PHOTOTROPIC TORSION 407 



taken of the movement of the leaf in a vertical plane. But 

 the responsive up-movement, induced by light acting from 

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

 torsional response, the leaf rests on the hooked glass 

 support and the movement is thus free from the compli- 

 cating factor of the weight of the leaf. Again the pulvinus 

 of Mimosa, for example, is sometimes subject to spontaneous 

 variation of turgor, on account of which it exhibits an auto- 

 nomous up or down movement. In the ordinary method of 

 record the true response to external stimulus may thus be 

 modified by 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 later&.H 

 stimulus is unaffected by the spontaneous movement of the 

 leaf. The torsional method, moreover, opens out possibilities 

 of inquiry in new directions, such as the comparison of 

 the excitatory effects of different stimuli by the Method of 

 of Balance, and the determination of the effective direction 

 of geotropic stimulus. 



THE TORSIONAL BALANCE. 



A beam of light falling on the left flank of the pulvi- 

 nus of Mimosa induces a torsion against the hands of the 

 clock. A second beam falling on the right flank opposes 

 the first movement ; the resultant effect is therefore 

 determined by the effective stimulation of the two flanks. 

 The pulvinus thus becomes a delicate index by which 

 two stimuli may be compared with each other. The follow- 

 ing experiment is cited as an example of the application of 

 the method of phototropic balance. 



Experimetit 155. — Parallel beam of light from a small 

 arc lamp passing through blue glass falls on the left flank 

 of the pulvinus ; a beam of blue light also strikes the 



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