Marsilea

Kessler, Michael & Smith, Alan R., 2017, Prodromus of a fern flora for Bolivia. XV. Marsileaceae, Phytotaxa 329 (1), pp. 93-96 : 94

Marsilea L., Sp. Pl. 2: 1099. 1753.

Unmistakeable by its clover-like leaves. Further characters of the genus include generally long-creeping, narrow rhizomes; dichotomously veined leaflets that are pulvinate at the base, the veins anastomosing regularly or near the margin to form a reticulum of elongate polygonal areoles; septate hairs often present on the rhizomes, leaves, and on sporocarps; spores borne in laterally attached, hardened, short-stalked globose or bean-shaped sporocarps that arise along the lower part of the petiole or from the petiole base or rhizome; and sporangia lacking an annulus. Marsilea grows in flooded or seasonally flooded habitats, especially along the edges of pools and marshes. Floating leaves produced during flooding can look quite different from the land leaves produced when the habitat is dry. The plants usually become fertile only when the habitat dries out. Where the habitat remains wet year-round, plants usually remain sterile, multiply through creeping rhizomes, and are often difficult to identify. A subcosmopolitan genus of about 45–60 species, with about 12 species in the Neotropics ( Johnson 1986).

Marsilea is likely sister to the clade Regnellidium + Pilularia ( Pryer 1999) . A molecular phylogeny produced by Schaefer et al. (2011) indicates that the Bolivian species fall within 2 main clades: 1) the M. mollis clade, represented by M. mollis , with sporocarps borne singly at bases of petioles (other species in this clade primarily in North America); and 2) the M. mutica clade, represented by M. crotophora and M. polycarpa , all largely tropical species with more numerous sporocarps borne along the petioles. Additional molecular studies of plastid genes by Whitten et al. (2012) provided evidence that many of the neotropical species of Marsilea may not be monophyletic. They suggested several possible reasons for this result: 1) extensive hybridization of species, which might have led to chloroplast capture of plastid types; 2) the existence of previously unrecognized cryptic species; and 3) a possibly inflated number of species within the sect. Notorhizae clade, which, however, is not represented in Bolivia.