AFTER-EFFECTS OF LIGHT 275 



a replica of the record of the periodic up and down move- 

 ments of the leaf. 



This method of torsional response has a very important 

 advantage over that of the ordinary method, since, the 

 petiole being supported by a loop of wire, the weight of 

 the leaf can have no effect on the curve of response. 



The Characteristic After-Effects of Light 



In the previous chapter it was shown (p. 265) : 



1. That the pre-maximum after-effect is a temporary 



continuation of the response induced by light 

 follow^ed by recovery ; 



2. That the after-effect at maximum is an immediate 



recovery to the normal equilibrium ; and 



3. That the after-effect at post-maximum is an over- 



shooting below the position of equilibrium. 



Consideration of these results will be found to explain 

 the various anomalies in the diurnal movement of the leaf of 

 Mimosa. 



Effect of Artificial Darkness 



Experiment 150. — Successive records w^ere taken of the 

 effect of artificial darkness for 2 hours, alternating with 

 exposure to light for the same time. The plant was sub- 

 jected to darkness by placing a piece of black cloth over 

 the glass case in which it was enclosed, from 12 to 2 p.m. ; 

 it was exposed to light from 2 to 4 p.m., and then subjected 

 to darkness once more from 4 to 6 p.m. 



The record given in fig. 160 shows that the leaf had 

 been moving upwards under the action of light (positive 

 phototropism) ; the moment of exposure to darkness is 

 marked with a thick dot in the record. The after-effect of 

 the withdraiml of light is seen to be a movement in the same 

 direction as that under exposure to light ; this persisted for 

 10 minutes, followed by recovery which was completed by 



