MATERIALS AND METHODS S. monoica Forssk ex. J. F. Gmel was grown in a greenhouse in a sand culture and irrigated with Hoagland solution with 100 mm NaCl.The structure of the three cell layers was investigated using an electron microscope. Leafsections were fixed in 2% glutaraldehyde in 0.15 M phosphate buffer (pH 7.2) and postfixed in 1% OS04 in the same buffer. After dehydration in ethanol the sections were embedded in Epon 812. Activity and localization of the carboxylating (PEP-C and RuBP-C) and of the decarboxylating (NADmalic enzyme) enzymes was tested in extracts of cell layers which ' Abbreviations: PEP-C: phosphoenolpyruvate carboxylase; RuBP-C: ribulose bisphosphate carboxylase; ME: malic enzyme; PGA: phosphoglyceric acid.were mechanically separated. Green succulent mature leaves taken from 3-to 4-year-old plants were used for celi layers separation.

Accumulation of sodium and rubidium by excised barley roots was investigated. The concentration isotherm yielded one absorption shoulder. Nevertheless, it is suggested that two mechanisms take part in the uptake of sodium and rubidium: One non‐metabolic mechanism with an apparent participation at low external salt concentrations (< 1 mM) and at high concentrations (> 20 mM). Such a mechanism is almost unaffected by low temperature conditions and by metabolic inhibitors. Rubidium possesses a high affinity toward this non‐metabolic system. The second mechanism is sensitive to metabolic inhibitors and to low temperature conditions. It dominates at intermediate external concentrations (1–20 mM). Sodium possesses high affinity towards this mechanism. The two mechanisms operate in a parallel manner beyond a diffusion barrier (= plasmamembrane) surrounding the cells. It is assumed that both the metabolic and the non‐metabolic mechanisms operate in the entire concentration spectrum, but their relative contribution to the total uptake varies at different ranges.