150 CHAP. XIV. THE PHOTOTROPIC CURVE 



of tropic curvature. The gradient at any point of the curve, 

 the increment of tropic curvature divided by the increment 

 of stimulation, gives the susceptibihty for that point. The 

 following table shows how the susceptibility undergoes 

 variation through the whole range of the curve. The 



Fig. 84. Simple characteristic curve of phototropic reaction. 

 Excitatory contraction increases slowly in the first part, 

 and rapidly in the second ; it is uniform in the third and 

 undergoes decline in the fourth part (Erythrina). 



average susceptibility for each point has been calculated 

 from the data furnished by the curve. 



The induced contraction which results in tropic curva- 

 ture is seen to be increased very gradually from the zero 

 point of susceptibility, at which there is no responsive move- 

 ment. After this, the increase of susceptibility is very rapid, 

 the maximum rate of increase being reached at the point 11 

 of the curve. In the median range the susceptibihty is 

 constant, each equal increment of stimulation inducing an 



