GEOTROPISM 4:>7 



cessation of light, tlif original rate of geotropic movement 

 is restored (Fig. 163). Application of light of increasing 

 intensity from below induces, on tlie other hand, a diminu- 

 tion, neutralisation, or reversal of geotropic movement. 



Light acting vertically from above induces a concavity 

 of the excited upper side in consequence of which the 

 organ moves, as it were, to meet the stimulus. The geo- 

 tropic response is precisely similar. In figure 162 the 

 arrow represents the direction of stimulus which may be 

 rays of light or vertical lines of gravity. 



ANALOGY BETWEEN THE EFFECTS OF STIMULUS OF 

 LIGHT AND OF GRAVITY. 



In geotropic curvature we may for all practical purposes 

 regard the direction of stimulus as coinciding with the 

 vertical lines of gravitj'. The analogy between the effects of 

 light and of gravity is very close* ; in both the induced 

 curvature is such that the organ moves so as to meet the 

 stimulus. This will be made still more evident in the 

 investigations on torsional geotropic response describetl in a 

 subsequent chapter. The tropic curve under geotropic 

 stimulus is similar to that under photic stimulus. The 

 tropic reaction, both under the stimulus of light and of 

 gravity, increases similarly with the ' directive ' angle. 

 These real analogies are unfortunately obscured by the use 

 of arbitrary terminology used in description of the geotropic 

 curvature of the shoot. In figure 163 records are 

 given of the effects of vertical light and of vertical 

 stimulus of gravity, on the responses of the horizontally 

 laid bud of Crimim. In both, the upper side undergoes 

 contraction and the movement of response carries the 



■ Exception to this will be found in page HHi-i. where esplanatiori is offered for the 

 difference. 



