Leaf sheaths of rattans are long, tubular and persistent and unlike many self-supporting palms, extend far from the apex of the plant. The mechanical role of the leaf sheath was investigated in eight rattan species of the subfamily Calamoideae. The main objective was to analyse its influence on the mechanical architecture and contribution to the climbing habit. Bending mechanical properties were measured along climbing axes before and after removal of leaf sheaths. Results were related to stem and leaf sheath geometry and mechanical properties. Contribution of the leaf sheath to axial flexural rigidity was high (c. 90%) in the early stages of growth and towards the apex of older climbing axes for all climbing palms tested. Senescence and loss of the leaf sheath strongly influenced axial stiffness. A nonclimbing species, Calamus erectus, showed a different mechanical architecture. Although lacking secondary growth, palms have been able to develop successful climbers with a mechanical architecture broadly analogous to, although developmentally different from, dicotyledonous lianas. The role of the leaf sheath in modulating mechanical properties during ontogeny ought not to be neglected in studies on monocotyledons, as it possibly contributed significantly to the ways in which different growth forms have evolved in the group.