Page:Researches on Irritability of Plants.djvu/52

Rh 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