UNIVERSALITY OF SENSITIVENESS IN PLANTS 35 



gives rise simultaneously to (a) contraction of the cells, with 

 concomitant negative turgidity-variation ; (b) negative me- 

 chanical response, or fall of the leaf; and (c) galvanometric 

 negative variation. Had the leaf been physically restrained 

 by any means, the mechanical response would have been 

 prevented, although the negative turgidity-variation con- 

 comitant to excitation would have taken place just the same. 

 But this internal change would have been imperceptible, and 

 in that case we could still have detected the effect of excita- 

 tion by means of the electromotive response. As a matter 

 of fact it is found that the electrical response always takes 

 place in answer to effective stimulation, even in cases where 

 the mechanical response is rendered impossible. We thus see 

 that galvanometric negativity is a certain indication of the 

 excitatory contraction of a cell,, whether or not the effect of 

 such contraction be outwardly manifested by mechanical 

 movement. The detection of the state of excitation by the 

 electric test is thus unfailing, and of universal application. 

 By the employment of this electric mode of investigation, I 

 have shown that not sensitive plants alone, but every plant, 

 and also every organ of every plant, is excitable. 



True excitatory negative versus hydrostatic positive 

 variation. — It has been supposed that the galvanometric 

 negativity consequent on stimulation may be due to mechani- 

 cal movement of water in the tissue. But I have shown that 

 this cannot be the case. For while it is true that the pro- 

 duction of water-movement by sudden forcing of water into 

 a tissue does cause electrical variation, yet it must be noted 

 that the sign of this electrical change is always one of 

 galvanometric positivity, which is opposite to that of the 

 true excitatory response. The intensity of the true negative 

 electrical response, moreover, varies with the physiological 

 activity of the tissue, and is abolished with its death. The 

 electrical variation due to mere water-movement, however, 

 may take place even in a dead tissue, and is, as has been 

 said, of positive sign. 



If a piece of living tissue be subjected to direct stimula- 



d 2 



