136 



ROOTS 



the absence of root-hairs by having their whole surface 



permeable. They can afford to have their whole surface 



soft better than soil roots can. Why? 



As you may already know, the absorption of food in 



your own body occurs in the small intestine. Here, too, 



we find an arrangement of ab- 

 sorptive hairs. The walls of 

 the small intestine are densely 

 covered with hairs called vil- 

 luses. Their structure is not 

 at all like that of root-hairs, 

 but they are like root-hairs 

 in that they permit absorp- 

 tion to go on much more 

 rapidly than if they were not 

 present. 



Root-hairs are very short 

 lived. They last only a few 

 days or weeks. Indeed the 



Fig. 46. -Seedlings of wheat the one .^j^^jg ^f ^^^ outermost layer 

 at the right being four weeks older 



than the one at the left. Note the of Cells of the yOUngeSt TOOtS 



regions of the root-hairs as indicated beCOmeS WOm off SOOn after 



by the adherent soil. ... , rx^i , 1 



it IS formed . The layer under 

 it becomes the outermost layer, and in this layer a sub- 

 stance called cutin is formed. It is this cutin which makes 

 the older surfaces of roots impermeable. 



As rapidly as the old hairs die, new hairs are formed. 

 All the while the root is growing forward through the soil. 

 Thus the root-hair region is constantly being moved ,into 

 new and fresher soil regions. These fresher regions may 

 be better for the work of the root-hair. This is not only 

 because they may be richer in the substances which the 



