A seed is the ripened ovule of a gymnosperm or angiosperm. A seed contains the embryo from which a new plant will grow under proper conditions. But it also contains a supply of stored food and is wrapped in a seed coat. The stored food begins as a tissue called endosperm that is supplied by the parent plant and 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 the stored food. At maturity, seeds of these species have no endosperm. Some common plant seeds that lack an endosperm are bean, pea, squash, sunflower, and radish. Plant seeds with an endosperm include the grasses, such as maize, and castor bean and coconut.
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 develop "naked" on the bracts of cones, although those of the yew have a fleshy coat called an aril. An example of a hard fruit layer surrounding the actual seed is that of the so-called stone fruits such as the peach.
Plants have evolved many ways for their seeds to disperse and spread the population. Some seeds are attached to feather-light fibre parachutes that may be blown by wind. Others have prickly burrs or spikes that will attach themselves to a passing animal's fur so that the animal will carry them away. Seedpods are often designed and shaped so that the seeds are flung away from the parent plant with great force when the seedpod springs open. And lastly, many seeds are contained within a sweet and juicy fruit that invites animals and birds to consume it. These seeds have a tough protective outer-coating so that while the fruit is digested, the seeds will pass through their host's digestive tract intact, and grow wherever they fall.