Seed

Seed, the fertilised ovule or macrosporocarp of Spermaphyta (Phanerogamia), of which it constitutes the most distinctive structure. It is a peculiar modification of a structure occurring in lower plants (Pteridophyta), which encloses a macrospore (embryo-sac) with its embryo and albumen, and becomes, when ripe, detached from the sporophyte. Its seed-coats or integuments being an outgrowth from the parent sporophyte, the embryo-sac being an oophyte, and the embryo an immature sporophyte, the seed contains structures belonging to three generations. When the structural and other changes that immediately follow fertilisation are complete, and the seed becomes "ripe," it enters upon what is generaly the most marked period of unchanging repose in the life-history of the plant, a period which may be of the most varied duration before germination begins. The typical seed consists of integument, embryo (q.v.), and albumen (q.v.). The integument may consist of one coat or testa, or there may be an inner one, the tegmen. The testa may be smooth, as in the bean or the horse-chestnut, where it is marked by a large scar or hilum at its point of attachment, or it may bear wrinkles or tubercles, wings or hairs. The seeds of firs and toadflax e.g. are winged; cotton (q.v.) is the hairs on the testa of Gossypium; and willows have a similar tuft or hairs or coma. Such wings and tufts occur only on the seeds of dehiscent fruits, serving, as do the similar structures on the fruits themselves, to disperse the seed beyond the shadow of the parent. The testa is usually thick, 1eathery, opaque, impermeable, bitter and indigestible, and is more often brown than any other colour. It serves to protect the contained embryo from premature germination by excluding damp, or from the action of sea-water, or the gastric juice of the ammal stomach. The testa of linseed is mucilaginous, that of the gooseberry and pomegranate is pulpy, and that of the Brazil-nut exceptionally woody, whilst orchids have a testa reduced to one layer of transparent cells. Brightly-coloured testas are confined to dehiscent fruits, as are also the fleshy outgrowths from the testa known as arils (q.v.). When present, the tegmen, or endopleura, is usually a delicate, cream-coloured coat, as in the almond, hazel, or walnut. In a few seeds, no albumen is formed; but in the majority of exalbuminous seeds, though formed, it is absorbed by the embryo before the seed ripens. [GERMTNATION.]