An apparatus is described to which the cut end of an excised root may be sealed and the developing root pressure measured by a transducer as a function of time. In addition to measurements of pressure under zero volume flow, flow rates at various applied pressures may also be obtained. By applying an instantaneous pressure change which is just sufficient to terminate flow, the true osmotic driving force within the root can be determined. This provides a measurement of the hydraulic conductivity of the root which is independent of the presence of unstirred layers and of osmotically active species whose reflection coefficients may not be one. The technique also separates the effects which changes in hydraulic conductivity (Lp) and solute flux (Js) exert on volume flow(Jv)