The Growth Form of Croton pullei (Euphorbiaceae)± Functional Morphology and Biomechanics of a Neotropical Liana

Croton pullei (Euphorbiaceae)is a woody climber of the lowland rainforest in French Guyana and Surinam. During ontogeny, a shift from a juvenile free-standing growth phase to an older supported growth phase is observed. The following bio- mechanical parameters were studied: structural Young s modu- lus, structural torsional modulus, flexural stiffness and bend to twist ratios. Changes in anatomical development were also ana- lysed for different stages of development of C. pullei which differ significantly in their mechanical properties. Free-standing plants show a nearly constant structural Young s modulus and structur- al torsional modulus during ontogeny, with flexural stiffness in- creasing proportionally with the axial second moment of area. These patterns are typical for ªsemi-self-supporting plantsº. In contrast, supported plants show a significant decrease in struc- tural Young s modulus in older stem parts, as well as a decrease in structural torsional modulus. Due to the decrease in structural Young s modulus, flexural stiffness does not increase propor- tionally with the axial second moment of area. These patterns are typical for non-self-supporting lianas. In all supported plants, a sudden transition occurs from early dense wood to a wood type with a much higher proportion of large diameter vessels. In contrast, only the dense wood type is present in all tested free-standing plants. The data are compared with results from other climbing species of the same study area and discussed with reference to observed features characterizing the growth form and life history of C. pullei.