Morphological, physiological, and growth characteristics of western poison oak,Toxicodendron diversilobum, T. & G.Green, in a Californian site suggest that the shrub vs. vine growth forms are determined by an environmental factors (the presence of absence of physical support), and not by ecotype. In a common garden, clones of shrubs and vines differed as a function of support in 11 variables (stem length, stem taper, internode length, mean and media vessel diameter, vessel lumen area/stem area, Huber value, specific hydraulic conductivity, and three indices of a leaf vs stem elongation) but differed as a function of plant origin (shrub or vine) in only one variable (vessel density). In the field, supported (viney) and unsupported (shrubby) parts of the same plants differed significantly for seven variables; stem taper, stem modulus of elasticity in bending. Huber value, specific hydraulic conductivity, vessel lumen area/stem area, and mean and median vessel diameters; differences by plant origin could not be tested. Although much of the variation was due to plasticity, common garden experiments showed that source plants also differed from one another in many of the measured characteristics. Individuals of T. diversilobum will function ecologically as vines if grown with support but as shrubs in the absence of support.