GROWTH 



289 



of development. At the tip, the cells are in the state of embryonic 

 growth, 7 to 8 mm. back of it they are in the condition of elongation, 

 and still farther back they are differentiating (Fig. 111). The zone 

 of growth in roots is usually exceedingly short, about 8 to 10 mm. 

 This apparently is conditioned by the necessity of the root of 

 making its way through the solid soil. This short region of growth 

 of a root is often compared to a short nail driven into a hard board, 

 where a long nail would bend. In stems as well as in aerial roots 



Fig. 111. — Vertical section of a growing root. A, root tip in meristematic stage; 



B, a higher portion in the stage of elongation; C, a still higher portion in the stage 



of differentiation (after Smith et al.). 



peculiar to certain plants, especially of the aroid family, the zone of 

 growth is considerably longer, often as much as 10 cm. 



These stages of growth, however, do not always follow each 

 other directly. Often they are separated by a considerable inter- 

 val of time, most frequently between the stages of meristematic 

 growth and of elongation. This condition is especially evident 



the cereals. During the first weeks of development, their 



in 



stems remain short, forming the so-called tillering node. Only the 

 leaves seem to be growing then. But when the internodes and 



