THE STEMS— A'/XDS AND FORMS; PRUNING 53 



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 grow- 

 ing-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 elon- 



7 • / ./ / Fig. 60. — Sprouts 



irate bv growing more or less tlirougn- 



&"■'" J <b 0> O ARISING FROM THE 



out the young or soft part or " between buds, or eyes, of a 

 joints" (Figs. 48, 49> But any part potato tuber. 

 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 mar- 

 gin 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 dicoty- 

 ledonous 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 



