320 LIFE MOVEMENTS IN PLANTS 



was the usual arrangement of inclined mirrors for illu- 

 mination of the plant. The flash of light from a single 

 spark is seen to induce a sudden retardation of rate of 

 growth which lasted for one and half minutes. The 

 record (Fig. 117) shows another interesting peculiarity of 

 acceleration as an after-efiect of moderate stimulation. 

 After the retardation which lasted for 90 seconds, there 

 is an acceleration of growth above the normal, which 

 persisted for 6 minutes, after which the rate of growth 

 returned to the normal. 



In order to show that the induced variation is due to 

 the action of light and not to any other disturbance, 

 I interposed a sheet of ebonite between the spark-gap and 

 the plant. The production of spark produced no effect, 

 but the removal of the ebonite screen was at once 

 followed by the characteristic response. 



MAXIMUM POSITIVE CURVATURE UNDER CONTINUED 

 ACTION OF LIGHT. 



The positive curvature is, as we have seen, due to the 

 contraction of the proximal side and expansion of the 

 distal side. The curvature will increase with growing 

 contraction of the proximal side ; a maximum curvature 

 is however reached since : 



(1) the contraction of the cells must have a limit, 



(2) the bending organ offers increasing resistance to 



curvature, and 



(3) the induced curvature tends to place the organ 



parallel to the direction of light when the 

 tropic effect is reduced to a minimum 



The pulvinus of Erythrina exemplifies the type of 

 reaction in which the positive curvature reaches a maxi- 

 mum, (see below Fig. 132) beyond which there is no further 



