440 LIFE MOVEMENTS IN PLANTS 



The leaf now erects itself till it becomes* almost parallel 

 to the stem. The response of the same pulvinus which 

 was formerly " dia-geotropic " now becomes " negatively geo- 

 tropic " ; but an identical organ cannot be supposed to 

 possess two different specific sensibilities. The normal 

 horizontal position assumed by the leaf i-<, therefore, due 

 to differential geotropic excitabilities of the two sides of a 

 dorsiventral organ. 



I have explained (p. 401) that when the pulvinus of 

 Mimosa is subjected to lateral stimulation of any kind, it 

 undergoes a torsion, in virtue of which the less excitable 

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

 ments will be described in a subsequent chapter which 

 show that geotropic stimulus also induces similar torsional 

 response. The results obtained from this method of enquiry 

 give independent proof : (1) that the lower half of the 

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

 the direction of incident geotropic stimulus is the vertical 

 line of gravity which impinges on the upper surface of 

 of the organ. 



SUMMARY. 



The stimulus of gravity is shown to induce an excita- 

 tory reaction which is similar to that induced by other 

 forms of stimulation. The direct effect of geotropic stimu- 

 lus is an incipient contraction and retardation of rate of 

 growth. 



The upper side of a horizontally laid shoot is more 

 effectively stimulated than the lower side, the excited 

 upper side becoming concave. Electrical investigation also 

 shows that it is the upper side that undergoes direct 

 stimulation. 



