During the past few years a great many studies have been made on the electromotive properties of biological systems. In general, the studies indicate that bacterial cultures as well as sterile bacteriological media are active oxidation-reduction systems. The effect produced by the bacteria on the potentials of the substratum on the one hand, and the effect of the potentials of the substratum on the growth of the organisms on the other, involve relations of very definite significance. In a

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... us communication, Allyn and Baldwin (1930) reported that marked effects on the growth of an aerobic form of bacteria were induced by certain changes in the oxidation-reduction character of the media. In continuation of these studies an endeavor has been made to study by electrometric measurements the oxidation-reduction potentials developed in certain bacteriological substrata and to interpret the same experimentally in terms of bacterial growth. Certain irregularities were encountered in these studies with aerobic organisms, rhizobia; yet some significant correlations, it is believed, were manifested between the potential-time curves of the substrata and the ability of the organisms to initiate growth therein. EXPERIMENTAL Methods A simple potentiometer circuit was found unsatisfactory in making potential measurements in these poorly poised biological 1 Published with the permission of the Director of the Agricultural Experiment Station. 369 on May 9, 2020 by guest