. Electro-physiology. Electrophysiology. 20 ELECTRO-PHYSIOLOGY CHAP. surfaces of each lobe of the leaf, external and internal (or upper and lower), give a different electrical reaction, so that 011 leading off from opposite points of the upper and lower surface, there is found to be current, either so that the latter is positive to the former (which Burdon-Sanderson at first held to be normal) or vice versa. The degree of positivity, and corresponding magnitude of and of the leaf-current, in the first case, depend, as soon appears, essentially upon the physiological state of the leaf, and
. Electro-physiology. Electrophysiology. 20 ELECTRO-PHYSIOLOGY CHAP. surfaces of each lobe of the leaf, external and internal (or upper and lower), give a different electrical reaction, so that 011 leading off from opposite points of the upper and lower surface, there is found to be current, either so that the latter is positive to the former (which Burdon-Sanderson at first held to be normal) or vice versa. The degree of positivity, and corresponding magnitude of and of the leaf-current, in the first case, depend, as soon appears, essentially upon the physiological state of the leaf, and above all upon the previous excitation. If, after compensating the current of rest, mechanical or other stimuli are sent in moderately rapid succession into a leaf of which the under surface is already positive, there is without. FIG. 142. exception a marked rise of positivity in the excited surface. It is only by degrees, during the subsequent period of rest, that the current diminishes, until with the same lead-off there is complete isoelectricity, and finally, as has been said, a current in the opposite direction, corresponding with negativity of the under (external), and positivity of the upper (internal), surface of the leaf—a state which, according to Burdon-Sanderson's later observa- tions, must be regarded as normal in the leaf which has remained a long time unexcited. In this instance the leaf-current must, in regard to the internal surface, be regarded as outgoing. Excitation in such a case is naturally followed by the opposite alterations, as in an initially ingoing leaf-current: the positive upper surface becomes suddenly negative to the lower surface, so that the leaf-current is once more ingoing. It should also be remarked that the lower surface of the leaf is the less positive and more negative, in proportion with the time that has elapsed since the last excitation. The current of rest, on the contrary, is at all times dependent on the previous excitation of.
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