Similarities in structural organization of the culm in Poaceae, Juncaceae, and Cyperaceae such as leaf sheaths and the presence of intercalary meristems at every node suggest the same mechanical properties and, accordingly, the same functionality. Meristems are zones of tissue formation, which constitute areas of weakness along the entire culm and provide the basis for rapid shoot elongation. Leaf sheaths clasp the culm preventing the shoot from breaking, ensuring the rigidity to grow erectly and to avoid damage of the meristematic tissue. The mechanical influence of leaf sheaths was investigated in members of Poaceae, Juncaceae, and Cyperaceae in the flowering stage. Mechanical properties of Poa araratica, Bromus erectus, Arrhenatherum elatius (Poaceae), Luzula nivea (Juncaceae), and Carex arctata (Cyperaceae) were determined in three-point bending before and after the removal of leaf sheaths. The presence of leaf sheaths results in smoothing the distribution of flexural rigidity and therefore avoids stress peaks. The achieved maxima of relative contribution of leaf sheaths to entire flexural rigidity ranged from 55% up to 81% for Poaceae, 72% for C. arctata, and 40% for L. nivea. Across the investigated families, the mechanical role of leaf sheaths could be verified as essential for culm stability during development and beyond. 1. Introduction One characteristic of the morphological feature of grass culms is the presence of intercalary meristems above every node. The growth is not restricted to one apical meristem; rather, the culm is able to grow at every internode [1, 2]. Therefore, the culm is showing frequent alternations of weak, nonreinforced and stiff, fully developed tissues below and above the nodes particularly during development. The increased vertical growth speed, enabled through many areas of growth, comes at a cost. Additional stabilizing structures to reinforce the less stable meristems have to be provided. The leaf sheaths envelope the culm in the most fragile section avoiding possible damage by providing the crucial stiffness [3–5]. Culms of Poaceae without regard to their leaf sheaths are studied concerning flexural rigidity, Young’s modulus, the relation between bending and torsional stiffness, and with regard to anatomy [2, 6–10]. The mechanical significance of leaf sheaths especially for necessity in development of culms was shown for Arundo donax [11] and Miscanthus [12]. Zebrowski [5] paid special attention on this aspect in Triticale as well as Niklas [3, 4] for Arundinaria tecta and Avena sativa. The contribution of leaf
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