Endosperm

Endosperm, a term applied to tissues within the seed of flowering plants, or in the macrospore of Selaginella (q.v.), which in eill cases serve to nourish the embryo; but which are probably not of homologous origin, so that the term is somewhat ambiguous or not morphologically precise. In Selaginella it is a large-celled tissue formed by free cell-formation below the diaphragm at the base of the prothallium, and it has been termed the secondary prothallium or metasperm. In Gymnosperms it is similarly formed within the embryo sac (whence its name), but before fertilisation, the archegonia being formed on it, so that it is apparently a primary prothallium or aichisperm. In Angiosperms it does not originate until after fertilisation, when the secondary nucleus of the embryo sac undergoes repeated division, and so forms this tissue, which seems here, as in Selaginella, to be a metasperm. None is formed in orchids, and but little in Alismaceee. In many seeds it is absorbed by the embryo before the seed is ripe (exalbuminous); but in others, including most monocotyledons, it remains (albuminous), and is only absorbed during germination. [Albumen.]