290 CHAP. XXVI. GEOTROPISM 



of obscuring analogies between phenomena. Until the exist- 

 ing terminology is revised, it would perhaps be advisable to 

 distinguish the geotropism of the shoot as Zenithotropism 

 and that of the root as Nadirotropism. 



Relation between the Directive Angle and 



Geotropic Reaction 



When the main axis of the shoot is held vertical, the 

 angle made by the surface of the organ with the lines of force 

 of gravity is zero, and there is no geotropic reaction. The 

 reaction increases with the directive angle ; theoretically, 

 the geotropic reaction should vary as the sine of the angle. 

 In Chapter XXXI, I will describe the very accurate electric 

 method which I have been able to devise for determining 

 the relative intensities of geotropic reaction at various angles. 

 Under perfect conditions of symmetry the intensity of 

 reaction is found actually to vary as the sine of the directive 

 angle. 



Differential Geotropic Excitation 



The geotropic excitability of a radial organ is the same on 

 all sides. It has been shown that when it is laid horizontal 

 it is the uppermost side that responds more effectively to the 

 geotropic stimulation. 



The two sides of a dorsiventral organ are unequally 

 excitable to different forms of stimuli, the lower side of 

 the pulvinus of Mimosa being far more excitable than the 

 upper side. Since the reaction to geotropic stimulation 

 is similar to that of other forms of stimulation, the lower 

 side of the pulvinus should exhibit a more pronounced 

 geotropic movement than the upper. 



Under ordinary circumstances the upper half of the 

 pulvinus is, on account of its favourable position, more 

 effectively stimulated by gravity ; in consequence of this, 

 the leaf assumes a more or less horizontal or ' dia-geotropic ' 

 position of equilibrium. But when the plant is inverted. 



