262 CHAP. XXIV. THE AFTER-EFFECT OF LIGHT 



Experiment 142. After-effect at post-maximum. — In this 

 case the light was continued till there was complete neu- 

 tralisation at c, the curve of response returning to zero-line ; 

 to all outer seeming the responsive indication of the organ is 

 the same as before excitation. But stoppage of stimulation 

 at c caused an overshooting at a rapid rate far below the 

 zero-line (fig. 150). 



The condit'on at post-maximum c is thus one of dynamic 

 equilibrium where two opposite activities, ' A ' and * D/ 

 balance each other ; for had the condition of the 

 ' neutralised ' organ been exactly the same as when it was 

 fresh, cessation of stimulus would have kept the galvano- 

 metric spot of light at the zero-position. 



The electric investigations described above indicate that 

 the after-effect is modified by the duration of stimulation, 

 and that : 



1. The after-effect of pre-maximum stimulation is the 



continuation of response in the original direction 

 (upwards, and away from the zero-line), followed 

 by recovery ; 



2. The after-effect of maximum stimulation is a recovery 



towards zero-position ; and 



3. The after-effect of post-maximum stimulation is an 



overshooting of response downwards, below the 

 zero-line. 



Tropic Response to Light and its After-Effect 



I next describe the after-effects of light exhibited by 

 mechanical response, the results of which will be found 

 to be parallel to those given by electric response. The 

 specimen employed was the terminal leaflet of Desmodium 

 gyrans, the pulvinus of which is very sensitive to light. 

 Pulvinated organs, generally speaking, exhibit a diurnal 

 variation of turgor, in consequence of which the position of 

 the equilibrium of the leaf or leaflet undergoes a periodic 

 change. The equilibrium position, however, remains fairly 



