86 RESEARCHES ON IRRITABILITY OF PLANTS 



exhibit the after-effect on the removal of the gas. It will 

 thus be understood how important it is to maintain the 

 external conditions constant for so long a period as two 

 hours. The method of maintaining the testing stimulus 

 constant has already been explained. With special care 

 the temperature of the plant-chamber can also be kept 

 uniform. The other factor which is liable to variation 

 is the intensity of light. I have often noticed a fluctuation 



Fig. 44. — Effect of sudden darkness on excitability of Mimosa. 

 First three responses, normal ; four succeeding responses 

 due to effect of darkness. Line below indicates period of 

 darkness. Vibration frequency of writer, five times per 

 second. 



in the uniformity of responses which was traceable to a 

 passing cloud. I soon found that a sudden change in the 

 intensity of light induces a marked variation of motile 

 excitability in Mimosa. Thus on bringing a highly sensi- 

 tive plant to a dark room its excitability is found to dis- 

 appear. This abolition of excitability is generally speaking 

 temporary, since the plant often regains its sensitiveness 

 after about an hour, though still kept in the dark. The 

 fact that under normal conditions it is the sudden diminu- 

 tion of light rather than darkness that induces depression 



