CHAPTER XXVI 



GEOTROPISM 



No phenomenon of tropic movement appears so inexplicable 

 as that of geotropism. There are two diametrically opposite 

 responsive movements induced by the stimulus of gravity : 

 in the root a curvature downwards, and in the shoot 

 a curvature upw^ards. The seeming impossibility of ex- 

 plaining effects so divergent by a single fundamental reaction 

 to stimulation has led to the assumption that the irrita- 

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

 however, endeavour to show that this assumption is quite 

 gratuitous and unnecessary. 



Beginning with the simple case of a horizontally laid 

 shoot, the geotropic up-curvature can be explained by one or 

 other of the two suppositions : either (i) that the stimulus of 

 gravity induces contraction of the upper side, or (2) that 

 it induces expansion of the low^er side. The second of these 

 two assumptions has found more general acceptance. 



In the parallel phenomenon of phototropic curvature, 

 light incident on one side of the shoot induces local con- 

 traction and concavity of the directly stimulated proximal 

 side of the organ. Since light is visible there can be no 

 difficulty in ascertaining the exact direction of the incident 

 stimulus and the induced curvature by which the organ 

 tends to place itself parallel to the raj^s with its apex 

 towards the source of stimulation. But in geotropism 

 the stimulus is invisible, and there is no definite knowledge 

 available about its effective direction in induction of the 

 responsive curvature. 



