STRUCTURE AN!) PHYSIOLOGY OF THE PLANT The organs of the root series are the least differentiated. We do not distinguish plants by means of their root characters, both because the roots are not clearly designative in most eases and because they are hidden. The most that we ordinarily do is to divide roots into fibrous-form and tap-form.
The parts of the root are distinguished as to their physiological functions rather than their taxonomic or descriptive The general form of the root is determined by the species ; but its details are conditioned on the particular soil in which it grows. It is often said of orchard trees that the roots extend as far as the branches of the top ; but the root system may be less or more than the top in horizontal and vertical extent, depend ing on circumstances. Yet there is a distinct root "habit" even as between varieties of apple trees. In the annual crops, the root habit is often characteristic, and it needs much more attention than it has yet received by cultiva tors (Fig. 4). The farmer may examine carefully the leaves and stalks of his grass and wheat, but he seldom examines the roots. Food for man and his animals is provided by many thickened roots, as the greater part of the substance of carrots, parsnips, turnips and beets.
The stem, as named by the botanist, is the framework on which the leaves and flowers are borne. The younger growing parts of it, containing chlorophyll, may function as foliage ; but the main office of the stem is to provide support. The stem may be very short and thick, as the "crown" of turnips and beets, carrying the leaves ; it may be exceedingly slender and light, as in the straw grains and grasses ; or it may be high and massive as in the trunks of trees. Sometimes the stem is subterranean, in which case it is distinguished from roots by its buds or " eyes," and rudimentary leaf-scales: the tuber of the Irish or round potato is an example, and also the creeping rhizomes of quack-grass and other grasses. It will be noted, from this discussion, that the botanist, by the word stem, means to designate the leaf bearing axis and its branches and modifications, and not the stalks of leaves and flowers. Thus, in the plantain and dandelion
(Figs. ii, 20), the stem is very short, bearing a rosette of leaves at the ground ; and from this arise the flower-stalks. In useful products, the stem provides timber, some of the fibers, and much of the forage ; and it also provides human food, as in the potato, asparagus, onion, kohlrabi, sugar-cane.
The leaves arise normally from the joints or nodes of the stem. Usually a bud is borne in the axil or upper angle made by the leaf with the stem. The bud is a very short and unde veloped branch. If the plant is dormant a part of the year in consequence of cold or dry, or because of other hereditary habit, the leaf usually falls and the bud remains quiescent till the growing season returns : it is then spoken of as a winter bud. Sometimes the bud remains quiescent, but alive, for a longer period, in rare cases even for years : it is then called a dormant bud (Fig. 6). The older the dormant bud, the less the likelihood that it will grow, in case necessity should arise. The common notion that old dor mant buds are readily forced into growth by pruning needs correction. In cases of heavy pruning, new shoots on old wood are more likely to arise from buds that are formed for the occasion, without reference to leaves and with out order ; these are known as adventitious buds (Fig. 7).
If the bud "grows,"—that is, if anything issues from it—it produces a branch. The branch may be exceedingly short, and bear only one or two leaves, or it may be several feet long and bear many leaves. If its destiny is to produce only foliage, it is known as a leaf-bud ; if to produce flowers, it is known as a fruit-bud or flower-bud. Peaches and apricots produce typical fruit-buds (Figs. 8, 9). Apples and pears bear both true leaf-buds, and fruit-buds that give rise to flowers and leaves (Figs. 6, 10)—for the flowers of these trees are in clusters or bouquets accompanied by foliage.