PHOTOTAXIS 695 



more energetic than that forwards. The consequence of this 

 will be, in the vegetable organism as in the case of a man 

 swimming, its forward propulsion. This result depends upon 

 the fact that the lower half of the anisotropic organ is in this 

 case the more excitable. Reversal of the relative activities 

 of the two halves of the dorsi-ventral organ was, however, 

 seen to occur in the case of the leaflets of Desmodium 

 when the intensity of stimulation was very great. Such a 

 reversal, under excessive stimulation, would give rise then 

 to a swimming movement in the opposite direction. We also 

 saw such reversals under continuous stimulation of light 

 becoming periodic (fig. 276). The corresponding swimming 

 response would thus consist of a movement to and fro. 



Supposing, however, that the excitability of the upper 

 half of the motile organ had been the greater, it is clear that 

 the normal excitatory response would have taken the form of 

 a backward or negative swimming movement. We thus see 

 the possibility of normal responsive swimming movements of 

 two different types, according to the particular half of the 

 anisotropic motile organ which is the more excitable. Taking, 

 again, that type of swimming in which the response is posi- 

 tive or forward, a stronger intensity of stimulation may give 

 rise to a reversal, or negative movement. And from what 

 has already been said, it will be seen that similar responsive 

 movements may also be expected under forms of stimulation 

 other than light. 



Similarity between swimming responses and the 

 ordinary heliotropic responses of radial organs. — Though 

 at first sight it would appear as if there were no con- 

 nection between the simple responsive curvatures of radial 

 organs and the apparently complicated responsive move- 

 ments of swimming, yet on a closer analysis we shall find 

 that there is little essential difference between the two ; 

 for we have seen that growth itself, or growth-curvature, 

 is simply a phenomenon of multiple responsive movements, 

 which, owing to the rapidity of the individual responses, 

 appears continuous. Hence, when, under moderate stimula- 



