112 



BOTANY. 



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. 



Fig. 91. 

 Sunflower plant removed from 

 darkness, leaves extending under 

 influence of light (diaheliotro- 

 pism). 



The leaves of plants with 

 a one-sided illumination, 

 as can be seen by trial, 

 are turned with their upper surfaces 

 toward the source of light, or per- 

 pendicular to the incidence of the 

 light rays. In this way light over- 

 comes for the time being the direc- 

 tion which growth gives to the leaves. 

 The so-called " sleep" of plants is 

 of course not sleep, though the leaves 

 f '£■ 9°- ' ' nod, ' ' or hang downward, in many 



Sunflower plant. Epinastic _,, 



condition of leaves induced dur- cases. 1 here are many plants in 



ine the day in darkness. .... . 



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 

 experiment to induce this condition during the day. The plant 

 which has been used to illustrate this is the sunflower. Some 

 of these plants, which were grown in a box, when they were 



