GEOTROPISM 431 



The spasmodic fall of the leaf towards evening is not due, 

 as has been suggested, to the increased mechanical moment 

 caused by the forward position of the sub-petioles, for after 

 removal of the sub-petioles the same abrupt fall of the 

 petiole occurs in the evening (p. 272). The evening fall 

 of the leaf is shown to be a post-maximum after-effect of 

 light, which causes an ' overshooting,' so that the leaf falls 

 to below the position of equilibrium (p. 278). 



Geotropism 



The obscurity surrounding the reaction to geotropic 

 stimulation arises from lack of definite knowledge in regard 

 to the exact direction of the incident stimulus, and as to 

 the character of the response, whether it is contraction or 

 expansion. The direction of the incident stimulus has been 

 definitely determined in two different ways : (i) by ob- 

 serving the effect of superposition of the stimulus of visible 

 light on the geotropically stimulated shoot ; and (2) by 

 observing the effect of geotropic stimulation in inducing 

 torsion in the pulvinus of Mimosa, the less excitable side of 

 the organ moving so as to face the stimulus. The results of 

 both lines of inquiry are fully concordant, proving that the 

 direction of incident geotropic stimulus coincides with the 

 vertical lines of force of gravity. According to the first 

 method, the geotropic up-movement is enhanced by light 

 acting from above, geotropic and phototropic effects being 

 concordant. Light acting from below induces diminution, 

 neutralisation, or reversal of the geotropic movement, the 

 two forces being now in opposition (p. 288). A similar 

 conclusion is arrived at by the method of geotropic torsion 

 (p. 308). This evidence, that in a horizontal shoot the upper 

 side becomes stimulated by gravity, is confirmed by the 

 excitatory reaction of that side which is an induced electro- 

 motive variation of galvanometric negativity (p. 315). 

 The electric response of the lower side is one of galvano- 

 metric positivity indicative of enhancement of turgor and 



