PHOTOTROPIC TORSION 403 



contractile stretched Imlia-rubber ; if such a strip be held 

 securely at one end in a clamp, and if the lateral flank, 

 consisting half of ebonite and half of India-rubber, be 

 subjected to radiation, and record taken in the usual 

 manner, it will be found that a torsional response takes 

 place which is similar to that of the pulvinus of Mimosa. 

 The above experiment was devised to offer an explanation 

 of the mechanics of the movement. It should, however, be 

 Itorne in niiiul 'u\ this connection that the torsional 

 response of pulvinus is brought about by differential 

 pliysinlogical contraction of the organ, the movement being 

 abolished at death. 



From the results given above, we arrive at the follow- 

 ing : — 



LAWS OF TORSIONAL RESPONSE. 



1. An ANISOTROPIC ORGAN, WHEN LATERALLY EXCITED 



by any stimulus, undergoes torsion by which the 

 less excitable side is made to face the stimulus. 



2. The intensity of torsional response increases 



WITH THE differential EXCITABILITY ; WHEN THE 

 ORIGINAL DIFFERENCE IS REDUCED, OR REVERSED, THE 

 TORSIONAL RESPONSE UNDERGOES CONCOMITANT DIMINU- 

 TION OR REVERSAL. 



Having thus established the laws that guide torsional 

 response, I shall try to explain certain related phenomena 

 which are regarded as highly obscure. I shall also des- 

 cribe the application of the method of torsional response 

 in various investigations. 



COMPLEX TORSION UNDER LIGHT. 



The leaves of the so-called "Compass plants" exhibit 

 very complex movements, these being modified according to 



