288 CHAP. XXVI. GEOTROPISM 



summation of the effect of geotropic with that of photic 

 stimulation whose direction is known ; (2) the relation 

 between the directive angle and geotropic reaction ; and 

 (3) the torsional response under geotropic stimulation. 



Effect of Algebraical Summation 



Experiment 158. — A flower-bud of Crinum is laid hori- 

 zontally and a record taken of its geotropic movement. On 

 application of light on the upper side (at L) the responsive 



C^ 



Fig. 169. Fig. 170. 



Fig. 169. Stimulus of light or gravity, represented by arrow, 

 induces up-curvature as shown in dotted figure (Crinum). 



Fig. 170. Additive efEect of photic stimulus. 



The first, third, and fifth parts of the curve give the normal 

 record under geotropic stimulation. Rate of up-curvature 

 enhanced under superposition of stimulus of light, l, acting 

 from above. 



upward curvature is enhanced, proving that gravity and 

 light are concordant in their effects. On the cessation of 

 light, the original rate of geotropic curvature is restored 

 (fig. 170). Apphcation of light of increasing intensity from 

 below induces, on the other hand, a diminution, neutrahsa- 

 tion, or reversal of the geotropic curvature, 



