SUMMARY Photosynthetic bicarbonate assimilation and cation transport by aquatic plants are reviewed. It is suggested that these two processes stand in a causal relation to one another. A number of observations of cation transport not directly associated with photosynthesis are also reviewed, and are related to current concepts of active transport in the plant kingdom. These characteristics of cation transport have been observed in many submerged aquatic plants: Cations are transported through the leaves in the light but not in the dark. Cations are accumulated at the abaxial leaf surface from the contiguous external medium and are excreted from the adaxial surface into the medium. Therefore accumulation and excretion can be studied separately. A new theory of cation transport in bicarbonate‐assimilating aquatic plants is presented. The theory entails the following propositions: Two active membranes pump cations through the leaf in the light. The membrane on the abaxial leaf surface pumps cations into the leaf; that on the adaxial surface pumps cations out of the leaf. The cell plasma membranes on the abaxial and adaxial leaf surfaces form these two active membranes. In the light–i.e. when the transport mechanism is functioning–the biochemical reactions of transport reach steady‐state levels. These levels are determined by the rates of reaction between the constituents. One can therefore alter the steady‐state levels by suitably altering the conditions of one or more of these reactions. In the dark–i.e. when the transport mechanism is not functioning–the biochemical constituents reach equilibrium levels. The equilibria can be altered by changing the environment of the leaf. The reactions of transport supply links of a respiratory chain. The reactions of transport synthesize a bicarbonate‐accepting compound at the plasma membrane. Evidence for each of these propositions is discussed. The theory is considered applicable, with appropriate modification, to cation transport in a variety of cells.

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