AFTER-EFFECT OF LIGHT 573 



(3) the after-effect of post-maximum stimulation is 

 an over-shooting downward below the zero line. 



TROPIC RESPONSE UNDER LIGHT AND ITS AFTER-EFFECT. 



I shall now describe the after-effect of light as seen in 

 mechanical response, and the results will be found parallel 

 to those given by the electric response. The specimen 

 employed is the terminal leaflet of Desjnodium gyi'ans, 

 the pulvinus of which is very sensitive to light. Pulvi- 

 nated organs, generally speaking, exhibit a diurnal varia- 

 tion of turgor in consequence of which the position of 

 equilibrium of the leaf or leaflet undergoes a periodic 

 change. But this equilibrium position of the organ 

 remains fairly constant for nearly two hours about mid- 

 day, the variation of temperatute at this period being 

 slight. We may therefore obtain the pure effect of light 

 by carrying out the experiment at this period, and 

 completing it within a short time to avoid complication 

 arising from the autonomous variation of turgor. 



The period of experiment of the plant may be short- 

 ened by a choice of suitable intensity of light ; a given 

 tropic effect induced by prolonged feeble light may thus 

 be obtained by short exposure to stronger light. The 

 source of light for the following experiment was a 50 c.p. 

 incandescent lamp. The intensity was increased to a suitable 

 value by focussing light on the upper half of the pulvinus 

 by means of a lens. The intensity was so adjusted that 

 the maximum positive curvature was attained in the course 

 of about G minutes, and complete neutralisation after an 

 exposure of 17 minutes. 



Pre-maxi?num after-effect : Experiment 217. — Light was 

 allowed to act on the upper half of the pulvinus for two 

 minutes and twenty seconds ; this induced an up-movement 



