254 



CHAP. XXIII. PHOTOTROPIC TORSION 



two factors — namely, the direction of incidence of the 

 stimulus, and the simultaneous but differential contraction 

 of both halves of the organ. 



Effect of Different Modes of Lateral 



Stimulation 



I proceed to show that torsional response is induced 

 not merely by the action of light, but by other modes of 

 stimulation as well. 



Fig. 147. 



Fig. 148. 



[Fig. 147. Clockwise torsion under stimulation of right flank^ 



by thermal radiation. 



Fig. 148. Anti-clockwise torsion under stimulation by thermal 

 radiation applied to left flank. (Mimosa.) 



Experiment 137. Effect of chemical stimulation. — 

 Dilute hydrochloric acid was at first applied on the left 

 flank of the pulvinus along the line of junction of the upper 

 and lower halves. This gave rise to a responsive torsion 

 against the hands of the clock. Chemical stimulation of the 

 right flank induced, on the other hand, a torsional movement 



