RESPONSE OF EPITHELIUM AND GLANDS 315 



effect was obtained as an after-effect of equi-alternating 

 shocks. I next took records of the direct effect of equi-alter- 

 nating shocks. The direction of the responsive current was 

 found, as before, from the inner to the outer. 



In the fruit of water-melon we obtain another specimen 

 whose interior cavity is filled with secretion. On making a 

 suitable preparation of this specimen, and arranging electrical 

 connections with the outer epidermal and inner secretory 

 surfaces, I found the respon'sive current, under equi-alter- 

 nating electrical shocks, to be from the inner secretory to the 

 outer epidermal. Fig. 193 gives a photographic record of 

 these responses. 



From the typical responsive effects thus obtained with 

 vegetable specimens under the simplest conditions, we are 

 enabled to see that the effect of 

 localised stimulus depends on 

 the characteristic response of 

 the surface layers of the organ. 

 When dealing with this question 

 of the electrical reaction of epi- 

 thelium and glands in animal 

 tissues, Biedermann rightly 

 came to the conclusion that it 



Was the surface epithelial layer Responsive current from internal 



which was, in an electro-motive 11^ l CXternal epidermal 



sense, most effective, the term, 



in its widest sense, including the epithelium of glands and 



papillae. 



One complicating factor present in the electrical reactions 

 of animal epithelia and glands, but relatively absent under 

 the simpler conditions of the plant, is the effect of injury caused 

 by the process of isolation. The very fact of making the neces- 

 sary section involves a stimulus of great intensity, and unless 

 the effect of this has thoroughly subsided, the after-effect of 

 such stimulation may be so strong as to reverse the normal 

 current of rest, and otherwise modify the excitability of the 

 tissue, This is seen, for example, in an experiment carried 





