256 CHAP. XXIII. PHOTOTROPIC TORSION 



Teleological advantage cannot, therefore, be the determining 

 factor which causes the directive movement. 



In all the instances given above, and under every mode 

 of stimulation, the responsive movement is such as to cause 

 the less excitable half of the pulvinus to face the stimulus. 



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 diminution or reversal. 



Advantage of the Method of Torsional Response 



The experimental study of torsional response not only 

 opens a new line of inquiry into the reactions of the plant 

 to various stimuli, but it also possesses certain special 

 advantages. For instance, in investigations on the response 

 of the leaf of Mimosa to light by the ordinary method, the 

 responsive movements in a vertical plane are recorded. 

 The responsive up-movement, induced by light acting from 

 above, is, however, opposed by the weight of the leaf. But 

 in the torsional response, where the leaf is held by a hooked 

 glass support, the movement is free from the complicating 

 factor of the weight of the leaf. The pulvinus of Mimosa, 

 again, occasionally exhibits an autonomous pulsation ; in 

 the ordinary method of record the true response to external 

 stimulation may thus be modified by the natural movement 

 of the leaf. But in the torsional method the autonomous 

 up or down movement is restrained by the hooked support, 

 and the response to lateral stimulation is unaffected by the 

 spontaneous movement of the leaf, The torsional method, 



