GEOTROPIC TORSIONAL RESPONSE 663 



3. An organ which, under lateral stimulation, responds by 

 torsion, is always physiologically anisotropic, and the side 

 which is made to face the stimulus is the less excitable. 



Demonstration of differential geotropic excitation in 

 a dorsi-ventral organ. — From this experimental demon- 

 stration, then, we have obtained a new means of discrimi- 

 nating the relative excitabilities of the two halves of an 

 organ, since that side which is turned by the responsive 

 movement to face the given stimulus is relatively the less 

 excitable to it. It will be remembered that we found that 

 the reason why certain dorsi-ventral organs showed a tendency 

 to assume a horizontal position under the action of vertical 

 light was not that they possessed dia-heliotropic sensibility, 

 but was due to the differential excitability of the two 

 halves under stimulus in general, including that of light. 

 Again, in the case of the action of gravity, it is found that 

 such organs exhibit a similar tendency to place themselves 

 horizontally ; and the assumption of a specific dia-geotropic 

 sensibility is not necessary, if it can be shown that the upper 

 and lower sides are unequally sensitive to this stimulus 

 also. We may first take an ideally simple case. We have 

 seen that in a radial apogeotropic stem, when laid hori- 

 zontally, it is only the upper half that is effectively 

 stimulated by geotropic action ; and there was reason to 

 believe that this was due to the fact that it was the inner 

 tangential wall of the upper side — in contrast to the less 

 excitable outer wall of the lower side — that was excited by 

 the statolithic or other influence of weight (pp. 495, 503). If 

 now, for any reason, the excitability of the upper half of the 

 horizontally placed radial organ be abolished, the geotropic 

 response of that effective half will disappear, and the organ 

 will remain horizontal, as if unaffected by stimulus of gra- 

 vity. This state of things we have already realised, when 

 the excitability of the upper half was artificially diminished 

 by the local application of cold, and geotropic response was 

 seen to be arrested (p. 504). Now, a horizontally placed 

 radial organ which has been rendered unequally excitable 



