A discontinuity of the ion efflux curve similar to that described by Pallaghy et al. (1970) for short maize root segments is also observed with more homogeneous materials such as root cortex cells, Mnium leaflets and others. The time course of 42K+ uptake by aged segments of isolated root cortex tissue also shows a discontinuity. The shoulder of the efflux curve cannot, therefore, be due to two overlapping efflux processes, i. e. via the cortex and the xylem respectively of the root segments. Although, at this stage, no model explaining all experimental findings can be presented, it seems clear that the three-compartmental model free space-cytoplasm-vacuole in series does not apply under all circumstances.