68 RESEARCHES ON IRRITABILITY OE PLANTS 



of the exciting current may, under these conditions, undergo 

 a change. This difficulty has been overcome by a con- 

 trivance for keeping the thread uniformly moist. This will 

 be understood from the diagram (fig. 31) of the electrolytic 

 contact-maker : Two small cells are made of cork ; the upper 

 cell is filled with very dilute saline solution, a little of which 

 also lies in the bottom of the lower cell. A bent piece of 

 silver-wire coated with a deposit of chloride, and fixed to the 

 horizontal metalHc-rod, pricks through two cells and dips 

 into the solutions above and below. The moistened thread 



coming through a hole near the 

 bottom of the upper chamber 

 makes one loop round that 

 portion of the plant speci- 

 men where electrical connec- 

 tion is desired, turns back 

 into the lower cell (which it 

 enters through the open aper- 

 ture), and dips into the saline 

 solution. It will be seen that 

 apart from capillary action, 

 owing to the upper cham- 

 ber being at a higher level, 

 the thread will be kept con- 

 stantly moist by the slow 

 streaming down of the solution. The current from the 

 coil, again, will have two entries by means of the 

 doubled thread, the resistance being thus halved. The 

 second electrode of the coil is connected with the other 

 contact-point on the plant in a similar manner. The 

 resistance offered by the plant tissue is relatively high, 

 being of the order of a million ohms. The resistance of 

 the electrolytic contacts, on the other hand, need be no 

 higher than a few thousand ohms. By thus making the 

 resistance of the moist contact relatively small, the total 

 resistance of the circuit remains practically the same, 

 especially since we guard against any variation that might 



Fig. 31. — Electrolytic contact. 



