IRRITABILITY. 



85 



which leaves assume under this influence is that of placing them 

 with the upper surface perpendicular to the rays of light which 

 fall upon them. Leaves, then, exposed to 

 the brightly lighted sky are, in general, 

 horizontal. This position is taken in direct 

 response to the 

 stimulus of light. 

 The leaves of plants 

 with a one-sided i 1 lu- 

 m in at ion, 

 as can be 

 seen by 

 trial, are 

 ' turned with 

 their upper 

 surfaces to- 

 ward the 

 source of light, or perpendicular to the in- 

 cidence of the light rays. In this way 

 light overcomes for the time being the 

 direction which growth gives to the leaves. 

 The so-called "sleep" of plants is of 

 course not sleep, though the leaves " nod," 

 or hang downward, in many cases. There 

 are many plants in which we can note 

 this 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 



Fig. 74- 



Dark chamber with opening at one side to show heliotropism. 

 (After Schleichert.) 



