706 TLANT RESPONSE 



organs, we find that its gradual evolution can be traced from 

 the simplest case, namely, the longitudinal response of a 

 radial organ. In such a radial organ, if one side be rendered 

 in any way the less excitable — say by the unilateral applica- 

 tion of cold or anaesthetics, or by the fatigue induced by 

 long-continued unilateral stimulation— we have an induced 

 anisotropy. On now subjecting the organ to diffuse stimula- 

 tion, the relatively more excited side will undergo the 

 greater contraction, and the response will thus be by that 

 movement which results from the concavity of the more 

 excited. We find many instances again of the various stages 

 through which this anisotropy passes before it culminates in 

 the dorsi-ventral pulvinus ; thus, for example, in a spirally 

 formed tendril, where concavity has been induced by the 

 unilateral excitation of that side, the concave surface is less 

 excitable than the convex ; and such a tendril, when 

 diffusely excited by strong electrical stimulus, exhibits the 

 excitatory effect by extraordinary writhing movements due 

 to the relatively greater excitation and concavity of the 

 originally convex side (p. 92). A plagiotropic stem, again, 

 whose upper side is fatigued by the action of sunlight, 

 exhibits on diffuse stimulation a downward movement, due 

 to the greater excitatory contraction of the more excitable 

 lower half (p. 86). The phenomenon of differential response, 

 then, whose various preliminary stages we have thus traced, 

 comes to its greatest perfection in the dorsi-ventral pulvinus 

 of such plants as Mimosa, for we have found that in the 

 last-named organ the characteristic responsive movement is 

 not brought about by the action of excitable cells restricted 

 to the lower half. That the upper half also is excitable is 

 shown by the fact that on applying to it localised stimulus, 

 say of light, its cells contract and raise the leaf (p. 63 1). The 

 fall of the leaf, on .the application of diffuse stimulation, is 

 thus the result of the greater excitability of the lower. This 

 fall, agajn, is not due to mere flaccidity caused by the ex- 

 pulsion of water from the excited organ, but to the actual 

 differential contraction of the lgwer half, whose activity may 



