SUMMARY 147 



Summary 



The normal effect of direct unilateral stimulation by light 

 is a positive curvature. 



Transverse conduction of excitation to the distal side 

 induces neutralisation, that is, a dia-phototropic attitude 

 of the organ. 



Stronger and long-continued action of light transforms 

 the positive into a negative curvature. An important con- 

 tributory factor in the reversal of response is the fatigue- 

 relaxation of the proximal side of the organ. 



Investigation of the action of light on the root shows 

 that the irritability of the root is in no way different from 

 that of the shoot. 



The effect of unilateral illumination of the tip of the 

 root is the indirect stimulation of the growing region by 

 transmitted impulse; this induces acceleration of the rate 

 of growth on the stimulated side which causes negative 

 curvature. The shoot gives a similar response to indirect 

 stimulation. 



In a thick root, in which there is no transmission of 

 excitation to the distal side, the response is a positive 

 phototropic curvature. 



In a thin root, such as that of Sinapis, the sequence of 

 response is positive, dia-phototropic, and finally negative 

 due to the transmission of excitation across the thin organ. 



It was want of knowledge of the preliminary positive 

 curvature of the root that led to the erroneous inference 

 that the root possesses an irritability specifically different 

 from that of the shoot. 



