128 



PHYSIOLOGY. 



which leaves assume under this infliieiKC is that of placing tlieni 

 with the upper surface perpendicular to the rays of light which 

 fall upon them. I,ea\"es, then, exposed to 

 the brightly lighted sky are, in general, 

 horizontal. This position is taken in direct 

 response to the 

 stimulus of light. 

 The lea\ es of plants 

 with a one-sided illu- 

 mination , 

 as can be 

 seen b )• 

 trial, are 

 turned with 

 , , their upijer 



Seedling uf castor-oil bean, bef'jre and after 



a une-iided illumination. SUrlaCeS tO- 



ward the 

 source of light, or perpendicular to the in- 

 cidence of the light rays. In this -way 

 light overcomes fur the time being the 

 direction which growth gives to the lea\'es. 

 The so-called "sleep" of plants is of 

 course not sleep, though thelea\"es " nod," 

 or hang downward, in many cases. There 

 are many plants in which we can note 

 tin's drooping of the leaves at nightfall, and in order to prove 

 that it is not determined by the time of day we can resort to 

 a well-known ex- 

 periment to induce 

 this condition dur- 

 ing the day. The 

 plant which has 

 been used to illus- 

 trate this is the sun- 

 flower. Some of 

 these plants, which 



FiR. U.S. 



Dark chamber with oiiening at one side to show heliotropism. 

 (After Schlelchcrt,) 



