i64 RESEARCHES ON IRRITABILITY OF PLANTS 



block under local application of cold, the fragments of ice 

 were removed and the cooled portion of the petiole allowed 

 to regain the temperature of the room, which must have 

 been accomplished in the course of 20 minutes. After this, 

 on taking the record of the transmitted effect of stimulus I 

 found that the block of conduction was still persistent. The 

 conducting-power of the benumbed tissue is thus paralysed 

 for a period which generally lasts for about 45 minutes. 

 I have, however, discovered the very suggestive fact that 

 the lost conducting-power can be very quickly restored by 

 subjecting the paralysed portion of the petiole to the action 

 of tetanising electric shocks. 



The Electrotonic Arrest of Excitatory Impulse 



If in a nerve-and-muscle preparation a constant current 

 be maintained in an intervening tract between the point of 

 stimulation and the responding muscle, this current is found 

 to act as a block to the passage of excitation. With moderate 

 intensity of current the block of conduction is due to the 

 depressing action of the anode. For demonstration of 

 electrotonic block of nerve-conduction, an intensity of 

 stimulus is found which is effective under normal conditions. 

 But during the continuation of the blocking current the 

 excitatory impulse due to the testing stimulus is found to 

 be arrested. The transmission is, however, renewed on the 

 stoppage of the current. It is of special interest to have 

 thus at our disposal a physiological block which can be put 

 ' on ' and ' off ' at will and many times in succession. The 

 alternate transmission and its arrest then affords a very 

 striking demonstration of the excitatory character of the 

 propagated effect. 



I have found a similar block of conduction induced in the 

 plant by electrotonus. Various forms of testing stimulus may 

 be employed. It is, however, more satisfactory to employ a 

 form of stimulus the intensity of which may be either gradu- 

 ally increased or maintained constant. These requirements 



