The potential difference across the protoplasm of impaled cells of two American species of Halicystis is compared. The mean value for H. Osterhoutii is 68.4 mv.; that for H. ovalis is 79.7 mv., the sea water being positive to the sap in both.

The higher potential of H. ovalis is apparently due to the higher concentration of KCl (0.3 M) in its vacuolar sap. When the KCl content of H. Osterhoutii sap (normally 0.01 M or less) is experimentally raised to 0.3 M, the potential rises to values about equal to those in H. ovalis.

The external application of solutions high in potassium temporarily lowers the potential of both, probably by the high mobility of K+ ions. But a large potential is soon regained, representing the characteristic potential of the protoplasm. This is about 20 mv. lower than in sea water.

The accumulation of KCl in the sap of H. ovalis is apparently not due to the higher mobility of K+ ion in its protoplasm, since the electrical effects of potassium are practically identical in H. Osterhoutii, where KCl is not accumulated.

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