This writeup is about biological seeds; for the Buddhist metaphor, see bija.
A seed is the ripened ovule of gymnosperm or angiospermplants. 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 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 a seed coat. 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 coniferss 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 (for example, peanut) or something more substantial.
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.
