i66 RESEARCHES ON IRRITABILITY OF PLANTS 



of the three records in the next diagram was taken without 

 this block and with a stimulus intensity of 2. It will be 

 seen (fig. 92) that excitation was transmitted as usual, the 

 velocity of transmission in this case being 29 mm. per second. 

 The specimen was exceptionally vigorous and the season 

 the height of summer, which facts account for the high 

 velocity. The blocking current was next introduced. In 

 order to prevent the excitation due to sudden make of 



Fig. 92. — Record of effect of electrotonic block. Uppermost 

 record normal ; lowest record shows block of trans- 

 mission of excitation ; middle record shows restoration 

 of conductivity on removal of block. 



current, the applied e.m.f. was gradually increased from 

 zero to 2 volts, by means of a potential slide. During the 

 passage of this constant current a second stimulus was 

 applied of the same value as before ; and it will be seen, 

 from the lowest record, that there was no response, the 

 transmission of excitation being effectively blocked. In 

 order to show that this block would only persist during 

 passage of the current, the latter was next reduced gradually 

 to zero by manipulation of the potential slide. This was 

 carefully done, to avoid the excitation due to sudden 

 cessation of current. On again repeating stimulation, the 

 block was found to be no longer operative (see intermediate 

 record) and response due to transmitted excitation took 

 place as usual. 



In the last figure records were taken on a fast-moving 



