are called nodes. The area or connection point between a leaf petiole and the main stem is called a leaf axil. One or more buds are usually found in the leaf axil at the node where the leaf is attached. These buds are called axillary buds. Nodes are quite active regions that generally have high levels of activity because of cell growth and division. Not only do leaves and axillary buds grow from leaf axils, but cuttings of most plants are most likely to root at nodes. The stem area between any two nodes is called an internode. Several environmental and genetic factors affect the length of internodes, and that length provides information about the plant that is useful in gardening.

AXILLARY VERSUS APICAL BUDEvery growing shoot ends in a bud called a terminal or apical bud. Apical buds differ from axillary buds because they are not in a leaf axil. (See Figure 1.) The growth of axillary buds is controlled by plant hormones. Apical buds contain plant hormones (auxins) that inhibit the growth of nearby axillary buds. Removing an apical bud reduces the auxin level and allows nearby axillary buds to begin grow, which influences many aspects of plant management. If axillary buds grow, they may become flowers or

shoots, depending on whether they are flower or vegetative buds.

APPLICATIONS TO GARDENINGThese basic facts about plant biology and terminology are vitally important in plant management. For example, knowing that removal of apical buds removes auxins that previously kept axillary buds from growing means that these apical buds are key points of management. We can remove terminal portions of shoots and be quite specific in the control of plant branching because axillary buds will grow to form branches immediately below the growing tip that was removed. Therefore, we can develop more compact, densely branched ornamental plants by pruning or pinching back the growing tips of plants. This information also guides our management of larger trees and shrubs. Pruning cuts should be made just above a bud where one wants a side branch to grow. Because these buds will produce important side branches, they should point away from the center of the plant to keep the center open. A more open center allows more light in the leaf canopy and improves air circulation to keep the plant most productive. Pruning cuts should be made on an angle about ⅛ inch above a bud. Cuts made closer to a bud may cause the bud to die if the tissue near the bud dries out during healing. It was mentioned that buds can be of the flower or vegetative type, so being able to identify each type is useful in plant production. The axillary buds of peaches and many other flowering plants are rounded if they are flowering buds and pointed if they are vegetative buds. Awareness of this difference helps determine the location for pruning cuts that will encourage side branches from vegetative buds. It can also help determine if flower buds are present on a dormant plant that will flower in the spring. Another benefit of recognizing the visual differences in buds is that flowering buds can be cut and examined for winter injury after heavy freezes. Other dormant plants such as dogwood and many viburnums also have dormant flower buds that are obvious.

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Knowledge of buds also impacts success in rooting cuttings. Gardeners should generally use rooting hormones on cuttings to promote rooting and should be sure that cuttings contain a node placed below the surface of the rooting medium. Propagation of plants by cutting, layering, and division allows the selection and then production of free or nearly free plants. However, growers must be knowledgeable and responsible about rooting cuttings because it is illegal to propagate patented plants without permission. Time, energy, and financial resources are required to develop new cultivars. As horticulturalists and gardeners, we need to respect the investment of businesses and individuals in creating new plant materials for our use by respecting plant patents. In most cases, the legal right to propagate these patented products must be purchased.

What a Master Gardener Should Know About StemsA detailed understanding of stem structure and function can aid in describing and organizing plants. Flowering plants are generally divided into two broad groups called monocots and dicots. These groups differ in many ways, but one of the most distinct differences is their stem structure. Figure 2 illustrates the internal stem structure of dicots and monocots.

CHARACTERISTICS OF TYPICAL MATURE DICOT STEMSMost woody plants in gardens are dicots. Mature woody dicot stems consist of rings of wood (xylem) surrounded by rings of bark (phloem). Where the xylem and the phloem meet is a layer of tissue called the cambium where cell division and expansion occurs, providing growth in girth of the stem. The cambium layer is thin and dry during the winter when plants are dormant and becomes much thicker and moist or even slimy when plants are actively growing. The cambium layer is thicker during the growing season because nearly all of the xylem and phloem cells in a plant are produced by the cambium layer.

CHARACTERISTICS OF TYPICAL MONOCOT STEMSThe monocot stem, illustrated in Figure 2, is very different from the dicot stem. Monocot stems contain bundles of vascular tissue (xylem and phloem) set in a generalized pithy tissue rather than rings. Furthermore, there is no cambium tissue in a vascular bundle of a mature monocot plant as there is in dicot stems. Because mature monocot stems lack cambium tissue, a monocot stem does not enlarge in diameter once it reaches the stage of initial maturity. Agave, asparagus, bananas, cannas, daylilies, hostas, iris, lilies, many flowering bulbs, onions, orchids, palm trees, pineapple, sedges,

Bundle system of vasculartissueina

monocot stem

Ring system of vasculartissueina

dicot stemBundle system ofvascular tissue

in a monocot stem

Ring system ofvascular tissuein a dicot stem

Cross-section of a stem

Phloem

Xylem

Phloem

Cambium

Xylem

Pith

Phloem

Xylem

Bundle system ofvascular tissue

in a monocot stem

Ring system ofvascular tissuein a dicot stem

Cross-section of a stem

Phloem

Xylem

Phloem

Cambium

Xylem

Pith

Phloem

Xylem

Cambium

Pith

FIGURE2Cross-section

of monocot and dicot stems.

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yuccas, and all of the grasses, including bamboo, sugar cane, and corn, are examples of monocots. Nearly all other plants in the garden are likely to be dicots.

STEM GROWTH CHARACTERISTICSDicots develop a new ring of xylem or wood from the cambium layer each year. This layer of xylem develops on the outside of the old layers of xylem where the cambium is located. Phloem tissue also develops from the cambium layer, but a plant develops a new phloem layer on the inside of the old layers each year. Because the woody plant increases in diameter each year and phloem tissue develops from the inside, old phloem layers of plants are unable to entirely cover the outer surface of the ever-expanding stem. The bark may become fissured, as may be seen in many trees, or it may peal or flake off, as occurs with a river birch, crepe myrtle, or oak leaf hydrangea. Many ornamental plants are grown partially for the appearance of their peeling or fissured stems. Since most mature monocot stems do not expand in diameter, they have no such problem and have a generally smooth surface.

FUNCTIONS OF VASCULAR TISSUESXylem and phloem, though differing in structure, both consist of tiny hollow tubes that transport liquids between roots and shoots. Water containing dissolved minerals is initially taken up by the roots. This liquid is then moved into the vascular system and up through the xylem to the green tissue of the plant where photosynthesis, or carbohydrate manufacturing, occurs. Liquid containing carbohydrates moves through phloem to the growing points or storage organs of the plant where the carbohydrates are stored or used in metabolism. This movement of liquids up through the xylem and out or down through the phloem is consistent in all plants. One other important aspect of vascular structure is that xylem tubes become somewhat thick-walled and strong, lending support to many of the tallest and strongest plants as wood tissue in tree trunks long after they cease to be living cells.

INTERNODE LENGTHThe length of plant-stem internodes varies considerably even among plants of the same species, and it can be altered in several ways. Three environmental factors that affect plant internode length are temperature, light intensity, and fertilization. Internode length tends to increase with low light intensity, at high temperatures, or with excessive nitrogen fertilization. Shorter internodes result from high light intensity, lower temperatures, and lower nitrogen fertilization. Extremes in growing conditions or micronutrient levels sometimes cause yellowing or dropping leaves and can also alter internode length. In commercial production of ornamental plants, internode length is an important quality factor and may be controlled chemically. Some plants have very short internodes because of their genetic makeup and are sometimes called genetic dwarfs. Dwarf plants can be recognized by their greatly shortened internodes, which can be as small as ⅛ inch. They are generally very compact plants and consequently do well in containers. Genetic dwarf plants often yield less because of their small size and can be difficult to prune, spray, and otherwise care for because of the dense foliage.

APPLICATIONS TO GARDENING

Plant Injury — GIrdlInG

Information about stem growth and function has many practical gardening applications. For instance, anything that completely removes a strip of phloem around a living plant stem is said to girdle the plant. Common examples include feeding by voles or beavers on stems and careless operation of string trimmers in the yard and garden. Girdling a stem just above the ground stops carbohydrates from reaching the roots of the plant. The roots then starve, leading to the death of the entire plant over a period from a few weeks to a couple of years. Early settlers knew well this practice of girdling and would sometimes remove a strip of bark from around trees (a process called ringing) to kill the trees when clearing fields for crop production. To prevent accidental ringing, string trimmers and lawn machinery should

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be used carefully, and only shallow layers of mulch should be applied around dicot plants to avoid providing shelter for small animals. The mulch around woody plants should be pulled away from the stem a couple of inches to reduce injury from voles. Since monocots have their phloem in vascular bundles throughout their stems, it is essentially impossible to girdle a monocot. An example of a place where monocots have an advantage is in coastal areas; palm trees survive well there, whereas blowing sand can girdle dicots. Winter injury or lawn-mower damage that removes significant amounts of phloem from a plant stem produces a dead area on the stem or trunk. A deep stem injury can remove the actively growing cambium. Living cambium tissue will then be found only at the edge of the dead area. Since new phloem cells arise only from cambium tissue, the wound will slowly heal from the edges over a period of several years. Meanwhile, the plant will continue to grow unevenly around the damaged area. Often this uneven growth weakens the stem or trunk over time, and the wound can be an entry point for disease.

GraftInG

In grafting of woody plants, a small portion of a dormant plant (a stem tip or bud) is removed and attached to a second plant where it can grow and mature. The part removed is generally called a scion, or bud, and the part to which the scion is attached is called the stock, or rootstock. When the graft grows, the top of the plant above the graft has the characteristics of the scion or bud while the stock retains its original characteristics. If a graft is to “take” (survive) and the two parts are to grow, the scion and the stock must come from closely related plants. For example, it is possible to graft between two pear varieties or between pear and quince but not between pear and apple or plum. Also, the graft union must be kept from drying out until callus tissue forms and xylem and phloem tissues unite and begin growth. That is why graft unions are covered with grafting wax, plastic parafilm, or a similar material that restricts the passage of water. Finally, since new phloem and xylem cells arise from the cambium tissue, the cambium of the scion and stock should be in contact over as broad an area as possible. Sometimes the graft will take, but the stock and the scion will not

VegetativeGraftingGrafting can be used to control growth or provide the scion with disease resistance or adaptation to local conditions found in the rootstock. It has been used for thousands of years in woody plants, such as fruit trees, but it can also be carried out on actively growing vegetative plants. In vegetative grafting (as in tomatoes), small plants are kept in a healing chamber that has high humidity and low light for a few days after grafting. These conditions allow the graft union to grow together without the severe stress of water loss and high light, which would likely kill the young plant. The photo on the next page shows a young tomato plant during grafting and immediately before being placed in a healing chamber. Planting depth of woody and vegetative grafted plants is critical to ensure that the shoot does not root and there are no shoots from the rootstock. The adjacent image illustrates a tree being planted in a way that ensures the graft union remains a few inches above the soil surface. Likewise, if grafted tomatoes are planted too deep and roots arise from the stem (adventitious roots), they will not have resistance to soil diseases that were present in the roots from the original rootstock.

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SOURCE: Ken Chamberlain, OSU CFAES Communications and the Vegetable Production Systems Laboratory, OARDC.

SOURCE: Anthony LeBude, NCSU, Bugwood.com

A young tomato plant being grafted

Example of adjusting planting depth to ensure that the graft union is at the proper height

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grow at the same rate or the scion will die after a few years. This is called partial or delayed graft incompatibility. Grafted apple trees often exhibit partial graft incompatibility, and the scion eventually breaks off unless the grafted tree is staked or otherwise supported. It is important to remember that tissue arising below a graft will have characteristics of the stock while tissue arising above the graft will have characteristics of the scion. In other words, grafting clones the plant from which the scion or bud is taken. Rose plants and certain other ornamentals may be produced by grafting a desired variety onto a vigorous wild rootstock. If the resulting plant freezes or dies back below the graft union and then sends up a shoot below this point, the result will be a plant similar to the original rootstock rather than the scion.

usInG stem CharaCterIstICs to seleCt transPlants

Short, stocky transplants look better and generally grow faster than stretched-out, yellowing transplants. Grow transplants under carefully selected growing conditions for optimum vigor and appearance. (See Chapter 11.) Use high light intensity, maintain temperatures that are not too warm (especially at night), and do not provide too much nitrogen fertilizer. Providing ample spacing between transplants also increases light intensity by reducing shading to help prevent stretching. When purchasing transplants, always keep in mind that their appearance is a telling sign about the conditions in which they were grown.

sPeCIalIzed stems In the Garden

In addition to the most common types, stems may take many specialized forms. Stems generally have buds present, and plants developing from these buds may be used to multiply garden plants. Examples of these specialized stems used in plant multiplication are stolons, or runners, tubers, rhizomes, and corms (Table 1). Asparagus is also largely stem tissue, and both kohlrabi and Irish potatoes are enlarged succulent stems that are the main edible portion of those crop plants.

What a Master Gardener Should Know About Roots Roots not only absorb and transport most plant nutrients (see Chapter 5) to above-ground portions of the plants but are also responsible for anchoring plants. Mature roots of the vast majority of plants do not contain chlorophyll and do not produce food through photosynthesis but are dependent on the transport system of the plant for their carbohydrates. Likewise, the above-ground portion of plants are generally dependent on the roots for most of their water and nutrients. Roots often also store carbohydrate or water, and aerial, green roots can even carry out photosynthesis. Many developing plants, such as nut trees, develop pronounced tap root systems before significant top growth appears. Communities of trees may help anchor and support each other as their roots intertwine over time.

DEVELOPMENT OF THE ROOT The three major zones of root growth are illustrated in Figure 3. The meristematic region is at the root tip and manufactures new cells. It is covered with a root cap of loose cells that are constantly rubbing away and being replaced as the root pushes through the soil. The root cap itself is covered by a layer of slick or slimy mucilage that helps to ease movement through the soil. Very little root elongation occurs in the meristematic zone. Root cell elongation is the primary function of the zone of elongation found just above the meristematic zone. Here the cells increase in size and push the root through the soil. The xylem and phloem also develop in this region and begin to carry out their vital functions of moving water, nutrients, and carbohydrates. Finally, there is the zone of maturation in which cells differentiate into specific tissues, such as epidermis or vascular tissue, and root hairs develop. Root hairs are tiny, tubular extensions of the epidermis that greatly increase the surface area for absorption of water and nutrients. Root hairs are constantly dying off and being replaced as the root extends itself through the soil.

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TABLE 1Specialized or Modified Stems

Crown Crowns are compressed stems with leaves and flowers on short internodes.

African violet, dandelion

Spur Spurs are short, stubby, side stems that arise from the main stem and are common on fruit trees where they may bear fruit.

Apple, pear, cherry

Stolon A stolon is a horizontal stem found growing along the top of the ground that is fleshy or semiwoody and has both nodes and buds or leaves.

Strawberry runner, spider plant, Bermuda grass

Rhizome Rhizomes are similar to stolons but grow underground. They can be compressed and fleshy such as iris, or slender with elongated internodes such as bentgrass.

Bentgrass, Johnson grass, iris

Above-groundModifiedStems ExamplePlants

Bulb Bulbs are shortened, compressed underground stems surrounded by fleshy scales (leaves) that envelop a central bud located at the tip of the stem.

Tulip, hyacinth, snowdrop, narcissus

Corm A corm is a solid, swollen stem whose scales have been reduced to dry, scalelike leaves. Corms have shapes similar to bulbs but do not contain fleshy scales.

Gladiolus

Tuber A tuber is an enlarged portion of an underground stem with nodes that produce buds. The eyes of a potato are actually the nodes, each of which contains a cluster of buds.

Irish potato

Tuberous stem A tuberous stem is shortened, flattened, enlarged, and underground. Buds and shoots arise from the top or crown, and fibrous roots are found on the bottom of the tuberous stem.

Tuberous begonia, cyclamen

Tuberous root Tuberous roots are underground storage organs often confused with bulbs and tubers. However, they are roots, not stems, and have neither nodes nor internodes.

Dahlia, sweet potato

Below-groundModifiedStems ExamplePlants

INTERNAL CHARACTERISTICS OF A TYPICAL MATURE ROOTThe internal structure of mature roots is much like the internal structure of the stem. Monocots have bundles of vascular tissue containing both xylem and phloem throughout the root. Dicots have xylem surrounded by phloem with cambium in between. However, the arrangement may be slightly different in dicot roots and appear more as an X or plus sign (+)

than in the rings found in the stem. Xylem and phloem function the same way in roots as they do in stems, moving liquids up through the xylem and down through the phloem.

TYPES OF ROOT SYSTEMSThe two most commonly described types of root systems are the tap root and fibrous systems. Plants with a taproot system, such as dandelions, pecans, and walnuts, have a taproot that grows almost straight down with very little branching.

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Zones of growth in the terminal portion of a root

Root Hair

Root Cap

Zone of Maturation

Zone ofElongation

MeristematicZone

Grasses generally have fibrous root systems in which numerous small roots spread outward in a matlike growth in the soil far beyond the leaf blades found above the soil surface. Most plants fit somewhere between these extremes. Roots of many mature shrubs and trees extend considerably beyond the dripline of the plants. A high percentage of nutrient and water absorption occurs at the outermost extent of the root system. If a tap-rooted plant has the tap root removed when small, the root generally does not grow back. Removal of a plant’s tap root causes most plants to develop a more fibrous root system, so removing part of the tap root is a common nursery practice for some woody species. However, if too much of the tap root is removed or the plant is too old when the tap root is removed, it may die.


edIble roots

We eat the storage roots of many vegetables, including beets, carrots, horseradish, parsnips, radishes, rutabagas, sweet potatoes, and turnips. Root vegetables tend to store well and contribute to the diets of many, especially in the winter when fresh produce is at a premium. Sugar beets contribute significantly to domestic

sugar production. Ginseng and certain other roots have been highly prized in many societies for their medicinal uses. Wild gathering of these roots has had a detrimental effect on some native populations. (See Chapter 2.)

roots for ProPaGatIon

Roots of plants such as blackberry and many ornamental shrubs initiate shoot development and may be used to propagate these plants. Stem sections or tips of blackberries, black raspberries, and many other plants will root and form new plants if buried shallowly over the winter while still attached to the parent plant. This process is called layering. Stem tip cuttings of many plants may also be rooted, some quite easily. After removal of some plants from the garden, a cluster of root suckers may sometimes be found at the previous site. This effect is common with crepe myrtle, sumac, and sassafras. Other roots, like sweet potato and dahlia, will not grow unless they contain a bit of stem tissue. While root cuttings may be used to propagate many plants, root cuttings of grafted plants of course produce plants like the rootstock of the plant rather than the scion.

root struCture and funCtIon durInG transPlantInG

Because they consist only of epidermis cells, tiny root hairs of plants are extremely fragile and likely to die when they dry out. Allowing plant root tips to dry during planting can kill the entire plant very quickly. It is usually best to keep bare-root woody plants in a container of water while planting until almost the moment they are placed into the planting hole. Leaving plant roots underwater for more than a few hours, however, will also kill many plants. Successfully transplanting a plant with a taproot may be very difficult. A pecan tree, for example, may have a taproot several feet long by the time it is a foot tall. One alternative is to plant seeds of plants known to develop extremely long taproots directly in containers where they can develop until they are permanently planted. Dicot roots are generally well branched, but monocot roots, like monocot stems, often exhibit little branching. Be careful not to

Root Hair

Root Cap

Zone of Maturation

Zone of Elongation

MeristematicZone

FIGURE 3Zones of growth in the terminal portion of a root.

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damage the roots of monocots during planting or transplanting unless it is known that the plant can initiate new roots or the old roots have the ability to regenerate by branching. If larger trees or shrubs are to be trans-planted, it is helpful to prepare the plant up to a year in advance by inserting a shovel into the soil at the outer edge of the expected root ball to prune the plant’s roots. This strategy forces the plant to grow more roots closer to the main stem and reduces transplanting shock. However, do not root prune the entire root ball at once if many roots are being severed. In that case, do one third of the root ball, wait a few weeks to do the second third, and finally do the last third a few weeks later. This will allow the plant to adjust gradually to the root pruning. Plants growing within reach of each other often intertwine their roots, which can then become grafted together. In the case of brambles (raspberries and blackberries), viral diseases can pass from plant to plant through these root grafts. If root grafting has occurred in a grove of trees, removing trees may reduce the support of those left and greatly increase the chances of those remaining being uprooted by severe weather. Thus an understanding of root growth can aid greatly in the understanding of plant performance and response to management in the lawn or landscape.

roots not found In the soIl

Aerial roots develop on a considerable number of plants and have several functions. Aerial roots of ivy and certain other vines attach to the growing surface and aid in supporting the plant. Some orchids and other epiphytes develop aerial roots to adsorb moisture from the air. Aerial roots of corn and mangrove plants prop up the plant. Aerial roots (adventitious roots) on the stem of a tomato plant indicate stress from too much or too little moisture that could be caused by environmental conditions or root disease or damage. Aerial roots of parasitic plants like mistletoe enter the supporting plant to absorb nutrients. When a cypress tree grows in standing water, its underwater roots cannot absorb enough oxygen, so the roots produce knobs that extend above the water level to absorb oxygen.

What a Master Gardener Should Know about LeavesLeaves grow to many shapes and sizes, providing a stunning combination of beauty and utility that can enthrall poets and physiologists alike. This section focuses on leaves that have one or more leaf blades, usually attached to a petiole that in turn is attached to a plant stem, as opposed to needles of evergreen trees, which are also leaves. Figure 4 illustrates a cross-section of a typical leaf blade and its main parts as would be seen through a microscope.

THE LEAF BLADEThe upper surface of a typical leaf consists of a single layer of cells referred to as the

The beauty of leaves speaks to scientists, gardeners, and artists alike

SOURCE: Natalie Bumgarner, University of Tennessee

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upper epidermis. This layer is generally quite transparent, contains few if any chloroplasts, and allows light to pass through to the underlying cells. Chloroplasts are plant structures that contain the pigments that absorb light as well as many other proteins, enzymes, and other materials that support photosynthesis. The surface of the upper epidermis is generally coated with a waxy, waterproof cuticle that enables the plant to absorb crucial light in the cells beneath while controlling moisture loss. Just below the upper epidermis, a typical leaf has one or more layers of cylindrical cells with their long axis perpendicular to the plane of the leaf. These layers of cells are referred to as palisade layers. They are filled with green chloroplasts and are vital to the manufacture of carbohydrate. The size and number of palisade cells varies greatly among plants and can be influenced by environment as well as genetics. Beneath the palisade layer is a spongy layer, often called the spongy mesophyll, where cells are not packed as tightly and there are much larger air spaces between the cells. The spongy layer has cells with fewer chloroplasts than found in the palisade cells. Together these two sections of the leaf, palisade and spongy mesophyll, make up the leaf mesophyll. The bottom of the leaf consists of a second epidermis. Although much like the upper epidermis, this lower layer of cells contains thousands of stomates, or stoma, per square inch. A stomate is a very tiny hole in the leaf epidermis that is flanked by two sausage-shaped cells called guard cells. Guard cells differ in shape from the other cells in the lower epidermis

and have the ability to alter their water content and change shape, enabling them to open or close the stomates. This control of the pores or openings in the leaf surface provide a means to control water loss and gas movement between the air and the leaf. The guard cells also differ from the other cells of the epidermis in that they contain chloroplasts. The only other part of the leaf to mention is the veins. Leaf veins contain both xylem and phloem and are surrounded by a sheath that adds strength to the leaf. Leaves contain a network of veins of many sizes such that a vein passes close to every cell of the leaf. Essentially, the leaves are an extension of the plant vascular system that allows movement of water, nutrients, and carbohydrates throughout the entire plant.

FUNCTIONS OF LEAF PARTSThrough a complex process called photosynthesis, leaves produce the carbohydrates that plants use for growth and other vital functions. Photosynthesis is the production of simple sugars from carbon dioxide and water using light for energy and releasing oxygen as a by-product. The vast majority of life on our planet derives its energy directly or indirectly from photosynthesis. The process by which plants produce and use sugars through photosynthesis and respiration is illustrated in Figure 5. Chlorophyll is a green pigment found in plant chloroplasts that gives plants their green color and has a crucial role in light collection. Most of the chloroplasts in a typical leaf are contained in the mesophyll of the leaf, and that

Structure of a typical leaf bladeCutin

Epidermis

Pallisadelayer

Spongyparenchymalayer

Stoma

EpidermisGuard cellMesophylllayer

Stoma

Cutin

Epidermis

Pallisade layer

Spongy parenchyma layer

Mesophyll layer

Stoma

Guard cell Stoma Epidermis

FIGURE4Cross-section view

of a typical leaf blade showing its

structure.

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is where photosynthesis generally occurs. Water is delivered to the mesophyll by the xylem cells. Carbon dioxide passes into the leaf through the open stomates of the lower epidermis. Likewise, the oxygen produced by the leaf mesophyll during photosynthesis passes out of the leaf and into the air through the stomates. If carbon dioxide and oxygen can travel in and out of the leaf through open stomates, then so can water vapor. If a plant loses too much water vapor (through transpiration), it will wilt. If it wilts too severely or for too long, part or all of it may die. Thus the guard cells are designed to close the stomates when plants lose too much water. This closure also shuts down photosynthesis but retains moisture and enables the plant to live to photosynthesize again. In addition, the guard cells reduce stomata openings and water loss at night since there is no light and therefore no photosynthetic activity. Under normal growing conditions, carbohydrates produced through photosynthesis move from the mesophyll to growing plant parts, storage organs, and the plant root system through the phloem tissue of the veins. The tough vein sheath provides support to the leaf throughout its life.

PHYSICAL VARIATION IN LEAVES Figure 6 illustrates several possible leaf variations called simple leaves and compound leaves. Simple leaves have a single leaf blade, while compound leaves have multiple leaf blades. Most monocots do not have a leaf petiole but merely parallel veins within a single leaf blade. (See Figure 7.) Dicots with simple leaves usually have a leaf petiole, a leaf midrib, and an arrangement of finer veins within the leaf blade. Some dicots with simple leaves have parallel veins extending from the midrib of the leaf blade like a bird feather, called pinnate venation. Other dicots with simple leaves have primary veins branching from the petiole like fingers from a hand, called palmate venation. Compound leaves have more than one leaf blade. If several leaflets are attached to a common petiole the leaf is said to be palmately compound. Buckeyes and horse chestnuts have palmately compound leaves. If leaflets are arranged in pairs along a midrib with or without a single leaflet at the end, the leaf is said to be pinnately compound. Many nut trees are pinnately compound. Occasionally one finds a plant with midribs and secondary midribs branching from the main midribs. Leaflets

FIGURE5Overview of essential plant processes.

How a Plant GrowsRespiration Photosynthesis

Water/CO2

Carbon Dioxide + Water(in sun + chlorophyll)Oxygen

Water

NitrogenPhosphorousPotashCalciumOther elements

SugarEnergy Released

(breakdown)

SugarOxygen

RESPIRATION PHOTOSYNTHESIS

Water/CO2

Oxygen

Energy Released

(breakdown)Sugar

Carbon Dioxide + Water{in sun + chlorophyll)

OxygenSugar

NitrogenPhosphorousPotashCalciumOther elements

Water

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are then attached in pairs from the secondary midribs. A plant such as this is said to have double pinnately compound leaves. Examples of this type of leaf may be seen on a mimosa tree or devils walking stick. Nandina is an example of a tripinnately compound leaf. One clue to help determine if one is viewing a leaf or a leaflet is to find the axial bud. Leaf arrangement also varies among plant species, as illustrated in Figure 8. A plant may have only a whorl or rosulate of leaves at the bottom, like a foxglove. Leaves may also alternate up the stem or be positioned in opposite pairs. Finally, sometimes leaves occur in whorls of three or more. All of these characteristics are important in the identification of plants.


InteraCtIon of leaves wIth the envIronment — survIval and ProduCtIvIty

Since photosynthesis is of such vital importance to plants, much effort is directed toward maintaining optimum photosynthetic activity for as long a time as possible. Horticulturalists select the site for plants based upon the USDA hardiness and heat zones the plant can tolerate

but also accounting for sun exposure and water availability. Experienced gardeners seek natural microclimates for plants of questionable hardiness, put up wind breaks, use mulches, and water plants as needed to provide optimal growing conditions. They may even protect plants from unseasonable frosts, use shade cloth to limit sun exposure, or attempt to raise the carbon dioxide level in greenhouses. All such practices are based on the fact that if plants freeze, wilt, sunburn, or otherwise cease normal photosynthetic activity, they will deteriorate, die, or lose their pleasing appearance or value as a food crop. Also keep in mind that leaves are essentially the “engines” of plant function; drastic pruning reduces sugar production ability and is detrimental to growth and survival. As was discussed with root pruning above, leaf pruning should be done in moderation to allow plants to adapt to change. Many plants have leaves that can slowly adapt to the conditions in which they find themselves, usually by thickening the waxy cuticle on the leaf surface. This is the principal behind “hardening off ” transplants. (See Chapter 11.) In hardening, plants are gradually exposed to more sun, fluctuating temperatures,

SimpleCompound

Palmate

Double PinnateCompoundCompound

PinnatePinnate Parallel

Palmate

Types of Leaves and Leaf VenationFIGURE6

Types of leaves and leaf venation.

Simple Palmate Palmate Compound

Pinnate Pinnate Compound Double Pinnate Compound

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and less water. Cuticles thicken and the plants become better able to withstand the extremes of the garden when finally placed there. Interactions of leaves with the environment can impact plant survival, but there are many ways leaves can alter productivity. It was noted earlier that genetic dwarf plants have reduced internode length, compact cucumbers being one example. The shortened internodes on a compact cucumber plant result in a smaller area of leaf cover. Recall, though, that sunlight is necessary for photosynthesis, so the smaller size of the genetic dwarf cucumber plant means that less sunlight falls on its leaves, less photosynthesis takes place, and less carbohydrate is manufactured. The result is fewer cucumbers being formed. In practical terms, if one wants a cucumber plant to put in a container to produce cucumbers for salads, compact cucumbers may be ideal. If, however, the intent is to preserve large amounts of pickles for the coming winter, full-sized cucumber plants with their higher yield are likely a better choice.

edIble leaves

Plant leaves comprise a significant portion of our vegetable and forage crops. Consider lettuce and all the other crops that can be added to a leafy salad. Consider also cabbage, Swiss chard, collards, turnip greens, and kale that are grown for their edible leaves. In addition, the main edible portions of celery and rhubarb are leaf petioles. If this example is expanded to include forage crops for animals, alfalfa and certain other hay crops are of higher nutritional quality if they retain significant amounts of leaf tissue. From these few examples, it is clear that edible leaves are quite important contributors to the nutrition of many species and are a key member of the food chain.

varIatIons In leaves and tyPes of modIfIed leaves

The variation of leaves in the plant world, including their size, color, shape, and arrangement, can be a never-ending source of delight to the gardener. Consider, for example, the English pea. The end of the leaf is a tendril that wraps around anything

AlternateRosulate

Opposite Whorled

FIGURE 8Types of leaf arrangement.

Rosulate Alternate

Opposite Whorled

FIGURE 7Parts of a grass leaf (monocot),

Leaf Sheath

Midvein

Leaf Blade

Collar

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it touches to support the plant. Numerous leaves support plants by floating on water. Cactuses and certain other plants have leaves that are modified into protective spines while the plant carries out most of its photosynthesis in the enlarged green stems. African violets have leaves covered with tiny trichomes, or leaf hairs, that give the leaves a fuzzy appearance and help modify extreme environmental conditions. Some leaves mimic flower petals and can easily pass as such to the novice. Some leaves are even adapted to catch insects, such as those found in pitcher plants, Venus flytraps, sundews, and certain others. Some plants have pigment other than chlorophyll in their leaves, giving the plant a color other than green. Consider the yellow- or purple-leaved plants popular today as well as plants with multicolored leaves such as coleus and hosta. While these various pigments all have specific defensive or other roles in the plant, they can also be a great visual asset. They can add variety and interest to our landscape and brighten shady spots where many plants flower poorly. The mixing of plants with different leaf shapes also adds texture to the landscape.

Some plants have leaves with other modifications that allow them to carry out activities that are typical of other plant parts. Leaf bracts are leaves that resemble flower petals in appearance and function. Dogwoods, for example, have these modified leaves called bracts that appear to be flower petals and attract pollinating insects as petals would. After filling this role for a few days, these modified leaves drop. Bracts of poinsettia also resemble flower petals because they contain pigments other than chlorophyll. Many people do not notice the actual poinsettia flower, and if the plant is placed in a warm room in bright, indirect light and watered as required, its bracts can remain attractive to keep the plant “in flower” for several weeks. Bougainvillea and many plants in the plant genus Euphorbia (such as the poinsettia) also have leaf bracts. Onions, lilies, and plants that grow from true bulbs have modified leaves that serve as storage organs. The fleshy leaves of aloe and many other succulents are examples of leaves that serve a storage function.

Modified leaves (bracts) of poinsettias in greenhouse production during the fall as bract red coloration increases


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What a Master Gardener Should Know About FlowersThe primary system of plant classification is based on flower structure, and thus knowledge of flowers is essential to plant classification and to gardening. A generalized complete flower has four main parts: sepals, petals, stamens, and a pistil. Not all flowers have all four parts. Flowers that lack one or more of these parts are called incomplete flowers. Figure 9 illustrates the parts of a complete flower, and the photo on page 3-22 clearly shows three stamens and one pistil with a background of petals.

THE FOUR MAIN PARTS OF A COMPLETE FLOWERSepals are small, generally green leaf-like structures at the base of a flower. They protect flower buds and are collectively called the calyx. Petals occur just inside of the calyx. They are frequently highly colored and may be scented. Flowers may contain many petals, which are collectively called the corolla. Sometimes differentiating between sepals and petals can be a challenge, and monocots, in particular, may have sepals that mimic petals. Monocots typically have flower parts in threes or multiples of three while dicots typically have flower parts in fours, fives, or multiples of four or five.

Stamens are the male reproductive parts of flowers. Flowers typically have a cluster of stamens just inside the petals. A stamen consists of a pollen sac called an anther, which produces pollen, and a large supporting filament. The pistil contains the female reproductive parts of the flower, and a single pistil is typically located in the flower center. The pistil is generally shaped like a miniature bowling pin and consists of a stigma, a style, and an ovary. The stigma, which is sticky or fuzzy to receive pollen, is typically at the top of the pistil and somewhat knoblike. It is connected to the ovary by the style. The ovary contains structures that develop into seeds after pollination. Flowers containing both functional stamens and pistils are called perfect flowers. If either of these parts is lacking, the flower is said to be imperfect.

POLLINATION AND FERTILIZATIONPollination is the process by which pollen is transferred from the anther to the stigma, enabling fertilization and reproduction. Pollen can be transferred mechanically in flowers whose anthers and stigma touch, or by wind or insects. Honeybees are the most widely recognized pollinating insect, but other insects, especially other kinds of bees, some types of flies, ants, moths, beetles, wasps, and butterflies also pollinate flowers. Bats, lizards, hummingbirds, and other organisms pollinate some flowers.

FIGURE 9Parts of a generalized flower.

Stigma

StylePlacenta Ovary

Ovules

AntherFilament

Petal (Corolla)Sepal (Calyx)

Stamen

Pistil or Carpel

Parts of a Generalized Flower

Stigma

Ovary

Style PistilorCarpelPlacenta

Ovules

AntherFilament

Stamen

Petal (Corolla)Sepal (Calyx)

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After pollination, a pollen tube grows down through the style and into the ovary where fertilization (union of sperm nucleus and egg) occurs and seed develops. Fertilization is a much more complex process than described here and serious students may wish to investigate it further.

SELF- VERSUS CROSS-POLLINATED PLANTSFor pollen tube growth and fertilization to occur, pollen must come from a plant of the same kind or one very closely related to the plant of the stigma where it lands. Specific requirements for pollination vary widely in the plant world. In some cases the pollen must come from the same kind of plant but one with a different genetic makeup. These plants are said to be cross-pollinated. Consider, for example, apples and peaches. Placing pollen from a red delicious apple flower on the stigma of a red delicious flower does not result in fertilization and the growth of an apple. Only pollen from a different variety of apple will produce fruit on a red delicious apple tree because apple trees are

cross-pollinated. Therefore, it takes two apple trees of different varieties or at least two sources of apple pollen to grow apples. In contrast, peaches are generally self-pollinated, and only a single peach tree is necessary to produce peaches. Obviously, these requirements are quite important for growers and gardeners alike. If fruit is the goal, then it is essential to know whether the plant is cross- or self-pollinated so a pollinator plant can be provided if needed.

MONOECIOUS PLANTSSome plants have pistils and stamens on separate flowers and may be said to have male and female flowers. They are called monoecious plants. Monoecious plants include corn, many trees and all vine crops, including cucumbers, squash, pumpkins, gourds, cantaloupe, and watermelon. Monoecious plants frequently develop flowers mostly of one sex early in the growing season. Flowers of both sexes are necessary for pollination to occur, and it is necessary to wait for them to occur if fruit set is desired.

SOURCE: Carol Reese, University of Tennessee

An open flower clearly showing three stamens and one pistil with a background of petals

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DIOECIOUS PLANTSDioecious plants have flowers of one sex on one plant and flowers of the other sex on another plant. That is, they have both male and female plants, and both are needed to produce fruit on the female. Dioecious plants include spinach, ginkgo, American persimmon, holly, and many others. Knowing if a plant is dioecious is quite important if the goal is to produce fruit because a female holly, for example, will not fruit unless a male pollinator is present.

APPLICATIONS TO GARDENINGSometimes plants are grown for the flowers, in which case the number of flowers and their appearance and quality are of utmost

importance. In other cases, the production of fruit, berries, or other results of pollination and fertilization are the objective. It is important to bear in mind that without fertilization there will be no seed development of any kind, and it is thus necessary to determine whether a plant is self-pollinated or cross-pollinated. If a plant is cross-pollinated, the grower must determine whether the main agent of pollination is an insect or wind, and whether a second plant of another variety — or even whether a specific plant or type of plant, as with pecans — is needed. Of course, cross-pollination requires that the plant varieties flower at the same time. With some plants, such as blueberries, fruit set will occur without cross-

How Do We Get Seedless Cucumbers? There are exceptions to general statements about flower types and pollination. For instance, some vine crops, such as cucumbers, can be bred to produce only (or primarily) female flowers, and they are called gynoecious. Some gynoecious cultivars require fertilization, so pollinator plants that produce male flowers are added to the field. However, many gynoecious cucumbers do not require fertilization to produce fruit, a characteristic called parthenocarpy. Parthenocarpy can be quite useful for production, and the fruit are often referred to as seedless. For instance, greenhouse cucumbers are generally gynoceious and parthenocarpic, and they can be quite productive in protected environments without the need to provide and manage pollinating insects.


A greenhouse cucumber plant that is both gynoecious and parthenocarpic

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pollination, but more and larger fruit will be produced if a pollinator is present.

PLANT PHYSIOLOGICAL AND ENVIRONMENTAL INTERACTIONS IN RELATION TO GARDENING

Plant Growth RequirementsEvery plant has temperature, water, nutrient, and light-intensity requirements for optimum growth. USDA plant hardiness zone maps (see Chapter 2) list the average low temperature for all areas of the US to help determine the likelihood of a specific plant surviving the winter in a given area. See Chapter 8 for additional information on climate factors and plant survival. It is likewise important to be aware of the degrees of shade (full shade, half shade, or dappled shade) in which a plant is grown to select the most favorable sites for plants. Half shade refers to shade for half the day. Among plants that prefer half shade, most prefer morning sun but shade from the hot afternoon sun. Dappled shade under a tree can frequently be obtained by removing the tree’s lower limbs until the sun shines through off and on throughout the day. Remember that a plant that will grow in extremely wet conditions, such as a bald cypress, will frequently tolerate drier conditions, whereas a plant that prefers dry conditions will likely be killed if placed in soil that is too wet. Interactions between light levels and moisture can occur as well. For example, many shade-loving plants, such as lily of the valley or hellebores, will tolerate more sun if given adequate water.

Plant HardinessLabels on plants sold by nurseries and garden centers commonly list a hardiness zone. For example, “hardy to zone 7” means that the plant should be hardy to temperatures as low as 0 to 10°F, which is the minimum temperature range of zone 7 on the USDA zone map. In real life, the situation is not so simple, so it is important

to consider some of the mitigating factors for plant hardiness in the residential landscape.

FACTORS AFFECTING PLANT HARDINESSActual plant hardiness is influenced by many factors, including general plant health, the extent to which a plant has become established in its current location, the degree of hardening it has undergone, water availability, wind conditions, and length of exposure to the minimum temperature. It is logical that a vigorous plant is more likely to survive cold stress than a barely living plant. Likewise, an established plant is likely to be hardier than one newly planted. Marginally hardy plants for the zone or climate should be planted in the spring rather than the fall to allow them to become well established before winter. Plant growth normally ceases as days shorten and temperatures fall. The normal transition into fall and winter involves plant tissues maturing, dormancy beginning, and hardiness increasing. Keep in mind that a plant’s winter hardiness is more a process than an event. Damage to plants from cold weather is worse in years with an extremely warm fall followed by a plunge to extremely low temperatures versus years with more gradually cooling weather. Nutrition and growth rate can also impact hardiness, so most outdoor plants should not be fertilized after July, thus encouraging them to cease growth and begin preparing for winter. Moisture stress can also impact hardiness in some plants. Evergreen plants retain their leaves during the winter and require significant moisture all year. Even during the winter they should be watered whenever the soil is dry until they are well established. Mulching these plants so that the soil around their roots does not freeze and remains moist may also increase the chances of survival. Wind can dry the foliage of evergreens and kill plants unable to take up moisture rapidly. Plants can be wrapped to reduce injury from wind or ice accumulation. Materials designed for wrapping evergreens during the winter are widely available from horticultural supply houses. Finally, cold damage to a plant can be cumulative. The longer extreme cold lasts, the

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more likely it is that tissues will freeze, adequate water will not be available in the frozen soil, and wind will dry out plant tissues. Judicious site selection can help. Certain places in a garden may be protected from north winds. Areas sloping to the east or south maximize exposure to the sun and can be a full zone warmer than the average in the garden. Savvy gardeners identify these microclimates and use them to their advantage.

HARDINESS VARIABILITY AMONG PLANT PARTSUnderground temperatures generally do not vary as much or change as fast as air temperatures. On the other hand, plant roots are frequently not as hardy as the above-ground portions of a plant. This sensitivity to cold can be a problem when plants are dug and shipped. Extreme cold during shipment can kill the roots of a plant without injuring the above-ground plant parts. A plant with dead roots may begin to leaf out in the spring and then die. Also, plant buds may also not be as hardy as the stems of a plant. Peaches, for example, may fail to bear because the pistol and stamens in the plant buds froze during the winter. Peach buds may be sliced and examined after a hard freeze to see if the bud parts are still alive.

LOSS OF HARDINESSOne might think that a plant hardy in New England would be fully hardy in Tennessee. After all, it can survive bitter northern winters, which are often far below zero. However, once again, the situation is not quite so simple. Cell changes occur in northern plants as temperatures cool, and the plants become ever hardier until they can withstand temperatures far below zero. In the spring, these plants quickly lose their hardiness and begin growth. Quite simply, they lose hardiness in the spring faster than they acquire it in the fall. When such a plant is brought south, it quickly loses its hardiness at the first winter warm spell and then freezes at the next cool spell. Since the southwest side of a plant receives the most winter sun, it often loses hardiness first, leaving a dead strip of bark on the southwest side of the trunk. These plants may or may not heal in future years from the edges of the cambium

where live cells remain. Painting the trunk of a young or marginally hardy tree with white latex paint can help prevent such damage or promote healing by reflecting sunlight and keeping the trunk cooler. Also, it may be best to purchase plants grown in the South because they tend to acquire and lose hardiness more slowly than northern cultivars.

RespirationPhotosynthesis, the process whereby green plants manufacture simple sugars for use in their growth processes (metabolism) was discussed earlier and illustrated in Figure 5. Metabolic activities continue as long as plants are alive, and they require energy obtained by breaking down carbohydrates manufactured during photosynthesis, a process called respiration. Essentially, respiration is photosynthesis in reverse. The constant need for carbohydrates for respiration is not generally problematic while photosynthesis is occurring because carbohydrates are often being manufactured more rapidly than they are needed for energy. But if continual darkness or environmental stresses stop photosynthesis, respiration still continues at some level. If such conditions last long enough and all the free carbohydrates in the plant are broken down through respiration, plant tissues will begin to be broken down for energy. Growth ceases, tissues yellow, leaves drop, and eventually the plant dies.

FlowerInitiation

LONG-DAY VERSUS SHORT-DAY PLANTSLong ago, when physiologists first began to study plants, it was discovered that some but not all plants respond to a specific critical day length by flowering (a characteristic called photoperiodism). Plants that flowered whenever days were longer than a critical length were referred to as long-day plants and those that flowered when days were shorter than a critical length were referred to as short-day plants. It was later discovered that it was the night length rather than the day length that was the critical factor. Not only did a short-day

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plant require a long night, but it required an uninterrupted long night. Consider, for example, the poinsettia, which is a short-day plant. When it gets long enough dark periods, plant chemical compounds initiate flowering. Once this process has begun, it is possible for light during this period to disrupt the flowering process. It also requires about two months for enough of this compound to be manufactured for the poinsettia’s leaf bracts to begin turning red. Hence, poinsettias can be made to flower again, but it is not a simple procedure. Chrysanthemum is also a short-day (long-night) plant, and spinach is a long-day plant.

EFFECT OF TEMPERATURE AND MOISTURE ON FLOWERINGTemperature and moisture are two other environmental factors that frequently affect flowering, although the relationship can be complex. The North American Clivia Society, for example, reports that one can coax a clivia into flowering by giving it no water from October 1 to January 22 and incorporating five weeks of chilling between 35° and 55°F during this time. That explains why a clivia may flower when left without water in an unheated garage over the winter. Likewise, Christmas cactus flowers when it experiences a certain period of chilling at 50° to 59°F or when exposed to temperatures of 60° to 68°F with uninterrupted nights of 13 hours or more. Amaryllis bulbs may also be dried in the fall until their foliage is dead. They can then be chilled and will flower when warmed and watered. Such detailed information was once almost unavailable without consulting complex scientific journals, but now it is readily available on the Internet. One merely types “flower initiation in Easter lily” or any desired plant into a search engine and numerous articles will be listed.

PLANT CLASSIFICATION AND TAXONOMY RELATED TO GARDENINGEstimates of the total number of plant species on our planet vary from 300,000 to 400,000.

With this volume of plant life, it is obviously important to have a uniform means of classifying and naming plants to enable accurate communication. Common names often vary from one area to another. Members of the Narcissus family, for example, are commonly known as daffodils on the West Coast, jonquils in the northern states, and buttercups in the south. If we want to communicate, purchase a specific plant, or use a specific plant in the development of products for human consumption, it is obvious we need a better, more uniform system for identifying it than its common name.

ClassificationBasedon LifespanThe simplest system of classifying plants is based on how long they live. Annuals live a single year, biennials take two seasons to flower and set seed, and perennials live more than two years. Even this simple system is useful to the gardener because biennials such as hollyhocks, Canterbury bells, sweet William, and many vegetables grow for a season and then flower and make seed after exposure to winter conditions. Gardeners would likely not be pleased if their cabbage, beets, lettuce, onions, or Swiss chard bolted and went directly to seed without making a tasty leaf or root crop. They might, on the other hand, be pleased if their flowering biennial plants flower the first year. Flowering may be managed to some extent by controlling the stress to which a plant is exposed. Small growing pots, early transplanting, cool temperatures, and even drought stress can result in earlier flowering and seeding during the first growing season, while better growing conditions can prevent bolting and flowering of vegetable crops. This concept can be very useful in gardening. Classifying plants based on lifespan is also a gross simplification of the real world. Herbaceous perennials, for example, have perennial roots but annual tops that die back to the ground each winter. Blackberries and raspberries (brambles) have biennial tops and perennial roots. The so-called evergreen raspberry has cane tips that are annual, lower

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We’ve met. You were wearing green.by Carol Reese I was young, single, and had taken a job several hours from my family. So that I could take my dogs and cats, I searched hard to find a secluded place in the country to rent. When you find places that landlords are willing to rent to pet owners, it is often because the houses are already in far from pristine condition. So the remote site and the rickety house whispered of neglect, even abandonment, and while I was not afraid, I did wonder if I could make it feel like my home. I needn’t have worried. While the cats were timid at first, the dogs took to it in just one night. Apparently, sleeping together in a pile with their chosen human anoints any place as an acceptable den. If anything, they were full of more zest than customary when we set out the next morning for an exploration of the surrounding fields. I found their canine “carpe diem” irresistible — that new scents, new sights, and new trails were to be explored, not feared. Not all was new. I recognized trees and fencerow flowers that also grew on the farm where I’d grown up. My granddaddy had taught their names to me, and even the names of the clouds that followed us overhead. I saw and heard the same birds that had been a part of my world as soon as I was old enough to notice them. It made me recall something I’d been told about the human need to have a name for things. I had been mewling about the amount of time I spend as a horticulturist identifying plants for people, often when their sole aim is to kill them. Most plants I could tell them how to kill without them knowing their names. This wise person said that knowing its name satisfied a need in the human psyche. In the future, a person wants to recognize it, even if detested — wants to be able to say, “Yes, I know you, we have met before.” I wholeheartedly get that, and it gives me the patience to deal with the numerous plant identification questions that come with the job. I only wish I could get that same patience from those very people when I give them the scientific name of the plant. One said to me irritably, “But what is its REAL name?” Of course, the scientific name is its real name. The common name can change from region to region, or even from family to family. Plus, the scientific name allows us to peg its exact place in taxonomic classification, so that we know how it fits into the whole plant kingdom. In other words, we know its kinfolks, and once again, this adds comfort to the human psyche. Think about how many conversations you have had with a new acquaintance, with the sole aim of figuring out who were their “people.”Univers

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canes that are biennial, and roots that are perennial, all on the same plant. Clearly, a still better plant classification system is needed.

ModernPlantClassificationToday’s system of plant classification is an ordered system based on plant relationships. This system grew out of the work of the Swedish botanist Carl Linnaeus, who developed parts of it during the 1700s. In this system, the plant kingdom is broken down into divisions, which are further divided into smaller groups called classes. This process continues through orders, families, genera (plural of genus), and finally species. Progressing downward from kingdom to genus and species, the degree of relationship increases until there is only a single type of plant represented by the combined genus and species. The genus and species names together comprise a plants’ scientific name. This two-word naming system is called binomial nomenclature and includes a scientific name for every identified distinct lifeform on the planet. Using scientific names, a person can communicate with anyone in any language. Other subgroupings are sometimes used in the classification system, but the scientific name does not require them and they will not be addressed here. Scientific names are always written in Latin because Latin is a dead, or unchanging, language. The genus name is always capitalized, but the species name is always lowercased. The two-word name is either written in italics or underlined. It may be followed by an initial representing the person who named the plant, by a variety or cultivar name, or both. Scientific names are understood the world over and eliminate all doubt as to the organism under discussion. They are frequently descriptive and help identify specific plant characteristics, regions of origin, or other details. New plants continue to be found even today, and previously named plants are sometimes reclassified as we learn more about them. In general, though, the system is now relatively fixed.

Taxonomic KeysBecause the system of plant classification is based on an ever-increasing degree of relationship, it is possible to develop keys that allow the identification of a specific plant by closely observing the characteristics of a sample. These keys are extremely complex because of the myriad of Latin terms they contain and are best used by botanists who understand the terminology. It is especially helpful if the samples to be identified contain a flower, as flower structure is at the basis of the taxonomic system. Figures 10 and 11 illustrate some of the terms describing leaf structure that are encountered when using a taxonomic key.

The Genuine Usefulness and Joys of Plant TaxonomyThe term taxonomy itself can seem intimidating, but this scientific discipline can actually be very interesting and helps shape the way we view the natural world and think about plants. Taxonomy is simply a system of learning how plants are related to one another, and in a larger sense, to the entire natural world. Obviously, there are many thousands of plant species, and they come in all sorts of shapes, sizes, and colors. It could be confusing, but one of the empowering traits of the human mind is to devise ways of putting things into an understandable order. It is really not that hard. (See Figure 12.) Perhaps it is easier to think about by examining an example pertaining to animals, as shown in Figure 13. For example, at the species level, consider that there are Siamese cats, Persians, Abysinnians, and others. This way of classifiying the types of cats is very similar to that as used for cultivars in plants: for example, the many different types of apples, such as ‘Fuji’, ‘Gala’, or ‘Granny Smith’. To make the process clearer, consider the following classification of oakleaf hydrangea:Kingdom: Plantae — PlantsSubkingdom: Tracheobionta — Vascular plantsSuperdivision: Spermatophyta — Seed plantsDivision: Magnoliophyta — Flowering plantsClass: Magnoliopsida — DicotyledonsSubclass: Rosidae

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Variations in Leaf Margin

Entire Sinuate Crenate Dentate Serrate Serrulate Doubleserrate

Incised Lacerate Pectinate Ciliate Lobed Cleft Parted

FIGURE 10Variations in leaf apex and base.

FIGURE 11Variations in leaf margins.

Variations in leaf apex and base

Acute Acuminate Aristate Cuspidate Mucronate Obtuse

Cordate Auriculate Sagittate Truncate

Cuneate Attenuate ObtuseRetuse Emarginate

Acute Acuminate Aristate Cuspidate Mucronate Obtuse

Retuse Emarginate Cuneate Attenuate Obtuse

Cordate Auriculate Sagittate Truncate

Entire Sinuate Crenate Dentate Serrate Serrulate Double Serrate

Incised Lacerate Pectinate Ciliate Lobed Cleft PartedUniv

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Order: RosalesFamily: Hydrangeaceae — Hydrangea familyGenus: Hydrangea L. (L. refers to this genus

being described by Carl Linnaeus)— hydrangea

In the nursery trade we often find these species:Species: Hydrangea arborescens L. — wild

hydrangea Species: Hydrangea macrophylla (Thunb.) Ser.

— French hydrangea Species: Hydrangea serrataSpecies: Hydrangea paniculata Siebold —

panicled hydrangea Species: Hydrangea radiata Walter — silverleaf

hydrangea Species: Hydrangea quercifolia W. Bartram —

oakleaf hydrangea Furthermore, there are dozens of selections of oakleaf hydrangea that have been made for special ornamental attributes, perhaps more compact, or that have very large panicles or double flowers, even one that has golden foliage. These are propagated vegetatively to clone that exact plant and not lose the special mutation. They earn the status of a named cultivar. Thus the proper way to write the full name of the oakleaf hydrangea cultivar ‘Ruby Slippers' is Hydrangea quercifolia ‘Ruby Slippers’. Note

that the genus is capitalized, the species is not, and the cultivar name is capitalized, not italicized, and framed in single quotes. Easy! Sometimes it is useful to know a bit about plant families, just one step back up the hierarchy. Many families have an easily recognizable feature. For example, all plants in the family Rosaceae have five petals in the flower. Flowers in the mint family Labiaceae have square stems, though plants in Verbenaceae do as well. Plants in the mint family also usually have a strong odor, sometimes pleasant, but not always. Fabaceae is the bean family, and its members bear the familiar fruiting pod, plus most are also nitrogen fixers. The fruit and floral characteristics are the most important features that demonstrate relationship, and a good example would be the redbud, which has a pealike flower and forms a bean. These features shows it is a member of the bean family, Fabaceae, as are garden string beans. Not only is the family relationship helpful with plant identification, but knowing that certain problems can be common in related plants can help with pest and disease diagnosis. For example, many plants in the family Rosaceae are susceptible to fire blight. Gardeners usually deal with plants at the genus and species level, and that very important

FIGURE12The organization

system that enables us to classify living

things.

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Increasing numbers

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KINGDOM - Animalia

PHYLUM - Chordata

CLASS - Mammalia

ORDER - Carnivora

FAMILY - Felidae

GENUS - Felis

SPECIES - Catus

Classification hierarchy of a cat

information is all they really need to know to be certain they are getting the right plant. A good example of why using common names can cause trouble is the case of the pin oak. For some people, pin oak is the tree known botanically as Quercus phellos, while other people would call this species by the common name willow oak, a highly recommended shade tree. However, another species is often called pin oak, namely, Quercus palustris. This tree is costing homeowners across Tennessee many thousands of dollars, as it is extremely susceptible to bacterial leaf scorch, a fatal and

untreatable disease. Use the Latin name to be certain! Sometimes gardeners are already using the Latin names and are not even aware they are Latin, such as magnolia, hydrangea, viburnum, zinnia, zoysia, or phlox. You can see that Latin is not intrinsically difficult, and often the Latin terms are quite accessible. For instance, the full Latin name for southern magnolia is Magnolia grandiflora, and it is easy to see that the species name would translate to mean “large flower.” The star magnolia is Magnolia stellata, and it does not take a stellar brain to make that connection.

FIGURE 13An example of the classification of cats.

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GLOSSARY

AAdventitious root A root that is not produced from a root organ. These roots are

commonly produced by underground stems but can be produced by stems that are not buried.

Annual A plant that completes its entire life cycle in a single year.Anther The pollen-bearing portion of the stamen.Apical bud The bud on the end of a shoot or branch.Asexual Without the fusion of egg and sperm.Auxin A chemical in apical buds that inhibits the growth of nearby axillary

buds.Axillary bud A bud located in a leaf axil.

BBiennial A plant that completes its life cycle in two years or seasons, usually

germinating in one year and flowering in the next year. Binomial nomenclature The modern two-word naming system founded by Carl Linnaeus.Bract A modified leaf resembling a flower petal.Bulb A specialized bud or underground storage organ with a greatly

shortened stem surrounded by fleshy leaves or scales.

CCambium A lateral meristem that gives rise to xylem and phloem.Calyx The sepals collectively. The calyx is the outer or lowest flower part and

is usually green.Chilling requirement The number of hours of exposure to cool temperature before seeds,

plants, or plant parts will grow after maturity or dormancy.Chlorophyll The green pigment in all green plants responsible for the absorption of

light to provide energy for photosynthesis.Chloroplast A specialized body in plant cells that contains chlorophyll.

Photosynthesis occurs in chloroplasts.Clone A group of genetically identical plants propagated vegetatively from a

single individual. Complete flower A flower in which all four parts (sepals, petals, stamens, and pistil) are

present.Corm A short, thickened underground stem covered with dried leaf bases but

without scales.Corolla A collective term to designate all the petals of a flower.Cotyledon An embryonic leaf that is the first to appear from a germinating seed.Cross-pollinate The transfer of pollen from the anther of one plant to the stigma of a

flower of a plant of another clone.Crown A compressed stem, often with a rosette of leaves where the shoot and

root meet.Cuticle The waxy coat of the epidermis.

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DDay-neutral plant A plant whose flowering is not affected by the length of the dark

period.Dicotyledon (Dicot) A flowering plant with two seed leaves or cotyledons.Dioecious Bearing male and female flowers on different plants.

EEpidermis The outermost layer of cells on leaves, stems, and roots.Epiphytes Plants that grow on another plant for support. Their roots are used only

for attachment because they get water and nutrients from the air and produce their own energy through photosynthesis.

FFertilization The union of sex cells to form a new living thing.Filament The stock of the stamen that supports the anther.

GGenus The taxonomic group between family and species.Germination The resumption of growth of an embryo.Girdle To remove a strip of phloem completely around a plant stem.Grafting The uniting of plant parts by placing them in close contact and

providing a favorable growth environment.Guard cell A specialized cell around a stomate responsible for opening and closing

the stomate.Gynoecious Monoecious plants that are bred and developed to produce primarily

female flowers.

IImperfect flower A flower missing either pistil or stamens.Incomplete flower A flower lacking one or more of the four flower parts.Internode The part of the stem between two nodes.

LLayering Rooting a stem portion to form a new plant while still attached to the

parent plant.Leaf axil The angle between the upper leaf petiole and the plant stem.Long-day plant A plant that flowers when it experiences short nights.

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MMeristematic region An area containing undifferentiated cells where cell multiplication

occurs.Mesophyll The interior cells of a leaf consisting of the palisade and the parenchyma

cells.Metabolism The physical and chemical processes occurring within a living cell that

are necessary for the maintenance of life.Microclimate The climate of a localized area that may differ in temperature, moisture,

wind, or other factors from the surrounding area. Microclimates could be small areas in the home landscape or large areas affected by large bodies of water or urban infrastructure.

Monoecious Bearing male and female flowers at different locations on the same plant.

Monocotyledon A plant that has only one cotyledon or seed leaf in its embryo.

NNode The part of the stem where leaves and axillary buds arise.

OOvary The bottom portion of the pistil that contains the ovules. Ovule A plant female reproductive structure in which seeds develop after

fertilization.

PPalisade cells The columnar, chloroplast-bearing cells making up the top layer(s) of

the mesophyll.Palmate Having leaves or veins radiating from a common point.Parthenocarphy The production of fruit without fertilization.Perennial A plant that lives three or more years.Perfect flower A flower with both stamen(s) and pistil(s).Petal One of the units of the corolla of a flower.Phloem The vascular tissue that conducts food.Photosynthesis The production of carbohydrate from carbon dioxide and water using

light energy and releasing oxygen.Photoperiodism Developmental response of plants to the relative length of light and

dark.Phototropism A growth movement in response to one-sided illumination.Pinnate Having parts arranged along two sides of an axis like a feather.Pistil The female part of a flower made up of the stigma, style, and ovary.Pollen Male cells produced in anthers capable of uniting with the female egg to

produce seed.Pollination The transfer of pollen from anther to stigma, enabling fertilization.Propagule A plant part or structure that can produce a new individual. It could

be a tuber or corm if propagating asexually or a seed or spore for sexual propagation.

Protoplasm The living material of a cell.

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RRespiration The release of energy from sugar and other organic compounds by

breaking them down into carbon dioxide and water.Rhizome An underground stem.Root hairs Tubular extensions of the root epidermis that greatly increase the surface

area of the root.

SScion The top portion of a graft.Self-pollinate The transfer of pollen from the anther to the stigma of the same flower,

another flower on the same plant, or within a clone.Sepals The lowest or outermost of the four flower parts.Shoot A young stem with leaves present.Short-day plant A plant that will flower only when it receives long, uninterrupted nights.Species A group of individuals that actually or potentially interbreed.Stamen Male part of the flower consisting of anther and filament.Stigma The end of the pistil that receives the pollen.Stock The bottom portion of a graft.Stolon A stem that grows on the surface of the ground.Stomate An opening through the epidermis through which gases are exchanged.Style The slender part of the pistil between the stigma and the ovary.

TTaxonomy The branch of science concerned with classification.Tendril Slender coiling organs of climbing plants, often modified leaves.Transpiration The loss of water from a plant in the form of vapor.Trichomes Hairlike extensions of plant cells.Trunk The main stem of a woody plant.Turgor The swollen condition of a cell caused by internal water pressure.Tuber A fleshy underground stem.

VVascular bundle A tissue made of xylem and phloem that conducts food, water, and

minerals within a plant.Vascular plant A plant with xylem and phloem.Vegetative propagation Propagation other than from seeds or spores.

XXylem The vascular tissue that conducts water and minerals.

ZZone of elongation The region of a root where cells enlarge, pushing the root tip deeper into

the soil.Zone of maturation The root zone in which tissues that will make up the mature root, such

as xylem, phloem, and epidermis, form.

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