PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 639 



downward folding of the leaflets, is exhibited in the diurnal 

 sleep movements of Oxalis and Biophytum. 



In Mimosa the application of moderate intensity of light 

 from above induces upward or positive heliotropic move- 

 ment. Strong and long-continued application of light on 

 the same upper surface, however, owing to the internal 

 diffusion of excitation, induces concavity of the lower and 

 more excitable side, or negative heliotropic movement. Both 

 these responses may be regarded as cases of the directive 

 action of light ; but if the organ be excited by stimulus 

 which is externally diffuse, the responsive movement is still 

 downwards. The directive action of light has thus passed 

 into non-directive. 



