Abstract

<jats:p> The tempo of water efflux from single Fagus grandifolia leaves has been measured. The resistance to pressure-driven water efflux has been measured for normal leaves, R<jats:sub>s</jats:sub>, and for leaves in which extracellular mesophyll spaces are infiltrated with water, R<jats:sub>s</jats:sub>*. The ratio R<jats:sub>s</jats:sub>*/R<jats:sub>s</jats:sub> is about 0.4. The resistance to water flow through the xylem, R<jats:sub>x</jats:sub>, was also measured and found to be a small part of R<jats:sub>s</jats:sub>, i.e., R<jats:sub>x</jats:sub>/R<jats:sub>s</jats:sub> = 0.08. The activation energies for water efflux from normal and infiltrated leaves are both about 26 ± 4 kJ/mol.After an analysis of our data, we conclude that the membrane hydraulic conductivity of F. grandifolia leaves is roughly 10<jats:sup>−6</jats:sup> cm s<jats:sup>−1</jats:sup> bar<jats:sup>−1</jats:sup> (1 bar = 100 kPa) and that water travels the shortest path between the cell sap and the nearest xylem vessel, flowing in and out of mesophyll cells through the areas in contact between adjacent cells along the pathway. </jats:p>