DIFFERENTIAL EXCITABILITY 255 



with the hands of the clock. Here also the direction of 

 stimulus is found to determine the direction of responsive 

 torsion. 



Experiment 138. Effect of lateral stimulation by thermal 

 radiation. — I next employed thermal radiation as the 

 stimulus ; the source of radiation was a length of electrically 

 heated platinum wire. It is advisable to interpose a shield 

 with a narrow horizontal slit, so as to localise the stimulus 

 at the junction of the upper and lower halves of the pulvinus. 

 The effectiveness of radio-thermal stimulus being great, 

 the response was very pronounced. Stimulus applied at 

 the right flank induced right-handed or clockwise torsion 

 (fig. 147) ; appHcation at the left flank gave rise to left- 

 handed or anti-clockwise torsion (fig. 148). 



Geotropic stimulus. — The stimulus of gravity induces 

 a similar responsive torsion which is determined by the 

 direction of the incident stimulus. This will be fully 

 described in a subsequent chapter. 



Effect of Differential Excitability on the 

 Direction of Torsion 



Under normal conditions the torsional response to light 

 places the upper surface of the leaf or leaflet at right angles 

 to the incident 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 and others — 

 induce similar responsive torsion. The torsional response 

 is determined not only by the direction of the incident 

 stimulus, but also by the differential excitability of the organ. 

 This latter may be reversed by the local application of various 

 depressing agents on the normally more excitable lower half 

 of the pulvinus. Under this treatment the lower half of 

 the pulvinus can be rendered relatively the less excitable. 

 Lateral stimulation by light is now found to induce a tor- 

 sional movement which is the reverse of the normal, so 

 that the upper surface of the leaf turns away from light. 



