A mature seed typically consists of a mature plant ovum containing a minute, partially developed young plant, the embryo, surrounded by an abundant supply of food and enclosed by a protective seed coat.
Seed plants are divided into two main groups: the gymnosperms, primarily cone-bearing plants such as pine, spruce, and fir trees, and the angiosperms, the flowering plants.
The gymnosperms have naked ovules which, at the time of pollination, are exposed directly to the pollen grains. Their food supply in the seed is composed of a female gametophyte, rather than the endosperm found in angiosperms.
In angiosperms, seeds develop from ovules that are enclosed in a protective ovary. The ovary is the basal portion of the carpel, typically a vase-shaped structure located at the center of a flower.
The top of the carpel, the stigma, is sticky, and when a pollen grain lands upon it, the grain is firmly held. The germinating pollen grain produces a pollen tube that grows down through the stigma and style into the ovary and pierces the ovule.
Two male sperm nuclei are released from the pollen grain and travel down the pollen tube into the ovule.One of the sperm nuclei fuses with an egg cell inside the ovule.
This fertilized egg divides many times and develops into the embryo. The second male nucleus unites with other parts of the ovule and develops into the endosperm, a starchy or fatty tissue that is used by the embryo as a source of food during germination.
Angiosperm seeds remain protected at maturity. While the seed develops, the enclosing ovary also develops into a hard shell, called a seed coat or testa, often enclosed in a fibrous or fleshy fruit.
In angiosperm seeds, the embryo may have either one or two cotyledons. Angiosperms with one cotyledon are plants are called monocots; those with two cotyledons are called eudicots (formerly dicots).
A typical example of a monocot is corn, or maize (Zea mays), whereas the bean (Phaseolus vulgaris) is a typical eudicot. In gymnosperms, the embryo may have between two and sixteen cotyledons; for example, the embryo of Scots pine (Pinus sylvestris) possesses eight cotyledons.
Immediately below the cotyledons is the hypocotyl, at the tip ofwhich lies the growing point of the root. Above the cotyledons lies the epicotyl, which consists of a miniature shoot tip and leaves.
Upon germination of the seed, the epicotyl develops into the stemand leaves of the new young plant. Almost all seeds carry with thema supply of food, which in angiosperms is the endosperm.
Although the embryo is usually surrounded by the endosperm, in some seeds (such as maize) embryos and endospermlie side by side. In the seeds of the pea family (Leguminosae), the food reserves of the endosperm are absorbed by the embryo, resulting in enlarged cotyledons.
Gymnosperm seeds differ from those of angiosperms in the origin of their stored food. In gymnosperms the stored food is provided by a female gametophyte housed with the embryo inside the seed, whereas in angiosperms the food reserve is the endosperm.
All seeds are surrounded by a seed coat, the testa. Variability in the appearance of the testa is considerable, and these variations are used by taxonomists as an aid in distinguishing among different genera and species.
The testa is of great importance to the seed; it is often the only barrier protecting the embryo from the external environment. The seed coats of some plants swell and produce a jellylike layer in response to contact with water.
The gel retains water needed by the seed for germination. Cotton fibers are formed as extensions from some of the outermost cells of the seed coat in cotton plants (Gossypium). The seed coats of nutmeg contain aromatic substances.
Size and Chemistry
The range of seed size is extreme—more than nine orders of magnitude. The largest known seed is that of the double coconut (Lodoiceamaldivica); the seed and fruit together weigh as much as 27 kilograms. At the other end of the scale, the dust like seeds of some orchids, begonias, and rushes weigh only about 5 milligrams per seed.
It is thought that the size of seed displayed by each species represents a compromise between the requirements for dispersal (which would favor smaller seeds that can be borne on wind or picked up by animals) and the requirements for establishment of the seedling (which would favor larger seeds that can adhere to a growth medium).
The chemical composition of seeds varies widely among species. In addition to the normal compounds found in all plant tissues, seeds contain unique food reserves that are used to support early seedling growth. About 90 percent of plant species use lipids (fats and oils) as their main seed reserves.
The cotyledons of soybeans and peanuts are rich in oil, whereas in other legumes such as peas and beans, starch is the reserve material. Sixty-four percent of the weight of a castor bean is derived from the oil stored in the endosperm.
In seeds of cereal crops, the endosperm stores much starch; in corn it can be up to 80 percent of the weight of the seed.All seeds, particularly legumes, also store protein as a reserve substance.
Successful reproduction depends on seed dispersal to places appropriate for germination to occur. During the evolutionary history of plants, seeds and fruits have developed a great variety of specialized structures that enhance seed dispersal.
Wind is onemajormeans of seed distribution. Very small seeds, such as the dust like seeds of orchids, heathers, and some rushes and grasses, are dispersed by wind. Such seeds have been recovered from the atmosphere by airplanes at elevations up to 1,000 meters.
Heavier seeds have evolved a variety of structures to ensure wind dispersal. For example, some members of the daisy family, such as dandelions, bear numerous one-seeded fruits to which are attached a feathery, tuft like structure that acts as a parachute.
Similar structures aid in the dispersal of the seeds of many other plants, such as cattails and milk weeds. Heavier seeds, such as those of ash, maple, and pine trees, have developed large, flat wings that allow the seeds to fly in a propeller-like manner for a considerable distance from the parent plant.
Often adaptations for wind dispersal of seeds can be seen not only in the seed’s structure but also in structures of the parent plant. Many plants offer their seeds to the wind by bearing them on long flower stalks that tower above the surrounding vegetation.
Tumbleweed bushes are small and almost spherical. When mature, they develop a weakness of the main stem at soil level. Wind can break the stem of the bush from which, as it rolls over the ground, the seeds are shaken loose and are scattered.
The seeds or fruits of many plants are dispersed by sticking to the outsides of birds or land animals. Seeds are transported in mud that sticks to the feet of animals. The large number of species whose seeds show no obvious special dispersal adaptation are probably spread in this manner.
Many seeds, however, can attach themselves to a passerby by means of adhesive substances, hooks (such as the fruit of bed straw), or burrs (such as the seeds of the burdock plant).
Some plant species have seeds that are adapted for dispersal by animals and birds that transport them internally. The attractive and tasty fleshy fruits and berries of plants can be considered an adaptation to aid in seed dispersal. The seeds of most fruits eaten by animals and birds have a digestion-resistant coat.
The animal deposits excrement containing seeds at a location at some distance from the parent plant, where the seeds grow into new plants. In some species, germination will not even occur unless the seed has passed through an animal’s digestive tract.
The presence of seeds in bird droppings is responsible for the appearance of some plants on remote, barren, volcanic islands. Various animal behaviors, including the collecting behavior of ants and the seed-burying activities of mice, squirrels, and jays, also aid in seed dispersal.
Several plants have evolved mechanisms that expel seeds explosively away from the parent plant. The pods of Impatiens, for example, develop strongly unbalanced tension forces as they ripen. When the pod is fully mature, the tension is so intense that the slightest disturbance causes the pod to split open, violently expelling the seeds.
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The delaying mechanism which prevents germination under adverse conditions is called dormancy. Seeds can remain in a condition of dormancy for varying lengths of time, depending on the species, until the correct balance of oxygen,moisture, and temperature triggers germination.
Viability varies greatly from species to species and may last only a few weeks or many years. Seeds of the cocoa plant are viable for only ten weeks. Some seeds, however, remain viable for decades or even hundreds of years. Seeds of the Indian lotus have been shown to remain viable for almost one thousand years.
No claims for long-term viability have surpassed those made for the Arctic lupine, however: Seeds of this species have been successfully germinated after having been buried in the Arctic tundra for ten thousand years.