402 LlE'E MOVEMENTS IN PLANTS 



responsive torsion, the direction of which is determine*! 

 by the direction of the incident stimulus. 



EFFECT OP DIFFERENTIAL EXCITABILITY ON THE 

 DIRECTION OF TORSION. 



Under normal conditions, the torsional response under 

 light places the upper surface of the leaf or leaflets at 

 right angles to light. That this movement is not due 

 to some specific sensibility to light is shown by the 

 fact that all modes of stimulation, chemical, thermal or 

 gravitational, induce similar responsive torsion. The tor- 

 sional response is, moreover, shown to be determined by the 

 direction of incident stimulus, and the differential excita- 

 bility of the organ. This latter may be reversed by the 

 local application of various depressing agents on the nor- 

 mally more excitable lower half of the pulvinus. Under 

 this treatment, the lower half of the pulvinus may be 

 rendered relatively the less excitable. Lateral applica- 

 tion of light now induces a torsional movement which is 

 the reverse of the normal, so that the upper surface of 

 the leaf moves away from light. The advantage of the 

 plant cannot, therefore, be the factor which determines 

 the directive movement ; the teleological argument often 

 advanced is, in any case, no real explanation of the 

 phenomenon. 



In all the instances given above, and under every 

 mode of stimulation, the responsive movement makes the 

 less excitable half of the pulvinus face the stimulus. The 

 torsional response is, in reality, the mechanical result of 

 the differential contraction of a complex organ, which is 

 fixed at one end and subjected to lateral stimulation. 

 I have been able to verify this, by the construction of 

 an artificial pulvinus consisting of a compound strip, the 

 upper half of which is ebonite, and lower half the more 



