NEGATIVE PHOTOTROPIC CURVATURE I37 



definite reactions which modify the rate of growth, i.e. a 

 contraction under moderate, and relaxation under intense 

 and prolonged, stimulation. Hence tropic curvature brought 

 about by one-sided action of light would likewise be subject 

 to modification, according 

 to the intensity and dura- 

 tion of the incident illu- 

 mination, by two factors : 

 (i) the transverse conduc- 

 tion of excitation to the 

 distal side, and (2) the 

 fatigue-relaxation at the 

 proximal side. This ex- 

 planation is based on the 

 following experiments. 



Negative Phototropic 

 Curvature 



Experiment 83. Effect ^ig. 75. Effect of continuous electric 



of varying intensity of HM. stimulation at arrow onwards in- 



•^ -/ o .y J o duces contraction followed by fatigue 



The growing organ was relaxation (Vigna Catjang). 



moderately thin ; it was 



subjected to unilateral action of light from a i6-candle-power 

 incandescent lamp placed at a distance of 10 cm. A 

 maximum positive curvature was induced in the course of 

 50 minutes. The intensity of light was then increased by 

 bringing the lamp nearer, the intervening distance being 

 reduced to 6 cm. After an exposure of 70 minutes to the 

 stronger light the specimen assumed a dia-phototropic 

 position of equilibrium. Sunlight was next applied, and 

 there was a pronounced reversal into negative phototropic 

 curvature in the course of 30 minutes. 



Experiment 84. Phototropic reversal under continued 

 action of strong light. — The change of response from positive 

 to negative, described in the previous experiment, was 

 brought about by successive increase in the intensity of 



