" dowu-hill," and in the latter case " up-hill." Electrical connections are so arranged that when the commutator is tilted to the right, the transmission is down-hill, when tilted to the left, up-hill.
The electrical resistance offered by the 95mm. length of stem and petiole will be from two to three million ohms. The intensity of the constant current flowing through the plant can be read by unplugging the key which short- circuits the micro-ammeter G. The choking coil C prevents the alternating induction current from flowing into the polarising circuit and causing direct stimulation of the palvinus.
Before describing the experimental results, it is as well to enter briefly into the question of the external indication by which the conducting power may be gaug>;d. Change of conductivity may be expected to give rise to a variation in the rate of propagation or to a variation in the magni- tude of the excitatory impulse that is transmitted. Thus we have several methods at our disposal for determining the induced variation of conductivity. In the first place the variation of conductivity may be measured by the in- duced change in the velocity of transmission of exciiation. In the second place, the transmitted effect of a sub-maximal stimulus will give rise to enhanced or diminished amplitude of mechanical response, depending on the increase or decrease of conductivity brought about by the directive action of the current. And, finally, the enhancement or , depression of conductivity may be demonstrated by the ineffectively transmitted stimulus becoming eflective, or the effectively transmitted stimulus becoming ineffective.
Exclusion of the factor of Excitability. — The object of the enquiry being the pure effect of variation of conductivity, we have to assure ourselves that under the particular
