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Chapter 4: Basic design I.

Adaptation basically means adjusting to or fitting a specific environment. Adaptive changes can occur in any part of the plant, and the outcome of this adjustment may be the development of unique or unusual morphological modifications. Theses natural morphological adaptations allow plants to exist in different environmental settings. Plants have become adapted to an amazing number of environmental settings and variations. Although plants are placed under stress by cold, drought, heat, and monsoons, plants found in these extreme areas are remarkably well designed, unable to move away from the inhospitable conditions. While the variability is great, the external morphology (overall form) of higher plants is composed of the same basic parts: roots, stems, and leaves.

Objectives

After completing this chapter, you should be able to:

* Use the terms that describe vegetative parts of the plant

* Name three types of roots

* List three functions for roots and for stems

* Describe a stem

* Name four types of stems

* List all of the parts of typical leaf

* Give three types of venation in leaves

* Explain the difference between an incomplete and a complete flower

* Identify all of the parts on a complete flower

* Describe the functions of the essential reproductive organs of the flower

* Name three types of flowering characteristics found in plants

* Discuss four ways flowers vary

* Name the two large categories of fruit

* List four terms used to describe fleshy fruits and four terms used to describe dry fruits

* Discuss how plant structure is used to classify plants

* Identify all of the parts of a typical seed

Key Terms

nonvascular plants

vascular systems

vascular plants

gymnosperms

angiosperms

anchorage

absorption

conduction

taproots

storage taproot

fibrous roots

branch roots

epidermal cells

root hairs

adventitious roots

prop roots

bromeliads

haustorial

rhizobium

mycorrhiza

symbiotic

nodes

internode

axil

lateral buds

terminal buds

bud scales

annual tree ring

alternate leaf arrangement

opposite leaf arrangement

whorled

basal rosette

rhizome

stolons

tuber

corms

cladophylls

convergent evolution

thorns

prickles

spines

tendrils

blades

petiole

pinnately compound

palmately compound

venation

netted

parallel

palmate

midrib

margins

entire

dentate

lobed

apex

peltate

sessile

sheath

pubescence

mesic

insectivorous plants

vegetative reproduction

budding

sexual reproduction

grafting

rootstock

scion

vascular cambium

stem cutting

layering

Vegetative Morphology and Adaptations

One of the consistent features common to plants is a lack of motility. Except for a rare example of motility in some aquatic plants, plants are stationary--fixed in their location and usually anchored by a root system. This immobility subjects plants to the dictates of nature. Like animals, the typical plant body is over 90% water. Thus, in addition to being able to withstand locations of light, temperature, space, and nutrient availability, plants do not have the ability to move to a place with more water.

Lower plants (those which are less complex) get water and dissolved materials to each cell of the plant body by being in contact with moisture. These single-cell or small multicellular organisms are known as nonvascular plants. Higher plants (those which are more highly evolved and complex) do have vascular systems in which water and solutes are transported to the different parts of the plant body, and these plants are known as vascular plants (see Figure 4-1).

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The two most visible, largest, and complex of vascular plants are the gymnosperms, notably represented by the cone-bearing trees such as pine, fir, spruce, and juniper, and the angiosperms, or flowering plants. Since angiosperms are very common and extremely important to human existence, our discussion centers on them more than the gymnosperms.

Root Morphology/Function

The four main functions of the plant root systems are: (1) anchorage, (2) storage of food, (3) absorption (uptake) of water and nutrients, and (4) conduction (movement) of water and nutrients dissolved in it to the aboveground parts of the plant. Anchorage holds the plant in position. Winds, which can buffer a tree at high velocities, exert an enormous force. The flexibility of the tree allows it to bend rather than blow over. But it is the root system that ultimately anchors the tree. Although grasses have a relatively shallow root system, it is so extensively intertwined among the soil particles that it forms a dense mat of roots, which provides an effective anchorage against grazing animals.

Some plants need extensive storage capabilities within their root systems. Sugar beets and carrots are examples of root adaptations for storage of water and carbohydrates. These materials are used for new shoot growth during the second growing season. Many such highly modified storage roots produce important food products.

The amount of water (and dissolved minerals) absorbed by roots can be extensive. Water absorption and conduction in a typical corn plant, for example, can surpass 2 liters per day. This requires an enormous root surface area through which water can be absorbed.

Root Structure

Figure 4-2 shows the three basic types of root systems: taproots, which have a main central root, and a storage taproot, which acts as a food reservoir to retain surplus food during the winter or adverse periods, fibrous roots, which have many roots of equal size. Each of these has extensive lateral branching, with the fibrous root system fashioning an interwoven mass of roots.

The depth that taproots penetrate the soil varies from only a few centimeters to a reported 35 m (for the mesquite). The taproot grows straight down, developing secondary branch roots (lateral roots) as it grows. These in turn have tertiary roots, which have quaternary roots, which successively branch and lengthen to provide increased surface area for water absorption. As the root system develops, the aerial portion of the plant body root and the increased volume of tissues demand a constant supply of water. Some trees are known to have taproots that extend far into the soil to ensure a supply of water even in periods of drought. Young pecan trees often have a taproot extending to a depth exceeding their height.

Fibrous root systems are generally much more diffused and closer to the surface than the taproot system. Each of the equal-sized roots develops secondary and tertiary branch roots, as in the taproot system. This root network can effectively prevent any other plant from becoming established. The grassland biome can be solid grasses without many other plants except where the grass turf has been thinned or removed.

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Having such a large fibrous root system close to the soil surface is very important for plants in environments with relatively little rainfall. What little moisture there is can be more efficiently used. Conversely, in such an environment, plants with taproot systems must develop the taproot very quickly to reach the soil moisture present below the fibrous root zone. Early development for the desert plants having a taproot system is much greater below the ground than above, and lateral roots do not develop as soon nor as close to the surface as they do in other plants.

Root Hairs

In most plants, many of the roots' epidermal cells elongate, forming long slender root hairs that increase the total root surface area many times, as shown in Figure 4-3. This increase allows much greater water absorption because these root hairs extend out among the soil particles not in direct contact with the root itself. On plants with actively growing root systems, new root hairs are continually being produced at the rate of many million each day (see Figure 4-4). Without root hairs, most plants would not be able to absorb even a fraction of the water they need. A newly germinated seeding is an excellent subject for observation of root hairs.

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Adventitious Roots

Some plants form roots on other parts of the plant body. These are adventitious roots, and they most commonly develop on a stem. The ability is propagated from leaf cuttings. Many new plants can be propagated from one adult Dieffenbachia (dumb cane) if its stem is cut into sections and rooted in moist soil or vermiculite.

Adventitious roots often augment the normal root function of anchorage, such as the mangrove plant in Figure 4-5. Developing from the lowest nodes of their stem, they extend from the stem into the soil and help brace the plant. These aptly named prop roots are found most commonly on plants growing in marshes and mud flats.

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Modified Roots

Structural modification of roots can affect any of their five primary functions.

Anchorage

Prop roots are modified adventitious roots proving anchorage and stability. Short adventitious roots of English ivy (Hedera helix) develop at the nodes of the stem. In Virginia creeper (Parthenocissus quinquefolia), pad-like appendages arising at the nodes secrete a sticky substance for anchoring the stem to surfaces. The substance cements the ivy to a surface so tightly that, when the ivy is removed, mortar and even the building's facing can break away. Although appearing root like, these are actually modified stems.

Plants that grow on other plants usually have modified roots. Epiphytes such as bromeliads, as shown in Figure 4-6, and more temperate ball moss (Tillandsia recurvata) and Spanish Moss (Tillandsia usneoides) (also bromeliads, not mosses), are anchored by their roots to the trunks and branches of trees.

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Water and Nutrient Absorption and Conduction

True parasite plants have a detrimental effect on their host. Mistletoe (Phoradendron) and dodder (Cuscuta), for example, produce haustorial roots, which not only anchor the parasite to its host but also penetrate the host's vascular system. In this way the parasite avails itself of water and nutrients from the host plant.

Since roots require an oxygen supply to carry on respiration, some plants that grow in poorly drained soil or in soil covered by stagnant water develop root modifications that grow out of the water to aerate the plant. Roots of the black mangrove (Avicennia nitida) develop pneumatophores, which stick straight out of the swampy water. The bald cypress (Taxodium distichum) has characteristic "knees," which may provide oxygen to the root system although there is some doubt about this function.

Storage

Many plants have extensive root storage capacities. Starch and other molecules are stored for growth or flowering and are a reserve against periods of harsh environmental conditions. Beets and carrots are well-known examples. The sweet potato (Ipomoea batatas) also has enlarged storage roots, as does the tropical cassava dish, poi, which is a staple in Polynesia and Southeast Asia made from crushed and fermented taro corm (Colocasia esculenta).

Nodulation

Some plants, notably members of the pea family (Fabaceae formally Leguminosae) are unique in their ability to increase the levels of soil nitrogen in a useful form. Nitrogen fixation involves the formation of a root nodule in response to an infection initiated by bacteria from the genus. Rhizobium bacteria convert, or fix, atmosphere nitrogen (N2 that is found in the air spaces of the soil). Plants cannot use atmospheric nitrogen, but ammonia can be used. In this way the modified roots and their bacterial guest effectively fertilize the soil with nitrogen.
ADAPTING TO A BIOME

In order to survive and thrive in a desert biome, organisms must
solve several problems that their relatives living in wetter
environments do not face. Chief among these are obtaining and
preventing water loss. For example, the grizzly bear cactus
(Opuntia polyacantha), is a member of the rose family (Rosaceae),
and is native to the American Southwest. Hoodia (Hoodia gordonii)
is a member of the milkweed family (Apocynaceae) and is native to
the Kalahari Desert of Africa. The hoodia is a stem succulent,
described as "cactiform" because of its remarkable similarity to
the unrelated cactus family.

These unrelated plants resemble each other in many ways. Both are
adapted to a desert biome. Through convergent evolution, both
plants developed characteristics that allowed them to survive a
desert biome; for example, both plants are essentially unbranched
cylinders. Their leaves are highly reduced compared to the
wetter-biome relatives. They are also succulents with thick, waxy
epidermis. All these adaptations prevent water loss.


Mycorrhiza

Mycorrhiza usually has short roots that form symbiotic, or mutually beneficial, associations with certain fungi found in the soil, as shown in Figure 4-7. The fungi actually enter the tissue of these modified roots, benefiting nutritionally from the roots. The roots, in turn, become more efficient in the absorption of certain minerals needed by the plant.

Stem Morphology/Function

Plant stems come in a great array of shapes and sizes, but they all share the same basic functions: (1) the attachment and support of leaves, flowers, and fruits, (2) the conductions of materials, (3) storage, and (4) growth.

Stems are the place of attachment for leaves, flowers, and fruits. Erect stems provide structural support for leaves, raising them to allow for adequate exposure to light. Similarly, flower stems allow more visibility to pollinators, and the resulting fruit and seeds are better dispersed from their elevated positions of attachment on the stem.

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A second function of the stem, in combination with roots, is conduction. The stem conducts water and minerals from the roots in all the aboveground parts, carries food produced by photosynthesis in the leaves to the roots, stem, flowers, and fruits and transports hormones from the tissue in which they are synthesized to those areas where the effects are produced.

Third, most stems store nutrients, organic molecules, water, and by-products; a large amount of storage occurs in certain modified stems, but even unspecialized stems contain some storage tissues.

Finally, the stem contains meristematic tissue. In general, meristematic tissues are areas of rapid cell proliferation; this results in growth elongation of stem tips, increase in stem diameter, and production of tissue and organs such as leaves and flowers.

Stem Structure

Regardless of whether stems are erect, prostate, or in some other position, all stems have areas of leaf and bud attachment called nodes. The portion of stem between one node and the next is termed the internode. Normally the leaf is attached to the stem at the node, with a bud (embryonic shoot) arising in the leaf axil (where the leaf connects to the stem). Because woody plants also have terminal buds, they are called axillary or lateral buds. See Figure 4-8 for a diagram of the parts of a leaf. All herbaceous and woody plants also have terminal buds at the end of each shoot. Since a bud is an undeveloped shoot, a new leaf, branch, or flower may arise from it when and if it becomes active. Not all axillary buds develop, and the hormonally controlled pattern of bud growth and inhibition, as well as the length of the internodes, is responsible for the vast array of plant shapes.

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Herbaceous stems are so called because they do not develop wood from year to year and in fact, normally live only one season. Some herbaceous stems have short life cycles, dying before winter; others are killed by the freezing temperatures of winter. Woody stems, on the other hand, do produce new growth each season, over wintering in a dormant state. Dormant lateral buds on most woody plants are protected by a number of overlapping bud scales. As the bud begins new growth, these scales fall off, leaving bud scale scars on the stem.

The dormant terminal bud is also protected by a series of scales that leave a complete ring of scars around the stem when they fall off at the onset of new growth in the spring. The length of a year's growth can be determined by measuring the distance between any two rings of terminal bud scale scars (see Figure 4-9). The total annual increase in length (new growth) of a stem varies from one species to the next, but it can also vary from year to year on the same plant in response to environmental conditions such as water availability and length of frost-free weather.

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Growth patterns in plants are entirely different from growth patterns in animals. In plants elongation is restricted to the tips of stems and roots, whereas in animals growth is not limited to specific tissues. Once a woody stem has completed a year's growth, new elongation will take place in the section. This can be illustrated by driving a nail into a stem at a measured distance above ground. The nail will remain at that height for the life of the tree, although the tree will certainly become taller. Annual increases in stem girth (thickness) also occur. Yearly growth is calculated by the analysis of the internal wood anatomy, measuring the thickness of each annual tree ring produced.

The most common pattern of leaf and axillary bud attachment is an alternating sequence of one leaf and bud per node. This is termed an alternate leaf arrangement. Two leaves (and their axillary buds) attached at the same node directly across the stem from each other produces an opposite leaf arrangement. Three or more leaves attached at each node produce a whorled arrangement. Plants such as the dandelion (Taraxacum officinale) have all leaves grouped in a basal rosette at ground level, which obscures whether they are alternate, opposite, or whorled. For other examples, see Figure 4-10.

[FIGURE 4-10 OMITTED]

Although most plants have only one bud in each leaf axil, some have several. In such case, the central bud is the true axillary bud that will develop into a new lateral branch; the others are called accessory buds. Buds that develop on the plant at positions other than the leaf axil are termed adventitious buds. These buds may develop on stems, roots, or leaves, producing new shoots from any of these positions. Adventitious buds are often produced on stems as a response to pruning or injury. As with axillary buds, they initiate new growth in response to hormonal controls, which are in turn linked to growth at the apex of each stem.

Modified Stems

Stem modifications include a variety of changes in both external morphology and internal anatomy. Minor adaptations include production of thicker bark, greater height, more flexible stems, basal buttressing, and increased photosynthesis. All enable the plant to adjust to some environmental stress. Major adaptations significantly alter the form of the stem. These specialized stems are still recognizable, however, in that they retain some or all of the typical stem structures, such as nodes, internodes, buds, and leaves.

Bulbs

Bulbs are cone-shaped stems surrounded by many scale-like leaves that are modified for food storage (see Figure 4-11). The conical stem produces a single aboveground shoot from the terminal bud and a new bulb from a lateral bud. Unlike corms, the food reserves of the bulb are in the modified leaves. These reserves are exhausted in the production of the leafy aboveground shoot, and food for storage in new bulbs is produced in the leaves by photosynthesis. Both onions and daffodils have bulbs. The food storage leaves of the onion bulb constitute the edible part.

[FIGURE 4-11 OMITTED]

Rhizomes

These are horizontal stems usually located beneath the soil surface. Superficially, they resemble roots, but like typical aerial stems they have nodes, internodes, buds, and often leaves. Adventitious roots develop at the bottom side of the rhizome in the area of the node, and shoots emerge above ground from the same location. The chief functions of rhizomes are food storage and vegetative reproduction. Most are perennial, increasing the length each year and sending up new plants at their nodes. Some plants, such as irises, produce new leaves and flowers only at the actively growing stem tip. If the rhizomes are divided between the nodes, new plants may be produced.

Stolons

Stolons, as shown in Figure 4-12a, are also horizontal stems that produce roots and shoots at the nodes, but they form above ground. Bermuda grass (Cynodon dactylon) and a number of other grasses spread vegetatively--they produce new growth at every node.

[FIGURE 4-12a OMITTED]

Tubers

These are short enlarged organs that develop at the end of slender rhizomes. The potato (Solanum tuberosum) is a well-known tuber (see Figure 4-12b). The "eyes" of the potato are actually groups of buds with underdeveloped internodes. The eyes are used for vegetative propagation.

Corms

Corms, as shown in Figure 4-12c, are short, thickened underground stems that act as food storage structures. Gladiolus is an example of a corm from which a single shoot develops by using the food stored there. Once leaves are produced above ground, photosynthesis provides the food necessary to continue growth, produce a flower, and develop a new corm with the next year's food store.

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Cladophylls

Cladophylls are green leaflike stems that not only look like leaves but also are actually photosynthetic. As a stem, however, they may also develop flowers, fruits, and even true leaves. The "spear" of the asparagus plant (Asparagus officinalis) is the edible shoot produced from underground rhizomes. Small scales on these spears are the true leaves. If allowed to continue growing; the shoots develop branches in the axils of their scales--the green, featherlike cladophylls. A few species of cactus (Opuntia) and some orchids (Epidendrum) have cladophylls.

Succulents

The succulent stems have tissue modified for water storage. Cacti are easily recognizable examples. There are two main types: those with jointed stems, such as the prickly pear and cholla, both of the genus Opuntia; and those with a single unjointed stem, the barrel cacti. Succulent stems are found not only in the cactus family (Cactaceae) of North and South American deserts but also in members of the Euphoribiaceae native to the African deserts. Members of these two families that are often remarkably similar in external morphology because of the analogous habitats is termed convergent evolution.

Thorns

As modified stem branches, thorns arise from the axils of leaves, as do regular branches. The honey locust (Gleditsia triacanthos), hawthorn (Crataegus) and fire-thorn (Pyracantha) are thorn-bearing plants (see Figure 4-13a). Rose (Rosa) "thorns" are not true thorns but are stem surface outgrowths called prickles, which Figure 4-13b shows. Spines and thorns are terms often used synonymously; however, most spines are modified leaves arising from below the epidermis. Both are stiff, sharp-pointed, woody structures that can be equally painful if encountered inadvertently, and thus the technical difference is understandably unimportant to the victim. Both are equally effective in reducing predation by herbivores (plant-eating animals).

[FIGURE 4-13a OMITTED]

[FIGURE 4-13b OMITTED]

Spines

As modified leaves, spines, as shown in Figure 4-13c, are technically different from thorns, which are modified stem tissue. The most familiar example of plants bearing spines are the members of the Cactaceae; whose stems are modified for water storage. The ocotillo (Fouquieria splendens) is another desert plant with true spines that are a modified petiole and midrib of the first season of growth for that stem.

In subsequent years, true leaves are produced in the axil of the spines after rain.

[FIGURE 4-13c OMITTED]

Tendrils

Tendrils can be modified leaves or stems. Grape plants (Vitis) and Virginia creeper (Parthenocissus) are two well-known examples of plants with stem tendrils. Grape tendrils twine around a support structure; Virginia creeper tendrils anchor the plant with a sticky substance. Once anchored, these tendrils coil or contract to pull the plant closer to its anchoring surface.

Leaf Morphology/Function

Green plants are green because of the light-trapping pigment chlorophyll. In the leaf, light energy is converted into food energy in the form of carbohydrates. Although many herbaceous stems are green and photosynthetic, the greatest amounts of photosynthesis carried on by terrestrial plants occur in their leaves. Leaves, then, can be thought of as energy factories providing food energy necessary to sustain all life on earth.

Gas Exchange

The process of photosynthesis involves the exchange of carbon dioxide (C[O.sub.2]) and oxygen with the atmosphere. Carbon dioxide is taken into the leaf, and oxygen is released into the atmosphere. During this gas exchange at the surface of the leaf, water is evaporated from the leaves. This water loss, the process of transpiration, is the end of the water movement through the plant, which began with the roots absorbing water from the soil. Thus, the interrelated processes of photosynthesis, gas exchange, and transpiration take place in the leaves.

Leaf Structure

Leaves are well designed to carry on their functions, usually having broad, flat, and thin blades that are connected to the plant stem by a stalk called a petiole. Leaf blades also come in an incredible array of sizes and shapes, and although sometimes consistent within a given plant group, the amount of variability displayed is often so great as to prevent use of leaf morphology as a diagnostic character for the identification of plant groups.

The leaf blade can be either simple, having only one unit attached to a petiole or stem node, or compound, having two or more separate leaflike subunits (leaflets) making up the blade. Even though some compound leaves are large enough that the individual leaflets might appear to be separate leaves, confusion can be avoided by remembering that buds always occur in the axil of leaves; leaflets do not have buds in their axils. The arrangement of the leaflets of a compound leaf can be pinnately compound (with leaflets arranged along the length of a central stalk) or palmately compound (with the leaflets attached to the end of the petiole like the fingers radiating from the palm of the hand). See the diagram in Figure 4-10.

Leaves also have a specific pattern of vascular arrangement in the blades. Leaf venation can be either netted (pinnate), parallel, or palmate, as shown in Figure 4-14. Netted, or pinnatifid, vein arrangement has a main central midrib with secondary veins branching from it. Tertiary and then quaternary lateral leaf venation arises from the primary branches, with each becoming progressively smaller. The midrib, then, is the most visible of all these levels of vein branching.

Parallel venation has no central midrib; rather, it has several equal-size veins running the length of the leaf blade parallel to each other. Between these major veins is a network of venation. Palmate venation is similar to parallel venation in that several equal-size veins exist, one going to each palmately arranged lobe of the blade. As in pinnate venation, each palmate vein is equivalent to a midrib before the leaf lobe, or leaflet, to which it runs.

[FIGURE 4-14 OMITTED]

Innumerable combinations of overall leaf shape (see Figure 4-15a), apex, margin, and base modifications are possible. Leaf margins are generally said to be entire (smooth), dentate (with sharp teeth), or lobed (with rounded intrusions). The apex (tip) and base, where the petiole attaches, can be variously rounded, angled, pointed, or indented. In addition, leaves can be smooth, rough, thin, thick, leathery, or succulent (fleshy and juicy with stored water), as shown in Figure 4-15b.

The petiole can also add to the overall leaf variability by being short or long, attached to the middle of the blade (peltate) or to the edge as is normal or even absent. When the petiole is absent, the leaf is said to be sessile on the stem and can further modify to partially wrap around (clasping) the stem or even form a sheath (very common in grasses). Additionally, the presence or absence of pubescence (hairs), which can be simple, branched, granular, barbed, long, short, fine, coarse, dense, sparse, and specifically localized or generally distributed, makes it evident how complex and variable a thorough discussion of leaf morphology would be.

Modified Leaves

Leaf surface-to-volume ratio is a factor in water loss; the greater the surface area of the leaf from which water can evaporate (transpiration), the greater the amount that is lost. Since the total leaf volume controls the amount of water available, a large surface area with a small volume can rapidly result in wilting. A large volume and a small surface area conserve water against loss. Desert plants usually have small, often thicker leaves with a small surface-to-volume ratio. Plants in more mesic (wetter) areas often have much larger and thinner leaves, resulting in a larger surface area for the leaf volume. Some desert plants have large thick leaves with heavy wax cuticles. These succulents have greater water storage capacities and are well designed to allow only minimal water loss.

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[FIGURE 4-15b OMITTED]

The century plant (Agave) and yucca have stiff, sharply pointed apical leaf spines, and Agave has sharp teeth on the edge of the leaf margin. Other desert species have marginal "ornament"; some exhibit extreme indentation, and others display sharp, stiff teeth, which are neither thorns nor spines. These effectively discourage predators (see Figure 4-16).

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Insectivorous Plants

Insectivorous plants display some of the most unusual adaptations of leaves to specialized functions. In general, the leaves are modified to entrap insects but occasionally trap small frogs and rodents. Enzymes secreted by the plant then digest the insects, providing necessary nutrients, especially nitrogen. Insectivorous plants grow in nitrogen poor areas such as bogs and swamps. Inadequate soil oxygen in these habitats results in little decomposition of organic material, the normal source of nitrogen for plants.

The manner of entrapment varies among the species. The Venus flytrap (Dionaea muscipula) has a folding leaf, the sundew (Drosera) a sticky surface, and the pitcher plant (Darlingtonia) columnar tubes. Pitcher plants differ from one species to the next in their insect-trapping modifications, as shown in Figure 4-17. Some have only water at the bottom of the column in which the insect drowns, others have stiff hairs on the inside of the tube pointing down to prevent the insect from crawling back out, and in others, sticky or gummy surfaces hold the insect.

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The misleading term carnivorous plant implies a general meateating capability. This occasionally has produced an astounding science fiction vision of giant man-eating plants with tendrils or vine-like stems that can reach out, entangle their hapless victims, and draw them into the plant's dark digestive interior. Such imaginary activities are, of course, just that. Insectivorous plants are hardly to be feared (unless you are a fly) but are remarkable examples of modified plant parts that result in increased adaptability to an otherwise inhospitable environment.

Vegetative Reproduction

Although most vegetative reproduction occurs by budding from the parent stem or from rhizomes or stolons, some plants are capable of vegetatively (asexually) reproducing via modified leaves. Bryophyllum leaves develop small plants at the notches along the dentate margin, which has foliar embryos. These small plants are complete with leaves and short stems; these baby plants fall from the still healthy and active parent leaf, root, and begin growing. Rex begonias produce new plants from the upper leaf surface once it is in contact with the soil. The process of propagating plants asexually is important in ornamental plant commercial operations, fruit tree and grapevine production, and other horticultural production.

Reproduction in plants is, for the most part, a completion of a sexual life cycle. The advantage of sexual reproduction is that it produces variations within the species (Chapter 12). Should the environment change, at least some members of the population might be adapted to the new condition. These individuals would survive, allowing for the successful continuation of the species.

On the other hand, it is sometimes advantageous to have an asexual reproductive system that produces genetically identical individuals. Individuals produced from a single plant are referred to as members of a clone. Although there might be short-term advantages, even in nature, to producing individuals that are identical and perfectly suited to the environment, it is difficult to see how long-term survival could be served by a species with no means of sexual reproduction. The advantages of genetically identical and uniform plants in agriculture are obvious. Mechanical harvesting, the timing of crops for market, and reducing perishability all depend on reliable and constant factors. Much the same results are achieved through traditional breeding however; genetic engineering is being used in agriculture to achieve the same and better goals as traditional breeders.

Although many plants reproduce asexually in nature, few of them have totally lost the ability for sexual reproduction. One plant that reproduces exclusively asexually is the commercially grown banana. As far as is known, there is no record of this plant having reproduced by seeds. Small, vestigial seeds are found in bananas, but they are not fertile. Thus all commercial banana propagation is accomplished by offshoots from the mother plant (see Figure 4-18). As the central stalk flowers and produces bananas, the stalk dies, but offshoots at the base create new plants, which can simply be pulled off and planted. Commercial banana plantations have generated their crops in this manner; however, banana plants are being micropropagated for commercial growers.

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Budding/Grafting

Specific adaptations of organs allow asexual propagation from essentially all vegetative parts of the plant: leaves, stems, buds, roots, and even single cells. Grafting (see Figure 4-19a) and budding (see Figure 4-19b) are vegetative methods used to propagate plants of a clone whose cuttings are difficult to root or to make use of a rootstock rather than having the plant on its own roots. The scion is that part of the graft combination that is to become the upper or top portion of the plant. Just as transplants in animals, genetic compatibility is essential, although it is not quite as specific. Often two species within the same genus can be grafted, and they are perfectly compatible. The critical factor in ensuring that the graft or bud "takes" is the matching of the vascular cambium of both the rootstock and the scion. If this tissue fails to join, the donor scion will die. For a graft, an entire stem piece is placed on a stock, according to the size of its stem, time of year, and various other factors. The art (as well as the science) of grafting ensures a high degree of success. In budding, a single bud is placed on a stem of the stock. Essentially all commercial roses are propagated in the manner.

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[FIGURE 4-19b OMITTED]

Stem Cuttings

Many ornamental and horticultural important species are propagated by stem cutting, which is shown in Figure 4-20. When stem cuttings are placed in a suitable propagating medium (sand, peat moss, vermiculite), adventitious roots form at the cut end of the cutting, after which the cutting can be potted and grown in a normal manner. Certain hormones promote the rooting process in most cases, and their use is standard practice in commercial operations.

[FIGURE 4-20 OMITTED]

A few ornamental species are propagated by single leaves (African violets and begonias are often propagated this way). The base of the leaf or the base of the petiole will root in a suitable medium and produce a new shoot from the leaf.

On rare occasions, roots may be cut into pieces, and adventitious buds will form on them. Sweet potatoes are propagated in this manner. The common perennial morning glory (Ipomoea purpurea) is spread during cultivation because it propagates from root cuttings; plowing increases the number of root sections, which then produce a new plant.

Layering

This is a method of propagating plants with long flexible stems, such as grapes or berries. The stem is pinned to the ground at a node and covered with soil, and adventitious roots form at the node. Once the roots have appeared, the stem may be cut from the parent plant, and the new plant is on its own. Layering, as shown in Figures 4-21a and 4-21b, is one of the surest methods of asexual propagation because the new part is still receiving nutrition from the parent plant; there is tip layering and simple layering.

[FIGURE 4-21a OMITTED]

[FIGURE 4-21b OMITTED]

Air Layering

Air layering is the process of packing a stem node with moist peat moss and wrapping it in plastic. Once adventitious roots form at the node, the top of the plant can be cut off just below the rooted node and planted as a new individual. The base of air layered plant can initiate new terminal growth (see Figure 4-22).

[FIGURE 4-22 OMITTED]

Summary

1. Higher plants, especially angiosperms, are adapted through their external morphology and survive the stresses of the environment, since plants do not have the advantage of motility.

2. Roots anchor plants, absorb and conduct water and minerals to the rest of the plant, and store starch as a food resource. Both taproot and fibrous root systems increase their surface areas many times by the production of root hairs. Adventitious roots include roots developing at stem nodes as well as prop roots, which help support the stem.

3. Root modification includes attaching to surfaces (as in ivy) for support and to other plants for nourishment. Parasitic plants drain their host so much that they may ultimately kill them. Roots also enlarge for storage. Nutrient uptake is improved through root nodulation for nitrogen fixation and by fungal mycorrhiza associations.

4. Stems conduct organic solutes, water and nutrients throughout the plant body, produce new growth; support leaves, flowers, and fruits; and store materials. Stems can be prostate, various upright shapes, or even twining on other structures. Stems are either herbaceous or woody and have leaves and buds attached in a variety of arrangements.

5. Stem modification includes rhizomes and stolons; both producing roots and shoots at the nodes for vegetative propagation. Enlarged storage tubers and reproductive corms and bulbs are all underground stems. Cladophylls are leaflike, succulent stems modified for water storage. Thorns protect the plant from herbivores.

6. Primary leaf functions are photosynthesis and gas exchange; huge quantities of water can be lost during the latter activity. Leaf structure is incredibly variable, with blade shape ranging from needlelike to flat and broad. Leaves can be simple or variously compound, and their margins can be smooth to deeply lobed or divided. Leaf surfaces are smooth, pubescent, prickly, or many other textures.

7. Leaf modifications include succulence for water storage, bulb leaves for storage, spines, and climbing tendrils. Insectivorous plants have leaves modified for insect entrapment and digestion to supplement nitrogen uptake.

8. Vegetative (asexual) reproduction by rhizomes, stolons, underground-modified stems, or leaves provide mechanisms for commercial plant propagation. There are a number of advantages to cloning plants although the variability resulting from sexual reproduction is lost. Budding and grafting are very common techniques for vegetative plant propagation, as are stem cuttings and layering.

Something to Think About

1. From where do the primary roots arise?

2. Where are the nodes found?

3. Which plants have stems?

4. Name three types of roots.

5. Identify four types of underground stems.

6. Describe four types of venation found in leaves.

7. Why is asexual reproduction included in this chapter?

8. List what leaf modifications are and how they function.

9. What are the vegetative reproduction structures?

10. Name root modifications and how they work.

Suggested Readings

Barden, J. A., R. G. Halfacre, and D. J. Parrish. 1987. Plant science. New York: McGraw-Hill.

Collinson, A. S. 1988. Introduction to world vegetation. London: Unwin Hyman Ltd.

Hartmann, H. T., A. M. Kofranek, V. E. Rubatzky, and W. J. Flocker. 1988. Plant science: Growth, development, and utilization of cultivated plants (2nd ed.). Englewood Cliffs, NJ: Prentice Hall.

Janick, J., R. W. Schery, F. W. Woods, and V. W. Ruttan. 1974. Plant science: An introduction to world crops (2nd ed.). San Francisco: W.H. Freeman and Company.

Janick, J. Ed. 1989. Classic papers in horticultural science. Englewood Cliffs, NJ: Prentice Hall.

Reiley, H. E., and C. L. Shry, Jr. 1997. Introductory horticulture. Albany, NY: Delmar Publishers.

Shroeder, C. S., E. D. Seagle, L. M. Felton, J. M. Ruter, W. T. Kelley, and G. Krewer. 1995. Introduction to horticulture: Science and technology. Danville, IL: Interstate Printers and Publishers, Inc.

United States Department of Agriculture. 1961. Seeds: The yearbook of agriculture. Washington, DC: United States Department of Agriculture.

Internet

Internet sites represent a vast resource of information. The URLs for Web sites can change. Using one of the search engines on the Internet, such as Google, Yahoo!, Ask.com, and MSN Live Search, find more information by searching for these words or phrases: primary roots, asexual propagation, sexual propagation, inflorescence, seed germination, leaf shapes, leaf modifications, imbibition, plumule, hypocotyl hook, dormancy factors, stem cuttings, budding, grafting, layering, bulbs, rhizomes, tubers, corm, herbaceous, perennials, annuals, terminal bud, compound leaves, and simple leaves.
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Title Annotation:PART 2: Form and Structure
Publication:Fundamentals of Plant Science
Date:Jan 1, 2009
Words:6699
Previous Article:Chapter 3: Biomes.
Next Article:Chapter 5: Design basic II: morphology and adaptations of reproductive structures.
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