COMPARATIVE ELECTRO-PHYSIOLOGY 



epidermal cells are, generally speaking, the least, and the 

 secreting cells the most, excitable. 



I shall show, moreover, that all the responsive character- 

 istics of these secretory cells are to be found repeated in those 

 admittedly glandular layers which occur in the lining of the 

 pitcher of Nepenthe, and cover the upper surface of the leaf of 

 Drosera. Before, however, entering upon the consideration of 

 these highly differentiated organs of Nepenthe and Drosera, 

 which are further characterised by some of the digestive 

 functions, I shall first discuss in detail the reactions of a 

 simpler type of vegetable organ. This is exemplified by 

 a single unripe carpel of Dillenia indica, already referred to. 

 When this is carefully removed from the inside of the 

 pseudocarp, and opened, the inside is found to be filled with 

 a gelatinous secretion. This is gently removed, and electrical 

 connections are made with the inner and outer surfaces. It 

 must be borne in mind that these vegetable organs, being not 

 highly excitable, admit of experimental preparations being 

 made with little or no excitatory effect of injury. Allowing 

 now for a period of rest after making the preparation, it will 

 always be found that the current of rest is from the outer 

 layer to the secreting inner layer, which latter is thus, 

 relatively speaking, galvanometrically positive. From the 

 fact which we have generally observed, that the natural 

 current of rest is from the less to the more excitable, it would 

 appear, then, that the inner layer is here the more excitable. 

 This conclusion, moreover, is in agreement with the inference 

 already arrived at, in connection with the tissues of the 

 Uriclis lily, that secreting cells as a rule are relatively the 

 most excitable. This inference may, however, be subjected 

 to the test of direct experiment. 



I first tested the response of the same specimen by means 

 of thermal shocks, applied to both surfaces simultaneously. 

 The definite direction of the responsive current, from the 

 inner to the outer, across the tissue, proved conclusively that 

 the inner surface was the more excitable, becoming as it did, 

 galvanometrically negative in relation to the outer. A similar 



