602 PLANT RESPONSE 



explains the positive heliotropic curvature exhibited by many- 

 roots. 



Intermediate phases between positive and negative 

 heliotropic response. — I shall next proceed to demonstrate 

 the induction of negative heliotropic movements in radial 

 shoots, a phenomenon which, for reasons already explained, 

 has no parallel in the case of geotropic action (p. 544). As it 

 has already been said that there is no specific sensibility which 

 determines the positive or negative character of the heliotropic 

 response, it would be interesting to trace out the transitions 

 by which the normal positive is gradually transformed into 

 the negative movement. We have seen that when stimulus 

 is applied unilaterally to a growing region, the positive 

 curvature at first induced is jointly due to the contraction 

 caused by direct stimulation of the proximal and the expan- 

 sion caused by the indirect stimulation of the distal ; but 

 when the stimulus is strong or long-continued, excitation is 

 transmitted from the proximal to the distal, the contraction 

 of which latter now neutralises the first effect. Hence the 

 normal positive curvature disappears. 



(a) Neutralisation by transverse transmission. — The con- 

 siderations just related explain the curious anomaly that has 

 been observed, by which, while feeble or moderate stimulus of 

 light, or interrupted light, gives rise to well-marked positive 

 heliotropic curvature, the continuous application of stronger 

 light induces a relatively feeble effect. Thus under moderate 

 lighting we often observe strong heliotropic curvature, which 

 disappears under strong sunlight. The curvature induced 

 is, as we have seen, due to the differential action of unilateral 

 stimulus, on the proximal and distal sides ; but when a strong 

 light is used the stimulus becomes internally diffused, and the 

 differential effect on the two sides is reduced in amount or 

 vanishes altogether. Such internal diffusion is due to the fact 

 that, owing to the weak transverse conductivity of the tissue, 

 while a feeble stimulus is not conducted across it, a stronger 

 stimulus is. This consideration, together with the fact that 

 the conductivity of a tissue undergoes seasonal variation, 



