Page:Researches on Irritability of Plants.djvu/91

68 of the exciting current may, under these conditions, undergo a change. This difficulty has been overcome by a contrivance 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 metallic-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 specimen where electrical connection is desired, turns back into the lower cell (which it enters through the open aperture), and dips into the saline solution. It will be seen that apart from capillary action, owing to the upper chamber being at a higher level, the thread will be kept constantly 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