CHAPTER XLIII 



NEGATIVE HELIOTROPISM 



Incomplete parallelism between actions of light and of gravitation — Theoretical 

 considerations — Recording microscope — Negative heliotropic curvature 

 induced by stimulation of the tips of root and shoot — Intermediate phases 

 between positive and negative heliotropic response : {a) neutralisation by 

 transverse transmission ; (b) neutralisation by transverse transmission, with 

 multiple response — Localised sensitiveness to light and transmission of 

 excitatory effect — Negative heliotropism of a radial organ — Gradual transition 

 from positive to negative, through intermediate phase of neutrality — Apparent 

 heliotropic insensitiveness of certain tendrils — Negative heliotropism of tendril 

 of Vitis. 



We have seen in the chapter on the response due to gravita- 

 tion, that the responsive curvature of the root is opposite in 

 character to that of the stem, this fact having led to the 

 assumption of specific sensibilities as characteristic of the 

 root-tip. It was there shown, however, that the responsive 

 characteristics of the root were not actually different from 

 those of the shoot, and that the differences in their observed 

 responses were simply a consequence of the fact that in the 

 one case the stimulus of gravity acted indirectly, and in the 

 other directly, upon the responding growing organ. This 

 assumption that the root possessed a definite sensitiveness 

 characteristically different from that of the stem, was ap- 

 parently supported by certain differences in heliotropic action 

 also, as between shoot and root ; for example, while the 

 hypocotyl of Sinapis bends towards the light, the root is 

 found to bend away from it. 



Incomplete analogy between action of light and 

 gravitation. — But I have already explained the fact that the 

 supposed analogy is false ; for while the stimulus of gravity 

 acts, in the case of the root, only on a restricted area of 



