It has frequently been demonstrated that when plant tissues of the same initial ionic status are transferred to solutions of a sufficiently wide range of concentration, two absorption isotherms are observed. They have been widely interpreted as evidence for the existence of two independent physiological mechanisms for the absorption of nutrients. The experiments here described on the uptake of phosphate and rubidium ions by excised barley roots and discs of beetroot tissue from solutions ranging in concentration between 0.001 and 50.0 mM were conducted by procedures similar to those on which the foregoing conclusions had been reached, except that sterile procedures were used to eliminate any effects of micro-organisms. The two absorption isotherms were evident under sterile conditions. However, evidence was obtained that only uptake observed at low concentrations (below 0.2–0.5 mM) is attributable to plant metabolism. The isotherm observed over the higher concentration range is accountable to passive diffusion and may thus be regarded as a consequence of the experimental procedures.