HELIOTROPIC TORSIONAL RESPONSE 



659 



face the source of stimulus. In the present case (fig. 267) 



we see that the responsive torsion took place during the 



application of light, and that on the cessation of stimulus 



there was a positive after-effect, followed by recovery. If 



light were applied laterally on the 



opposite flank, the torsion would be 



found to take place in the opposite 



direction, the law which determines 



such movement being, as said before, 



that the less excitable is always 



turned towards the stimulus. In 



connection with this, we have to 



notice, in the first place, that the 



responsive torsion takes place not 



when the lamina, but when the 



pulvinus, is acted on laterally by 



light ; and, secondly, the particular 



torsion in question results from the 



differential action of stimulus, by its 



lateral application to a complex 



organ, the lower half of which is the 



more excitable. 



Torsional response to other forms of lateral stimu- 

 lation. — The supposition that this torsional response is due 

 to a specific sensibility to light, evolved for the advantage of 

 the plant, will be found entirely untenable if it can be shown 

 that the same movement is manifested under the same 

 conditions in response to other forms of stimulus. Thus, on 

 bringing a heated wire near that side which was previously 

 excited by light, I obtained exactly the same torsional 

 response of the same sign. Still more interesting is the 

 excitation of the same lateral side by chemical stimulus, say 

 a strong solution of common salt. In the record given 

 (fig. 268) we see how similar in every way are this response 

 and that evoked by light. The upward arrow (t) indicates 

 the application of chemical stimulus to the same flank as had 

 previously been stimulated by light, of which the record is 



U U 3 



Fig. 267. Torsional Re- 

 sponse of Leaf of Mimosa 

 when Laterally Stimu- 

 lated by Sunlight 



The ordinate gives the ab- 

 solute angular movement 

 in degrees. The dotted 

 line shows positive after- 

 effect and recovery on 

 stoppage of light. 



