214 



GROWTH OF STEMS 



reach maturity, are easily broken off just above the node. Fur- 

 thermore, in having this meristematic 

 zone, stems of the Grasses, when blown 

 down, are able to become partially erect 

 by bending in the region of the node due 

 to a more rapid growth of this region on 

 its lower side. 



Since the stem segments are added 

 in succession at the apex, a stem soon 

 comes to have segments in various stages 

 of development, for while those at the 

 apex are just beginning to elongate, those 

 at the stem's base may have completed 

 their elongation and formation of tissues. 

 This feature is illustrated in Figure 195, 

 although none of the segments are yet 

 mature. 



Primary and Secondary Growth. In 

 both stems and roots, apical growth, 

 since from it the tissues of the stem and 

 root first originate, is called primary 

 growth, while growth from the cambium 

 is known as secondary growth because it 

 is chiefly concerned with adding more 

 tissues of the same kind to those already 

 formed from the apical meristems. Tis- 

 sues are also called primary or secondary 

 according to whether they originated 

 from the primary meristems or from the 

 cambium. 



Character and Rate of Growth in 

 Stems. Since elongation or enlarge- 

 ment is the most conspicuous phase of 

 growth, it is employed in determining 

 the character and rate of growth. Al- 

 FIG. 195. Lengthwise though the most conspicuous, neverthe- 



section through the stem 



of a Corn plant, the plant being about two feet high. I, leaves; t, tassel; 



r, region of stem where internodes have not elongated; a, fnternodes which 



have undergone the most elongation; 6, meristematic region at the base of 



the internodes. 



