Dicotyledons. The fact that the majority of young seedlings exhibit two small seed-leaves or cotyledons on germination must have been familiar from time immemorial, while the classificatory importance of this character in broadly separating the obviously flowering plants into two great groups of dicotyledons and monocotyledons was thus naturally one of the earliest conceptions of systematic botany, having been adopted by Linnæus from Ray, and doubtless by him in turn from earlier writers. We now distinguish phanerogams into Gymnosperms (cycads, conifers, and gnetums), and Angiosperms characterised by the possession of a closed ovary: of these the monocotyledons and dicotyledons are the main alliances, the latter being on the whole the more evolved. The embryo of dicotyledons possesses, with rare exceptions (usually due to parasitic degeneration—e.g. Dodder, q.v.), two cotyledons facing each other, and more or less embracing the plumule, while the albumen is generally proportionally less in quantity than in monocotyledons, and, in fact, is frequently absorbed altogether; these differences being explained through a more precocious acceleration of development (see OVULE, SEED). The cotyledons usually develop equally, but one may be more or less completely arrested.
The fibro-vascular bundles of dicotyledons are open and collateral, with few exceptions arranged in a reticulated cylinder, which thus presents a single circle in transverse section. The parenchyma in which they lie is thus divided into two main regions—the cortical layer or cellular envelope, and the pith; while the intermediate spokes of parenchymatous tissue connecting the pith and cortex through the meshes of the cylinder are the medullary rays. While the fibro-vascular bundles of dicotyledons arise like those of cryptogams and monocotyledons, by the differentiation into wood and bast of a special strand of primitive embryonic tissue continuous with that of the growing point, they differ in a marked tendency to embryonic persistence of a central layer of this as the Cambium (q.v.), and hence in so many groups of dicotyledons there arises that association of perennial habit and continuous growth of stem to which we owe our familiar shrubs and trees. See VEGETABLE PHYSIOLOGY, WOOD, BARK, BAST. Since the cambium lies between the wood and bast, the new layers of wood are external to the older ones, but those of bast internal to their predecessors: hence Lindley's name exogens was apt to be misleading. The softness and delicacy of the cambium layer leads, when force is applied, to the easy separation of the bast elements of the fibro-vascular bundles, along with the cortical parenchyma and epidermic layer, as the bark—a structure, or rather juxtaposition of structures, unknown in Monocotyledons (q.v.).
The reticulated character of the bundles of the stem is similarly present in the leaf, which thus presents a marked contrast to the usually parallel venation of the monocotyledons. The leaf also presents far greater variety of form and adaptation (see LEAF).
The flower has most frequently its parts arranged in whorls of five, although four is also a not uncommon number; hence another very obvious distinction from monocotyledons, in which a 3-merous symmetry prevails. The outer perianth whorl is most frequently green, as a calyx. See FLOWER, PHANEROGAMS, and VEGETABLE KINGDOM; also ANGIOSPERMS, and MONOCOTYLEDONS.