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112 left. A necessary precaution is to increase the current gradually by means of a suitable potentiometer slide, to its full value. The reason for this will be given later. The intensity of the constant current employed was 1.4 micro-amperes. Now on exciting the petiole by the previous stimulus, the conducting power was found to be greatly enhanced. The excitatory impulse now reached the end of the petiole, and caused six pairs of leaflets to fall.

Transmission of excitation 'Down-hill': Experiment 39.—In continuation of the previous experiment, the constant electric current was reversed, its directions being now from left to right. Transmission of excitation was now in a down-hill direction. On applying the induction shock stimulus of the same intensity as before, the conducting power of the petiole was found to be abolished, none of the leaflets exhibiting any sign of excitation. This modification of the conducting power persists during the passage of the constant current. On cessation of the current the original conducting power is found to be restored. It will thus be seen that the power of conduction is capable of modification, and that the passage of an electric current of moderate intensity induces enhanced power of conduction in an 'up-hill' and diminished conductivity in a 'down-hill' direction.

In my 'Researches on Irritability of Plants' I have shown how intimately connected are the various physiological reactions in the plant and in the animal, and I ventured to predict that the recognition of this unity of response in plant and animal will lead to further discoveries in physiology in general. This surmise has been fully justified, as will be seen in the following experiments carried out on the nerve-and-muscle preparation of a