vi ELECTROMOTIVE ACTION IX VEGETABLE CELLS 27 



galvanic effects of excitation at the actual point excited. Yet an 

 experiment, in which excitation and leading-off are absolutely 

 simultaneous, would encounter almost insuperable difficulties, not 

 the least being to determine exactly the real commencement of 

 the variation. 



Burdon-Sanderson's assumption that the electrical variation 

 does not coincide with the beginning of excitation, may be met 

 with the same arguments that were brought forward above, in 

 discussing the relations between excitation and contraction in 

 muscle. If the galvanic alteration is really the expression of 

 the chemical change induced by the stimulus, it must commence 

 simultaneously with it, i.e. at the moment when the stimulus 

 takes effect. 



The preceding observations show beyond doubt that the 

 primary phase of the excitatory variation accompanies and is the 

 direct consequence of excitation of the protoplasm of the excitable 

 parenchyma cells of the leaf comparable throughout its manifest- 

 ation with the galvanic effects of excitation in excitable animal 

 tissues, i.e. the negative variation of muscle, nerve, or gland 

 currents. It is therefore to be expected that the time-relations 

 between the galvanic and mechanical effects in the leaf of Dioncea 

 would be similar to those exhibited in muscle always remember- 

 ing the different causes of movement in the two cases. 



In the first place it is evident that the excitatory variations 

 of the " current of rest " are quite independent of the coarsely 

 perceptible movements of the lobe, and are demonstrable 

 both on the fixed and open leaf, and on that which is entirely 

 closed. The next point, as Burdon - Sanderson has correctly 

 pointed out, is to ascertain " whether the interstitial movement 

 of the fluid discharged from the cells on excitation (which in 

 all excitable plant-organs is the active cause of the change of 

 form) may not occur without any perceptible change in the 

 curving of the lobe, no matter how fine the means of observation 

 employed." This entails the presumption that every excitation 

 implying an electrical alteration must also cause a perceptible 

 discharge of water from the excited cells. That there is further 

 a perceptible interval between the moment of excitation and the 

 consequent closure of the leaf of Dioncea may as a rule be verified 

 from direct observation, since, when the temperature is not too 

 high, the mechanical latent period is actually macroscopic, e.g. at 



