200 



COMPARATIVE ELECTRO-PHYSIOLOGY 



the outer convex. Electric contacts were now made 

 through non-polarisable electrodes with equal and opposite 

 areas on the two sides. These adjustments were made in a 

 heating chamber containing electrical arrangements by which 

 the temperature could be raised continuously. This was 

 satisfactorily accomplished by an incandescent electrical 

 lamp which was placed in a second chamber, vertically below 

 the plant-chamber. There was a wooden partition between 

 the two, by which the light of the lamp was excluded from 

 the specimen (fig. 131). For the radiation itself will be 



FIG. 131. The Thermal Chamber 



Electric lamp in the lower compartment raises temperature of the upper. 

 E, E', electrodes making contacts with the specimen ; T, thermometer. 



shown to constitute stimulus, and the object in the present 

 case was to eliminate all exciting factors except death itself. 

 By means of side-openings, the heated air was enabled to 

 pass into the plant-chamber, thus raising the temperature. A 

 rheostat included in the lamp-circuit made it possible to 

 adjust the rate of this rise of temperature, its average being 

 about i per minute. 



The natural current through the petiole is, under normal 

 circumstances, from the less excitable outer to the more 

 excitable inner surface: that is to say, the inner is galvano- 



