Although the characteristics of different plant seeds vary greatly, some structural features are common to all seeds. Each seed contains an embryo with one, two, or several cotyledons. 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 of which 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 stem and leaves of the new young plant. Almost all seeds carry with them a 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 endosperm lie 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.
See also: Size and Chemistry, Dispersal, Dormancy
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