Canny's compensating pressure theory for water transport (American Journal of Botany 85: 897-909) has evolved from the premise that cavitation pressures are only a few tenths of a megapascal negative (approximately -0.3 MPa). In contradiction, "vulnerability curves" indicate that xylem pressures can drop below -3 MPa in some species without causing a loss of hydraulic conductivity. Canny claims these curves do not measure the limits to negative pressure by cavitation, but rather the limits to the compensating tissue pressure that otherwise quickly refills cavitated conduits. Compensating pressure is derived from the turgor pressure of the living cells in the tissue. To test this claim, we compared vulnerability curves of Betula nigra stems given three treatments: (1) living control, (2) killed in a microwave oven, and (3) perfused with a -1.5 MPa (10% w/w) mannitol solution. According to Canny's theory, the microwaved and mannitol curves should show cavitation and loss of conductance beginning at approximately -0.3 MPa because in both cases, the turgor pressure would be eliminated or substantially reduced compared to controls. We also tested the refilling capability of nonstressed stems where compensating pressure would be in full operation and compared this with dead stems with no compensating pressure. According to Canny's interpretation of vulnerability curves, the living stems should refill within 5 min. Results failed to support the compensating tissue theory because (a) all vulnerability curves were identical, reaching a -1.5 MPa threshold before substantial loss of conductance occurred, and (b) killed or living stems had equally slow refilling rates showing no significant increase in conductivity after 30 min. In consequence, the cohesion theory remains the most parsimonious explanation of xylem sap ascent in plants.