This study investigated the effect of mechanical support on water transport properties and wood anatomy of stems of western poison oak, Toxicodendron diversilobum T. & G. Greene. This plant grows as a vine when support is present but as a shrub when support is absent. I compared vines and shrubs growing naturally in the filed and those produced from cuttings of 11 source plants in a common garden. Huber value (xylem transverse area/distal leaf area) was lower but specific conductivity (water volume .cntdot. time-1 .cntdot. xylem transverse area-1 .cntdot. pressure gradient-1) was higher in supported than unsupported plants both in the field and the common garden. The opposing effects of Huber value and specific conductivity resulted in the same values of leaf-specific conductivity (LSC, water volume .cntdot. time-1 .cntdot. distal leaf area-1 .cntdot. pressure gradient-1) for supportred and unsupported shoots at a given site. Therefore, for the same rates of evapotranspiration, supported and unsupported shoots will have the same pressure gradients in their stems. Vessel lumen composed a higher proportion of stem cross-section in supported than unsupported plants (due to slightly wider vessels and not to greater vessel density). These results suggest that the narrow stems of supported plants are compensated hydraulicity by the production of wider vessels: at a given site, poison oak plants co-ordinate their leaf and xylem development such that their stems achieve the same overall conductive efficiencies (LSCs), regardless of support conditions.