26 



GROWTH AND WORK OF PLANTS 



day to day show that the elongating part of the root occupies a 

 certain area back of the tip. This elongation is due to the 

 elongation of individual cells which are constantly being formed 

 in the growing point of the tip, and are left behind. These cells 

 elongate slowly at first, then rapidly and then slowly again until 

 they cease to elongate. All of the cells in one cross area of the 

 root grow at about the same rate at the same time. Their united 

 action then is manifest in the slow elongation of the root just 

 back of the tip, its more rapid elongation further back, followed 

 by the slow elongation again until elongation finally ceases. 



40. Region of elongation in stems. The region of elonga- 

 tion in stems is determined in a similar way. It occurs just back 

 of the growing point, but covers usually a greater area than in the 

 root. 



41. The motor zone in roots, or region of curvature. 

 After a seedling or a plant has been growing in one direction for 



a time, if its position be 

 changed so that the 

 root and stem are in a 

 horizontal position, or 

 at any angle from the 

 upright, the root and 

 stem will curve so that 

 the root grows down- 

 ward and the stem up- 

 ward. The region of 

 curvature of the root 

 under these circum- 

 stances corresponds 

 with the region of elongation. This curvature is made possible 

 because the cells in the region of the root are all elongating. 

 Those on the upper side of the root elongate more than those 

 on the underside and bring about the curvature. 



42. The perceptive zone in roots. The perceptive zone in 

 roots is that part of the root which receives the stimulus causing 

 it to turn downward. It is, therefore, a sense organ. 



Fig. 31. 



Bean seedling placed horizontally and marked to 

 show where the root bends. 



