Biomechanics and anatomical investigation on the nodes and internodes of an arborescent monocotyledon

Abstract

Premise

Bamboos have extremely long internodes, making it possible to measure tensile properties and anatomical features of discrete nodes and internodes and understand how the plants endure mechanical stress and how the tissues function in commercial products. We compared upper, middle, and lower parts of culms in black bamboo, Phyllostachys nigra, to determine whether the nodes comply with the Niklas spring-like joint model.

Methods

We measured flexural stiffness, modulus of elasticity (E), load at failure (F_max), modulus of rupture (MOR), and density and fiber cap size of vascular bundles and assessed bundle orientations. Microcomputed tomography (microCT) during bending tests were used to determine sites of failure.

Results

Regardless of shoot position, nodes were less stiff (lower E) and weaker (lower MOR) than adjacent internodes. However, F_max was similar or greater in the nodes, suggesting nodes absorb considerable strain energy. Vascular bundle density was highest in the top internodes and nodes. Apparently, in lower nodes, sustained primary growth thickened the ground tissue between bundles, while size of the fiber caps also increased. Whole-shoot bending experiments and microCT indicated that failure occurred first in hollow internodes parallel to the vertical vascular bundles, not at nodes.

Conclusions

Vascular bundle positioning and composition allows nodes to bend and absorb considerable strain energy while resisting breakage, consistent with the spring-like joint model of the node. Sustained primary growth allows stems to thicken near the base to remain mechanically adequate. Bamboo nodes are thus areas of remarkable resilience; they will not break before internode tissue.