Seed is a fertilized or ripened ovule, protecting the dormant embryo within and serve the function of perennation, dispersal and reproduction of the parent plant.
In angiosperms, the seeds are enclosed within fruits, whereas in gymnosperms seeds are exposed or naked.
Seeds Are Enormously Variable In Shape, Size, Structure And Period of Viability.
General Structure of a Seed:
A seed is generally made up of seed coat and embryo.
(a) Seed Coat:
It is the protective covering of the seed derived from one or both integuments of the ovule. Usually seed coat is two layered. The outer thick and hard layer is called testa, while the thin inner membranous layer is called tegmen. The exterior of a seed coat contains prominent structures like hilum, raphe, micropyie and chalaza
(b) Embryo:
It represents the dormant future plant that remains enclosed within the seed coat. Embryo consists of an embryo axis or tigellum and cotyledons.
The embryo axis consists of plumule, epicotyl, cotyledonary node, hypocotyle and radicle. Plumule represents the embryonic shoot while radicle represents embryonic root. The embryo axis has a node called cotyledonary node that bears one or two cotyledons (= seed leaves). The part of embryo axis between plumule and cotyledonary node is called epicotyle and a similar region between cotyledonary node and radicle is called hypocotyle. The seed contains reserve food either in cotyledons or in a special tissue called endosperms.