S S symmetry Article A Superellipse with Deformation and Its Application in Describing the Cross-Sectional Shapes of a Square Bamboo Weiwei Huang 1, Yueyi Li 1, Karl J. Niklas 2, Johan Gielis 3,* , Yongyan Ding 1, Li Cao 1 and Peijian Shi 1,* 1 Bamboo Research Institute, College of Biology and the Environment, Nanjing Forestry University, Nanjing 210037, China;
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[email protected] (P.S.); Tel.: +86-25-8542-7231 (P.S.) Received: 23 November 2020; Accepted: 9 December 2020; Published: 14 December 2020 Abstract: Many cross-sectional shapes of plants have been found to approximate a superellipse rather than an ellipse. Square bamboos, belonging to the genus Chimonobambusa (Poaceae), are a group of plants with round-edged square-like culm cross sections. The initial application of superellipses to model these culm cross sections has focused on Chimonobambusa quadrangularis (Franceschi) Makino. However, there is a need for large scale empirical data to confirm this hypothesis. In this study, approximately 750 cross sections from 30 culms of C. utilis were scanned to obtain cross-sectional boundary coordinates. A superellipse exhibits a centrosymmetry, but in nature the cross sections of culms usually deviate from a standard circle, ellipse, or superellipse because of the influences of the environment and terrain, resulting in different bending and torsion forces during growth.