GEOTROPIC TORSION 



505 



applied at L : the rate of movement is seen to become 

 greatly enhanced. Light is next cut off, and the en- 

 hanced rate induced by it is also found to disappear, the 

 response-curve being now due solely to geotropic action. 

 The etfect of geotropism in opposition to phototropism 

 will be found in the following experiments, where the 

 opposing action of light of different intensities is seen to 

 give rise to a partial, to an exact, or to an over-balance. 



BALANCE OF (iEOTROPIC BY PHOTOTROPIC ACTION. 



PliDto-geotropic balance : Experiment 195. —I shall here 

 describe in detail the procedure for obtaining an exact 

 l)alance. A parallel beam of light from a small arc lamp is 



Fig. 180. Fig. 181. 



Fin. 180. — Additive effect, of stituulus of gravity G, and of Hj^ht L. Ap. 

 plication of light at — L increases torsional lesponse. Removal of light restores 

 original geotropic torsion. 



Fig. 181. — Algebraical summation of geotropic and phototropic actions Light 

 ai)plied below at — L, oppose? geotropic action. Cessation > f light restores geo- 

 irojiic torsion. Cessation of light is indicated by L within a circle. 



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

 pulvinus below. An iris diaphragm regulates the intensity 



Of- . 



ot A 



