ADAPTATIONS TO CONDITIONS OF LIFE 33 



43. Fitness of the root for its position and work. The dis- 

 tribution of material in the woody roots of trees and shrubs and 

 their behavior in the soil show many adaptations to the condi- 

 tions by which the roots are surrounded. The growing tip of 

 the root, as it pushes its way through the soil, is exposed to 

 bruises ; but these are largely warded off by the root cap. The 

 tip also shows a remarkable sensitiveness to contact with hard 

 objects, so that when touched by one it swerves aside and thus 

 finds its way downward by the easiest path. Eoots with an 

 unequal water supply on either side grow toward the moister 

 soil ; when unequally heated they grow in the direction of the 

 most desirable temperature, and they usually grow away from 

 the light. Eoots are very tough, because they need to resist 

 strong pulls, but not as stiff as stems and branches of the same 

 size, because they do not need to withstand sidewise pressure, 

 acting from One side only. The corky layer which covers the 

 outsides of roots is remarkable for its power of preventing evapo- 

 ration. It must be of use in retaining in the root the moisture 

 which otherwise must be lost on its way from the deeper root- 

 lets (which are buried in damp soil), through the upper portions 

 of the root system, about which the soil is often very dry. 



