XXXIX.— GEOTROPISM 

 By 

 Sir J. C. BosE. 



No phenomenon of tropic movement appears so inex- 

 plicable as that of geotropism. There are two diametrically 

 opposite effects induced by the same stimulus of gravity, 

 in the root a movement downwards, and in the shoot a 

 movement upwards. The seeming impossibility of explain- 

 ing effects so divergent by the fundamental reaction of 

 stimulus, has led to the assumption that the irritability of 

 stem and root are of opposite character. I shall, however, 

 be able to show that this assumption is unnecessary. 



The difficulty of relating geotropic curvature to a detinite 

 reaction to stimulus is accentnated by the fact that the 

 direction of the incident stimulus, and the side which 

 responds effectively to it are not clearly understood ; nor 

 is it known, whether the • reaction to this stimulus is a 

 contraction, or its very opposite, an expansion. 



Taking the simple case of a horizontally laid shoot, 

 the geotropic up-curvature is evi<lently due to differential 

 effect of the stimulus on upper and lower sides of the 

 organ. The up-curvature may be explained by one or the 

 other of two suppositions : (1) that the stimulus of gravity 

 induces contraction of the upper side ; or (2) that the 

 fundamental reaction is not a contraction but an expansion 

 and this of the lower side. The second of these two 

 assumptions has found a more general acceptance. 



Tropic curvatures in general are brought about by the 

 differential effect of stimulus on two sides of the organ 



o2a 



