6lO PLANT RESPONSE 



Jieliotropic effect, thus conclusively proving that the induction 

 of these three effects is not due to the presence of three 

 distinct and characteristic sensibilities. These different 

 responsive movements are thus traced to a single pheno- 

 menon of contractile response. 



This may be made equally clear from another point of 

 view, as illustrations namely of the general law that response 

 takes place by the contraction of the more excited, or the 

 relatively more excited. With moderate stimulation it is the 

 proximal side of the organ that is excited, and, becoming 

 concave, gives rise to a positive heliotropic curvature. On 

 the application of somewhat stronger stimulation, however, 

 when excitation is transmitted to the distal side, there is a 

 case in which the excitation of the proximal and distal are 

 equal, and the differential excitation being thus zero, there is 

 no resultant response. The organ, standing thus at right 

 angles to the light, and moving neither towards nor away 

 from it, will now appear to be either dia-heliotropic or else 

 irresponsive to heliotropic stimulus. 



But under still stronger stimulation two effects are in- 

 duced simultaneously: (i) a physiological anisotropy, by the 

 fatigue and loss of excitability of the proximal, in conse- 

 quence of which the distal becomes the more excitable ; and 

 (2) the internal diffusion of the stimulus, which, now acting 

 on the physiologically anisotropic organ, induces concavity of 

 the distal, that is to say a negative heliotropic curvature. 



1 shall now proceed to give further examples of that 

 transverse transmission of excitation, in consequence of which 

 an organ appears either to be irresponsive to heliotropic 

 stimulus or to give negative response. 



Organs apparently insensitive to light. — Many vegetable 

 organs exhibit no resultant movement under the action of 

 light, and are therefore supposed to be insensitive to it ; but 

 this inference could be accepted only on the theory of a 

 specific sensibility, in which case some organs would be 

 without it, while in others it would be characterised by 

 certain inherent positive or negative properties. We have 



