732 PLANT RESPONSE 



from the fact that, on localised cooling of the upper side, the 

 movement of the organ in response to gravity is abolished, 

 whereas cooling of the lower side has little or no effect on 

 the responsive movement. This experiment incidentally sup- 

 ports the view that it is the inner tangential wall of the 

 cells which is relatively effective in responding to the stimulus 

 of gravity. In turning to the geotropic response of the root, 

 on the other hand, we find that it is the distant tip which 

 is the perceptive region for gravitational stimulus. Hence 

 it is only the indirect effect of stimulus which acts on the 

 responding growing region. But we have seen that moderate 

 stimulation of the tip, by any form of stimulus whatsoever, 

 always induces a movement at the responding region, of 

 opposite sign to that which is the result of direct stimulation, 

 and from this the opposite geotropic responses of shoot and 

 root follow as a matter of course. This fact entirely negatives 

 the assumption that shoot and root are possessed of any 

 polar difference of sensibility, or that any specific geotropic 

 sensibility has been evolved in the radicle for the advantage 

 of the plant. 



Heliotropic action in radial organs. — We shall find 

 similarly, in studying the various movements of the plant in 

 response to heliotropic stimulus, that, diverse as they seem, 

 they are characterised by an underlying unity, being in fact but 

 so many expressions of the universal law that response takes 

 place by the contraction and concavity of the more excited. 



The fundamental effect of light was demonstrated by 

 showing that, in a growing organ, diffuse stimulation induces 

 a contraction and retardation of the rate of growth. This 

 was also shown to be true of all other forms of stimulation, 

 including those of thermal and electrical radiation. The 

 incidence of radiation may, it is true, cause a rise of tem- 

 perature ; and this would, as we know, have the effect of 

 enhancing the rate of growth. In order, therefore, to 

 discriminate the effect of radiation as such from that of 

 temperature, an experiment was described in which the 

 circumstances were so arranged that no rise of temperature 



