Biomechanics and anatomical investigation on the nodes and internodes of an arborescent monocotyledon: flexure and breakage data sets

ABSTRACTPremise: Bamboos have extremely long internodes, making it possible to measure tensile properties and anatomical features of discrete nodes and internodes. We compared the upper, middle, and lower parts of culms in the black bamboo, Phyllostachys nigra, to determine whether the nodes comply with the Niklas “spring-like joint” model.Methods: Mechanical parameters included flexural stiffness (EI), Modulus of Elasticity (E), load at failure (Fmax), and Modulus of Rupture (MOR). Anatomical features included vascular bundle density, orientation, and fiber cap size. CT scans were used to determine where failure occurred upon breakage of nodes and internodes.Results: Regardless of region within the shoot, nodes were less stiff (had lower E), with weaker tissue (lower MOR) than adjacent internodes. However, Fmax was similar or greater in the nodes, suggesting nodes could absorb considerable strain energy. Vascular bundle density was highest in the top internodes and nodes. Apparently, sustained primary growth thickened the parenchyma matrix between bundles while the size of the fiber caps also increased. Bending experiments and microCT scans indicated that failure occurred first in hollow internodes parallel to the vertical vascular bundles and later in nodal diaphragms horizontal to the long axis.Conclusions: Vascular bundle positioning and composition allow nodes to bend, absorbing considerable strain energy, while resisting breakage, consistent with the spring-like joint model of the node. Sustained primary growth allowed the stems to thicken near the base to thus remain mechanically adequate. The unique anatomical and morphological features contribute to the usefulness of bamboo stems for many products.