REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 707 



be gauged by the tension it can be made to exert on a 

 spiral spring (p. 26). As regards mechanical response, then, 

 a single law — that response is always by concavity of the 

 more excited side — will be found universally applicable. In 

 a dorsi-ventral organ, like the pulvinus of Mimosa, we have 

 seen that response to diffuse stimulation is always by the 

 contraction of the more excitable lower half. The same 

 law, however, is also applicable even in the case of a radial 

 organ excited unilaterally, for here the side acted on is 

 relatively the more excited, and response is by concavity of 

 that side. 



A radial organ, acted upon from one side, undergoes 

 concavity of that side, and consequent movement towards 

 the stimulus. The same is true of pulvinated organs, when 

 either the upper or lower half is acted upon locally by 

 stimulus. As in radial organs, then, so also here, under these 

 circumstances, we obtain instances of the directive action of 

 stimulus ; but when stimulus is either internally or externally 

 diffused, we obtain from a dorsi-ventral pulvinus the greater 

 contraction of the more excitable half, and movement is 

 thus made, for anatomical reasons, to occur in the direction 

 at right angles to the plane which separates the two aniso- 

 tropic halves. 



Even in Mimosa, the contraction of the pulvinus itself is 

 not very great, but the contractile movement is highly 

 magnified by the attached petiolar index. When a radial 

 organ is diffusely stimulated there is no lateral movement at 

 all, and from this fact it has been erroneously inferred that 

 ordinary plants are not sensitive. In fact, however, every 

 plant is sensitive, and exhibits contractile response, which 

 is shown, in the case of radial organs, by longitudinal con- 

 traction under diffuse stimulus, and by lateral response under 

 unilateral stimulus. 



The direct effect of stimulus on the pulvinus of Mimosa, 

 causing a negative turgidity-variation of the organ, finds 

 appropriate expression in the responsive movement of fall. 

 But we have seen that the expression of the indirect effect of 



z z 2 



