SUMMARY 291 



the more excitable lower half of the organ then occupies 

 the favourable position for geotropic excitation. The leaf 

 now erects itself till it becomes almost parallel to the stem 

 (r/. fig. 179). The response of the same pulvinus which 

 was formerly ' dia-geotropic ' now becomes ' negatively 

 geotropic ' ; but an identical organ cannot possess two 

 different specific sensibilities. The different effects in the two 

 positions are in reality due to differential excitability of the 

 two sides of the dorsi ventral organ. 



It has also been explained that when the pulvinus of 

 Mimosa is subjected to lateral stimulation of any kind, it 

 undergoes torsion, in virtue of which the less excitable 

 half of the organ is made to face the stimulus. Experi- 

 ments \vill be described (Chapter XXVIII) which will show 

 that geotropic stimulation induces similar torsional response. 

 The results obtained from this method of inquiry give 

 independent proof (i) that the lower half of the pulvinus 

 is geotropically the more excitable, and (2) that the 

 direction of incident geotropic stimulus is that of the 

 vertical lines of force of gravity w^hich impinge on the upper 

 side of the organ. 



Summary 



The stimulus of gravity is shown to induce an excitatory 

 reaction w^hich is similar to that induced by other forms of 

 stimulation. The immediate effect of geotropic stimulation 

 on a horizontal growing organ is an incipient contraction 

 and a retardation of the rate of growth of the upper side 

 on which the stimulus is incident. This is followed by an 

 up-curvature. 



That the upper side of the stem undergoes excitatory 

 contraction is demonstrated not only by the concave 

 curvature of that side, but also by the excitatory reaction 

 of galvanometric negativity of that side (p. 315). 



Tropic reactions are said to be positive when the directly 

 stimulated side undergoes contraction, with the result that 



