Plants in which there are two cotyledons formed in the embryo are dicotyledonous. The two cotyledons thus formed are oppo site to each other, but are not always of the same size. Thus, in the Nyctaginaceae, one of them is smaller than the other (often very small), and in Carapa guianensis there appears to be only one, in consequence of the intimate union which takes place between the two. The union between the cotyledonary leaves may continue after the young plant begins to germinate. The texture of the cotyledons varies. They may be thick, as in the pea, exhibiting no traces of venation, with their flat internal surfaces in contact, and their backs more or less convex; or they may be in the form of thin and deli cate laminae, flattened on both sides, and having distinct venation, as in Euonymus, etc. The cotyledons usually form the greater part of the mature embryo, and this is remarkably well seen in such exalbumi nous seeds as the bean and pea.
Cotyledons are usually entire and sessile. But they occasionally become lobed, as in the walnut ; or petiolate, as in Geranium molle; or auriculate, as in the ash. Like leaves in the bud, cotyledons may be either applied directly to each other, or may be folded in various ways, the same terms be ing applied as to the foliage leaves. The radicle and cotyledons are either straight or variously curved. Thus, in some cruciferous plants, as the wall flower, the cotyledons are applied by their faces, and the radicle is folded on their edges, so as to be lateral ; the cotyledons are here accumbent. In others, as Hesperis, the cotyledons are applied to each other by their faces, and the radicle is folded on their back, so as to be dorsal, and the cotyledons are incumbent. Again, the cotyledons are conduplicate when the radicle is dorsal, and enclosed between their folds. In other divisions the radicle is folded in a spiral manner, and the coty ledons follow the same course.
In many gymnosperms more than two cotyledons are present, and they are ar ranged in a whorl. This occurs in Conif
erae, especially in the pine, fir, spruce and larch, in which six, nine, twelve and even fifteen have been observed ; they resemble in their form and mode of development the clustered or fasciculated leaves of the larch. In species of Streptocarpus the coty ledons are permanent, and act the part of leaves. One of them is frequently largely developed, while the other is small or abortive. In those plants in which there is only a single cotyledon in the embryo, hence called monocotyledonous, the embryo usually has a cylindrical form more or less rounded at the extremities, or elongated and fusiform, often oblique. The axis is usually very short compared with the cotyledon, which in general encloses the plumule by its lower portion, and exhibits on one side a small slit which indicates the union of the edges of the vaginal or sheathing portion of the leaf. In grasses, by the enlargement of the embryo in a particular direction, the endosperm is pushed on one side, and thus the embryo comes to lie outside at the base of the endosperm. The lamina of the cotyledon is not developed. Upon the side of the embryo next the endo sperm and enveloping it is a large shield shaped body, termed the scutellum. This is an outgrowth from the base of the cotyledon, enveloping more or less the cotyledon and plumule, in some cases, as in maize, completely investing it ; in other cases, as in rice, merely sending small prolongations over its anterior face at the apex. (See GRASSES.) In many aquatic monocotyledons (e.g., Potamogeton) there is a much-developed hypocotyl, which forms the greater part of the embryo and acts as a store of nutriment in germi nation. In some grasses, as oats, a projec tion of cellular tissue is seen upon the side of the embryo opposite to the scutellum, that is, on the anterior side. This has been termed the epiblast. This by some was considered the rudimentary second cotyledon; but is now generally regarded as an out growth of the sheath of the true cotyledon.