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.