- This writeup is about biological seeds; for other meanings see Seed (disambiguation).
A seed is the ripened ovule of gymnosperm or angiosperm plants. The importance of the seed relative to more primitive forms of reproduction and dispersal is attested to by the success of these two groups of plants in dominating the landscape.
A fertilized seed contains the embryo from which a new plant will grow under proper conditions. It also contains a supply of stored food and is wrapped in the seed coat or testa. The stored food begins as a tissue called endosperm derived from the parent plant. Endosperm becomes rich in oil or starch, and protein. In some species, the embryo is imbedded in the endosperm, which the seedling will use upon germination. In others, the endosperm is absorbed by the embryo as the latter grows within the developing seed, and the cotyledons of the embryo become filled with this stored food. At maturity, seeds of these species have no endosperm. Some common plant seeds that lack an endosperm are bean, pea, oak, walnut, squash, sunflower, and radish. Plant seeds with an endosperm include all conifers and most monocotyledons (e.g. grasses and palms), and also e.g. brazil nut, castor bean.
The seed coat develops from tissues (called integument) originally surrounding the ovule. The seed coat in the mature seed can be a paper thin layer (as for example, in the peanut) or something more substantial (as for example, thick and hard in honey locust and coconut). The seed coat helps protect the embryo from mechanical injury and from drying out. In order for the seed coat to split, the embryo must imbibe (soak up water) which causes it to swell, splitting the seed coat. However, the nature of the seed coat determines how rapidly water can penetrate and subsequently initiate germination. For seeds with a very thick coat, scarification of the seed coat may be necessary before water can reach the embryo. Examples of scarification include: gnawing by animals, freezing and thawing, battering on rocks in a stream bed, or passing through an animal's digestive tract. In the latter case, the seed coat protects the seed from digestion, while perhaps weakening the seed coat such that the embryo is ready to sprout when it gets deposited (along with a bit of fertilizer) far from the parent plant. In species with thin seed coats, light may be able to penetrate into the dormant embryo. The presence of light or the absence of light may trigger the germination process, inhibiting germination in some seeds buried too deeply or in others not buried in the soil. Abscisic acid is usually the growth inhibitor in seeds.
The seeds of angiosperms are contained in a hard or fleshy (or with layers of both) structure called a fruit. Gymnosperm seeds begin their development "naked" on the bracts of cones, although the seeds do become covered by the cone scales as they develop. An example of a hard fruit layer surrounding the actual seed is that of the so-called stone fruits (such as the peach).
Unlike animals, plants are limited in their ability to seek out favorable conditions for life and growth. Consequently, plants have evolved many ways to disperse and spread the population through their seeds (see also vegetative reproduction). A seed must somehow "arrive" at a location and be there at a time favorable for germination and growth. Those properties or attributes that promote the movement of the next generation away from the parent plant may involve the fruit more so than the seeds themselves. The function of a seed typically is one of serving as a delaying mechanism: a way for the new generation to suspend its growth and allow time for dispersal to occur or to survive harsh, unfavorable conditions of cold or dryness or both. In many if not most cases each plant species achieves success in finding ideal locations for placement of its seeds through the basic approach of producing numerous seeds. This is certainly the approach used by plants, such as ferns, that disperse by spores. However, seeds involve a considerably greater investment in energy and resources than do spores, and the payoff must come in achieving similar or greater success with fewer dispersal units.