CHAPTER XIV 



THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 



The characteristics of the phototropic curve will be studied 

 in greater detail in this chapter, any point in the curve 

 exhibiting the relation between the stimulus of light and 

 the effect induced by it. The effective intensity of stimula- 

 tion has been shown to depend on the duration of exposure 

 (p. 112). It has been further shown (i) that in the simpler 

 cases the reaction under continued unilateral stimulation by 

 light is one of increasing phototropic curvature ; (2) that 

 the tropic curvature is modified by the transverse conduction 

 of excitation across the organ from the proximal to the 

 distal side. 



I will first consider the contractile reaction induced in 

 a growing organ under continued action of diffuse external 

 stimulation, either electric or photic. The reaction-curve 

 is obtained by making the plant record, by means of the 

 High Magnification Crescograph, the increasing retardation 

 of its growth (incipient contraction). Two records of 

 the effects of continuous electric and photic stimulation 

 have already been given (cf. fig. 36), in which the ordinate 

 of the curve represents the incipient contraction and the 

 abscissa the duration of application of the stimulus. The 

 contraction was seen to be slight at the first stage ; it 

 increased rapidly in the second, after which it declined and 

 reached a limit at the third stage. The excitator\^ con- 

 traction is thus not constant throughout the whole curve, 

 but undergoes very definite and characteristic variation. 

 To facilitate explanation of certain characteristic effects, 



