Structure of the ligule in American species of Selaginella P. Beauv. (Selaginellaceae)

The ligule is an important character in the taxonomic delimitation of Selaginella species. Here, the morphology of this organ is described in 11 American species. Four of these species are xerophytic: Selaginella convoluta, S. microphylla, S. peruviana, and S. sellowii, and seven are mesophytic: S. anceps, S. diffusa, S. flagellata, S. flexuosa, S. mollis, S. silvestris, and S. sulcata. 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.