MECHANICAL RESPONSE 263 



constant for nearly 2 hours about midday, the variation 

 of temperature at this period being slight. The pure effect 

 of light can be obtained by carrying out the experiment 

 during this period, and thus complications which may arise 

 from autonomous pulsation are also avoided. 



The duration of the experiment may be shortened by the 

 choice of a 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 in the 

 following experiment w^as a 50-candle-power incandescent 

 lamp. The intensity was increased to a suitable value by 

 focusing the light on the upper half of the pulvinus by means 

 of a lens. The intensity was so adjusted that the maximum 

 positive curvature occurred in the course of about 6 minutes, 

 and complete neutralisation was attained after an exposure 

 of 17 minutes. 



Experiment 143. After-effect atpre-maximum. — Light was 

 allowed to act on the upper half of the pulvinus for 2 minutes 

 and 20 seconds ; this induced an up-movement, i.e. a posi- 

 tive curvature. On the stoppage of light the up-movement 

 continued for i minute and 20 seconds, after which the 

 down-movement of recovery was completed in 6 minutes 

 and 20 seconds (fig. 151). The immediate after-effect is 

 thus a movement upward, away from the zero-line of 

 equilibrium. The result is seen to agree with the electric 

 after-effect of pre-maximum stimulation. 



Experiment 144. After-effect at maximum. — Application 

 of light for 5 minutes and 20 seconds induced a maximum 

 positive curvature. Stoppage of light was followed at 

 once by recovery, which was completed in about 10 minutes 



(fig- 152). 



Experiment 145. After-effect at post-maximum. — As 

 the plant was fatigued by previous experiments, a fresh 

 specimen was taken and light was applied continuously on 

 the upper half of the pulvinus. This gave rise first to a 

 maximum positive curvature ; neutralisation took place 

 after application of light for 17 minutes. On the stoppage 



