\ 



NEGATIVE HELIOTROPISM 



601 



tip is probably due to the higher degree of receptivity 

 possessed by that part of the organ. In taking a third record 

 in a case in which both tip and growing region were simul- 

 taneously subjected to unilateral stimulation of light, I found 

 that a resultant responsive movement was induced, which was 

 away from light. 



That this negative movement, induced by stimulation of 

 the root-tip, is not due to any specific sensitiveness of the 

 root as such, is seen from the fact that on local stimulation of 

 the tip of the shoot, e.g. the flower-bud of Crocus, I obtained 

 a responsive movement 

 away from, whereas uni- 

 lateral stimulation of the 

 growing region of the pe- 

 duncle induced a move- 

 ment towards, light. 



Apart from this pos- 

 sible factor, however, of 

 the greater receptivity of 

 the tip, there is another, 

 which tends to make the 

 positive curvature of the 

 growing region of the 

 root relatively ineffec- 

 tive. This region, being 

 acted on unilaterally by 



light, the proximal excitation often passes by conduction 

 to the distal side, thus neutralising the first positive action. 

 Instances of this will be given in greater detail presently. 



The negative curvature induced by the action of the tip, 

 depending as it does on the indirect transmission of stimula- 

 tion, requires as a condition the relative non-conductivity of 

 the intervening tissue to the passage of true excitation. 

 Hence, if the conductivity of such a tissue be not sufficiently 

 feeble, or if the intensity of stimulus be too great, we shall 

 find that the direct effect of stimulus is transmitted to the 

 growing organ, and a positive curvature is induced. This 



Fig. 245. Record of Response of Root of 

 Sinapis nigra 



The curve seen to the left shows the negative 

 response due to stimulation of root-tip. 

 The curve to the right exhibits positive 

 response on stimulation of growing region. 



