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DICOTYLEDONS , in See also:botany, the larger of the two See also:great classes of See also:angiosperms, embracing most of the See also:common See also:flower-bearing See also:plants. The name expresses the most universal See also:character of the class, the importance of which was first noticed by See also:John See also:Ray, namely, the presence of a pair of See also: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 development to See also:form a seedling when the seed germinates; frequently, however, as in See also:pea or See also:bean and their See also:allies, the whole of the nourishment for future use is stored up in the cotyledons themselves, which then become thick and fleshy. In germination of the seed the See also:root of the embryo (radicle) grows out to get a holdfast for the plant; this is generally followed by the growth of the See also:short See also:stem immediately above the root, the so-called " hypocotyl," which carries up the cotyledons above the ground, where they spread to the See also:light and become the first See also:green leaves of the plant. Protected between the cotyledons and terminating the See also:axis of the plant is the first stem-bud (the plumule of the embryo), by the further growth and development of which the aerial portion of the plant, consisting of stem, leaves and branches, is formed, while the development of the radicle forms the root-See also:system. The See also:size and manner of growth of the adult plant show a great variety, from the small See also:herb lasting for one See also:season only, to the See also:forest See also:- TREE (0. Eng. treo, treow, cf. Dan. tree, Swed. Odd, tree, trd, timber; allied forms are found in Russ. drevo, Gr. opus, oak, and 36pv, spear, Welsh derw, Irish darog, oak, and Skr. dare, wood)
- TREE, SIR HERBERT BEERBOHM (1853- )
tree living for centuries. The arrangement of the conducting See also:tissue in the stem is characteristic; a transverse See also:section of the very See also:young stem shows a nunber of distinct conducting strands—vascular bundles—arranged in a See also:ring See also:round the See also:pith; these soon become See also:united to form a closed ring of bast and See also:wood, separated by a layer of formative tissue (cambium). In perennials the stem shows a See also:regular increase in thickness each See also:year by the addition of a new ring of wood outside the old one —for details of structure see PLANTS: See also:Anatomy. A similar growth occurs in the root. This increase in the See also:diameter of See also:stein and root is correlated with the increase in See also:leaf-See also:area each season, due to the continued See also:production of new leaf-bearing branches. A characteristic of the class is afforded by the complicated network formed by the leaf-See also:veins,—well seen in a See also:skeleton leaf, from which the sof t parts have been removed by maceration. The parts of the flower are most frequently arranged in See also: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 arrangement in fours is less frequent, while the arrangement in threes, so common in monocotyledons, is rare in dicotyledons. In some orders the parts are numerous, chiefly in the See also:case of the stamens and the carpels, as in the See also:buttercup and other members of the See also:- ORDER
- ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
- ORDER, HOLY
order See also:Ranunculaceae. There is a very wide range in the See also:general structure and arrangement of the parts of the flower, associated with the means for ensuring the transference of See also:pollen; in the simplest cases the flower consists only of a few stamens or carpels, with no enveloping sepals or petals, as in the See also:willow, while in the more elaborate type each See also:series is represented, the whole forming a complicated structure closely correlated with the size, form and habits of the pollinating See also:agent (see FLOWER). The characters of the See also:fruit and seed and the means for ensuring the dispersal of the seeds are also very varied (see FRUIT).
End of Article: DICOTYLEDONS
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