STRUCTURE AND GROWTH OF ROOTS 131 



elongates most rapidly. You can do it readily with peas, 

 beans, or corn. You sprout the seeds and when the young 

 roots.have become about an inch long, you rule the surface 

 off into small equal spaces. The rul- 

 ing should be done with India ink 

 and a fine-pointed brush. The lines 

 should be as close together as you can 

 get them and still keep them distinct. 

 After one or two days, you will find 

 the spaces between the lines are no 

 longer equal. That part of the root Fig- 43 — Seedlings of 

 in which these spaces have become ^'^l ^^'^ ^T T'' 



^ marked so as to show 



widest is the region of elongation, the "region of eionga- 

 (See Figure 43.) The cells elongate *'°°"" 

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

 they cease to elongate. What would happen if the cells 

 on one side elongated more rapidly than those on the 

 side directly opposite? 



A . Root Curvature. — As the root grows it is almost 

 sure to curve. The young tips curve around the soil 

 grains. They curve toward moisture. It is evident that 

 this curvature can be more easily accompKshed by the 

 region of elongation than by any other part. If the cells 

 elongate more rapidly on one side than upon the opposite 

 side, a curve is sure to result, whereas if the cells have 

 finished their growth in length, and have become somewhat 

 more rigid, curvature evidently would not be so easy to 

 accomplish. So it does not surprise us to find that the 

 region of elongation is also the region of curvature. 



Suppose that the growing tip meets an obstacle. It is 

 always meeting obstacles. The tip itself cannot curve 



