Abstract
When the pressure gradient across a root alters, there is often an apparent change in the permeability of the root to water. Fiscus suggested that this can be explained by a simple 2-compartment model which took into account rates of solute uptake into the xylem. A method of testing actual data against the Fiscus model was proposed; this showed that in some cases the apparent changes in permeability cannot be explained by the model. The model was not adequate to predict the amounts of solute reaching the xylem by passive drag: a 3-compartment model would be more realistic.