PHOTOTROPIC TORSION 401 



of the left, the responsive torsion is found to be reversed, 

 the direction of movement being clockwise. Here also the 

 responsive movement is such that it is the less excitable 

 upper half of the organ that is made to face the stimulus. 

 It will thus be seen that the torsion, anti-clockwise or 

 clockwise, depends on two factors, namely the direction of 

 stimulus, and the differential excitability of the organ. 



EFFECT OF DIFFERENT MODES OF LATERAL STIMULATION. 



I shall now proceed to show that the torsional response 

 is induced not merely by the action of light, but by all 

 forms of stimulation. 



Effect of cliemical stimulation : E tperiment 150. — Dilute 

 hydrochloric acid was at first applied on the left flank 

 of the pulvinus along the narrow strip of junction of 

 the upper and lower halves. This gave rise to a respon- 

 sive torsion against the hands of a clock. Chemical 

 stimulation of the right flank induced, on the other band, 

 a torsional movement with the hands of a clock. Here 

 also the direction of stimulus is found to determine the 

 direction of responsive torsion. 



Effect of thermal radiatio?i : Experiment 151. — 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 narrow horizontal 

 slit, so as to localise the stimulus at the junction of the 

 upper and lower halves of the pulvinus. Stimulus applied 

 at the left flank induced left-handed or anti-clockwise 

 torsion ; application at the right flank gave rise to right- 

 handed torsion. 



Geotro]nc stimulus. — The stimulus of gravity induces, 

 as I shall show in a subsequent chapter, a similar 



