738 TLANT RESPONSE 



The fact that it is the differential excitability of the 

 organ which under lateral stimulation causes this torsional 

 movement was further demonstrated, when the difference 

 was artificially increased by the local application of chloro- 

 form to the upper half of the pulvinus. The torsional 

 response was then found to take place, with a corresponding 

 enhancement of rate, in the same direction as before. But 

 when this natural difference of excitability was reversed, by 

 the abolition through local application of chloroform of the 

 excitability of the lower half of the pulvinus, the direction 

 of the responsive torsion was found to undergo reversal. 



By carrying out a similar series of experiments, with 

 special reference to the lateral action of gravitational 

 stimulus on a dorsi-ventral organ, it was shown that such 

 an organ as a whole exhibited neither a positive nor a 

 negative, but a differential geotropic action. The investiga- 

 tion showed that the upper half of a pulvinus was less 

 excitable than the lower half under geotropic stimulus. An 

 artificial increase of the existing difference between the 

 excitabilities of the two halves enhanced the rate of the 

 normal torsional response, and the reversal of these natural 

 excitabilities reversed the direction of the torsional response 

 to geotropic stimulus (p. 664). 



Phototactic movements. — A leaflet of Desmodium in a 

 state of standstill resumes its pulsatory beats when stimu- 

 lated by light. Owing to the anisotropy of the motile 

 organs, one half of the beat is more rapid than the other. 

 Too strong an intensity of light, however, by causing greater 

 fatigue of the more excitable half of the organ, may cause a 

 reversal of the relative rapidities of the up and down beats. 

 In Desmodium, under the continuous stimulation of strong 

 light, these reversals are often recurrent. The downstroke, 

 which was at first the quicker, becomes less quick than the 

 upstroke, and this reversal may take place again and again, 

 in alternation with its opposite. These effects, seen in a 

 pair of anisotropic motile organs in Desmodium, afford an 

 explanation of the swimming movements of certain ciliated 



