EFFECTS OF DIFFERENT GASES 87 



of excitability, is borne out by the fact that the plant is 

 fully sensitive at night. 



Effect of Sudden Darkness 



In order to demonstrate the variation of excitability 

 induced by sudden diminution of light, I first took a set 

 of three normal responses in diffuse daylight. The plant- 

 chamber was then suddenly darkened by means of an opaque 

 screen. It will be noticed (fig. 44) that the next two responses 

 were nearly abolished ; the excitability of the plant was 

 however beginning to be restored after 45 minutes' exposure 

 to darkness. After an hour in darkness the excitability was 

 fully restored, the response here being even larger than in 

 light. 



In order to guard against the disturbing effect of varia- 

 tion of light it is advisable to carry out the following 

 experiments in an open veranda, the plant being kept in a 

 chamber with frames of ground glass. In this way the 

 plant is maintained under diffuse light of fairly uniform 

 intensity. 



Effect of Absorption of Water 



Another peculiarity I noticed in Mimosa was a depression 

 of excitability on rainy days. This effect I was afterwards 

 able to trace to the absorption of water by the pulvinus. 

 The variation of motile excitability by absorption of water 

 is very clearly exhibited in the accompanying record (fig. 

 45). A pair of normal uniform responses were first taken. 

 A drop of water was then applied on the pulvinus, when the 

 leaf was recovering from the second stimulus. It will be 

 noticed that the period of recovery became very much 

 protracted in consequence of absorption of water. The 

 usual time for complete recovery is about 15 minutes. In 

 the present case it was prolonged to 45 minutes. Testing 

 stimuU were applied at the usual intervals of 15 minutes, 

 the moments of application being represented by thick 



