The theoretical aspects of diffusion of Rb+ ions in the free space of young barley roots and their concomitant absorption by the epidermal and cortical cells were studied on the basis of a simplified geometrical model. It is concluded that participation of the cortical cells in the process of Rb+ uptake is irreconcilable with the kinetic features of this process as determined experimentally. Limitation of uptake to the root surface is considered to pertain to other macronutrient ions and other objects as well.