<p>Petiole mechanics is essential for displaying leaf lamina efficiently in a shaded forest understory. We investigated the structure, anatomy, and mechanics of the petioles of 25 coexisting woody species in a warm-temperate forest understory and related them to interspecific differences in leaf size, habit (evergreen vs deciduous leaves), and form (simple vs compound leaves). Flexural stiffness of the petioles was greater in large leaves than in small leaves, in evergreen leaves than in deciduous leaves of similar area, and in compound leaves than in simple leaves of similar area. Greater second-order moment of area of petioles was responsible for greater flexural stiffness for species with large leaves and evergreen species. In contrast, the petioles of compound leaves showed a greater modulus of elasticity, but a smaller second-order moment of area compared to those of simple leaves of similar leaf area. Anatomical properties were related to the flexural stiffness and resulted in different biomass costs of the petioles. These results are consistent with the idea that the petioles of compound leaves are analogous to laterally growing branches, in which the increase in density is theoretically efficient in terms of the mass required to produce a branch of a given length. Therefore, different factors constrained the flexural stiffness of petioles among coexisting species of different leaf groups.</p>