METHODS OF STIMULATION 29 



use non-polarisable electrodes for making the connections 

 with the plant tissue. Such non-polarisable electrodes 

 may be of the usual U-tube type. The shorter limb of the 

 glass U-tube is filled with kaolin paste in normal saline. 

 A cotton thread moistened in saline protrudes from this 

 and makes the connections with the tissue. The longer 

 limb is filled with zinc sulphate solution, into which dips 

 a zinc rod. For ordinary purposes, however, a much simpler 

 contrivance is found effective. A narrow cork is partially 

 hollowed out and paraffined. The well thus formed is 

 filled with dilute saline solution. The bottom is pierced 

 for the entry of a cotton thread into the saline. A thick 

 silver-wire, whose surface has been covered electrolytically 

 with a film of chloride, pierces the side of the cork and 

 dips into the saline solution. The silver wire forms one 

 of the two electrodes, and the cotton thread makes the 

 necessary electrical connection with the tissue. 



For the purpose of excitation we may make the two 

 electrical connections, one at or near the pulvinus itself, and 

 the other on the petiole at a short distance. It will be shown 

 later that when the electrical current leaves the tissue by 

 the pulvinus, that point becomes the seat of excitation. 

 Thus by making the pulvinus the point of exit of current, 

 or kathode, we may cause direct excitation. Or we may 

 have the pulvinus included between the two electrodes, so 

 that the electrical current passes through it (fig. 9, a). This 

 connection we may designate the intra-electrodal. Here, in 

 certain circumstances, the excitation throughout the tract 

 becomes diffuse and practically instantaneous. And lastly, 

 the two electrical connections may be made side by side, 

 say about 1 cm. apart on the petiole, at a moderate distance 

 from the pulvinus. The excitation thus caused in the 

 petiole reaches the pulvinus, as I have already said, by 

 conduction. This connection we may call extra-electrodal 



(fig- 9, b). 



I give below a series of records (fig. 10) of response to 

 stimulation by condenser discharge. The plant was highly 



