=Nodes and Internodes.=--The parts of the stem at which buds grow are called =nodes= or =joints= and the spaces between the buds are =internodes=. The stem at nodes is usually enlarged, and the pith is usually interrupted. The distance between the nodes is influenced by the vigor of the plant: how? [Illustration: FIG. 59.--RHIZOME OR ROOTSTOCK.] =Stems vs. Roots.=--Roots sometimes grow above ground (Chap. VII); so, also, _stems sometimes grow underground_, and they are then known as =subterranean stems=, =rhizomes=, or =rootstocks= (Fig. 59). =Stems normally bear leaves and buds, and thereby are they distinguished from roots=; usually, also, they contain a pith. The leaves, however, may be reduced to mere scales, and the buds beneath them may be scarcely visible. Thus the “eyes” on a white potato are cavities with a bud or buds at the bottom (Fig. 60). Sweet potatoes have no evident “eyes” when first dug (but they may develop adventitious buds before the next growing-season). The white potato is a stem: the sweet potato is probably a root. [Illustration: FIG. 60.--SPROUTS ARISING FROM THE BUDS, or eyes, of a potato tuber.] =How Stems elongate.=--_Roots elongate by growing near the tip. Stems elongate by growing more or less throughout the young or soft part_ or “between joints” (Figs. 48, 49). But any part of the stem soon reaches a limit beyond which it cannot grow, or becomes “fixed”; and the new parts beyond elongate until they, too, become rigid. When a part of the stem once becomes fixed or hard, it never increases in length: that is, _the trunk or woody parts never grow longer or higher; branches do not become farther apart or higher from the ground_. =Stems are modified in form= by the particular or incidental conditions under which they grow. _The struggle for light_ is the chief factor in determining the shape and direction of any limb (Chap. II). This is well illustrated in any tree or bush that grows against a building or on the margin of a forest (Fig. 4). In a very dense thicket the innermost trees shoot up over the others or they perish. Examine any stem and endeavor to determine why it took its particular form.