The effect of direct current, of controlled direction and density, across the protoplasm of impaled cells of Halicystis, is described. Inward currents slightly increase the already positive P.D. (70 to 80 mv.) in a regular polarization curve, which depolarizes equally smoothly when the current is stopped. Outward currents of low density produce similar curves in the opposite direction, decreasing the positive P.D. by some 10 or 20 mv. with recovery on cessation of flow. Above a critical density of outward current, however, a new effect becomes superimposed; an abrupt reversal of the P.D. which now becomes 30 to 60 mv. negative.

The reversal curve has a characteristic shape: the original polarization passes into a sigmoid reversal curve, with an abrupt cusp usually following reversal, and an irregular negative value remaining as long as the current flows. Further increases of outward current each produce a small initial cusp, but do not greatly increase the negative P.D. If the current is decreased, there occurs a threshold current density at which the positive P.D. is again recovered, although the outward current continues to flow. This current density (giving positivity) is characteristically less than that required to produce reversal originally, giving the process a hysteretic character.

The recovery is more rapid the smaller the current, and takes only a few seconds in the absence of current flow, its course being in a smooth curve, usually without an inflection, thus differing from the S-shaped reversal curve.

The reversal produced by outward current flow is compared with that produced by treatment with ammonia. Many formal resemblances suggest that the same mechanism may be involved. Current flow was therefore studied in conjunction with ammonia treatment. Ammonia concentrations below the threshold for reversal were found to lower the threshold for outward currents. Subthreshold ammonia concentrations, just too low to produce reversal alone, produced permanent reversal when assisted by a short flow of very small outward currents, the P.D. remaining reversed when the current was stopped.

Further increases of outward current, when the P.D. had been already reversed by ammonia, produced only small further increases of negativity. This shows that the two treatments are of equivalent effect, and mutually assist in producing a given effect, but are not additive in the sense of being superimposable to produce a greater effect than either could produce by itself.

Since ammonia increases the alkalinity of the sap, and presumably of the protoplasm, when it penetrates, it is possible that the reversal of P.D. by current flow is also due to change of pH. The evidence for increased alkalinity or acidity due to current flow across phase boundaries or membranes is discussed. While an attractive hypothesis, it meets difficulties in H. ovalis where such pH changes are both theoretically questionable and practically ineffective in reversing the P.D.

It seems best at the present time to assign the reversal of P.D. to the alteration or destruction of one surface layer of the protoplasm, with reduction or loss of its potential, leaving that at the other surface still intact and manifesting its oppositely directed potential more or less completely. The location of these surfaces is only conjectural, but some evidence indicates that it is the outer surface which is so altered, and reconstructed on recovery of positive P.D. This agrees with the essentially all-or-none character of the reversal. The various treatments which cause reversal may act in quite different ways upon the surface.

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