HELIOTROPIC TORSIONAL RESPONSE 66 1 



leaf, the less contractile ebonite being turned so as to face 

 the light (fig. 269). 



Modification of torsional response by artificial varia- 

 tion of the relative excitabilities of the two halves. — 

 I find it necessary to go still further into this subject of 

 torsional response, as by its means I have been enabled to 

 solve a question of very great importance, that, namely, of the 

 different excitabilities of the two halves of the anisotropic 

 organ to geotropic stimulus. The fact that it is the differential 

 character of the excitabilities of these two halves that brings 

 about the torsion of a dorsi-ventral organ under lateral stimu- 

 lation may be still further established in a very interesting 

 manner by inducing artificial variation in the existing excita- 

 bilities. The torsion depends, as said before, on the difference 

 of excitability as between the two. If, then, we could render 

 the lower and more excitable half gradually less and less 

 excitable, till its differential excitability had disappeared, the 

 organ would thus have been rendered virtually radial. The 

 torsional effect might then be expected to vanish, and a 

 simple curvature towards stimulus to result, without torsion, 

 as in the case of other radial organs. Let us next suppose 

 the reduction of excitability to be carried still further, till the 

 lower half of the pulvinus have been rendered less excitable 

 than the upper. On the theory of torsional response which 

 has just been advanced, it is the less excitable lower half 

 which should now twist round to face the stimulating light. 

 In other words, there would then be a reversal of the original 

 torsion. Thus, as the excitability of the lower half becomes 

 gradually reduced, the intensity of the normal positive 

 torsional response by which the upper half was made to face 

 the stimulus would be gradually first decreased to zero, and 

 then reversed to negative, as the excitability of the lower 

 became first equal to, and then less than, that of the upper half. 



If, on the other hand, the excitability of the upper half 

 be reduced, the existing differential excitability as between 

 upper and lower would then be still further increased, and 

 the intensity of the positive torsional response would be 



