Ascent of Sap in Trees: Effect of Negative Absolute Pressure on Flow Resistance

Abstract

IN considering the ascent of sap in trees, it is generally accepted1 that the transpiration process at the leaves somehow ‘pulls’ the sap up through the xylem vessels in the tree trunk during active transpiration periods. This ‘cohesion theory’ postulates that negative absolute pressure (liquid tension) must be present in trees more than 34 ft. high, or in any plant where the combined effects of hydrostatic head and flow resistance produce a pressure drop of more than one atmosphere. Investigators have had little success, however, in applying traditional resistance laws to the calculation of viscous pressure drop. Theoretical estimates of flow resistance to sap movement have varied by several orders of magnitude2 and experimental checks of flow resistance through sections of trees frequently show the flow rate decreasing with time at constant pressure3. It appears that the present resistance ‘law’ is incorrect, is being misinterpreted, or the viscosity of sap is not what it is assumed to be from measurements made at positive pressures. This communication is relevant to the latter possibility and reports, in particular, on whether negative absolute pressures have any detectable effect on flow resistance.