A technique is described for precisely measuring the drop in water potential, Δψ, between the soil and the leaf xylem of wheat seedlings. The technique was used to explore the relation between transpiration rate and Δψ at various times during the monotonic drying of the soil in which the plants were growing. When the soil was wet, the relation was linear, but, as the soil dried, nonlinearities appeared which were, in the main, explicable in terms of simple soil physical models describing the flow of water through the soil to the roots. There was no sign of the major hydraulic resistance at the root: soil interface that other people have recently found.