Twist-to-Bend Ratios and Cross-Sectional Shapes of Petioles and Stems

Two structural properties, resistance to twisting (torsional rigidity or torsional stiffness, GJ) and resistance to bending (flexural rigidity or flexural stiffness, El), were measured on a variety of herbaceous stems and petioles. Specimens with non-circular cross-sections had higher values of the ratio of El to GJ that is, such specimens were relatively more flexible in twisting than in bending. But both kinds had higher ratios than those that characterize simple, isotropic materials, and thus both structural and material factors contribute to achieving a high twist-to-bend ratio. The composite property expressed as the dimensionless variable EI/GJ appears to be a functionally relevant parameter in many biological situations.