The structure that develops from the fertilized ovule in seed plants. It usually contains one *
embryo together with a food supply, which may be contained in a specially developed *
endosperm or in the *
cotyledons of the embryo. The whole is surrounded by a protective coat, the *
testa. In gymnosperms the seed remains naked and unprotected but in angiosperms it is enclosed within the ovary wall. A seed may germinate immediately on ripening or may have special *
dormancy mechanisms to prevent germination under unfavourable conditions. Although seed production is only observed today in angiosperms and gymnosperms, there are extinct forms possibly intermediate between these and the nonseed-bearing vascular plants (see
Pteridospermales). The development of the seed-bearing habit has released seed plants from dependence on the availability of water for their reproductive phase, thus opening up new habitats for colonization. In addition the seed allows for wide dispersal of the plant and, in annual and ephemeral plants, serves as an organ of perennation. Some plants produce seed without fertilization taking place (see
apomixis).
