Petiole- Pulvinus Preparations of Mimosa pudica. 227 



marked diminution. Conversely, by increasing the resting interval, first to 

 12 and then to 15 min., the extent of fatigue was reduced and then abolished. 



9. The Influence of Constant Electric Current on Recovery. 



From the above experiment it would appear that since the incompleteness 

 of recovery induces fatigue, hastening of recovery would remove it. With 

 this idea I tried various methods for quickening the recovery of the excited 

 leaf. The application of a constant electric current was found to have the 

 desired effect. Two electrodes for introduction of current were applied, 

 one on the stem and the other on the petiole, at some distance from the 

 pulvinus. In order to avoid the excitatory effect of sudden application, 

 the applied current should be increased gradually ; this was secured by 

 means of a potentiometer slide. In my experiment a current having an 

 intensity of 1*4 micro-ampere was found to be effective. Eesponses at 

 intervals of 10 min., as we have seen, exhibit marked fatigue. Two responses 

 were recorded on a fast-moving plate, N before, and C after, the application 

 of the current. It will be seen (fig. 11) how the application of current has, 

 by hastening the recovery, enhanced the amplitude of response and brought 

 about a diminution of fatigue. In connection with this, I may state that the 

 tonic condition is, in general, improved as an after-effect of the passage of 

 current. This is seen in some cases by a slight increase in excitability ; 

 in others, where the responses had been irregular, the previous passage of a" 

 current tends to make the responses more uniform. 



10. The Action of Light and Darkness on Excitability. 



In taking continuous records of responses I was struck by the marked 

 change of excitability exhibited by the intact plant under variation of light. 

 Thus the appearance of a cloud was quickly followed by an induced depression, 

 and its disappearance by an equally quick restoration of excitability. This 

 may be explained on the theory that certain explosive chemical compounds 

 are built up by the photosynthetic processes in green leaves, and that the 

 intensity of response depends on the presence of these compounds. But 

 the building up of a chemical compound must necessarily be a slow process, 

 and it is difficult on the above hypothesis to connect the rapid variation of 

 excitability with the production of a chemical compound, or its cessation, 

 concomitant with changes in the incident light. 



In order to find out whether photosynthesis had any effect on excitability, 

 I placed an intact plant in a dark room and obtained from it a long series of 

 responses under uniform test-stimulus. While this was being done the green 

 leaflets were alternately subjected to strong light and to darkness, care being 



