552 COMPARATIVE ELECTRO-PHYSIOLOGY 



stimulus could not well have taken place. This will be clear 

 if we consider the case of a radial organ, such as the stem, 

 unilaterally acted on by light. Here a positive heliotropic 

 movement is induced, by which the growing organ is placed 

 in the most favourable position as regards illumination. The 

 peculiarity of this phenomenon lies in the responsive 

 contraction of the side acted on by stimulus, with consequent 

 concavity and curvature towards light. This heliotropic 

 movement continues until the organ has placed itself in the 

 direction of incident radiation. When this orientation has 

 become perfect there is no further movement, because the 

 proximal and distal sides are now equally stimulated. Had 

 the cortical tissue, on whose differential responsive action the 

 curvature depended, been as highly conducting as the 

 vegetable nerve, this particular-directioned movement would 

 have been an impossibility, for the stimulus, instead of 

 remaining localised on one side, would in that case have 

 become diffused, with the result of inducing antagonistic 

 effects on the proximal and distal sides, under which there 

 could have been no resultant curvature. Indeed this neutral- 

 ising action of conduction, in nullifying responsive curvature, 

 is seen even when unilateral stimulus is excessively strong. 

 For under these circumstances stimulus is conducted 

 transversely, through the imperfectly conducting tissue, with 

 the result of undoing the previous curvature. And it is 

 obvious that had the conductivity of the tissue been higher, 

 this neutralisation would have taken place, even under feeble 

 stimulation. 



I have also shown elsewhere that, in the responsive 

 movements of leaves, conduction through nervous elements 

 plays little or no part. For the blade of the leaf may be 

 acted on by light without showing any responsive movement. 

 Hence the lamina is not to be regarded as the perceptive 

 organ. The organ by which, on the contrary, the responsive 

 movements of the leaves are determined is the pulvinus or 

 pulvinoid. This is at once perceptive and motile. When 

 such an organ, then, is acted on directly and locally by light, 



