238 



The Living Plant 



supplied to a plant, it will turn to that side to a considerable 

 degree; but the turning is never without limit, for, generally 

 speaking, the farther it turns the more reluctant, so to speak, 

 it is to turn any farther. If, on the other hand, the plant is so 

 grown that it does not receive a one-sided stimulation, which is 



Fig. 82.— a typical example of the sleep of plants. Both are Acacias, identical in kind and 

 age, but the one on the right has been covered for an hour from light. 



easiest accomplished by keeping the plant in continual rotation 

 by aid of an instrument (called a clinostat) designed for the 

 purpose (figure 83), then it always develops wdth remarkable 

 symmetry, determined, very obviously, by internal and hereditary 

 causes. The plant, accordingly, is born with an internal tendency 

 to symmetrical form, but likewise with a considerable though not 

 unlimited margin of possible deviation therefrom; and it is 

 within this margin that the irritable responses take place. But 



