DICOTYLEDONS, in botany, the larger of the two great classes of angiosperms (q.v.), embracing most of the common flower-bearing plants. The name expresses the most universal character of the class, the importance of which was first noticed by John Ray namely, the presence of a pair of seed-leaves or cotyledons, in the plantlet or embryo contained in the seed. The embryo is generally surrounded by a larger or smaller amount of foodstuff (endosperm) which serves to nourish it in its develop ment to form a seedling when the seed germinates; frequently, however, the whole of the nourishment for future use is stored in the cotyledons themselves, which then become thick and fleshy. In germination of the seed the root of the embryo (radicle) grows out to get a holdfast for the plant ; this is generally fol lowed by the growth of the short stem immediately above the root, the so-called "hypocotyl," which carries up the cotyledons above the ground, where they become the first green leaves of the plant. Protected between the cotyledons and terminating the axis of the plant is the first stem-bud (plumule of the embryo). The size and manner of growth of the adult plant show great variety, from the small herb lasting for one season only, to the forest tree living for centuries. The arrangement of the conducting tissue in the stem is characteristic ; a transverse section of the very young stem shows a number of distinct conducting strands—vascular bundles—arranged in a ring round the pith; these soon become united to form a closed ring of phloem and wood, separated by a layer of cambium. In perennials the stem shows a regular in crease in thickness each year by the addition of a new ring of wood outside the old ene—for details of structure see PLANTS : Anatomy. A similar growth occurs in the root. This increase in the diameter of stem and root is correlated with the increase in leaf-area each season, due to the continued production of new leaf-bearing branches. A characteristic of the class is afforded by the complicated network formed by the leaf-veins,—well seen in a skeleton leaf, from which the soft parts have been removed by maceration. The parts of the flower are most frequently arranged in fives, or multiples of fives; for instance, a common arrangement is as follows,—five sepals, succeeded by five petals, ten stamens in two sets of five, and five or fewer carpels. An ar rangement in fours is less frequent, while the arrangement in threes, so common in monocotyledons, is rare in dicotyledons. In some families the parts are numerous, chiefly in the case of the stamens and the carpels, as in the buttercup and other mem bers of the Ranunculaceae and in the Rosaceae. The characters of the flower arid fruit are described in the articles FLOWER, FRUIT and SEED.