66 COMPARATIVE ELECTRO-PHYSIOLOGY 



by the loss of excitability of the tissue. Similarly, the 

 transmission of true excitation may be selectively blocked, 

 in a tissue, by the application of various depressing agents, 

 such as anaesthetics, without appreciably affecting the 

 passage of the wave of increased hydrostatic tension. 



For the present experiment I took a leaf of fern, and 

 made the proximal electrical connection with the petiole, and 

 the distal with an indifferent point on a leaflet. By means 

 of the thermal stimulator (p-38), I now applied successive 

 uniform stimuli at intervals of one minute, on a point 

 i '5 cm. below the proximal contact. The first three 



FIG. 49. Photographic Record of Electrical Response of Petiole of Fern 



First part of record shows normal negative responses ; second part shows 

 positive response unmasked by selective physiological block of chloro- 

 form ; in the third part is seen the abolition of response when stimulus 

 is applied on anaesthetised area itself. 



responses of the series are seen (fig. 49) to be more or less 

 uniform, and negative. Such resultant response is, as has 

 been pointed out, due to the summation of two antagonistic 

 effects, of which the excitatory is predominant. In order 

 therefore to eliminate from this resultant response its excita- 

 tory component, I applied chloroform on a narrow belt inter- 

 mediate between the point of stimulation and the proximal 

 contact. From the next three responses it will be seen that 

 the hydrostatic effect of positivity, represented by down- 

 responses, was thus successfully unmasked. The application 

 of chloroform is thus seen to act here as a selective block, 



