The Mechanical Roles of Clasping Leaf Sheaths: Evidence fromArundinaria técta(Poaceae) Shoots Subjected to Bending and Twisting Forces

Abstract

Representative shoot segments of the grass speciesArundinaria téctaconsisting of one intact internode and its subtending node and clasping leaf sheath were tested to determine the mechanical influence of the leaf sheath on the ability of stems to resist bending and twisting forces. These segments were also used to measure shoot morphometry and composite tissue Young's and shear moduli (EandG,respectively) to simulate the global deformation patterns attending bending and twisting by means of finite element analyses. On average, leaf sheaths contributed 33% of the overall bending stiffness and 43% of the overall torsional stiffness of stem segments. Comparisons betweenEandGof isolated internodes and leaf sheaths indicated that sheaths were composed of stiffer tissues measured either in bending or twisting. Thus, leaf sheaths could act as an external cylindrical brace composed of stiffer materials than those of the internodes they enveloped. The magnitudes of internodalEandGwere correlated with internodal shape such that the ability of internodes to resist twisting relative to the ability to resist bending forces decreased as internodes became more slender or developed thinner walls (both of which occur in an acropetal direction from the base to the tip of shoots). Finite element simulations predicted that, in bending, the leaf sheath laterally braces internodal walls as they tend to ovalize in cross section and push against its inner surface which ovalizes to a lesser extent in the plane normal to the curvature of shoot flexure. In twisting, the successive ovalized transections of internodal walls assumed a helical pattern along the length of shoot segments. This helical deformation pattern was attended by an inner lateral contraction of internodal walls that was less developed in the leaf sheath that thus provided decreasing mechanical support to the internode as the lateral contraction of internodal walls amplified. The twisting of internodes and sheaths was also predicted to concentrate tensile and shear strains in the nodal diaphragm. Here stress intensities sufficient to produce tissue shear failure were concentrated at two opposing points on the surface of the diaphragm. Finite element analyses thus identified a potential weak point in the mechanical construction of hollow, septate shoots that are, nevertheless, more than adequately stiff to support their own weight, yet sufficiently flexible to twist without irreparable damage in normal winds.