1 84 TEXTBOOK OF BOTANY 



position of the roots does not explain how the roots come to 

 grow in this particular way. A plant cannot choose to do a 

 thing because that thing will help it. The direction in which 

 a root or stem or other part grows is determined by the effect 

 of outside influences upon the plant. An outside influence 

 which can thus affect the growth or any other activity of a 

 plant is called a stimulus. 



The stimulus that has most effect in determining the direc- 

 tion of growth of roots is gravity. Most primary roots and 

 adventive roots tend to respond to the stimulus of gravity 

 by growing directly or almost directly downward. But 

 branch roots coming from a vertical root do not as a rule 

 grow straight downward ; if they did, they would lie close 

 beside the main root. As a matter of fact, branch roots 

 grow downward but at an angle with the main root. Ex- 

 periment shows that this difference in the direction of 

 growth of branch roots is due to a different response on their 

 part to the stimulus of gravity. The exact angle at which 

 a branch root tends to grow varies with its position upon the 

 plant and differs also in different species of plants. As a 

 result of this tendency on the part of branch roots, they 

 spread out from the main root and from one another, and the 

 root system comes in contact with much more of the soil than 

 it would if all the roots grew straight downward ; the roots 

 are thus in a position to absorb more water and other foods 

 and the plant is more firmly anchored. However, if part of 

 the main root is cut or broken away, one or more of the 

 branch roots nearest the break will turn and grow straight 

 downward. That is, the removal of part of the main root 

 causes the branch roots to respond differently to the stimulus 

 of gravity from the way in which they other^^dse would 

 respond. 



205. Responses to Other Stimuli. — While gravity is the 

 stimulus chiefly concerned in most cases in determining 

 the direction in which roots are to grow, it is by no means the 



