ROOTS AND THEIR USES 185 



only one. The actual position of a root is probably seldom 

 exactly what it would be if gravity were the only stimulus at 

 work. Most roots tend to grow away from the light. Most 

 of them tend also to grow toward a supply of water, and some 

 grow away from the air. Various substances in the soil may 

 serve as chemical stimuli to roots ; in some cases the roots 

 grow toward the substance in question ; in other cases they 

 grow away from it. The pressure caused by contact with a 

 solid object, such as a stone, serves as a stimulus ; the root 

 bends, not necessarily at the point where it touches the hard 

 body, but in the region in which its cells are growing ; and the 

 result of the bending often is that the root grows around an 

 object that is in its way. The roots of parasitic plants like 

 dodders and mistletoes, when they push into the tissues of 

 a host plant, respond to a set of stimuli quite different from 

 those which affect the growth of ordinary roots. Probably 

 some of these are chemical stimuli and some are contact 

 stimuli. 



