38 COMPARATIVE ELECTRO-PHYSIOLOGY 



two cups of mercury, thus closing the electrical circuit for a 

 brief and definite time. When a second press-key, not shown 

 in the figure, is open, the circuit is incomplete, and there 

 is no thermal stimulation. The observer then presses this 



key, and counts, say, 

 five strokes of the 

 metronome, after 

 which the press-key 

 is again opened. In 

 this way, the sum- 

 mated effect is ob- 

 tained, of five equal 

 thermal shocks. This 



FIG. 27. Stimulation by Thermal Shocks process is repeated 



as often as desired, 



at intervals of, say, one minute, by which time the tissue is 

 generally found to have completely recovered from its ex- 

 citatory electrical variation. 



In the case of the experimental arrangements of which 

 the diagram is given in fig. 27, stimulation is confined to one 

 contact of the responding circuit. The method by which 

 excitation was here prevented from reaching the distal 

 contact is important. I shall, in the course of the present 

 work, show that the parenchymatous tissue of the lamina of 

 a leaf or leaflet is a bad conductor of excitation. Hence if 

 the second contact of the circuit be made with this tissue, 

 the stimulus does not reach the distal point. It is true that 

 a certain small proportion might conceivably be conducted 

 through the attenuated fibre-vascular channel of the midrib. 

 But even so remote a contingency is provided against by a 

 transverse cut across the midrib on the hither side of the 

 contact. 



The arrangements, then, being made in the manner de- 

 scribed, the tissue may be subjected to the action of successive 

 uniform stimuli. How regular the resulting responses may be 

 rendered will be seen from fig. 28, in which is given a series 

 of responses obtained from the petiole of a fern (fig. 28) 



