THE rHOTOTROPlC CURVE AND ITS CHARACTERISTICS 361 



uniform ; and in the last part the curve rounds off and 

 the rate of ascent becomes very small. 



The susceptibility for excitation is feeble at the begin- 

 ning ; it increases very rapitUy with increasing stimulus ; 

 finally it undergoes a fall, increase of stimulus inducing 

 no further enhancement of excitation. 



In a complete phototropic curve the first part is nega- 

 tive ; this is due lo the physiological expansion induced by 

 sub-minimal stimulus. The curve then crosses the abscissa 

 upwards, and the positive curvature reaches a maximum. 

 This is followed by neutralisation and reversal into nega- 

 tive ; the curve crosses the zero line and proceeds in the 

 negative direction. 



Weber's law is not applicable for the entire range of 

 stimulation. The quantitative relation fails in the region 

 of sub-minimal stimulus, where the physiological reaction 

 is qualitatively different. 



28 A 



