IRRITABILITY. 



87 



flower plant in the experiment detailed above. The day position of the 

 leaves on the other hand, which is more or less horizontal, is induced because 

 of their irritability under the influence of light, the inherent downward or 

 epinastic growth is overcome for the time. Then at nightfall or in darkness, 

 the stimulus of light being removed, the leaves assume the position induced 

 by the direction of growth. 



In the case of the cotyledons of some plants it would seem that the growth 

 was hyponastic even after they have opened. The day position of the coty- 



Position of cotyledons in 

 the dark. 



Fig. 77. Fig. 78. 



Squash seedling. Position of cotyledons in Squash seedling, 

 light. 



ledons of the pumpkin is more or less horizontal, as shown in fig. 77. At 

 night, or if we darken the plant by covering with a tight box, the leaves 

 assume the position shown in fig. 78. 



While the horizontal position is the general one which is assumed by 

 plants under the influence of light, their position is dependent to a certain 

 extent on the intensity of the light as well as on the incidence of the light 

 rays. Some plants are so strongly heliotropic that they change their posi- 

 tions all during the day. 



203. Leaves with a fixed diurnal position. — Leaves of some plants when 

 they are developed have a fixed diurnal position and are not subject to 



