Mechanical role of the leaf sheath in rattans.

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.

Mechanical architecture and development in Clematis: implications for canalised evolution of growth forms.

• Mechanical architectures of two Clematis species, the herbaceous perennial Clematis recta and the woody liana, Clematis vitalba, were investigated and compared with the woody rhizomatous sand dune plant Clematis flammula var. maritima. • Bending mechanical properties of stems from various developmental stages were compared and related to stem geometry and relative proportions of tissues during development. • Clematis vitalba and C. flammula var. maritima showed mechanical architectures with reductions in structural Young's modulus of the stem during ontogeny. Irreversible loss of stem rigidity was mediated by disruption, separation and eventual loss of primary phloem fibres via secondary growth of the periderm and cambial activity. Each species showed variations of non-self-supporting mechanical architecture relating to specific habitat preferences. In aerial stems of C. recta the structural Young's modulus remained approximately constant during ontogeny, a mechanical signal characteristic for semi-self-supporting architectures. • Woody aerial plant stems are extremely rare in the Ranunculaceae and seldom, if ever, show self-supporting characteristics. Growth form evolution in the group may have been canalised by evolution of rhizomatous geophytic growth forms with secondary growth confined to underground stems specialized for water conduction, storage and perennation. Variation of this ground plan includes climbing, straggling or rhizomatous architectures but not self-supporting shrubs or trees with secondary growth generating requisite self-supporting mechanical properties. Certain body plan organisations appear to have inbuilt mechanical constraints which may have profound effects on the subsequent evolution of growth forms.