=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. =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_. [Illustration: FIG. 60.—SPROUTS ARISING FROM THE BUDS, or eyes, of a potato tuber.] =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 the 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 endeavour to determine why it took its particular form. [Illustration: FIG. 61.—CRACKING OF THE BARK ON AN ELM BRANCH.]