DICOTYLEDONS, a subdivision of the angiosperms, or plants with encased ovules, which is characterized by the fact that the embryo has at least two (sometimes more) cotyledons or seed leaves, whereas the mono cotyledons (q.v.), the other subdivision of the angiosperms, have only one. Besides this almost invariable characteristic, others of nearly equal constancy are the structure of the stem, which consists in the mature plant of a wood layer surrounding a pith, and separated from the last layer enclosing it by a zone of active !growth, the cambium; the forked, anastomos mg venation of the leaves, with free veinlets ending in the margin, which may be lobed or toothed; and the prevalently quaternary or quinary symmetry of the flower, as opposed to the usual ternary symmetry of the monocotyle dons. There is a very interesting difference between the development of the embryo of the dicotyledons and of the monocotyledons. In both cases, the cell formed by the union of the two gametes eves rise to a filament of cells placed end to en the so-called proembryo. In the dicotyledons, the terminal cell by dividing generates the entire embryo except the tip of the hypocotyl, which is formed from a portion of the next cell. In the monocotyledons, the terminal cell forms only the cotyledon, while the whole remainder of the plant is formed from a part of the next cell. The dicotyledons group themselves into two great series: the Chors petala, with free petals and sepals; and the Sympetake, with the petals coherent. Those Choripetake which lack a corolla together with a few related forms are known as the Mono chlamydee, as opposed to the Dialypetale. The orders of Monochlamydee are the Verticillatee, containing the cassowary, trees; the Juplandi flora, or walnuts, Quercsflore, or relatives of the oak, Saliciora, or willows and poplars, and Urticince, containing the elms, mulberries, net tles, etc., all of these orders being characterized by the possession of catkins consisting of uni sexual, wind-fertilized simple flowers, the Loranthifiore, containing the mistletoe; the Piperina or peppers; the Hamamelidina or relatives of the witch-hazel; the Tricocce, typi fied by the spurge; and the Centrosperma, such as the rhubarb, spinach, pink, cactus, etc. The
orders of are the Polycarpice, to which belong the water-lilies, crowfoots, mag nolias, etc., usually with free carpels and a spiral arrangement of some parts of the flower; the Rheradince, to which belong the poppies, mustards, etc., the Cisitiflora, among which are the violets and the tea plants; the Columni f ere, often with a large number of stamens in a single whorl, containing the mallows, cocoa tree, etc.; the Gruinales, containing the allies of the geranium; the Sapindia, among which are the sumachs, maples and horse-chestnuts; the Frangulina, with the grape vine and its allies; the Saxifragina, with the currants, gooseber ries, etc.; the kosiflora, containing the rose, apple, strawberry, plum, etc.; the Leguminosa, with a pod for a fruit and with flowers usually bilaterally symmetrical, to which belong such forms as the bean, pea, clover and lentil; the Passiflorina, such as the begonia, papaw and passion-flower; the Thymekeincr; the Myrti flora, with the myrtle and eucalyptus; and the Umbelliflorcr, or dogwoods, ginsengs, carrots, parsleys, etc., in most of which the flowers are borne in simple or compound umbels. The Sympetala are divided into the Pentacyclicce, with two whorls of stamens, and the T etracy clue, with one. The Pentacyclicce consists of the Ericine, or heath allies; the Diospyrina, containing the persimmon, ebony tree, naseberry, etc.; and the Primulina, typified the prim rose. The orders of the Tetracyclicez are the Consorter, to which belong the olive, lilac, gen tian, milkweed, etc.; the Tubiflore, such as the morning-glory and mint; the Personate, among which are the potato, jimsonweed, foxglove, plantain, etc.; the Rubiner, containing the coffee plant, elder and valerian; the Companuline, or l-fl belowers, lobelias and gourds; and the vast assemblage of the Aggregate, characterized by the grouping of the flowers in heads with a common involucre and the reduction of the calyx of the separate flowers. This last order, which contains daisies, asters, thistles, sun flowers, tansies, yarrows, etc., possesses at least I2,500 species. The total number of dicotyled onous species is not less than 103,000 whereas there are only about 25,000 monocotyledons.