EFFECT OF ALTERNATION OF LIGHT AND DARKNESS 545 



The terminal leaflet of Desmodium gyrans exhibits 

 a diurnal movement which is very similar to that of Cassia. 

 It begins to open early in the morning and remains out- 

 spread during the v^hole day ; the leaflet exhibits a rapid 

 dowu-movement after 5 p.m. and becomes closely pressed 

 against the petiole in the course of about two hours. 



The midday sleep of leaflets of Mimosa and Averrhoa 

 is due to the excitatory action of strong sunlight on the 

 pulvinule, the more excitable half becoming contracted 

 under excitation. In Mimosa leaflets it is the upper, and 

 in Averr/ioa, it is the lower half of the pulvinule that 

 is the more excitable. It is in consequence of this that 

 the diffuse excitation of strong sunlight causes the leaflets 

 of Mimosa to fold upwards, those of Averrhoa to fold 

 downwards. 



