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Seeds

Seeds

In seed-bearing plants, a seed is the end product of sexual reproduction. It is a mature ovule , comprising an embryo or miniature plant along with food reserves, all within a protective seed coat. Seed plants first appeared during the Devonian period some 400 million years ago and rapidly became the dominant vegetation. Up to that point, plants relied on spores for dispersal and were heavily dependent on water for reproduction.

Seeds develop by fertilization of ovules, both the exposed ovules of gymnosperms like the conifers and the enclosed ovules of the angiosperms (flowering plants). The seeds of gymnosperms are virtually naked and exposed to the elements, whereas those of the flowering plant develop within a protective structure: the fruit. In both groups, the egg within the ovule is fertilized by a male nucleus arriving via a pollen grain. From this, a miniature plant or embryo develops that will later resume development in a process termed "germination," utilizing energy stores laid down in the seed.

Flowering plants differ from gymnosperms in that seed development in angiosperms starts with double fertilization. Male and female gametes fuse to form the diploid zygote , which develops into the embryo, while a second male nucleus fuses with two other nuclei of the ovule to give rise to a triploid endosperm . The endosperm is a nutritive tissue that provides food material for the developing embryo. In some flowering plant seeds it remains throughout seed development, storing the reserves that the embryo will require for germination. Such endospermic seeds are produced by cereals like wheat, as well as dicotyledonous plants like castor bean.

In nonendospermic seeds the endosperm virtually disappears, all the food reserves being transferred during seed development to the embryo itself. In such seeds, the cotyledons or first seed leaves become quite large and accumulate the reserves that will be mobilized later in germination. Reserves may take the form of intracellular oil droplets (for example, the sunflower), protein bodies (beans), and starch grains (cereals), or combinations of these. Some seeds also store polysaccharide reserves as massively thickened cell walls (some leguminous plants and date palm) that will later be hydrolyzed . The exception to this general pattern is the family of flowering plants known as orchids. They produce the smallest seeds known. These dustlike seeds contain just a few cells, often not even organized into a recognizable embryo, and contain absolutely no food reserves. Their germination relies on symbiotic associations with fungi to provide the fuel for germination.

Seeds often exhibit dormancy, meaning they fail to germinate even when provided with adequate water and suitable temperature conditions. Dormancy acts to prevent germination until conditions are right. This dormancy may be broken by proper exposure to light or darkness. Alternatively, a hard seed coat may physically prevent water uptake and embryo expansion or even gas exchange, with germination only proceeding following physical damage to the seed coat. Last, chemical inhibitors present in the seed may cause dormancy, and these must first leach out into the soil before germination can take place. Seeds of crop plants can often be stored for years under cold, dry conditions, and some plants show extreme seed longevity under natural conditions (for example, the sacred lotus germinates after hundreds of years buried in lake mud).

see also Angiosperms; Flowers; Fruits; Grain; Gymnosperms; Pollination and Fertilization

C. M. Sean Carrington

Bibliography

Kaufman, Peter B., et al. Plants: Their Biology and Importance. New York: Harper & Row Publishers, 1989.

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Seeds

Seeds

The seed is the dispersal stage of the life cycle of angiosperms and gymnosperms . It contains the embryo, the next generation of plant in miniature. Many seeds are dry when shed from their parent plant and are thus adapted to withstand harsh environments until conditions suitable for germination are achieved.

The evolution of plants to produce seeds is poorly understood because the fossil evidence is incomplete. The advantage of reproducing through seeds is apparent, however: The embryo is encased in a protective coat and is provided with a source of nutrients until, as a young seedling following germination, it becomes established as an independent photosynthetic (autotrophic ) entity.

Seeds account for 70 percent of food consumed by humans, and are also the major feeds for domestic animals. Their importance cannot be overstated. World seed production is dominated by the cereals, and even the production of wheat, maize, or rice alone by far exceeds that of all the other crops. As a concentrated source of carbohydrate, cereals provide for the human diet, livestock feed, and industrial raw materials. They are also an important source of protein, oil, vitamins, and fiber. Grain legumes , particularly soybeans and groundnuts (peanuts) are an important source of proteins and vegetable oils, which are used in margarine and cooking fats, and have applications in paints, varnishes, and plastics, as well as the manufacture of soaps and detergents. An understanding of seeds is therefore an essential prelude to human attempts to improve their quality and yield, whether it be by conventional breeding techniques or the novel approach of genetic engineering.

Seed Structure

A seed is a combination of maternal tissues, embryo tissues, and (in angiosperms) endosperm tissue. Seeds of different species are variable in size and internal structure at the time they are shed from their parent plant. They may be barely visible to the naked eye (for example, orchids), weigh a few micrograms to milligrams (for example, poppy, tobacco, and many annual weeds), weigh up to several hundred milligrams to grams (for example, soybean, maize, pea, and bean) or even several kilograms (coconut and Lodoicea maldivica ).

Non-Maternal Tissues

The seed develops from the ovule after the egg cell within has been fertilized by a male gamete from a germinated pollen grain. The resulting diploid zygote cell then undergoes extensive mitotic divisions to form the embryo. In angiosperms, the process of double-fertilization occurs when a second male gamete from the pollen tube fuses with two female nuclei in the ovule, yielding a triploid nucleus containing one set of paternal genes and two maternal sets. This also undergoes extensive mitotic divisions to produce the endosperm, usually a storage tissue that may (cereals, castor bean) or may not (peas, beans) persist in the mature seed, or it may be reduced to a thin layer of cells (lettuce, tomato, soybean).

Maternal Tissues

The seed coat (testa) develops from the outer layers of the ovule, the integuments, and is a diploid maternal tissue. In many angiosperm species the ovary wall surrounding the ovule also divides and develops at the same time as the seed to form an enclosing fruit. While many species form a fleshy fruit, in others the fruit tissues (pericarp) develop as only a few layers of cells, become dry, and adhere to the outside of an equally thin or thinner seed coat. Strictly, by botanical definition, such dispersal structures are fruits (for example, cereal grains, lettuce, sunflower, oak). Nonetheless, for convenience, and because they contain the embryo, they are usually called seeds.

In some angiosperm seeds, following the completion of fertilization, another part of the ovule, the nucellar tissue, may divide mitotically and grow to produce a nutritive perisperm (sugar beet, coffee). In gymnosperm (conifer) seeds, the tissue surrounding the mature embryo is the megagametophyte, a haploid maternal tissue into which the fertilized zygote grows during seed development; it is still substantially present in the mature seed as a source of stored reserves.

The Embryo

The embryo is made up of an axis bearing one or more cotyledons. The axial region contains a hypocotyl to which the cotyledons are attached, a radicle that will become the primary root following germination, and the plumule, the shoot axis bearing the first true leaves. These parts are usually easy to discern in dicot angiosperm seeds and in those of the polycotyledonous (many cotyledons) gymnosperms. But in seeds of monocot plants, particularly the cereal grains, the single cotyledon is much reduced and modified to form the scutellum, an absorptive structure that lies against the endosperm and absorbs material from it. The basal sheath of the cotyledon is elongated to form a coleoptile , which covers the first leaves.

The shapes of the embryos and their position within the seed are variable between species. In those dicot species that have a substantial endosperm (endospermic seeds) the embryo occupies proportionately less of the seed than when the endosperm is rudimentary or absent (compare castor bean with lettuce or runner bean). In contrast, the cotyledons of nonendospermic seeds are much bulkier and are the storage tissues, and in peas and beans account for over 90 percent of the mass of the seed.

Several variations on this general theme occur. In the Brazil nut the cotyledons are much reduced and the bulk of the seed is occupied by a storage hypocotyl. Because the Brazil nut is primarily a single hypocotyl, it does not split in two like most other nuts made from two enlarged cotyledons. Cotyledons are absent from the seeds of many parasitic species. In orchids, seeds are shed when the embryos are extremely small and contain only a few cells, and completion of development occurs afterward.

Non-Embryonic Storage Tissues

In most species, the maternally derived perisperm fails to develop and is quickly absorbed by the developing embryo. Where it does persist, it is a major storage tissue, sometimes in conjunction with an endosperm, or the cotyledons (for example, sugar beet).

As noted, seeds are categorized as endospermic or nonendospermic in relation to the presence or absence of a well-formed endosperm within the mature seed. The relatively massive endosperm is the major source of stored seed reserves in species such as the cereals, castor bean, date palm, and endospermic legumes (carob, fenugreek). In the cereal grains and seeds of some endospermic legumes (for example, fenugreek) the storage cells of the endosperm are nonliving at maturity, and the cytoplasmic contents have been replaced entirely by the stored reserves (starch and protein in cereals; hemi-cellulose cell walls in fenugreek). But on the outside of the endosperm there remains a living tissue of one to a few cell layers in thickness, the aleurone layer, whose role is to synthesize and secrete enzymes to mobilize those reserves following germination.

The Seed Coat (Testa)

The anatomy of the seed coat is highly variable, and differences among species have been used for taxonomic purposes. The coat is of considerable importance to the seed because it is a protective barrier between the embryo and the outside environment (in some species the fruit coat may augment or be a substitute for this role). Protection by the seed coat is aided by the presence of an inner and outer cuticle , impregnated with fats and waxes, and lignified cell walls. Phenolics or crystals (of calcium oxalate, for example) may be deposited in the coat to discourage predation by insects. Mucilage-containing cells may be present that burst on contact with water, retaining and absorbing moisture as a supply to the germinating embryo. Rarely, hairs or wings develop on the seed coat to aid dispersal (willow, lily); more frequently the dispersal structures are a modification of the surrounding fruit coat.

Quiescence and Dormancy

The completion of seed development and the acquisition of the mature structure is marked in many species by a loss of water, so that the mature seed can be dispersed in the dry state. The water content of a dry seed is usually 5 to 15 percent, versus 70 percent or more for the plant as a whole. When dry, a seed can withstand extremes of temperature that would rapidly result in death in the hydrated state. Not surprisingly, then, dry seeds are more or less in a state of suspended animation, with little or no metabolic activity. As such, they are said to be quiescent. When introduced to water again, under favorable conditions such seeds will rapidly resume metabolism and complete germination.

The phenomenon of seed quiescence is very different from that of dormancy. The latter is when seeds in a hydrated state fail to complete germination even when conditions are favorable; that is, temperature, water and oxygen supply are not limiting. Dormant seeds are metabolically active, in fact as active as their nondormant counterparts, but there exists within the seed a block (or blocks) that must be removed before germination can be completed. To be released from dormancy, a seed must experience a particular stimulus, or undergo certain metabolic changes. The cause of dormancy is not clearly understood, but at least one factor is the growth regulator hormone abscisic acid (ABA), which is imported from the parent plant into the seed during its development.

Many seeds lose dormancy (while remaining quiescent) while still in the mature dry state, in a process called after-ripening, which may extend over several weeks to many years. Dormancy of hydrated seeds in the soil may be broken by one or more environmental cues, whose effectiveness depends on the species. These cues include: 1) light, usually for a short duration, with sunlight being the most effective; 2) low temperature, around 1 to 5°C for several to many weeks; 3) fluctuating temperatures, usually day-night

CONDITIONS THAT TERMINATE SEED DORMANCY
Species Common Name Light Chilling Alternating Temperatures After-ripening
Acer pseudoplatanus Great maple + +
Avena fatua Wild oat + +
Betula pubescens Birch + + +
Hordeum species Barley + +
Lactuca sativa Lettuce + + +
Nicotiana tabacum Tobacco + +
Pinus sylvestris Scot's pine + +
Prunus domestica Plum + +
Triticum aestivum Wheat + +

fluctuations of 5 to 10°C; and 4) chemicals, of which nitrate is the most important in the soil.

Dormancy is a mechanism to ensure the optimum distribution of seed germination in time and space. For example, seeds that require weeks of cold temperatures to break their dormancy cannot complete germination immediately after being shed from their parent plant in early fall, but will do so only following the cold winter months. This ensures that they are not in the delicate seedling stage at the onset of winter, which would be detrimental to their survival. Dormancy of light-requiring seeds will be removed only when seeds are at the soil surface, a mechanism that prevents germination at too great a depth. This is crucial for small seeds whose stored reserves are insufficient to support growth through the soil to carry the seedling leaves into the light to begin photosynthesis. Seeds on the forest floor receive light that is poor in the red wavelengths, since this is absorbed by the leaves of the overarching canopy. Thus, seeds in this environment must wait for the appearance of gaps in the canopy (tree fall or logging) before their dormancy can be broken, and they can then emerge in situations where there is reduced competition for resources from established plants. Phytochrome is the light-perception system in dormant seeds and is activated by wavelengths rich in red.

While the significance of dormancy can best be understood in an ecological context, it is important in agriculture too. Prolonged dormancy in crop species is undesirable since germination could be spread out over several years, resulting in unpredictable and low annual yields. On the other hand, lack of at least a temporary dormancy can be harmful also because, for example, mature seeds of barley or wheat could germinate on the ear if wetted by rain before harvest, resulting in crop spoilage.

see also Embryogenesis; Flowers; Fruits; Fruits, Seedless; Germination; Germination and Growth; Grains; Phytochrome; Pollination; Reproduction, Fertilization and; Reproduction, Sexual; Seed Dispersal; Seed Preservation.

J. Derek Bewley

Bibliography

Bewley, J. Derek, and Michael Black. Seeds: Physiology of Development and Germination, 2nd ed. New York: Plenum, 1994.

Bradbeer, J. W. Seed Dormancy and Germination. Glasgow and London: Blackie, 1988.

Chrispeels, Maarten J., and David E. Sadava. Plants, Genes and Agriculture. Boston and London: Jones and Bartlett, 1994.

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Seeds

Seeds

Biology of seeds

Dissemination of seeds

Uses of seeds

Seeds as food

Other uses of seeds

Resources

Seeds are the products of the sexual reproduction of plants, and for this reason the genetic information of seeds is influenced by both of the parents. Sexual reproduction is important for two reasons. The first involves the prevention of the loss of potentially important genetic information, a process that occurs when non-sexual means of propagation are prevalent. The other benefit of sexual reproduction is associated with the provision of new genetic combinations upon which natural selection acts, so that species continue to evolve populations that are favorably adapted to a dynamically changing environment.

Plants have evolved various mechanisms for the dissemination of their seeds, so that new plants can be established at some distance from their parent. The dispersal of seeds is important in expanding the range of plant species, especially if species are to take advantage of habitat opportunities that may be created by disturbances and other ecological processes.

The seeds of some plant species are important to humans, as sources of food, while other seeds are important as raw materials for the manufacture of industrial chemicals, and other products.

Biology of seeds

Seeds develop from the fertilized ovules of female (pistillate) floral parts, following fertilization by pollen released from the male (staminate) floral parts. If ovules and pollen come from different individual plants, then the genetic makeup of the seed represents a mixture of the two parent plants, and sexual reproduction is said to have occurred.

In some plant species (known as monoecious plants), pollen from a plant may fertilize its own ovules, a phenomenon that is known as self-pollination. This can occur when flowers contain both pistillate and staminate organs (these are known as perfect flowers). Self-fertilization can also occur when the same flowers on the same plant are either male or female. Although self-pollination results in genetic mixing, the degree of mixing is much less than in true, sexual reproduction. If self-fertilization occurs frequently, the eventual result is a loss of genetic variation through inbreeding, which may have deleterious consequences on the evolutionary fitness of the plant.

Most plant species avoid self-pollination, and encourage cross-pollination among genetically different individuals of the species. One such adaptation involves individual plants that produce only male

flowers or only female flowers (these are known as dioecious plants). In addition, many plant species have pollination systems that encourage out-crossing, such as pollination by the wind. Other plants are pollinated by insects or birds that carry the pollen to the receptive stigmatic surfaces of other plants of the same species. The benefit of out-crossing is to reap the evolutionary benefits of sexual reproduction by producing genetically diverse seeds.

A seed is more than just a fertilized ovule; it also contains the embryonic tissues of the adult plant, including a rudimentary root, shoot, and leaves. These structures are surrounded by tissues containing starch and/or oil that are intended to provide nourishment for germination and the early growth of the seedling. The walls of the ovule develop into a hard seed coat, intended to provide protection for the tender, internal tissues.

The above description gives an idea of the basic, anatomical structure of seeds. However, the actual proportion of the various tissues in the seed varies according to species. Orchids (family Orchidaceae), for example, have tiny, dust like seeds that consist of little more than core embryonic tissues, with very little in the way of energy reserves. In contrast, the gigantic seeds of the Seychelles Islands coconut (Lodoicea mal-divica ) can weigh more than 11.5 lb (25 kg ), most of which is nutritional reserve surrounded by fibrous, protective husk.

The seeds of many plant species are dispersed as individual units throughout the environment, while those of other species are encased as groups of seeds inside of fruits of various sorts. These fruits are usually intended for ingestion by animals, which then disperse the seeds widely (see below ).

Dissemination of seeds

A plant seed is a unique genetic entity, a biological individual. However, a seed is in a diapause state, an essentially dormant condition, awaiting the ecological conditions that will allow it to grow into an adult plant, and produce its own seeds. Seeds must therefore germinate in a safe place, and then establish themselves as a young seedling, develop into a juvenile plant, and finally become a sexually mature adult that can pass its genetic material on to the next generation. The chances of a seed developing are generally enhanced if there is a mechanism for dispersing to an appropriate habitat some distance from the parent plant.

The reason for dispersal is that closely related organisms have similar ecological requirements. Consequently, the competitive stress that related organisms exert on each other is relatively intense. In most cases, the immediate proximity of a well-established, mature individual of the same species presents difficult environment for the germination, establishment, and growth to maturity of a seed. Obviously, competition with the parent plant will be greatly reduced if its seeds have a mechanism to disperse some distance away.

However, there are some important exceptions to this general rule. For example, the adults of annual species of plants die at the end of their breeding season, and in such cases the parent plants do not compete with their seeds. Nevertheless, many annuals have seeds that are dispensed widely. Annual plants do well in very recently disturbed, but fertile habitats, and many have seeds with great dispersal powers. Annual species of plants are generally very poor competitors with the longer-lived plant species that come to dominate the site through the ecological process of succession. As a result, the annuals are quickly eliminated from the new sites, and for this reason the annual species must have seeds with great dispersal capabilities, so that recently disturbed sites elsewhere can be discovered and colonized, and regional populations of the species can be perpetuated.

Plants have evolved various mechanisms that disperse their seeds effectively. Many species of plants have seeds with anatomical structures that make them very buoyant, so they can be dispersed over great distances by the winds. Several well-known examples of this sort are the fluffy seeds of the familiar dandelion (Taraxacum officinale ) and fireweed (Epilobium augustifolium ). The dandelion is a weed species, and it continuously colonizes recently disturbed habitats, before the mature plants are eliminated from their rapidly-maturing habitats. Favorable disturbances for weed species such as dandelions are often associated with human activities, such as the demolition of old buildings, the development of new lawns, or the abandonment of farmland. In the case of the fireweed, it is recently burned forests or clear-cuts that are colonized by aerially-dispersed seeds. The adult plants of the dandelion and fireweed produce enormous numbers of seeds during their lifetime, but this is necessary to ensure to that a few of these seeds will manage to find a suitable habitat during the extremely risky, dispersal phase of the life cycles of these and other aerially dispersed species.

The seeds of maple trees (Acer spp. ) are aerially dispersed, and have a one-sided wing that causes them to swirl propeller like after they detach from the parent tree. This allows even light breezes to carry the maple seeds some distance from their parent before they hit the ground.

Some plants have developed an interesting method of dispersal, known as tumbleweeding. These plants are generally annual species, and they grow into a roughly spherical shape. After the seeds are ripe, the mature plant detaches from the ground surface, and is then blown about by the wind, shedding its seeds widely as it tumbles along.

The seeds of many other species of plants are dispersed by animals. Some seeds have structures that allow them to attach to the fur or feathers of passing animals, who then carry the seeds some distance away from the parent plant before they are deposited to the ground. Familiar examples of this sticking sort of seed are those of the beggar-ticks (Bidens frondose ) and the burdock (Arctium minus ). The spherical fruits of the burdock have numerous hairs with tiny hooked tips that stick to fur (and to clothing, and were the botanical model from which the inspiration for velcro, a fastening material, was derived.

Another mechanism by which seeds are dispersed by animals involves their encasement in a fleshy, edible fruit. Such fruits are often brightly colored, have pleasant odors, and are nutritious and attractive to herbivorous animals. These animals eat the fruit, seeds and all. After some time, the animal defecates, and the seeds are effectively dispersed some distance from the parent plant. The seeds of many plants with this sort of animal-dispersal strategy actually require passage through the gut of an animal before they will germinate, a characteristic that is referred to as scarification. Some familiar examples of species that develop animal-dispersed fruits include the cherries (Prunus spp. ), tomato (Lycopersicon esculen-tum ), and watermelon (Citrullus vulgaris ).

After seeds have been dispersed into the environment, they may remain in a dormant state for some time, until appropriate cues are sensed for germination and seedling establishment. Especially in forests, there can be a large reservoir of viable but dormant seeds, known as a seed bank, within the surface organic layer of the soil. The most prominent species in the seed bank are often particularly abundant as adult plants during the earlier stages of forest succession, that is, following disturbance of the stand by a wildfire, windstorm, or clear-cut. These early-successional species cannot survive as adult plants during the earlier stages of forest succession, that is, following disturbance of the stand by a wildfire, windstorm, or clear-cut. These early-successional species cannot survive as adult plants beneath a mature forest canopy, but in many cases they can continue to exist in the forest as living but dormant seeds in the surface organic layer, often in great abundance. Species that commonly exhibit this strategy of a persistent seed bank include the cherries (Prunus spp. ), blackberries, and raspberries (Rubus spp. ).

Uses of seeds

The seeds of some species of plants are extremely important for human welfare. In some cases, this is because the seeds (or the fruits that contain them ) are used as a source of food, but there are some other important uses of seeds as well.

Seeds as food

There are numerous examples of the use of seeds as food for humans. The seeds may be eaten directly, or used to manufacture flour, starch, oil, alcohol, or some other edible products. The seeds of certain agricultural grasses are especially important foodstuffs, for example, those of wheat (Triticum aestivum ), rice (Oyza sativa ), maize (Zea mays ), sorghum (Sorghum bicolor ), and barley (Hordeum vulgare ). Other edible seeds include those of the legumes, the second-most important family of plants after the grasses, in terms of providing foods for human consumption. Examples of legumes whose seeds are eaten by people include the peanut (Arachis hypogaea ), soybean (Glycine max ), lentil (Lens esculenta ), common pea (Pisum sativum ), and common bean (P. vulgaris ). Other edible seeds include those of the coconut (Cocos nucifera ), walnut (Juglans regia ), pecan (Carya illinoensis ), and sunflower (Helianthus annua ).

Many other seeds are eaten with their fruits, although it is generally the encasing fruit walls that are the sought-after source of nutrition. A few examples of

KEY TERMS

Dioecious Plants in which male and female flowers occur on separate plants.

Dispersal Here, this referring to the spreading of propagules outward from their point of origin, as when seeds disperse away from their parent plant, using wind or an animal vector.

Germination The beginning of growth of a seed.

Monoecious Referring to cases in which individual plants are bisexual, having both staminate and pistillate floral parts.

Perfect flowers Referring to cases in which individual flowers are bisexual, having both staminate and pistillate organs.

Pollination The transfer of pollen from its point of origin (that is, the anther of the stamen ) to the receptive surface of the pistil (i.e., the stigma ) of the same species.

Scarification The mechanical or chemical abrasion of a hard seedcoat in order to stimulate or allow germination to occur.

Seed bank The population of viable seeds that occurs in the surface organic layer and soil of an ecosystem, especially in forests.

Succession A process of ecological change, involving the progressive replacement of earlier communities with others over time, and generally beginning with the disturbance of a previous type of ecosystem.

edible fruits include those of the pumpkin or squash (Cucurbita pepo ), bell pepper (Capsicum anuum ), apple (Malus pumila ), sweet cherry (Prunus avium ), strawberry (Fragaria vesca ), raspberry (Rubus idaeus ), and sweet orange (Citrus sinensis ).

Other uses of seeds

The seeds of some plants have other uses, including serving as resources for the manufacturing of industrial chemicals, such as grain alcohol (ethanol ), derived from a fermentation of the seeds of corn, wheat, or some other plants. The seeds of some plants are used as attractive decorations, as is the case of the Jobs tears (Coix lachryma-jobi ), a grass that produces large, white, shiny seeds that are used to make attractive necklaces and other decorations, often dyed in various attractive colors.

Resources

BOOKS

Fenner, Michael. Seeds: The Ecology of Regeneration in Plant Communities Cambridge, MA: CABI Publishing, March 2001.

Herberd, Nicholas. Seed to Seed: The Secret Life of Plants. Ashland, OR: Bloomsbury USA, May 2006.

Kessler, Rob. Seeds: Time Capsules of Life. Richmond Hill, ON: Firefly Books, September 2006.

Bill Freedman

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Seeds

Seeds

Seeds are the products of the sexual reproduction of plants, and for this reason the genetic information of seeds is influenced by both of the parents. Sexual reproduction is important for two reasons. The first involves the prevention of the loss of potentially important genetic information, a process that occurs when non-sexual means of propagation are prevalent. The other benefit of sexual reproduction is associated with the provision of new genetic combinations upon which natural selection acts, so that species continue to evolve populations that are favorably adapted to a dynamically changing environment.

Plants have evolved various mechanisms for the dissemination of their seeds, so that new plants can be established at some distance from their parent. The dispersal of seeds is important in expanding the range of plant species, especially if species are to take advantage of habitat opportunities that may be created by disturbances and other ecological processes.

The seeds of some plant species are important to humans, as sources of food, while other seeds are important as raw materials for the manufacture of industrial chemicals, and other products.


Biology of seeds

Seeds develop from the fertilized ovules of female (pistillate) floral parts, following fertilization by pollen released from the male (staminate) floral parts. If ovules and pollen come from different individual plants, then the genetic makeup of the seed represents a mixture of the two parent plants, and sexual reproduction is said to have occurred.

In some plant species (known as monoecious plants), pollen from a plant may fertilize its own ovules, a phenomenon that is known as self-pollination. This can occur when flowers contain both pistillate and staminate organs (these are known as "perfect" flowers). Self-fertilization can also occur when the same flowers on the same plant are either male or female. Although self-pollination results in genetic mixing, the degree of mixing is much less than in true, sexual reproduction. If self-fertilization occurs frequently, the eventual result is a loss of genetic variation through inbreeding, which may have deleterious consequences on the evolutionary fitness of the plant.

Most plant species avoid self-pollination, and encourage cross-pollination among genetically different individuals of the species. One such adaptation involves individual plants that produce only male flowers or only female flowers (these are known as dioecious plants). In addition, many plant species have pollination systems that encourage out-crossing, such as pollination by the wind . Other plants are pollinated by insects or birds that carry the pollen to the receptive stigmatic surfaces of other plants of the same species. The benefit of outcrossing is to reap the evolutionary benefits of sexual reproduction by producing genetically diverse seeds.

A seed is more than just a fertilized ovule; it also contains the embryonic tissues of the adult plant, including a rudimentary root, shoot, and leaves. These structures are surrounded by tissues containing starch and/or oil that are intended to provide nourishment for germination and the early growth of the seedling. The walls of the ovule develop into a hard seed coat, intended to provide protection for the tender, internal tissues.

The above description gives an idea of the basic, anatomical structure of seeds. However, the actual proportion of the various tissues in the seed varies according to species. Orchids (family Orchidaceae), for example, have tiny, dust-like seeds that consist of little more than core embryonic tissues, with very little in the way of energy reserves. In contrast, the gigantic seeds of the Seychelles Islands coconut (Lodoicea maldivica) can weigh more than 11.5 lb (25 kg), most of which is nutritional reserve surrounded by fibrous, protective husk.

The seeds of many plant species are dispersed as individual units throughout the environment, while those of other species are encased as groups of seeds inside of fruits of various sorts. These fruits are usually intended for ingestion by animals, which then disperse the seeds widely (see below).


Dissemination of seeds

A plant seed is a unique genetic entity, a biological individual. However, a seed is in a diapause state, an essentially dormant condition, awaiting the ecological conditions that will allow it to grow into an adult plant, and produce its own seeds. Seeds must therefore germinate in a safe place, and then establish themselves as a young seedling, develop into a juvenile plant, and finally become a sexually mature adult that can pass its genetic material on to the next generation. The chances of a seed developing are generally enhanced if there is a mechanism for dispersing to an appropriate habitat some distance from the parent plant.

The reason for dispersal is that closely related organisms have similar ecological requirements. Consequently, the competitive stress that related organisms exert on each other is relatively intense. In most cases, the immediate proximity of a well-established, mature individual of the same species presents difficult environment for the germination, establishment, and growth to maturity of a seed. Obviously, competition with the parent plant will be greatly reduced if its seeds have a mechanism to disperse some distance away.

However, there are some important exceptions to this general rule. For example, the adults of annual species of plants die at the end of their breeding season, and in such cases the parent plants do not compete with their seeds. Nevertheless, many annuals have seeds that are dispensed widely. Annual plants do well in very recently disturbed, but fertile habitats, and many have seeds with great dispersal powers. Annual species of plants are generally very poor competitors with the longer-lived plant species that come to dominate the site through the ecological process of succession . As a result, the annuals are quickly eliminated from the new sites, and for this reason the annual species must have seeds with great dispersal capabilities, so that recently disturbed sites elsewhere can be discovered and colonized, and regional populations of the species can be perpetuated.

Plants have evolved various mechanisms that disperse their seeds effectively. Many species of plants have seeds with anatomical structures that make them very buoyant, so they can be dispersed over great distances by the winds. Several well-known examples of this sort are the fluffy seeds of the familiar dandelion (Taraxacum officinale) and fireweed (Epilobium augustifolium). The dandelion is a weed species, and it continuously colonizes recently disturbed habitats, before the mature plants are eliminated from their rapidly-maturing habitats. Favorable disturbances for weed species such as dandelions are often associated with human activities, such as the demolition of old buildings, the development of new lawns, or the abandonment of farmland. In the case of the fireweed, it is recently burned forests or clear-cuts that are colonized by aerially-dispersed seeds. The adult plants of the dandelion and fireweed produce enormous numbers of seeds during their lifetime, but this is necessary to ensure to that a few of these seeds will manage to find a suitable habitat during the extremely risky, dispersal phase of the life cycles of these and other aerially dispersed species.

The seeds of maple trees (Acer spp.) are aerially dispersed, and have a one-sided wing that causes them to swirl propeller-like after they detach from the parent tree . This allows even light breezes to carry the maple seeds some distance from their parent before they hit the ground.

Some plants have developed an interesting method of dispersal, known as "tumbleweeding." These plants are generally annual species, and they grow into a roughly spherical shape. After the seeds are ripe, the mature plant detaches from the ground surface, and is then blown about by the wind, shedding its seeds widely as it tumbles along.

The seeds of many other species of plants are dispersed by animals. Some seeds have structures that allow them to attach to the fur or feathers of passing animals, who then carry the seeds some distance away from the parent plant before they are deposited to the ground. Familiar examples of this sticking sort of seed are those of the beggar-ticks (Bidens frondose) and the burdock (Arctium minus). The spherical fruits of the burdock have numerous hairs with tiny hooked tips that stick to fur (and to clothing, and were the botanical model from which the inspiration for velcro, a fastening material, was derived.

Another mechanism by which seeds are dispersed by animals involves their encasement in a fleshy, edible fruit. Such fruits are often brightly colored, have pleasant odors, and are nutritious and attractive to herbivorous animals. These animals eat the fruit, seeds and all. After some time, the animal defecates, and the seeds are effectively dispersed some distance from the parent plant. The seeds of many plants with this sort of animal-dispersal strategy actually require passage through the gut of an animal before they will germinate, a characteristic that is referred to as scarification. Some familiar examples of species that develop animal-dispersed fruits include the cherries (Prunus spp.), tomato (Lycopersicon esculentum), and watermelon (Citrullus vulgaris).

After seeds have been dispersed into the environment, they may remain in a dormant state for some time, until appropriate cues are sensed for germination and seedling establishment. Especially in forests, there can be a large reservoir of viable but dormant seeds, known as a "seed bank," within the surface organic layer of the soil . The most prominent species in the seed bank are often particularly abundant as adult plants during the earlier stages of forest succession, that is, following disturbance of the stand by a wildfire , windstorm, or clear-cut. These early-successional species cannot survive as adult plants during the earlier stages of forest succession, that is, following disturbance of the stand by a wildfire, windstorm, or clear-cut. These early-successional species cannot survive as adult plants beneath a mature forest canopy, but in many cases they can continue to exist in the forest as living but dormant seeds in the surface organic layer, often in great abundance. Species that commonly exhibit this strategy of a persistent seed bank include the cherries (Prunus spp.), blackberries, and raspberries (Rubus spp.).


Uses of seeds

The seeds of some species of plants are extremely important for human welfare. In some cases, this is because the seeds (or the fruits that contain them) are used as a source of food, but there are some other important uses of seeds as well.


Seeds as food

There are numerous examples of the use of seeds as food for humans. The seeds may be eaten directly, or used to manufacture flour, starch, oil, alcohol , or some other edible products. The seeds of certain agricultural grasses are especially important foodstuffs, for example, those of wheat (Triticum aestivum), rice (Oyza sativa) maize (Zea mays), sorghum (Sorghum bicolor), and barley (Hordeum vulgare). Other edible seeds include those of the legumes , the second-most important family of plants after the grasses, in terms of providing foods for human consumption. Examples of legumes whose seeds are eaten by people include the peanut (Arachis hypogaea), soybean (Glycine max), lentil (Lens esculenta), common pea (Pisum sativum), and common bean (P. vulgaris). Other edible seeds include those of the coconut (Cocos nucifera), walnut (Juglans regia), pecan (Carya illinoensis), and sunflower (Helianthus annua).

Many other seeds are eaten with their fruits, although it is generally the encasing fruit walls that are the sought-after source of nutrition . A few examples of edible fruits include those of the pumpkin or squash (Cucurbita pepo), bell pepper (Capsicum anuum), apple (Malus pumila), sweet cherry (Prunus avium), strawberry (Fragaria vesca), raspberry (Rubus idaeus), and sweet orange (Citrus sinensis).


Other uses of seeds

The seeds of some plants have other uses, including serving as resources for the manufacturing of industrial chemicals, such as grain alcohol (ethanol ), derived from a fermentation of the seeds of corn, wheat, or some other plants. The seeds of some plants are used as attractive decorations, as is the case of the Job's tears (Coix lachryma-jobi), a grass that produces large, white, shiny seeds that are used to make attractive necklaces and other decorations, often dyed in various attractive colors.


Resources

books

judd, walter s., christopher campbell, elizabeth a. kellogg, michael j. donoghue, and peter stevens. plant systematics: a phylogenetic approach. 2nd ed. with cd-rom. suderland, md: sinauer, 2002.

klein, r.m. the green world: an introduction to plants and people. new york: harper & row, 1987.


periodicals

white, j.a., et al. "expressed sequence tags from developing seeds. the metabolic pathway from carbohydrates to seed oil." plant physiology 124 (december 2000): 1582-1594.


Bill Freedman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dioecious

—Plants in which male and female flowers occur on separate plants.

Dispersal

—Here, this referring to the spreading of propagules outward from their point of origin, as when seeds disperse away from their parent plant, using wind or an animal vector.

Germination

—The beginning of growth of a seed.

Monoecious

—Referring to cases in which individual plants are bisexual, having both staminate and pistillate floral parts.

Perfect flowers

—Referring to cases in which individual flowers are bisexual, having both staminate and pistillate organs.

Pollination

—The transfer of pollen from its point of origin (that is, the anther of the stamen) to the receptive surface of the pistil (i.e., the stigma) of the same species.

Scarification

—The mechanical or chemical abrasion of a hard seedcoat in order to stimulate or allow germination to occur.

Seed bank

—The population of viable seeds that occurs in the surface organic layer and soil of an ecosystem, especially in forests.

Succession

—A process of ecological change, involving the progressive replacement of earlier communities with others over time, and generally beginning with the disturbance of a previous type of ecosystem.

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