Serpocaulon (Polypodiaceae), a new genus segregated from Polypodium

Abstract

We describe the neotropical fern genus Serpocaulon (Polypodiaceae), segregated from Polypodium, and make new combinations for the forty known species. Serpocaulon has been recognized previously as a putatively natural group by several authors, based on morphological evidence, but never accorded generic rank. The monophyly of Serpocaulon is strongly supported in chloroplast DNA‐based phylogenetic analyses, using the genes rbcL, rps4, rps4‐trnS intergenic spacer, and trnL‐F intergenic spacer on 29 samples from 22 species. Serpocaulon is readily separated from other subclades within Polypodiaceae, and especially from Polypodium, which is less closely related to Serpocaulon than are several other genera. Serpocaulon differs from other New World polypods in the combination of usually long‐creeping, sparingly branched rhizomes with clathrate (sometimes strongly so), usually peltately attached scales; regularly anastomosing (goniophlebioid) veins with individual areoles chevron‐shaped and each with a single, free, included excurrent veinlet; and non‐paraphysate sori in one to 10 rows between costae and pinna margins. In Polypodium, the rhizome scales are not clathrate, ± concolorous, and invaginated at the base, with somewhat overlapping basal lobes, and rhizomes are generally shorter‐creeping and more frequently branched, the veins are free or forming a single row of areoles, and the sori uniseriate on each side of the costae. Most species of true Polypodium are north‐temperate, Mexican, or Mesoamerican in distribution, while Serpocaulon is entirely neotropical or subtropical, with the greatest number of species in South America. There appears to be no convenient infrageneric division of the species within Serpocaulon, although the most widespread species, S. triseriale, is somewhat isolated at the base of the clade. Species with the thinnest rhizomes also have the fewest scales, and generally grow at high elevations. Species having the scaliest rhizomes, with scales spreading and strongly overlapping, form a weakly supported subclade, as do species with multiseriate rows of areoles between costae and pinna margins.