The shoreweed Littorella uniflora (L.) Aschers. possesses two life forms differing from each other both in morphological and in biochemical characteristics. The submerged (isoetid) form showed marked fluctuations in the levels of malic acid, as well as in the total titratable acidity, thus meeting the most important biochemical requirement of Crassulacean Acid Metabolism (CAM). In contrast, the terrestrial form, which depends on atmospheric sources of CO2, showed negligible variations in the levels of malate. The isoetid form has wax-covered, functionless stomata, whereas the stomata of the terrestrial form are operative. Comparison of the two genetically uniform types of Littorella strongly supports the hypothesis that the CAM-like carbon pathway is selected in submerged plants as an adaptation to the carbon shortage which they experience in daytime.