696 PLANT RESPONSE 



tion, the organ moves towards the light, or exhibits a positive 

 response, this means that the resultant of its multiple move- 

 ments is towards the stimulus, like the resultant movement 

 of the ciliated organism towards light. Similarly, under 

 strong photic stimulation, the negative heliotropic movement 

 of the organ corresponds to the swimming of the ciliated 

 organism away from light. In the intermediate case, again, 

 where the stimulated organ shows no resultant curvature, but 

 oscillates about a mean position, we have an instance which 

 is paralleled, in the case of the ciliated organism, by alter- 

 nate swimming backwards and forwards. 



Again, just as in the heliotropic response of a radial 

 organ, the minor pulsations by which it is brought about are 

 too rapid and minute to be easily detected, and we can per- 

 ceive only the resultant movement of the organ as a whole, 

 so in the case of the ciliated organism the individual beats 

 cannot easily be perceived, and we infer their presence from 

 the resultant motion of the organism as a whole. 



Phototactic movements. — From the fundamental de- 

 monstrations which I have already given of the character- 

 istics of multiple response, and its modification by relative 

 variations of contractility, as between the upper and lower 

 halves of the responding organ, it will be found that the 

 multifarious responsive movements of ciliated organisms, 

 under various forms of stimulus of differing intensities, will 

 have been elucidated. 



(a) Two natural types of responsive movements.- -The two 

 opposite types of movement, positive and negative, which we 

 have now theoretically anticipated, are found to be com- 

 pletely illustrated in the case of swarm-spores under the 

 action of light. Thus, for example, in the case of Botrydium 

 granulatum, they respond by a positive swimming move- 

 ment, or motion towards the light. Again, while certain 

 varieties of Ulothrix exhibit the positive effect, by movement 

 towards light, there is another variety which gives the nega- 

 tive response, by swimming away from it. This .opposition 

 of effects is obviously due, as we have anticipated, to a 



