258 CHAP. XXIII. PHOTOTROPIC TORSION 



The responsive torsion is determined by the direction of 

 the incident stimulus, and by 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 in the demonstrated laws of 

 torsional response. 



The direction of an incident stimulus may be determined 

 from the responsive torsion of a dorsiventral organ. 



The Method of Torsional Balance permits of comparison 

 of the excitatory efficiency of two different stimuli which 

 act simultaneously on opposite flanks of the organ. 



