DESPITE two centuries of research, the mechanism of water transport in plants is still debated1-8.The prevailing cohesion-tension theory2,3, which states that water is pulled upwards by capillarity in cell-wall pores, remains vulnerable to challenge because its corollary is difficult to prove: that large negative pressures exist in xylem conduits4-7. Recent xylem pressure-probe and z-tube experiments suggest that cavitation limits xylem pressures to above -0.5 MPa, despite the much more negative pressures predicted by the cohesion-tension theory and measured with the standard pressure-chamber method4,5,9,10. Here we show, using centrifugal force to induce negative pressure between -0.5 and -3.5 MPa in intact stems, that xylem conduits remained water-filled and conductive to species-specific pressures ranging from -1.2 to below -3.5 MPa. Results were consistent when stems were air-dried or injected with air. Agreement among these techniques demonstrates that xylem can support large negative pressures, that the pressure chamber reliably measures these pressures, and that cavitation is nucleated by air entry through conduit wall pores.