284 CHAP. XXVI. GEOTROPISM 



to the active responsive expansion of the lower side. The 

 crucial test of excitatory reaction under geotropic stimula- 

 tion is furnished by the geo-electric response. When a shoot 

 is displaced from the vertical to the horizontal position, 

 the upper side of the organ is found to undergo an excitatory 

 electric change of galvanometric negativity indicative of 

 diminution of turgor and contraction (p. 315). The tropic 

 effect of geotropic stimulation is thus similar to that of any 

 other mode of stimulation, i.e. a contraction of the directly 

 stimulated side, which in the present case is the upper side. 

 The vertical lines of force of gravity impinge on the upper 

 side, and the effective direction of geotropic stimulus is 

 therefore the same as the vertical lines of force indicated by 

 the movement of falling bodies from above to the centre of 

 the earth. The effective direction of geotropic stimulus 

 inferred from the above considerations is fully confirmed 

 by experimental results (p. 308). 



It has been explained that excitatory electric response is 

 manifested even in the absence of mechanical expression of 

 excitation ; and under geotropic stimulation a firmly held 

 shoot gave the response of a galvanometric negativity of 

 the upper side (p. 317). Hence the fundamental reaction to 

 geotropic stimulation is excitatory contraction, as under other 

 modes of stimulation. 



Determination of the Latent Period 



' As regards the interpretation of the record of geotropic 

 movement, it should be borne in mind that after the per- 

 ception of stimulus a certain time must elapse before the 

 induced growth- variation will result in curvature. There 

 is again another factor which causes delay in the exhibition 

 of true geotropic up-movement : the up-movement of 

 a shoot, in response to the stimulus of gravity, has to over- 

 come the opposition offered by its weight. On account 

 of the bending due to weight there is a greater tension on 

 the upper side, which, as already mentioned (p. 60), enhances 



