TEMPERATURE ON POLAR EXCITATION 247 



response ; this was found to be 2 volts, the temperature 

 of the room being 30° C. 



The point k was now alternately raised and lowered in 

 temperature. This was effected by means of a stream 

 of hot or cold water applied at the point. It is essential 

 that the warming or cooling should be effected gradually, 

 for, as stated above, sudden variation of temperature of 

 itself causes excitation. 



Fig. 123. — Record showing abolition of polar excitation at high 

 temperature : n, normal response ; h, c, alternate ineffec- 

 tive and effective excitations at high and low temperatures 

 respectively. Testing stimulus of kathode-make was kept 

 constant. 



A series of automatic records are shown in fig. 123, 

 made by the plant. The first record of the series, N, gives 

 the response to the descending make-current ; the amplitude 

 of this is only moderate. The petiole was locally warmed 

 to about 37° C, and an identical stimulus of the make of 

 descending current was applied at a moment marked H. It 

 will be noticed that the stimulus which was formerly effective 

 has now become ineffective. 



The petiole was next locally cooled to about 10° C. and 



