508 LIFE MOVEMENTS IN PLANTS 



from outside, induces phototropic change earlier than 

 variation in geotropism. 



SUMMARY. 



Under lateral action of geotropic stimulus, a dorsiven- 

 tral organ undergoes torsional response by which the less 

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



The direction of incident geotropic stimulus is the same 

 as the direction of vertical lines of gravity. Under geo- 

 tropic stimulus it is the upper side of the organ that 

 undergoes effective stimulation. 



The effects of gravity and of light become algebrai- 

 cally sumniated under their simulianeous action. Light may 

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

 By suital)le adjustment of the intensity of light, the two 

 torsions become exactly balanced. 



This state of balance is upset by any slight variation in 

 one of the opposing stimuli. 



The relative modification of geotropic and phototropic 

 excitabilities by an external agent, is determined by the 

 resulting upset of the photo-geotropic balance. 



