PHOTO-GEOTROPIC BALANXE 



309 



intensities gives rise to a partial balance, to an exact balance, 

 or to an overbalance. 



Fig. 185. 



Fig. 186. 



Fig. 185. Additive effect of stimulation by gravity g, and by 

 light L. Application of light at l increases torsional response. 

 Removal of light (l in circle) restores original geotropic 

 torsion. 



Fig. 186. Algebraical summation of geotropic and phototropic 

 action. Light applied below at-L opposes geotropic action. 

 Cessation of light (-l within a circle) restores geotropic 

 torsion. 



Balance of Geotropic by Phototropic Reaction 



Experiment 171. PJwto-Geotropic Balance. — I describe 

 in detail the procedure for obtaining an exact balance. A 

 parallel beam of light from a small arc-lamp is reflected by 

 means of an inclined mirror, so as to act on the junction of 

 upper and lower halves of the pulvinus from below. An iris- 

 diaphragm regulates the intensity of incident light. The 

 first part of the curve is the record of normal geotropic 

 torsional movement. Light of a given intensity was apphed 

 from below at a point marked -L (fig. 186) ; this is seen 

 to produce an overbalance, the phototropic effect being 

 slightly in excess. The intensity of the incident light was 



