102 CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT 



effect being positive. But when the leaflets are acted on 

 by hght from below, then they also exhibit an upward 

 folding, the phototropic effect being now negative. Precisely 

 opposite effects are exhibited by the leaflets of Biophytum 

 and Averrhoa. They fold downwards whether light acts 

 from above or from below. 



In these circumstances the hypothesis of the positive or 

 negative irritabihties of different organs is quite untenable. 

 Phototropic reactions are complex, for they represent the 

 summated effects of numerous factors. The individual 

 effect of these can only be ascertained by careful inspection 

 of the curves of response recorded continuously under the 

 action of light. The mechanism of tropic curvature will 

 be dealt with in detail in the next chapter. 



Positive Phototropic Curvature 



In this chapter I describe : 



1. The positive curvature of pulvinated and growing 

 organs exposed to light. 



2. Determination of the latent period. 



3. The immediate and the after-effect of light. 



4. The relation between the quantity of light and the 

 responsive curvature. 



I have already shown (p. 68) that there is no essential 

 difference between the responses of pulvinated and those of 

 growing organs, direct stimulation inducing contraction in 

 both. The experimental investigation of the tropic effect 

 of hght has been carried out with both pulvinated and 

 growing organs. 



Phototropic Recorder 



A very sensitive method had to be devised for obtaining 

 accurate record of phototropic movements of pulvinated 

 and of growing organs. The writing lever is made of fine 

 glass fibre (fig. 50). The fulcrum-rod supported on jewel- 



