Cotyledon

Cotyledon, from the Greek kotuledon, is the name applied to the embryonic or primary leaves of the higher plants. There is no such structure among mosses; but in ferns one portion of the embryo (q.v.), resulting from the division of the fertilised oosphere (q.v.), develops into a distinct leaf or cotyledon. In the yew and other gymnosperms (q.v.), as in most dicotyledonous angiosperms, there are two opposite simple cotyledons with the plumule (q.v.) or bud of the stems between them; but in Pinus and some other gymnosperms the cotyledons are so deeply lobed as to appear like many arranged in a whorl, and the gymnosperms have in consequence been termed Polycotyledones. In the class Monocotyledones (q.v.), as its name indicates, there is but one cotyledon, and it generally appears as if terminating the axis of the embryo. In exceptional dicotyledons, such as the oak, we may have three cotyledons, and in the parasitic genus Cuscuta, the dodder (q.v.), there is hardly a trace of any. In most plants the cotyledons are thick, fleshy, and colourless at first, serving as storehouses of food for the seedling. This is especially the case in those exalbuminous seeds in which they remain within the seed during germination (q.v.). The cellular tissue of the cotyledons may be oily, as in walnut and almond, or mealy, as in the bean; in many cases, as in the barberry and linden, veins are distinctly traceable in them; they may have petioles; and in Pinus they develop chlorophyll (q.v.) before germination. They may be of unequal size; and, though always turned towards the chalaza (q.v.), may be variously folded on themselves and with reference to the radicle. Thus they may be parallel, with the radicle against their edges, when they are called accumbent; or the radicle may rest against the flat side of one of them, when they are termed incumbent. Such characters are used to subdivide the order Cruciferae (q.v.) into tribes. When in germination they rise above ground as the first green leaves they often differ much in shape from the foliage-leaves subsequently produced, as in the case of the seedling plants of mustard and cress, commonly eaten as salad.