Leaves of Potamogeton lucens, surrounded by a 1 mM rubidium solution, take up rubidium ions at their lower surface and release these ions into the upper solution. Only small amounts of the ions taken up remain fixed within the leaf tissue, so that the greater part of the rubidium ions are transported across the leaf in one direction. The above-mentioned polar transport depends on light and the presence of bicarbonate. However, it also occurs when rubidium bicarbonate is only applied to the lower surface of the leaves. The results so far obtained are explained by the following simple hypothesis. Concentration gradients are developed owing to the bicarbonate assimilation in the light, causing the bicarbonate ions to be taken up and the hydroxyl ions formed to be released. These anions are accompanied by rubidium ions. So, according to the hypothesis a completely inactive movement of cations as well as anions is involved. The polarity of the process is due to a difference in anion permeability between the lower and upper layers of the leaf tissue.