THE PLANT AND THE ANIMAL 273 



galvanometer goes back : this is the negative variation 

 of the current. The same phenomenon takes place in 

 the case of the catchfly. At the moment when the 

 two parts of the leaf fold together a negative variation 

 in the current becomes apparent. Again a very- 

 small but always measurable interval of time passes 

 between the moment of irritation and the contraction 

 of a muscle : this is known as the latent period of 

 excitation. A similar though more considerable in- 

 terval is noticed in the case of the leaf between the 

 moments of irritation and movement. 



Thus the similarity between the movements of the 

 catchfly and of animals does not lie solely in the out- 

 ward manifestations of these movements but also in 

 the internal processes which accompany them. As 

 we have seen, it is true that the movements of vegetable 

 organs resolve themselves in the most obvious cases 

 into the exudation of water from the cells of the irritable 

 tissue overfilled with it, while the movements of animals 

 result from the contraction of muscles, a change in 

 their actual form ; but the contraction of muscles is 

 not an elementary phenomenon. Will it not likewise 

 reduce itself ultimately into the mutual rearrangement 

 of the elementary solid particles and liquids which 

 enter into the composition of the muscle ? ^ 



Thus the main wall of partition breaks down which 

 used to differentiate the vegetable kingdom from the 

 animal. Movement is not exclusively confined to 

 animals ; it also takes place among plants. But if 

 this distinction does not hold good, can we not find 

 something else in its place ? Let us consecutively 

 review the characteristics of the life of organisms, 



1 It is possible that the overfilling of cells with water, its sudden 

 exudation, the presence of a current and its variation — aU these details 

 of the process of motion in vegetable organs will find an explanation in 

 the phenomena of electro-difiusion. It would be out of place to enter 

 into greater details about them at present. 



S 



