PHOTOTROPISM 327 



change. This is due to absence of transverse conductivity 

 in the organ. The modifying effect of transverse conducti- 

 vity on response will be dealt with in the next chapter. 



SUMMARY. 



The positive phototropic curvature is brought about by 

 the joint effects of the directly stimulated proximal, and 

 indirectly stimulated distal side. 



The phototropically curved organ undergoes recovery 

 after brief stimulation. 



The recovery after moderate stimulation is hastened by 

 the previously stimulated side exhibiting an acceleration of 

 the rate of growth above the normal. The after-effects 

 of photic and mechanical stimulation are similar. 



The latent period of photic reaction is shortened with 

 the increasing intensity of light. The seedling of wheat 

 responds to a flash of light from an electric spark, the 

 duration of which is about a hundred thousandth part of 

 a second. 



Tissues in which the power of transverse conduction 

 is negligible, the positive phototropic curvature under 

 continued action of light attains a maximum without 

 subsequent neutralisation or reversal. 



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