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

Abstract

Summary• 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.

Paedomorphosis, Secondary Woodiness, and Insular Woodiness in Plants

“…2a). Very young stems have a ring of lignified primary fiber bundles similar to those found in species of Clematis (Isnard et al, 2003). Older stems have dense wood with small groups of relatively narrow diameter vessels, and conspicuous large unlignified rays.…”

“…Although wood is generally understood to mean the secondary xylem of gymnosperms and dicotyledons (Evert, 2006), when applied to growth forms woodiness can mean a variety of things. For instance, a plant may have a woody stem, a woody root system, or both (Isnard et al, 2003). Woodiness in growth forms may also exist in varying degrees, from a small amount to a very substantial amount.…”

“…Secondary woodiness is best described as the evolution of secondary xylem in plants with herbaceous ancestors (Carlquist, 1974(Carlquist, , 1992Lens et al, 2005a). The term itself suggests that woodiness was lost, and then reappeared during later evolution (Isnard et al, 2003). It also implies that primary woodiness, a term rarely used in the literature, exists.…”

Interrelations between hydraulic and mechanical stress adaptations in woody plants

“…[1][2][3]5,[9][10][11][12]25 Modifications of the xylem towards mechanically reinforcing stressed or strained areas simultaneously impacted the conductive physiology of the trees studied, and drought adaptations resulted in stiffer and stronger tissue. This provides further evidence that in order to fully understand plant physiology and ecology it is necessary to consider the various functions of wood simultaneously and attempt to unravel causal relationships between e.g., the hydraulic and mechanical functioning of the tissue.…”

Inheritance of growth form and mechanical characters in reciprocal polyploid hybrids of Rosa section Caninae—implications for the ecological niche differentiation and radiation process of hybrid offspring

“…In the group of semi-self-supporting species E struct and G struct remain approximately constant during ontogeny. Typical semi-self-supporting plants (such as some rattan palms and Strychnos-species, or Clematis recta) become mechanically unstable when they reach a certain height, often form arching stems that lean on the surrounding vegetation and/or form dense monotypic stands where the individual plants provide mutual support (Gallenmüller et al 2001, Isnard et al 2003, Speck 1991, 1994, Speck and Rowe 1999). In the two tested rose species and their hybrids E struct and G struct increase during ontogeny, and therefore EI and GI do increase overproportionally with I ax and I pol , a characteristic for selfsupporting species.…”