The ligule is an important character in the taxonomic delimitation of species. Here, the morphology of this organ is described in 11 American species. Four of these species are xerophytic: , and , and seven are mesophytic: , and . Morphological data were obtained by traditional histological techniques. Observations were made by light and scanning electron microscopy. Variations in ligule size in vegetative leaves and sporophylls were determined using a correlation analysis with Spearman's coefficient. The ligule was found to have five body shapes: campanulate, linguiform, elliptic, fan-shaped, and rectangular. Each shape can have a lobed (bi-, tri-, or multi-lobed), rounded, or truncate apex. Size, shape, and apex are characteristic of each species and remain constant in young and mature leaves. No significant differences were observed between xerophytic and mesophytic species. The ligule of all species studied is described for the first time. Micrographs of all the species studied are presented, and the morphological results of this study have been mapped into the recent phylogeny of the genus.

中文翻译：

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美洲卷柏 P. Beauv 叶舌的结构。 （卷柏科）

叶舌是物种分类学界定的重要特征。此处描述了 11 种美洲物种的该器官的形态。其中四种是旱生植物： 、 和 ， 七个是中生植物： 、 和 。形态学数据通过传统组织学技术获得。通过光学显微镜和扫描电子显微镜进行观察。使用斯皮尔曼系数的相关分析确定营养叶和孢子叶中叶舌大小的变化。叶舌有五种形状：钟状、舌状、椭圆形、扇形和矩形。每种形状都可以有叶形（双叶、三叶或多叶）、圆形或截头顶点。大小、形状和顶端是每个物种的特征，并且在幼叶和成熟叶中保持不变。旱生植物和中生植物之间没有观察到显着差异。首次描述了所有研究物种的叶舌。提供了所有研究物种的显微照片，并且本研究的形态学结果已被映射到该属的最新系统发育中。