PHOTOTROPISM 427 



tendril round a support. The after-effect of stimulation 

 of short duration is an acceleration of growth of the stimu- 

 lated side above the normal, in consequence of which the 

 recovery becomes hastened. Stimulation of one side of the 

 tendril induces expansion of the indirectly stimulated distal 

 side, even in cases where the contractility of the stimulated 

 side is feeble. Hence the response to stimulation of the 

 more excitable side of the tendril may be inhibited by 

 stimulation of the opposite side (p. 99). 



Phototropism 



Quantitative relation. — It has been shown that the 

 amount of phototropic curvature depends (i) on the intensity 

 of light ; (2) on the duration of exposure ; and (3) on the 

 sine of the directive angle. The intensity of phototropic 

 action is therefore determined by the quantity of the inci- 

 dent light (p. 114). 



Dia-phototropism and negative phototropism. — Transverse 

 conduction of excitation to the distal side induces 

 (i) a neutralisation or dia-phototropic response, and sub- 

 sequently (2) a negative phototropic curvature. An im- 

 portant contributory factor in the reversal of response is 

 the fatigue-relaxation of the proximal side (p. 137). 



The effect of light on the root shows that its irritability 

 is in no way different from that of the shoot. In a thick 

 root, in which there is no transmission of excitation to the 

 distal side, the response is positively phototropic, but in a 

 thin root transverse conduction of excitation transforms the 

 positive curvature into negative. Thus, in the root of 

 Sinapis, the sequence of response is positive, dia-phototropic, 

 and finally negative. It was w^ant of knowledge of the pre- 

 liminary positive curvature that led to the wrong inference 

 that the root possessed an irritability specifically different 

 from that of the shoot (p. 145). 



The complete phototropic curve of leaf and of stem consists 

 of four parts : (i) the stage of subminimal stimulation ; 



