636 TLANT RESPONSE 



retaining permanently the induced polarity. Tropceolum 

 majus, on the other hand, behaves like soft iron, assuming a 

 definite but temporary polarity, which disappears when the 

 influence of light, like that of the magnet, is withdrawn. This 

 illustration, however,, though vivid, is likely to be misleading, 

 for it suggests that, under strong stimulus, the normal pro- 

 perties of the organ pass into a polar or opposite condition. 

 But there is no such change. Light merely induces a 

 difference of excitability in the two sides. That there is no 

 reversal of heliotropic sensibility is shown by the fact that 

 either side, when excited by moderate intensity of stimulus, 

 gives positive response. The necessary condition for the 

 exhibition of negative heliotropic response is not only the 

 differential excitability of the two sides of the organ, but also 

 the internal diffusion of stimulus. 



Owing, however, to the permanence of this differentiation 

 in a pulvinated organ, there will be certain conditions under 

 which the action of light will appear to be non-directive, 

 for it is the diffuse stimulus, however produced, that brings 

 about the responsive concavity of the more excitable lower 

 half of the pulvinus, and this internal diffusion will take 

 place just the same, whatever be the flank of the organ on 

 which the strong external stimulus may have been applied. 

 In the case of the radial organ, on the other hand, the 

 differentiation between the less excitable proximal and more 

 excitable distal is not fixed, but changes according to the 

 side acted upon at the time by light. 



The different responsive movements induced by light — 

 positive, negative, dia-heliotropic, and para-heliotropic — have 

 hitherto been ascribed, as I have already had occasion to 

 point out, to as many specific sensibilities possessed by the 

 plant. They have now, however, been demonstrated to be 

 but so many examples of the general law that plant-organs 

 respond to stimulus, by the induced concavity of the relatively 

 more excited. 



There is thus no fundamental discontinuity between the 

 responses of radial and of dorsi-ventral organs, or between 



