Abstract
From a mechanical viewpoint, petioles of foliage leaves are subject to contradictory mechanical requirements. High flexural rigidity guarantees support of the lamina and low torsional rigidity ensures streamlining of the leaves in wind. This mechanical trade-off between flexural and torsional rigidity is described by the twist-to-bend ratio. The safety factor describes the maximum load capacity. We selected four herbaceous species with different body plans (monocotyledonous, dicotyledonous) and spatial configurations of petiole and lamina (2-dimensional, 3-dimensional) and carried out morphological-anatomical studies, two-point bending tests and torsional tests on the petioles to analyze the influence of geometry, size and shape on their twist-to-bend ratio and safety factor. The monocotyledons studied had significantly higher twist-to-bend ratios (23.7 and 39.2) than the dicotyledons (11.5 and 13.3). High twist-to-bend ratios can be geometry-based, which is true for the U-profile of Hosta x tardiana with a ratio of axial second moment of area to torsion constant of over 1.0. High twist-to-bend ratios can also be material-based, as found for the petioles of Caladium bicolor with a ratio of bending elastic modulus and torsional modulus of 64. The safety factors range between 1.7 and 2.9, meaning that each petiole can support about double to triple the leaf’s weight.
Highlights
The petioles of foliage leaves fulfill various functions with sometimes contradictory demands
To analyze the form-function relationship of the selected petioles, we focused on four key aspects: geometry, size, shape and biomechanics
High twist-to-bend ratios allow the petiole to be stiff enough to withstand bending loads caused by the selfweight of the leaf and to be flexible enough to twist away from damaging influences such as wind loads
Summary
The petioles of foliage leaves fulfill various functions with sometimes contradictory demands. Of particular importance is the mechanical compromise of high flexural rigidity, which allows the petiole to be stiff enough under the bending load of the self-weight of the leaf (Vogel, 1992; Niklas, 1999), and a comparatively low torsional rigidity, which enables the petiole to be flexible enough to avoid damaging influences by wind loads (Vogel, 1989; Niklas, 1996) This biomechanical trade-off is reflected in the dimensionless twist-to-bend ratio (EI/GJ) (Vogel, 1992), which describes the flexural (bending) rigidity (EI) compared with the torsional rigidity (GJ) (Wainwright et al, 1976; Vogel, 1992; Etnier, 2003).
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