RESPONSE OF ANIMAL AND VEGETAL SKINS 295 



I shall, however, be able to adduce facts and considera- 

 tions from which it will be possible to arrive at a simpler and 

 more conclusive explanation of these phenomena, on the 

 basis of the differential physiological excitability of the two 

 surfaces. I shall show, moreover, that the difference between 

 animal and vegetable protoplasm, thus assumed to exist, has 

 nothing to do with the question. 



We have seen that when the physiological activity of a 

 tissue is in any way impaired, its normal excitatory re- action 

 of galvanometric negativity is depressed. This may even go 

 so far as to cause an actual reversal of the response, to 

 galvanometric positivity, as we found in the case of depressed 

 tissues (p. 84). Taking a vegetable specimen, then, say a 

 hollow petiole or peduncle, we find that the outer surface, 

 which is habitually exposed to the manifold influences of the 

 environment, becomes histologically modified, being much 

 more cuticularised than the inner. Thus these outer and 

 exposed cells generally become reduced in size, and thick- 

 walled, with little protoplasmic contents. Hence, as regards 

 functional activity, these epidermal cells are in a physio- 

 logical sense very much degraded. We should then expect 

 their excitability to be proportionately lowered in comparison 

 with, say, the inner surface of the same tube, protected as 

 that has been from outside influences. And the variation of 

 physiological excitability thus induced may involve not only 

 the surface, but also the subjacent layers to a certain extent. 



Theoretically, then, the induced galvanometric negativity 

 of the outer would be less than that of the inner surface, 1 

 and simultaneous excitation of both, by whatever means 

 produced, should give rise to a resultant responsive current 



protoplasm is in major degree an instrument of analysis and emission, in minor 

 degree the seat of synthesis and accumulation. The vegetable, in most immediate 

 contact with inert things, combines, organises, and accumulates, The animal, 

 in less immediate contact with inert matter, disrupts, utilises, and dissipates 

 in their fragments organic compounds that it has received ready made from 

 other animals and from plants.' Waller, Signs of Life, p. 85. 



t 1 This refers to normal skin, and not to that in which the surface is typically 

 landular. 



