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The effect of increasing pressure difference (ΔP) on intervessel pit membrane porosity was studied in two angiosperm tree species with differing pit architecture. Fraxinus americana L. possesses typical angiosperm bordered pit structure while Sophora japonica L. exhibits well-developed vestures in intervessel pit chambers. It was hypothesized (a) that large ΔP across intervessel pits would cause the deflection of pit membranes in the stems of F. americana resulting in significant increases in porosity and thus lower cavitation thresholds, and (b) that the presence of vestures would prevent the deflection of pit membranes in S. japonica. To determine if the porosity of pit membranes increased under mechanical stress, suspensions of colloidal gold, 5 nm and 20 nm in diameter, were perfused across intervessel pit membranes at ΔP ranging from 0.25 MPa to 6.0 MPa. The effect of increasing ΔP on membrane porosity was also tested by comparing air seeding thresholds (Pa) in stems perfused with water or a solution with lower surface tension. Air seeding and colloidal gold experiments indicated that pit membrane porosity increased significantly with ΔP in F. americana. In S. japonica, increases in permeability to colloidal gold with ΔP were small and maximum pore diameters predicted from Pa were independent of ΔP, suggesting that vestures limited the degree to which the membrane can be deflected from the centre of the pit cavity. This provides the first experimental evidence that vestures reduce the probability of air seeding through pit membranes.