noun, plural: cotyledons
(1) (botany) The primary or rudimentary leaf of the embryo of a seed plant that either remains in the seed or emerges upon seed germination; a modified leaf that is part of the plant embryo within the seed; a seed leaf.
(2) (plant taxonomy) A genus of the Crassulaceae family first described by Linnaeus, which include plants with succulent leaves that are opposite, stiff, and persistent, corolla of five petals fused into a tube, flowers that are tubular and bell-like, and stamens that are ten in two whorls, such as the species Cotyledon tomentosa commonly known as Bear's Paw.
Botany: In plants, the cotyledons are involved in the storage or absorption of food reserves. The number of cotyledons is one of the plant characteristics used in grouping angiosperms into monocots and dicots.
Anatomy: In humans, a full term placenta contains about 20 cotyledons that are surrounded with maternal blood which contact the villous trees from the fetal circulation. Exchanges of gases (oxygen and carbon dioxide) and nutrients between the mother and her fetus take place here.