GROWTH 



Ul 



lichiiul this is the zone of growth or cell enlargement, where the 

 cells which have been formed at the growing-point stretch and 

 elongate. It is in this zone (a few millimeters in length), and 

 here only, that growth of the root in hnigth takes place (Fig. 72) ; 

 and it is the force exerted here by cell elongation which drives the 

 root tip through the soil. Just back of this region, in turn, is the 



Fig. 72. — Growth of the root in length. Two squash seedlings, the one at the 

 right a day or two older than the one at the left. The change in length of the 

 zones between the markings, originally equidistant, shows that growth in length 

 takes place only very near the tip of the root. 



zone of maturation, where the cells, now having attained their 

 full size, assume their mature characteristics. Here differentia- 

 tion begins, the central cells developing into wood and bast, 

 those farther out into cortex, and the outermost ones into epider- 

 mal cells and root hairs. It should be noted that the width of the 

 young root is determined by the width of the meristem, and that 

 no lateral growth occurs here at this time. The root grows 



zone of enlargement, the cells are no longer dividing but are rapidly elongating. 

 Vacuoles or small sap-cavities are beginning to appear in the cytoplasm. In the 

 zone of maturation the cells have attained their final size, and the center of each 

 is now occupied by a large sap-cavity, surrounded by a thin layer of cytoplasm. 

 In this zone (only a part of which is shown) the cells are beginning to assume their 

 mature characteristics and differentiation of the tissues is taking place. A, B, 

 and C: cells from these three zones, much enlarged. 

 10 



