264 CHAP. XXIV. THE AFTER-EFFECT OF LIGHT 



of light there was a sudden overshooting below the zero-line 

 (fig. 153) ; the rate of the movement on the cessation of 

 light was nearly twice as quick as during the process of 

 neutralisation. 



I also obtained a very interesting record of the post- 

 maximum after-effect of light with Cassia alata. 



Fig, 151, 



Fig. 152, 



Fig. 153. 



Fig. 151. Light applied at arrow, and stopped at the second 

 arrow within a circle. After-effect of pre-maximum stimula- 

 tion is continuation of positive curvature followed by 

 recovery. 



Fig. 152. After-effect of stimulation at maximum ; recovery 

 towards zero-position of equilibrium. 



Fig. 153. After-effect of post-maximum stimulation is a rapid 

 overshooting below the position of equilibrium. 



Light was applied in all cases on upper half of pulvinus of 

 terminal leaflet of Desmodium gyrans. 



Experiment 146. — In Cassia, as in Mimosa, light acting 

 from above induces at first an erectile movement of the leaf 

 which reaches a maximum, after which there follow neutral- 

 isation and reversal. In the record given (fig. 154), light 

 from a small arc-lamp acting on the upper half of the 

 pulvinus for 48 minutes induced the maximum positive 

 curvature ; this was completely neutralised by further 

 exposure to light for 20 minutes ; cessation of light was 

 followed by a rapid fall of the leaf beyond the position of 



