GEO-ELECTRIC RESPONSE 439 



under the action of gravity, undergoes excitatory contraction. 

 It follows that, as an indirect effect of such contraction, the 

 water expelled from the upper will reach the lower side of 

 the shoot, and cause an increased turgidity and expansion of 

 that side. The apogeotropic curvature is thus brought about 

 directly by the contraction of the upper, and indirectly by the 

 expansion of the lower, side of the horizontally-laid shoot. 

 This being so, we may expect, in accordance with our pre- 

 vious investigations on the true excitatory and hydrostatic 

 effects respectively, that that side will become galvano- 

 metrically negative which is excited by geotropic stimulus. 

 And it is found that it is in fact the upper side of the hori- 

 zontally-laid shoot which exhibits galvanometric negativity ; 

 the lower, in which the positive turgidity-effect had been 

 indirectly induced, being found to show galvanometric posi- 

 tivity. 



But this result is only obtained simultaneously with the 

 induction of the normal apogeotropic curvature. And, before 

 this, other disturbing effects may occur. We take an erect 

 stem and make two galvanometric contacts, one on each side 

 of its growing region. The stem is next laid horizontally. 

 As regards the electrical effects which are now exhibited 

 three distinct phases are to be distinguished. First, in con- 

 sequence of the mechanical disturbance due to the laying of 

 the organ on one side, there will be an excitatory electrical 

 effect on both upper and lower sides, and the resultant re- 

 sponsive current will be determined by the difference of 

 excitability which may happen to exist, in this particular 

 individual, between the two. This variable stage is termi- 

 nated in the course of about ten minutes. We next have a 

 steady condition, brought about by the effects of tension and 

 compression already described as acting on the upper and 

 lower sides respectively. This gives rise, as we have seen, to 

 increased growth, with its attendant positive turgidity, on the 

 .upper as compared with the lower side. 



Hence, during this stage, when the curvature is proceed- 

 ing downwards, we may expect that the upper side will 



