Dicotyledons

Dicotyledons, the larger, and, in most respects, the more highly organised class of angiospermous, or fruit-bearing flowering plants. In their embryos they have in almost all cases two cotyledons (q.v.), or seed-leaves, and a primary root or radicle which is commonly protruded in germination and develops into a tap-root. The fibro-vascular bundles in the stem are open, i.e. retain a layer of cambium (q.v.), or growing tissue, between their wood and their bast. The bundles form a ring of wedge-shaped masses round a central pith or medulla, from which there radiate bands of tissue between the bundles, known as medullary rays. Part of these rays, the interfascicular cambium, in conjunction with the cambium within the bundles, which is termed fascicular, forms a complete ring of this soft protoplasmic tissue round the stem, from which additions are made to the wood on the inside, and to the inner bark or bast on the outside. Thus the wood is in more or less distinct seasonal (generally annual) rings, and the whole "bark" (q.v.), both the inner or bast, which is fibrous, and the outer or periderm, which is often corky, is separable. The leaves often have a pair of stipules (q.v.) at their base, have commonly distinct stalks or petioles, are often compound or divided, have not seldom hairy surfaces, and have a complex venation or system of vascular bundles, branching into many degrees of fineness of the so-called veins and generally forming an irregular network, whence they are termed reticulate. The leaves of the perianth, or calyx and corolla, are commonly in whorls of five, or, in reduced types, of fours cr twos; the stamens, two, four, five, ten, or indefinite in number; and the carpels, one, two, five, or indefinite. If both calyx and corolla are present, they commonly differ in texture and colour, the outer whorl being green, leaf-like, and often hairy externally, the inner commonly delicate and coloured. In a fossil state dicotyledonous angiosperms are not certainly known below the middleof the Cretaceous system. The class is divided into the three sub-classes Incompletce, Gamopetaliv, and Polypetalct. The largest Natural orders among dicotyledons are the Compositcv, Leguminosec, Bubiacccv, Etiphovbiacecc, and Labiatcv. [Monocotyledons.]