6o8 PLANT RESPONSE 



mediate phase of neutralisation. We have seen that at a 

 certain definite intensity of illumination the excitations 

 of proximal and distal sides, balancing each other, cause 

 neutralisation. If now the intensity of stimulating light be 

 further increased, it is easy to see that while the stimulation 

 transmitted to the distal side, with the concomitant contrac- 

 tion, is being increased, the excitatory contraction of the 

 proximal will be at the same time decreased, owing to the 

 fatigue brought on by over-stimulation. The result then 

 will be the greater contraction of the distal side, with a 

 consequent negative heliotropic curvature of the organ. 



These considerations show the mechanical action of 

 heliotropic stimulus in causing : (i) positive heliotropic curva- 

 ture under moderate illumination ; (2) the neutralisation of this 

 action under stronger illumination ; and (3) the conversion 

 of the normal positive into negative when illumination is 

 excessive. Thus the observation made by Oltmanns, on 

 young seedlings of Lepidiiim, subjected to varying intensities 

 of light, an observation of which there has been hitherto no 

 satisfactory explanation, is fully accounted for. Oltmanns 

 subjected a row of seedlings of Lepidium to the action of 

 sunlight, diverging from the focus of a lens. The seedlings 

 nearest the focus were thus subjected to the strongest stimu- 

 lation, those further from this point being under gradually 

 decreasing intensities of light. It was then found that 

 the seedlings nearest the focus, which were subjected to the 

 strongest degree of light, exhibited negative heliotropic 

 curvature, while others, further away, and therefore subjected 

 to less intense illumination, did not show any effect at all 

 (neutralisation), and others again, which were still further 

 away, and therefore under only moderate intensity of 

 illumination, exhibited positive curvature. 



Gradual transition from positive to negative, through 

 intermediate phase of neutrality. — I shall, however, give a 

 still more conclusive verification of the theoretical inferences 

 which I have just set forth regarding the gradual transition 

 of positive heliotropic response into negative, through the 



