Chapter 5: Design basic II: morphology and adaptations of reproductive structures.
After completing this chapter, you should be able to:
* Identify the different floral parts
* Distinguish the male and female parts of the flower
* Know flower classifications
* Recognize different fruit types
* Relate flower production to fruit production
* Learn morphology of different seeds
* Understand seed germination
* Discover the difference between monocots and dicots
* Link plant science to everyday life
Although the flower has deservedly been made the object of poetry and often symbolizes beauty, love, peace, and happiness, the basic biological functions of the flower is sexual reproduction. Exotic colors and shapes of flowers are actually devices to attract specific pollinators. This ensures their reproductive success, since pollen will be carried among flowers of the same species. Not all flowers are large or beautifully showy, but even the plainest of flowers functions successfully. Some flowers are not conspicuous at first glance, but close observation reveals remarkable, complex, colorful, and beautiful design.
A typical flower possesses four different floral parts that are attached to the receptacle in the following order, from outside to inside: sepals, petals, stamens, and pistil (see Figure 5-1). The sepals are collectively referred to as a calyx, which encloses and protects the bud as the flower develops within. The corolla (crown), the collective name for the petals, is frequently colorful and exhibits diverse sizes and shapes. Together, the calyx and corolla are termed the perianth.
The actual production of sex cells, and the gametes, takes place in the stamens and ovary of the pistil. Stamen produces pollen in its anthers, which are saclike structures attached to the end of the slender filaments. These filaments raise the anthers to a position where the pollen is more accessible to visiting pollinators.
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The filament of a stamen is sometimes fused to the corolla tissue, especially in corollas with petals joined to form a tuber section. Some flowers have nonfunctional, sterile stamens called staminodia, which often have flattened petaloid filaments. In fact, petals originally evolved from stamen through such modifications.
At the center of the flower is the pistil, which often resembles a pharmacist's pestle. The base of the pistil is the ovary, which contains one or more ovules. The slender neck-like portion of the pistil is the style, which elevates the stigma to a favorable position relative to contact with pollen. Whether air born or brought by a visiting pollinator, the process of pollen landing on the stigma is pollination. Many stigma surfaces are sticky, pubescent, or otherwise modified to help ensure successful pollen attachment. In addition, pollen grains of many species are "ornamented" with spines, ridges, and barbs to further aid pollination.
A pistil can have a single placental surface to which one or more ovules are attached, or it can have two or more placentas. When an ovary has more than one, a wall usually separates them. Each separate reproductive unit composed of a placental surface and ovules is called a carpel. Each carpel also has its own style and stigma; however, these are usually fused together and appear as a unit of the total pistil. When a pistil is composed of a single carpel, as in the flowers of peas and beans, it is called a simple pistil. When there are multiple carpels, such as those found in tulips, lilies, grapefruits, and poppies, it is termed a compound pistil. The sections of a cut grapefruit are the carpels of the compound ovary. The ovules of each of these carpels are where sexual fertilization (gamete fusion) takes place in flowering plants. The resulting plant embryo is housed in the seed (mature ovule) within the fruit (mature ovary).
In most flowers, the pistil is attached to an enlarged apical portion of the stem called the receptacle. Usually it is located above the attachment points of the other floral parts (stamen, petals, and sepals). Where so attached, it is termed a superior ovary. In some flower groups, however, the ovary portion of the pistil has a modified receptacle or perianth tissue surrounding it and fused to it with the other floral parts attached above the ovary at the top of this tissue. Such flowers are said to have an inferior ovary because of the lower position of ovary attachment relative to the other floral parts. The position of the ovary is an important taxonomic characteristic.
Flowers can occur singly or in an inflorescence of several to many flowers, as you can see in Figure 5-2. Inflorescence can be as simple as only a few flowers attached near one another on the flowering stem or as complex as the head of a sunflower, which is composed of hundreds of tightly grouped flowers with the head actually being in the shape of a single larger flower. The range of inflorescence complexity and shape and the modifications of the component flowers within some inflorescence are great.
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All these arrangements and modifications in some way ensure pollination and fertilization and thus the reproduction success of the species. Unlike leaf morphology, the reproductive parts of a given plant species are much less affected by fluctuations in the environment such as temperature, wind, and availability of water. Flower appearance is usually consistent because pollinator-flower specificity often exists. If a pollinator fails to recognize the flowers of some individuals of that species, reproduction cannot occur. Failure to successfully reproduce sexually could ultimately result in few offsprings, reduced genetic variability, less adaptability, and finally extinction.
A complete flower, such as the one in Figure 5-3, has sepals, petals, stamens, and pistil all present. Many successful species, however, lack one or more of these four basic floral parts. Such flowers are termed incomplete flowers. Flowers having both stamens and pistil (that is, both sexes) are called bisexual, or perfect, flowers whereas unisexual, or imperfect, flowers contain either stamens or pistil but not both. Those containing only stamens are logically called staminate flowers, and those lacking stamens but having carpels are termed either pistillate or carpellate.
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Plants having both staminate and pistillate flowers on the same plant are monoecious, literally meaning "one house" (see Figure 5-4). Dioecious (two houses) plants, such as the holly shown in Figure 5-5, have the staminate flowers on one plant and the pistillate flowers on another. Corn (Zea mays) and squash (Cucurbita) are monoecious; willow trees (Salix) are dioecious. Historically, the designation "male" and "female" parts have often been applied to the stamens and carpels, respectively, and thus with unisexual flowers one might see reference to the male or female flower. But such designations should be made only with a full understanding of what is meant, and that is best accomplished with the more descriptive terms staminate (male) and pistillate (female).
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Flower shapes are either regular (radially symmetrical), as a sunflower (see Figure 5-6) or irregular (bilaterally symmetrical), as the snapdragon in Figure 5-7. A regular flower can be cut through the center in more than one plane and results in identical halves. An irregular flower is bilateral, having only one plane through which it may be cut to result in mirror-image halves.
Additionally, the petals can be either district (not fused to one another) or partially to completely fused into a single structure. Sepals, stamens, and carpels also may be distinct or variously fused. Nor is fusion limited to members of the same floral part. The filaments of many species fuse to the corolla, and there are isolated examples of other floral parts fused to each other.
The array of flower shapes, sizes, color, and arrangement seems endless. The variability that exists, however, is not haphazard. Flower modification heightens the likelihood that pollination agents, or vectors, may be general visitors, or they may be attached to specific flowers. They are enticed by a unique floral presentation that result from alternation of the flower parts. The changes include different sizes, shapes, fusion, colors, patterns, odors, and edible materials, as well as relocation of stamens and stigma. Thus, flower modifications promote reproductive success.
Petal color, size, shape, and fusion are the most common variables in flower morphology. Each such modification exists as a result of increased reproductive success through pollinator visitation. Some flowers are successful exclusively through visual attraction (see Figure 5-8). Others must additionally produce nectar as a food reward for a visiting pollinator or have a strong, detectable odor. Odors can be sweet and fragrant or foul. An aroma similar to rotting meat attracts flies as the pollinator vector in certain plants. Some petal surfaces are marked to signal to appropriate insects which way to approach, where to land, and which way to enter the corolla tube. These markings often change after successful pollination. This signals to other potential visitors that the flower has already been visited; significant efficiency results.
The human eye cannot appreciate all these petal markings, but certain insects can. Reflections of ultraviolet wavelengths of light from a given portion of the petal produce a specific pattern for bees and wasps, which can be seen in the ultraviolet range (see Figure 5-9). Such insects, however, do not discriminate colors at the opposite end of the spectrum, making them essentially red color blind.
Other floral and nonfloral parts display modifications involved in pollinator attraction. In some flowers, the sepals appear petaloid; in others, highly modified staminodia are petal-like. The stamens are visually attractive through elongated and colorful filaments or large, colorful, and showy anther sacs (Caesalpinia and Tradescantia). The bright red poinsettia "petals," shown in Figure 5-10, are in fact colorful leafy bracts located below the cluster of small relatively inconspicuous flowers. Bougainvillea also has petaloid bracts and less conspicuous flowers.
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There are only a few examples of modified flowers and flower parts, whose evolution increased pollinator visitation and reproductive success. The complex mosaic of natural beauty that has resulted from these modifications is intellectually intriguing but also enjoyable to simply observe and appreciate.
There are two general categories of pollination strategy: plants that are self-pollinated have their own pollen land on their own stigma; those that are cross-pollinated receive pollen from the flowers of other individuals of the same species.
Most self-pollinating plants have bisexual flowers; their flower design allows their own pollen to be shed onto their stigma (or a visiting insect can initiate this). Since all the flowers of a single plant have the same genetic origin, pollen transfer to any flower on the same plant is still considered to be a self-pollinating event. It does not increase genetic variability.
When genetic information from two different plants is mixed, the seeds have a larger pool of genetic information. The resulting progeny will display variations in form and function. This cross-pollination is also called outcrossing. Because greater genetic variability is desirable, outcrossing has been ensured in many plants by the development of genetic self-incompatibility.
This can be physical or chemical in nature. Some flowers are structurally designed so that the pollen of that flower cannot be shed onto its stigma. In some the style is elongated more than the stamen filaments, positioning the stigma well above the anther sacs. In others, the pollen is not released from the anthers when the stigma is receptive; it is shed either before the stigma is ready or after the stigma has already been pollinated from a different plant.
This type of incompatibility is less easily determined. Certain proteins in the outer layer of the pollen grain are involved in a recognition reaction with the surface of the stigma, which ensures that pollen from the same plant will be rejected.
For a plant to be successfully cross-pollinated or outcrossed, there must be a dependable source of pollen from other plants of the species. Wind-pollinated plants generally have many small, inconspicuous flowers that produce large quantities of lightweight pollen grains. Unlike much of the pollen of insect-pollinated plants, the pollen grains are usually smooth and do not stick together. Since wind carries the pollen, colorful petals, edible tissues, attractive odors, and nectar production are generally lacking; these adaptations are necessary only for plants that attract insect visitors. Wind-pollinated flowers are usually modified, having long styles with well-exposed stigmas that are often feathery or branched and fairly large. Their stamens are also well exposed to the wind, sometimes hanging down away from the flower on long, slender, flexible filaments to better shed their pollen when the wind blows across them. Since they are usually very small flowers, they most often occur in inflorescence; their great numbers and density further increases the chances of pollination. Most of these flowers have only a single ovule and produce a single-seeded fruit, such as a grass grain, the winged fruit of the elm, or the acorn of an oak.
Because wind pollination is chancy, copious amounts of pollen are shed from each flower (see Figure 5-11). The vast majority never lands on a stigma but end up on the ground not far from its source. Many wind-pollinated species are unisexual (monoecious) with small staminate flowers grouped together on one part of the same plant, and pistillate flowers together on a different part of the same plant. Other wind-pollinated species, such as cottonwood and honey locust, are dioecious and have staminate and pistillate flowers on different plants.
Since wind pollinate is inefficient, it is most successful where many plants of the same species grow close together in open areas. The tropics, therefore, have few wind-pollinated species. They are found mostly in temperate zones. All grasses and deciduous hardwood trees are wind-pollinated, the latter flowering primarily in the early spring before the leaves develop.
Gymnosperms are also wind pollinated. Most of the cone-bearing trees (conifers) have small pollen-producing male cones and much larger female cones on the same tree. It is probable that angiosperms evolved from gymnosperms, so for a long time plant scientists theorized that, since gymnosperms are wind pollinated, wind-pollinated angiosperms must be relatively primitive. Now it is generally accepted that they are not primitive but in many cases fairly advanced, having evolved from insect-pollinated groups.
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During the flower season, many areas have so much pollen shed by wind-pollinated trees (both angiosperms and gymnosperms) and grasses that the air appears hazy. Ponds and shorelines of lakes and still streams have a layer of billions of yellow pollen grains floating on the surface. Hay fever and many other allergic reactions are common ailments during these periods.
By far the most common pollinators are insects. The only group of organisms that displays as great a diversity in numbers of species, distributional range, and overall complexity as the flowering plants are the insects. The origins and developments of such variability and numbers in these two groups have been processes of co-evolution.
In most plants, after fertilization, the ovary of the flower matures into fruit. Fertilization actually takes place in the ovules, which in turn develop into seeds. Fruit development, therefore, is normally triggered by pollination-fertilization-seed development, and if there is no fertilization or if the seeds fail to develop, the ovary will not mature into a fruit (see Figure 5-12).
As with most other "normal" events, there are exceptions; some fruits, bananas for example, develop without fertilization. This process is called parthenocarpy, and occurs in fruits that are seedless. Crop scientists specializing in fruit production have studied seedless grapes and oranges found in natural stands. In some unusual cases, the ovary does not enlarge, but the receptacle grows and surrounds the ovary. Such is the case in the apple.
Fruit play an important role in the reproductive cycle of flowering plants, providing continued protection for the enclosed seed and siding in their dissemination. The step from naked seed (in gymnosperms) to enclosed seed (in angiosperms) was a major one in the evolution of plants.
As the ovules begin development into seeds, the ovary wall matures into the fruit wall, which is then called the pericarp. The pericarp usually has an outer exocarp, a middle mesocarp layer, and an inner endocarp. The distinctiveness of these three subunits of the fruit wall varies from plant to plant. On the outside of the developing ovary are shriveled stamens, corolla, and calyx. The calyx, in fact, is often not only still visible, but in some species remains healthy and even enlarges as the fruit matures.
Kinds of Fruit
Simple fruits are fruits that develop from a single ovary, whether it is composed of only one or several fused carpels, such as the peach in Figure 5-13.
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Fruits can be further subgrouped as either fleshy or dry, depending on whether the pericarp is soft and juicy. Additionally, dry fruits can be either dehiscent, where the dry pericarp splits open at maturity, releasing the seeds, or indehiscent, where the seeds remain with the fruit and the entire fruit falls from the parent plant at maturity (see Table 5-1).
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These are formed by the development of a group of simple fruits. There are two general kinds of compound fruit.
Aggregate fruits are formed from many separate carpels (ovaries) of a single flower, as in the strawberry, raspberry and blackberry. Strawberries differ from the latter two in having small, dry, individual fruits or achenes attached to an enlarged juicy receptacle. Blackberries and raspberries are an aggregation of separate fleshy fruits (drupes) attached to a common receptacle.
Multiple fruits are the result of the development of the ovaries of several separate flowers that have fused on the axis of the inflorescence. Pineapples and figs are both multiple fruits (also called accessory fruits).
As among flowers, the difference among fruits has evolved directly as a result of increased reproductive success. The role that fruits play is directly a result of increased reproductive success. The role of fruits in the reproductive cycle of plants is seed dispersal. The modifications therefore reflect the methods by which the fruit are transported--by wind, water, or animal. If the fruit (and the seed within) dispersed to areas away from the parent plant, each new plant seedling has a better chance for adequate space, water, and nutrients, which it needs to survive. Additionally, the species thereby has the opportunity to increase its total distributional range. The genetic variability of new generations contributes to their success in new environments.
Very small, lightweight fruit can be carried some distance from the parent plant by the wind. Larger fruit must be modified in some way for effective wind dispersal. Dandelion fruit, with their parachute tuft of soft fine bristles and the winged fruit of maple trees are wind-borne (see Figure 5-14).
Some fruits have thick, fibrous outer coverings that provide buoyancy and protection from salt water. The coconut's husk has ensured it dispersal to virtually every tropical sandy shoreline in the world (see Figure 5-15). Freshwater streams and lakes also are dispersal agents for buoyant fruit of many of the plants found only in these habitats. Some fruits float because of low-density buoyant tissue, whereas others have air inside them. Other fruits are not particularly buoyant or water-resistant but can float long enough to be carried at least a short distance from their source.
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Rainwater can act as a dispersal agent. A reasonable heavy downpour will help knock some fruits from the parent plant and wash them away if there is any incline available to produce a runoff.
When ripe, many fruits are bright colored, thin walled, and juicy, especially the red fruits. Although not attractive or even visible to most insects, red fruits are highly visible to birds and other animals. Many such fruits are also sweet, making them even more attractive to animals. When the fruit is ripe, the enclosed seeds are dry and hard enough to pass through the digestive tract of the animals. The animals distribute the seed effectively throughout their range, as shown in Figure 5-16. Some fruits even taste bitter before complete ripening discouraging animals from eating them before they are ripe.
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Although the seeds pass through the digestive tract intact, the digestive acid acting on them alters the seed coats. Many actually could not germinate without this scarification. Immature seeds cannot survive the action of these juices; thus, most fruits are green when unripe, camouflaged against the green leaves of the plant. The level of sugar in these fruits remains low until maturity, further reducing their attractiveness to animals before the enclosed seeds are mature.
Not all fruit that depends on animals for dispersal are juicy and sweet; many hard-shelled nuts and dry fruits, such as grass grains, are gathered and stored away for the winter by squirrels and other small mammals such as packrats and field mice. Since not all of these seeds are eaten, some may germinate when conditions are appropriate.
Still others are dispersed externally by hitching a ride on a person or an animal. Such fruits are externally modified with barbs, hooks, bristles, or sticky surfaces by which they adhere to the fur, hair, feathers, or skin of animals. These fruits can also be carried by humans on their clothing, cars, camping equipment, and tires of their vehicles.
Sexual reproduction in flowering plants culminates when ovules develop into seeds. Some fruits have only one seed and others thousands; some seeds are very large (palm) and others microscopically small (orchids); some must germinate immediately, and others can remain dormant for many years. All seeds, however, have certain common characteristics.
In flowering plants there are two-gamete fusions in the ovule. Thus double fertilization is unique to angiosperms and results in the formation of a fertilized egg, and a nutritive tissue, the endosperm. The endosperm tissue developed first and is allowed by divisions of the zygote, which produces the embryo, as shown in Figure 5-17. The embryo and endosperm are enclosed within the seed coat, and when both reach maturity the seed often becomes dormant or inactive until the germination process begins.
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Seeds that display adaptive features do so for improved dispersal, and most of their modifications are similar to those found in fruit. Seeds release from their surroundings by dehiscent fruit fall either to the ground or into water. Those, which land in water, are often as buoyant as fruit and are dispersed as readily. Small lightweight seeds can be carried away by the wind, whereas larger seeds develop wings to allow them to spin or flutter through the air. It is possible for the seeds to land far away from the parent plant. Some seeds are actually edible. Some like that of Erythrina seeds possess brightly colored coats that are displayed openly to entice passing birds. Seeds also develop a full array of surface modifications to promote adherence to animals. Some others have been found economically useful, such as the long surface fibers on the cottonseed.
One of the most interesting dispersal mechanisms is the physical ejection of seeds by the fruit. Several species are explosively dehiscent, propelling seeds up to several meters away from the parent plant. Some split open and eject violently when a certain degree of dryness is reached. Other seeds actually germinate while still attached to the parent plant. These viviparous seeds later fall to root in the soft soil or mud. Once dispersed, seeds next must successfully germinate and grow to maturity. Although not normally considered modifications, the different strategies that have evolved controlling seed germination and seedling growth are responses to environmental factors.
ECONOMIC IMPORTANCE OF SEEDS Many seeds are edible. In fact, the majority of food calories for humans come from seeds, especially from cereals, legumes, and nuts. Seeds also are the source of most cooking oils, for example soybean oil, canola oil, corn oil, and safflower oil. Seeds or extracts are used in many beverages and used as spices and as food additives. Some seeds are also poisonous. One of the deadliest poisons, ricin, comes from seeds of the castor bean (Ricinus communis). Another seed poison is strychnine from the seeds of the Strychnos nux-vomica tree, an evergreen tree native to Southeast Asia. Other poisonous seeds are those of the yew, wisteria, apple, horse-chestnut, and peach. The world's most important clothing fiber grows attached to cottonseed (Gossypium spp.). Other seed fibers are from kapok (Ceiba pentandra), used as an alternative to down as filling and for insulation, and milkweed (Asclepias spp.), used as a substitute for kapok during World War II. Important nonfood oils are extracted from seeds, for example flax or linseed (Linum usitatissimum) oil is used in paints. Two plant seeds produce an oil with characteristics similar to whale oil: jojoba (Simmondsia chinensis), a shrub native to the Sonoran and Mojave deserts of Arizona, California, and Mexico, and crambe (Crambe abyssinica), native to southwest and central Asia and eastern Africa. Castor oil from the castor bean is a nonfood oil. Other seed uses include: toys and beads; resin from Clusia rosea; nematicide from milkweed seeds; animal feed from cottonseed meal, soybean meal, canola meal, and others; and birdseed.
The germination of the seed begins a new plant life. Even though the embryo is formed and has its primary tissue well developed, the mature seed may be stored for varying periods of time and still retains its viability--the ability to germinate. Only when the proper environmental conditions are provided does the seed revitalize and produce a seedling. For seeds such as the maple (Acer), such longevity may be only 1 week; but other seeds, such as Lotus, may retain viability for hundreds of years under proper storage conditions. Most common cereal plants retain viability for about 10 years.
The environmental requisites for seed germination include suitable oxygen concentrations, temperatures, moisture, and in some cases light. Mature seeds are dry for germination to begin; these dry tissues must be hydrated. It may be difficult to tell that a dry and dormant seed is really living, but respiration and metabolism continues throughout dormancy at a much-reduced level.
If moisture enters the seed coat, a strictly physical process called imbibition causes the tissue to swell with enormous expansion forces. This process will occur even in dead seeds, or in sticks of wood that become wet. Dry seeds can be placed in plaster of paris and hardened into a block. When the block is wetted, the imbibing seeds will swell so dramatically that they will shatter the plaster. It is said that the Egyptian pyramid laborers drove pegs of wood into holes bored in rock, poured water around the pegs, and thereby split huge boulders with ease.
The amount of moisture required for seed germination varies greatly among species. Some seeds germinate in what might be considered a very dry soil. Modifications of the seed coats allow some surfaces to act as a wick and absorb more water. Some seeds have mucilaginous coatings that attract eaters and enhance the imbibition process.
Good drainage and aeration are just as essential for seed germination as they are for obtaining enough water to begin imbibition. Since imbibition triggers the onset of high rates of metabolism and respiration, adequate amounts of oxygen are absolutely critical. If the soil is flooded, oxygen levels may be so low that respiration is impossible, and the seed will rot.
Once water has penetrated the seed, the tissues of the endosperm consisting of stored foods (macromolecules of starch, protein or lipids) and cotyledons (embryonic leaves) start the metabolic process called digestion. The breakdown of these macromolecules into their simpler components. For starch, the breakdown product is glucose; for protein, the breakdown product is amino acid; and for lipids, it is fatty acids.
These simpler molecules become the "fuel" or substrate for respiration. Respiration goes on in a dormant seed at a reduced rate; the rate accelerates greatly when new substrate becomes available, and the enzymes of metabolism begin to function. This extra energy results in the synthesis of new compounds necessary for growth and development, and the embryo begins its enlargement.
To the naked eye, the imbibition process produces a swollen seed, with a large increase in volume and weight simply from the uptake of water. The next observable step is usually the protrusion of the radicle, the embryonic root. It may protrude hours, or even days, before the first sign of the shoot.
The growing point of the shoot above the point of cotyledon attachment is called the epicotyl, and the section of stem below the cotyledons is called the hypocotyl. At the base of the hypocotyl, the transition zone separates the shoot from the root (see Figure 5-18). The growing tip (apex) of the epicotyl is usually referred to as the plumule.
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Some seedlings emerge with the cotyledons rising above the ground and participating for a short while in photosynthesis before they shrivel and fall off. Others, such as the pea, emerge with the cotyledons still underground, in approximately the same place where the dry seed was placed. The difference is strictly a genetic one and is determined by whether the hypocotyl elongates sufficiently to elevate the cotyledons above the soil. In such seeds, like the bean, the hypocotyl is U shaped at the time the seedling emerges from the soil, and the bent shoot is literally pulled up through the soil. This mechanism is an adaptation for exerting force at the soil surface to penetrate the soil crust while protecting the delicate shoot. This apparatus is referred to as the hypocotyl hook.
As the ovule matures, the cell layers that make up the seed coat become rather impermeable to water, and this ensures that during the long period of dormancy and storage the seed will not lose too much water and that respiration will remain at a low rate. On the other hand, it is essential for the seed coat to retain sufficient permeability to begin the absorption of oxygen wherever renewed growth begins. Some seeds, such as many beans, become so impermeable to both water and oxygen that germination cannot proceed. These seeds must go through an abrasion of the seed coat (scarification) to allow penetration by water and gases.
Even though the physical process of imbibition may occur; internal factors that control the metabolic processes are responsive only at suitable temperatures. Again, the critical temperature varies. Some species germinate in the winter and others only during very hot weather. The adaptive strategy of each species determines the appropriate timing for the greatest chance of survival. Within certain limits, the rate of chemical reactions in living systems doubles for each 10-degree (Celsius) increase in temperature. Therefore, germination tends to be more rapid at higher temperatures, although temperatures that exceed certain limits may be inhibitory.
Light can influence germination in some species. Since most seeds germinate underground, the general trend is for germination to occur in darkness. However, some seeds, particularly those that are very small and germinate right at the soil surface, may be light sensitive. Certain types of lettuce seeds, for example, will germinate only if they are exposed to light. Researchers studying this phenomenon discovered that red light at a wavelength of about 660 nm stimulated germination. Interestingly, if the same seeds were exposed to light of a slightly longer wavelength, about 730 nm (far red), germination was effectively inhibited.
Most adaptive strategies, including those for germination, do not depend on a single environmental factor. More likely, several factors may interact to trigger the germination. In the desert, for example, ephemerals complete their life cycle in a matter of a few weeks. Their chance of success is dictated almost entirely by an interaction of temperature and rainfall. Moisture is needed for imbibition to initiate germination, but a sudden summer thundershower could spell doom for a seed that began to develop at that time of year. Fortunately, summer temperatures are too high for germination to begin; the seedling could never survive the intense summer heat. Germination must wait until early spring or fall. At that time there is adequate moisture and cooler temperatures.
The same situation holds true for species in alpine meadows, where the growing season is very short because of temperature restrictions.
Water may be always plentiful, but germination too early might cause a young seedling to die because of frost. Even though a few warm days may occur in early spring, the moisture-temperature combination signals the delay of germination until later in the season, when all danger of frost is past. Once the seedling has been established, the life cycle must be completed rapidly before the onset of cold weather in the late summer or early fall.
Many times during ovule development the cells of the seed coat become so lignified (impregnated with lignin) or otherwise become so hard that uptake of water and oxygen is essentially impossible. As the seed ages, various chemicals and physical forces gradually break down the seed coat so that it finally does become penetrable. This might come about by freezing and thawing, by the seed passing through the digestive tract of animals, or by wind abrasion and water erosion. Not all seeds from the same plant, even if produced in the same year, will have the same coat thickness.
Some of them may germinate the first year, those of medium thickness may germinate the second year, and the very hard ones may germinate several years later. Such variability increases the chances for survival, even if no seeds at all are produced in certain years.
Occasionally, seeds appear to be mature, but in fact the development of the embryo is not yet sufficiently advanced to allow germination to proceed. Whenever this happens, it is necessary to delay germination until embryo maturity is completed. Even under the microscope, the embryo may appear to be mature, but certain chemical adjustments are necessary before germination will proceed.
Sometimes germination is inhibited by the accumulation of chemicals. This appears to be an adaptive strategy to spread out the germination process over time. As the seed ages, certain chemicals may be broken down until the concentration is so low that germination can proceed. More often, the chemical inhibitors are water soluble, and with rainfall they are simply leached from the seed. Sometimes the concentration is such that very little leaching is needed; other seeds may have very high concentrations, which requires years of leaching before the seed will germinate. Again, the adaptive strategy is clear: if all seeds were to germinate at one time, and it happened to be the wrong time, the species could become extinct.
Development: Seedling to Adult
The mature seed contains a new embryo complete in every detail- an embryonic axis (the stem and root) and one or more leaves. In the germination process, the new plant simply expands its existing tissues and adds new ones. The digestion of storage molecules (carbohydrates, proteins, or lipids) found in either the endosperm or cotyledon(s), provides the energy necessary for building these new parts.
Environmental factors are critical in the germination process and in the establishment of the seedling. It is not strictly by chance that the radicle begins to emerge and grow faster than the shoot. Although proper temperature and oxygen are obviously important in the process, proper moisture is probably the most critical factor in the success or failure of the new plant. An emerging root system must remain in contact with moist soil, or the embryo will die. It is absolutely essential that the root system grow rapidly enough to penetrate soil to depths that provide additional moisture. In many situations, seeds may germinate on a moisture layer even if the surface layers are dry. This is one of the critical factors in dryland farming (no supplemental irrigation). The same principle also holds in natural ecosystems. Not only is rainfall critical to germination per se, but stored soil moisture also is essential to seedling success. Some genetic stains are particularly adapted for seedling vigor and are thus better able to survive under stress conditions.
If the embryo does become established, it proceeds through its life cycle as a rapidly growing seedling, reaching vegetative maturity, passing into reproduction maturity (the ability to flower and set seed), and finally into senescence (literally old age). Plants that complete this process in one growing season are called annuals. Most crops like corn are annuals. In some cases the entire growing season (early spring to fall) may be required to accomplish all these processes. Other annuals do so in a very short period. These are adapted to environmental conditions that force them to reproduce quickly if they are going to survive.
A second group of plants, the biennials, require two growing seasons to complete their life cycle. Many members of the cabbage family (Cruciferae), among others, require one season of vegetative growth followed by a cold period to induce flowering the following year. Apparently some chemical stimulus required for flowering, perhaps a flowering hormone, is synthesized only under low temperature conditions. During the first year, growth is strictly vegetative, and many leaves are produced on short internodes of stem; thus the leaves are "telescoped" close together and near the ground. Such plants are referred to as rosettes because the leaf placement resembles the petals of a rose. In the spring of the second year, the stem begins to elongate rapidly and produces flowers.
Interestingly, humans handle biennials as "annual" crops. When cabbage is grown as a vegetable crop, it is necessary to use only the first part of the life cycle. The rosette in this case is a group of overlapping leaves that make up the head, and the plant is strictly vegetative. On the other hand, when cabbage is grown from seed, it is absolutely essential to allow the plant to proceed through normal development into the second year of flowering and seed maturity.
The third group of plants is perennials. They complete their life cycle in more than 2 years and often continue to live for many years, producing flowers, fruits, and seeds each year. All woody plants are perennials and many are specially adapted to survive harsh winters and revive in the spring. Many perennials go through a long period of vegetative growth before they begin to flower. It is not known why these plants require such lengthy leaf production. Citrus trees, for example, may need 8 to 10 years of vegetative growth prior to the onset of flowering. If farmers could hasten the beginning of reproduction activity, they could bring orchards into production sooner. Some perennials, such as chrysanthemums, die back to the ground level each winter but produce new growth each spring from an underground crown of stem tissue. Such plants are called herbaceous perennials.
Botanists categorize plants as annuals, biennials, or perennials according to life cycle. However, there are complications where certain plants are concerned. Consider the century plant (Agave americana) so named because it remains vegetative for many years, flowers once, and dies. Is this plant truly perennial, or should it be considered an annual or some modified form of biennial? One way to simplify the problem is to classify plants only according to whether they flower once or more than once. Since botanists often refer to fruits as ripened ovaries or carpels, we can call all plants that flower once monocarpic plants. Those plants, which flower over and over again, are called polycarpic plants. The later scheme is definitely simpler, but the terms annuals, biennial, and perennial will probably continue to be used, since they are readily understood.
The vegetative phase of plant growth consists of the addition of more roots, stems, and leaves. Certain hormonal relationships, to be discussed later, determine whether the axillary buds at the nodes initiate active growth. If they do begin growth, then each bud develops into its own new shoot with stems and leaves. The overall effect of such development is to produce a bushy plant with a rounded head. If, on the other hand, the lateral (axillary) buds fail to develop, the plant continues growing from the terminal bud and becomes much taller with very little lateral spread. Such plants have strong apical dominance, and the terminal bud exerts control over all growth. The bushy, round-head plant has weak apical dominance.
The plant continues its vegetative activities--adding new photosynthesis surface, and storing organic materials--until some internal or external mechanism initiates a changeover to reproductive activities. When this happens, internal biochemical forces cause the terminal bud, and perhaps the lateral buds, to stop producing leaves and begin producing flowers. Exactly what triggers the meristematic cells (those capable of division) at the shoot apex to stop producing leaves and begin producing flowers is one of the mysteries of botanical research.
Monocots and Dicots
One of the impressive development phenomena of angiosperms is that the two major subgroups, monocotyledonae (monocots) and dicotyledonae (dicots) as shown in Figure 5-19, are so easily distinguished throughout their life cycles. As seeds, the monocots (mono, "one"; cot, "cotyledon") have only a single cotyledon, or embryonic leaf; the dicots (di, "two") have two. As they develop into seedlings and adult flowering plants, their morphological distinctiveness continues to be evident. Table 5-2 summarizes the major differences between these two groups of flowering plants.
Additionally, within each of these two groups there are several categories of gradually less inclusive groupings of plants. The next useful level of inclusiveness is the plant family.
[FIGURE 5-19 OMITTED]
Two of the main plant groups discussed so far are the angiosperms and gymnosperms. As mentioned, angiosperms are either monocots or dicots, and within each of these two categories plants are grouped by relationship into families. The family level is usually a recognizable and coherent group that also has subgroups within it.
There are both scientific names and common names for most plant families. For example, the rose family is Rosaceae and the nightshade family is Solonaceae. Although the use of common names predominates among nonscientists, the need for scientific names is very real, especially at more specific levels, such as genus and species. Every organism has a unique scientific binomial consisting of the genus and species; but not every one of these organisms has a unique common name. For accuracy and ease of classification, therefore, the scientific name is essential.
Early in human history, small populations of predominately nomadic people gathered plants for food or medicine. In time humans evolved from food gatherers to food growers. Members of the primitive farming communities recognized the value of some of the plant modifications and were able to capitalize on them. Wild strains that processed beneficial variations could be selectively propagated. These adaptations might have included plants with larger edible roots, increased seed yields, and the capacity to withstand unusually adverse environmental conditions.
Once humans began domesticating plants, they were able to select individual plant species systematically according to desirable traits, and develop them through traditional hybridization and plant breeding. These techniques are employed today in the development of many commercially valuable consumer products. Although these products have a wide variety of applications, certainly the most important is as a food source.
The vast majority of food for human consumption comes from the flowering plants. Most of us are familiar with the broad selection in the canned fruit and vegetable aisles of the supermarket. The commercial designation of these plant products does not always reflect their botanical function. Squash, cucumber, pepper, tomatoes, and corn are commonly called vegetables ("Eat your vegetables, dear"), even though they are reproductive not vegetative, parts of the plant. Many grocery store "vegetables" are misnamed. Beans and peas, for instance, are actually fruits (green beans) or seeds (lima, kidney, pinto, navy, soybeans, and green and black peas). In addition, many of these consumer items belong to the same plant family.
Table 5-3 lists some of the more common edible plant materials by family and identifies the actual part of the plant that is eaten. Scientific binomials (genus and species) are also included. Note how frequently several species of a single genus (for example, Brassica) are commercially developed for the consumer.
1. The flower, the site of sexual reproduction in angiosperms, has four male parts: sepals, petals, stamen, and pistil. The stamens produce pollen; the ovary of the pistil contains the ovules. A simple pistil contains only a single carpel; a compound pistil is made up of two or more carpels. The ovary, which can have a superior or inferior position, matures into the fruit, and the ovules mature into the seeds. Flowers occur singly or in an inflorescence and can be bisexual or unisexual. Unisexual flowers are either monoecious or dioecious.
2. Flower shape is highly variable, but is either regular (radially symmetrical) or irregular (bilaterally symmetrical). Floral parts can be fused or distinct and have a wide variety of colors, patterns, sizes, and other modifications. All flower modifications exist for a pollinator's attraction. Most pollination is by animals, especially insects, but wind pollination is also common in nonshowy flowers.
3. Flowering plants are either self-pollinated or cross-pollinated. Self-incompatibility ensures outcrossing. Flower-insect specificity can be very highly developed and results from long periods of co-evolution.
4. Fruits develop only if fertilization occurs in the ovules. Parthenocarpic fruits develop without fertilization and are normally seedless. A mature fruit usually has three layers to its pericarp or fruit wall. The pericarp can be fleshy or dry; when dry, it can be either dehiscent or indehiscent.
5. There are simple fruits and compound fruits, which include aggregate and multiple fruits, according to how they develop. Most fruit modifications are dispersal mechanisms. Small light fruit are wind dispersed, whereas other fruits can be water dispersed or animal dispersed.
6. Seeds contain the embryo and nutritive tissue either in the endosperm or cotyledon(s). The embryo and endosperm are formed because of double fertilization of the embryo and the polar nuclei. Seed modifications are also a result of developing successful dispersal mechanisms.
7. Seed germination can occur immediately following seed dispersal or up to many years later. The first step is imbibition, the absorption of water. The emergence of the radicle precedes the emergence of the cotyledons, epicotyl, hypocotyl, and plumule.
8. Several mechanical and environmental components affect seed germination. The thickness of the seed coat or the presence of chemical inhibitors produced dormancy until scarification or washing breaks the dormancy. Oxygen, temperature, and light all can affect the germination of seeds, either individually or in combination.
9. The development of a seedling until it is an adult plant involves many physiological and anatomical changes over time. Annuals develop much more quickly than biennials and perennials, the latter being woody or herbaceous. The terms monocarpic and polycarpic refer to the number of times a given plant flowers in its lifetime.
10. Angiosperms contain two natural subgroups, the monocots and dicots. Each has several sequentially less inclusive groups, including families, genera, and species. The supermarket name for a plant part often differs from the botanical name for the part. Many commercially useful food plants reflect vegetative and reproductive modifications.
Something to Think About
1. Why is the flower so important in people's lives?
2. What is sexual propagation?
3. What are considered female flower parts and male flower parts?
4. What role does pollination have on the production of fruit?
5. What role does fertilization have in the production of fruit?
6. What environmental components affect seed germination?
7. What are some of the physiological and anatomical changes that occur within the seed?
8. What are the differences in a monocot and dicot?
9. How are angiosperms and gymnosperms different?
10. What are scientific binomials?
Bewley, J. D., and M. Black. 1994. Seeds: Physiology of development and germination. New York: Plenum Press.
Bird, R., et. al. 1998. The complete book of plant propagation. Newtown, CT: Taunton Press.
Bowes B., and B. Bowes. 1999. A colour atlas of plant propagation and conservation. Australia: Blackwell Publishing.
Hartmann, H. T., et. al. 2002. Plant propagation principles and practices. Upper Saddle River, NJ: Prentice Hall.
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 Goggle, Yahoo!, Ask.com, and MSN Live Search, find more information by searching for these words or phrases: incomplete flower, complete flower, pistil, vascular system, root structure, modified roots, meristems, nitrogen fixation, mycorrhiza, apical dominance, botanical names, morphology, dry fruits, fleshy fruit, angiosperms, and gymnosperms.
Table 5-1 Types of Simple Fruits Type Development Examples Simple Fruits: Dry and Dehiscent Follicle Develops from Columbines, magnolia, single-carpel ovary, milkweeds splits open down one side Legume Develops from Pea family; all peas and single-carpel ovary, beans (Fabaceae) splits open along both sides Silique Develops from two-carpel Mustard family ovary, halves fall (Cruciferae) Capsule Develops from compound Cotton, poppy, primrose, ovary with two or more pinks carpels; capsules dehisce in many different ways Simple Fruits: Dry and Indehiscent Achene Small, one-seeded fruit; "Dry" fruits of pericarp easily separable strawberry, buckwheat, from seed coat, although and sunflower family closely encasing it (Asteraceae) Samara Winged one- or two-seeded Elms, ash, on seeded achenelike fruit; wings maples form from outgrowth of ovary wall Schizocarp Two- (or more) carpel Maples considered samaras ovary that, on maturity, or winged schizocarps splits into separate, one-seeded sections that fall Caryopsis One-seeded, usually small "Grain" of all grass fruit with pericarp family (Graminacea); completely united to seed includes wheat oats, coat rice, corn, barley, oats, and other important grasses Nut One-seeded fruit with Walnut, hazelnut, hard pericarp (shell) chestnut, acorns Fleshy Two- (or more) carpel Tomatoes, grapes, dates berry ovary, each usually having many seeds; inner layer of pericarp (mesocarp and endocarp) is fleshy Hesperidium Berry with thick leathery Oranges, grapefruit, "peel" (exocarp and lemons, limes; all mesocarp) and juice; citrus fruits pulpy endocarp arranged in sections; rind has oil glands Pepo Berry with outer wall or Gourd family rind formed from (Cucurbitaceous) receptacle tissue fused cucumbers, watermelons, to excocarp; fleshy squash, pumpkin interior is mesocarp and endocarp Drupe Usually only one-carpel Many members of the rose ovary and with only one family (Rosaceae), seed developing; endocarp including cherry, peach, is hard and stony, plum, almond, apricot; fitting closely around not in the Rosaceae, seed: mesocarp is fleshy, olive and coconut are and fruit is thin skinned also drupes (coconut has (thin, soft exocarp) a fibrous outer coat rather than fleshy) Pome From compound, inferior Apples and pears, both ovary in surrounding members of the Rosaceae receptacle or perianth family tissue (one embedded); fleshy edible part is ripened tissue surrounding ovary, which matures into "core" and contains seeds Aggregate and Multiple Fruits Aggregated Development of numerous Strawberry, blackberry, fruits simple carpels from a raspberry single flower; some are dry fruits, attached to fleshy receptacle, others an aggregation of simple fleshy fruit (drupe) Multiple Individual ovaries; Mulberry, pineapple fruits nutlets on enlarged fleshy receptacle or group of berries Courtesy of Rick Parker. Table 5-2 Monocot and Dicot Characteristics Monocots Dicots One cotyledon Two cotyledon Flower parts in threes or Flower parts in fours or fives multiples of three or multiples of these numbers Herbaceous almost never woody Can be woody or herbaceous Usually linear leaves with Leaves with netted parallel venation (reticulate) venation Scattered vascular bundles in Vascular bundles in a ring the stem Table 5-3 Common Food Plants Found in the United States Family Name Common Name Scientific Name Monocots Poaceae (grass family); Wheat Triticum aestivum all cereal grains Rice Oryza sative belong to this family Corn Zea mays of plants, as does Barley Hordeum vulgaris sugarcane, a great Oats Avena sativa source of granulated Rye Secale cereale sugar Sorghum Sorghum bicolor Sugarcane Saccharum officinarum Lilliaceae (lily Onion Allium cepa family) Garlic Allium sativa Asparagus Asparagus officinalis Bromeliaceae (pineapple Pineapple Ananas comous family); a family of mostly epiphytic plants; pineapples rooted in soil Dicots Brassicaceae (mustard Mustard Brassica alba family); most food Broccoli Brassica oleraceae plants in this family Cabbage Brassica oleraceae have a tangy, sharp Cauliflower Brassica oleraceae taste Brussels sprouts Brassica oleraceae Turnips Brassica rapa Radish Raphanus sativus Watercress Nasturtium officinale Fabaceae (bean family); Broad bean Vica faba a very large family Green bean Phaseolus vulgaris having many members Pinto bean Phaseolus vulgaris with high protein Navy bean Phaseolus vulgaris levels and nitrogen Kidney bean Phaseolus vulgaris fixation in roots Lima bean Phaseolus lunatus nodulated with the Black bean Phaseolus mungo bacterium Rhizobium Soybean Glycine max Green pea Pisum sativum Black-eye pea Vigna sinensis Peanut Arachis hypogaea Rosaceae (Rose family); Cherry Prunus avium a large family with Apple Malus sylvestris many different kinds of Pear Pyrus communis fruits Peach Prunus persica Plum Prunus domestica Apricot Prunus armeniaca Blackberry Rubus canadensis Strawberry Fragaria virginiana Solanaceae (nightshade Tomato Lycopersicon family); also contains esculentum other economically Potato Solanum tuberosum important species, Pepper Capsicum (jalapeno, including tobacco and a bell, cayenne,etc) number of poisonous Eggplant Solanum melongena members Cucurbitaceae Squash Cucurbita (gourd family) Pumpkin Cucurbita pepo Cucumber Cucumis sativus Gherkin Cucumis anguria Watermelon Citrullus vulgaris Honeydew melon Cucumis melo Chenopodiaceae Beet Beta vulgaris (goosefoot family) Spinach Spinacia oleracea Asteraceae (sunflower Sunflower Helianthus annus family); although one Artichoke Cynara scolymus of the largest Lettuce Lactuca sativa families, it contains Carrot Daucus carota very few economically Celery Apium graveolens important plants Parsley Petroselinum crispum Apiaceae (carrot family) Convolvulaceae (morning Sweet Potato Ipomoea batatas glory family) Family Name Fruit Part Monocots Poaceae (grass family); Fruit (grain) all cereal grains Fruit (grain) belong to this family Fruit (kernel) of plants, as does Fruit (grain) sugarcane, a great Fruit (grain) source of granulated Fruit (grain) sugar Fruit (grain) Stem Lilliaceae (lily Bulb family) Bulb Young stem Bromeliaceae (pineapple Fruit family); a family of mostly epiphytic plants; pineapples rooted in soil Dicots Brassicaceae (mustard Seeds and leaf family); most food Inflorescence plants in this family Leaves have a tangy, sharp Young inflorescence taste Lateral buds Root Root Leaves Fabaceae (bean family); Seed a very large family Seed (pod) having many members Seed with high protein Seed levels and nitrogen Seed fixation in roots Seed nodulated with the Seed bacterium Rhizobium Seed Seed and pod Seed Seed Rosaceae (Rose family); Fruit (Drupe) a large family with Fruit (Pome) many different kinds of Fruit (Pome) fruits Fruit (Drupe) Fruit (Drupe) Fruit (Drupe) Fruit (Berry) Fruit (Achene) Solanaceae (nightshade Fruit (Berry) family); also contains Tuber other economically Fruit important species, including tobacco and a Fruit number of poisonous members Cucurbitaceae Fruit (gourd family) Fruit (Melon) Fruit Fruit Fruit (Melon) Fruit (Melon) Chenopodiaceae Root (goosefoot family) Leaves Asteraceae (sunflower Seeds family); although one Inflorescence of the largest Leaves families, it contains Root very few economically Petiole important plants Leaves and stem Apiaceae (carrot family) Convolvulaceae (morning Tuberous root glory family)
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|Title Annotation:||PART 2: Form and Structure|
|Publication:||Fundamentals of Plant Science|
|Date:||Jan 1, 2009|
|Previous Article:||Chapter 4: Basic design I.|
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