DIURNAL MOVEMENT OP .\flMOSA 593 



antl reversal. In ihe record given in figure 216, light 

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

 pulvinus for 48 minutes gave the maximum positive 

 curvature ; this was completely neutralised by further 

 exposure to light for 20 minutes. Light was cut off at 

 neutralisation and there was a sudden fall beyond the 

 equilibrium position, which was more rapid than the 

 movement under light. The after-effect of prolonged 

 exposure is thus an 'over-shooting' beyond the normal posi- 

 tion of equilibrium. 



RESPONSE OF Mimosa TO DARKNESS AT DIFFERENT 

 PARTS OF THE DAY. 



I now tried the effect of darkness on the movement of 

 Mimosa, and was surprised to find that while artificial 

 darkness caused a sudden fall of the leaf in the after- 

 noon, it had no such effect in the forenoon. 



Experiment 226. — Successive records were taken of the 

 effect of artificial darkness for two hours, alternating with 

 exposure to light for two hours. The plant was subjected 

 to darkness by placing a piece of black cloth over the 

 glass cover from 12 to 2 p.m., it was exposed to light from 

 2 to 4 p.m. and darkened once more from 4 to 6 p.m. 



The record given in figure 217 shows that the leaf had 

 been moving upwards under the action of light (positive 

 phototropism); darkness commenced at the point marked 

 with a thick dot. The after-effect on the stoppage of light 

 is seen to he in the same direction as under light ; this 

 persisted for ten minutes followed by recovery which was 

 complete by 2 p.m., as seen in the horizontal character 

 of the curve. On restoration of light (at the point 

 marked with the second thick dot) the leaf moved up- 

 wards till the positive phototropic movement attained a 

 maximum in the course of an hour and twenty minutes, 



