CHAPTER XI 



STEMS AND THEIR FUNCTIONS 



Stems vary more than roots, both in form and in details of structure. 

 They may be thick, strong, and relatively rigid, like those of trees; delicate 

 and succulent, like those of many herbs; long, slender, flexible, and tough, 

 as in vines; rootlike and subterranean, as in dewberry and Solomon's- 

 seal; or they may be modified in a variety of other ways. Although they 

 somewhat resemble roots in the arrangement of their tissues and share 

 with them the functions of support and transportation, they show numer- 

 ous striking differences from roots. They normally have no absorptive 

 function and lack epidermal structures corresponding to the root hairs; 

 the apical growing point is not protected by a cap of cells; their out- 

 growths (leaves and branches) are restricted to definite parts of the stem 

 separated by leafless and branchless intervals, and these outgrowths 

 develop from surface buds instead of pushing out from the pericycle. The 

 arrangement of the vascular tissues in the stele is also somewhat different 

 from that of roots. 



Certain important characteristics of stems may be most easily com- 

 prehended by study of deciduous trees or shrubs, i.e., those which shed 

 their leaves at the end of the growing season. While the leaves are still 

 on the tree, it may be noted that they are all attached to the short ter- 

 minal portions of the branches; such leaf-bearing shoots are called twigs. 

 All parts back of the twigs constitute the trunk and its branches. When 

 the leaves are shed in the fall, a leaf scar marks the former position of 

 each, and it can then easily be seen that just above each leaf scar there 

 is a bud. The buds situated along the sides of the twig occur singly or in 

 pairs or whorls, according to the particular habit of the tree; they are 

 called axillary buds, because each arises in an axil, or angle between a 

 leaf petiole and the twig (cf. Latin, axilla, "armpit"). At the tip of the 

 twig is a terminal bud. Each bud contains a rudimentary twig, already 

 bearing rudimentary leaves. This delicate embryonic structure is en- 

 closed and protected by overlapping, waxy bud scales, which are regarded 

 as modified leaves. 



With resumption of activity by the tree in the spring, the terminal 

 bud and one or more of the axillary buds (generally those nearest the tip) 



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