PHOTOTROPIC TORSION 409 



The responsive torsion is determined by the direction 

 of incident Htiniulus, and the differential excitability of two 

 halves of the organ, the torsion being such that the less 

 excitable half of the organ is made to face the stimulus. 



The twist exhibited by various leaves and leaflets under 

 light finds its explanation from the demonstrated laws of 

 torsional response. 



The direction of incident stimulus may be determined 

 from the responsive torsion of a dorsiventral organ. 



The Method of Torsional Balance enables us to compare 

 the excitatory efficiencies of two different stimuli which 

 act simultaneously on the two flanks of the organ. 



31 A 



