312 CHAP. XXVIII. GEOTROPIC TORSION 



It is the upper side of the organ that is directly 

 stimulated by geotropic action. 



The effects of gravity and of light are algebraically 

 summated when in simultaneous action. Light may be 

 made to act in opposition to the stimulus of gravity. By 

 suitable adjustment of the intensity of light, the two torsions 

 can be exactly balanced. 



This state of balance is upset by any slight variation in 

 the intensity of the opposing photic stimulation. 



The relative modification of geotropic and phototropic 

 reaction by an external agent can be determined by the 

 resulting upset of the Photo-Geotropic Balance. 



