DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM 337 



algebraical sunmiation of the effects of direct and indirect 

 photic Btimulus. 



SUMMARY. 



The normal positive phototropic curvature is modified by 

 transverse conduction of true excitation to the distal side 

 of the organ. 



The extent of neutralisation or reversal due to internal 

 conduction of excitation from the proximal to the distal 

 side of the organ depends : (a) on the intensity of the inci- 

 dent stimulus, (b) on the conductivity of the organ in a 

 transverse direction, (c) on the thickness of the intervening 

 tissae, and (d) on the relative excitability of the distal as 

 compared to the proximal side. 



The dia-phototropic position is not one of indifference, 

 but of balanced antagonistic reactions of two opposite 

 sides of the organ. 



The supposition that direct sunlight is phototropically 

 ineffective is unfounded. The response is fully vigorous, but 

 the first positive curvature may in certain cases be neutra- 

 lised by the transmission of excitation to the distal side. 



Under light of strong intensity and long duration, the 

 transmitted excitation to the distal side neutralises, and 

 finally reverses the positive into negative curvature. 



The positi ue-phototroTpic, the d/a-phototropic, and the 

 negative phototropic curvatures are not due to three distinct 

 irritabilities but are brought about by a fundamental 

 excitatory reaction remaining localised or increasingly 

 transmitted to the distal side. 



