Sodium stimulated ATPase activity in the nuclei of the meristematic cells, while potassium stimulated it in the mycorrhizae between root cap cells. The detection of these 2 mutually exclusive cation-stimulated ATPases, which both require magnesium-ATP in equivalents, which have a similar optimum pH of about 5.5, and which are located in entirely different parts of the root tip, suggests that particular enzyme systems can only be activated by a specific cation and that one cation cannot substitute for the other. Such a feature may explain the capacity of plants to differentiate between ions as closely similar as sodium and potassium. The similarities between the enzyme system described for salt transport in animal tissues and that depicted here cytochemically in pine roots at the most active site of salt uptake in roots indicate this may be a carrier mechanism for salt entry into plant roots. The presence of the potassium-activated enzyme only in the mycorrhizae may relate to the dependence of pine trees on mycorrhizae for growth.