It would be natural to suppose that potassium enters Valonia as KCl since it appears in this form in the sap. We find, however, that on this basis we cannot predict the behavior of potassium in any respect. But we can readily do so if we assume that it penetrates chiefly as KOH. We may then say that under normal conditions potassium enters the cell because the ionic activity product (K) (OH) is greater outside than inside. This hypothesis.leads to the following predictions:

1. When the product (K) (OH) becomes greater inside (because the inside concentration of OH- rises, or the outside concentration of K+ or of OH- falls) potassium should leave the cell, though sodium continues to enter. Previous experiments, and those in this paper, indicate that this is the case.

2. Increasing the pH value of the sea water should increase the rate of entrance of potassium, and vice versa. This appears to be shown by the results described in the present paper.

It appears that photosynthesis increases the rate of entrance of potassium by increasing the pH value just outside the protoplasm. In darkness there is little or no growth or absorption of electrolytes.

The entrance of potassium by ionic exchange (K+ exchanged for H+ produced in the cell), the ions passing as such through the protoplasmic surface, does not seem to be important.

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