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.
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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.
The stem is cylindrical, the outer part being bark and the inner part being wood or woody tissue. In the dicotyledonous plants, the bark is usually easily separated from the remainder of the cylinder at some time of the year; in monocotyledonous plants the bark is not free. Growth in thickness takes place inside the covering and not on the very
