Selaginella boomii (Selaginellaceae – Lycopodiophyta): A new and widely distributed spikemoss from South America IVÁN A. V ALDESPINO Departamento de Botánica, Facultad de Ciencias Naturales, Exactas y Tecnología, Universidad de Panamá, Apartado Postal 0824-00073, Panamá, Panamá; e-mail: iavaldespino@gmail.com Abstract. Selaginella boomii is described as a new species and compared to the similar S. roraimensis. The new species is widely distributed in Venezuela, Guyana, Suriname, and Brazil where it grows on granite outcrops and boulders in forests and savannas. Selaginella boomii is illustrated with Scanning Electron Micrographs of stem sections, leaves, and spores, and its conservation status is discussed based on IUCN Categories and Criteria. This new species is classified within subgenus Stachygynandrum because of its heteromorphic vegeta- tive leaves and monomorphic sporophylls that are arranged in quadrangular strobili. Finally, a brief overview on the current number of Selaginella species known from South America is provided to highlight that our knowledge of the diversity of this genus in the region is far from complete. Key Words: Boulders, granite outcrops, lycophyte, savanna. Resumen. Selaginella boomii se describe como una especie nueva y se compara con la similar, S. roraimensis. La nueva especie está ampliamente distribuida en Venezuela, Guayana, Surinam y Brasil, donde crece sobre afloramientos rocosos de granito y rocas en áreas de bosques y de sabanas. Selaginella boomii es ilustrada con micrografías del Microscopio Electrónico de Barrido de secciones del tallo, de las hojas y de las esporas, mientras que su estado de conservación se discute con base a las Categorías y Criterios de IUCN. Esta nueva especie se clasifica en el subgénero Stachygynandrum ya que posee hojas vegetativas heteromórficas y esporofilos monomórficos que se disponen en estróbilos cuadrangulares. Finalmente, se provee una sinopsis sobre el número actual de especies de Selaginella que se registran en Suramérica para subrayar que nuestro conocimiento de la diversidad de éste género en dicha región es todavía incompleto. Selaginella P. Beauv. is the largest lycophyte genus and one of the oldest seedless vascular plant lineages with fossils dating back 333–350 Myr ago (Banks, 2009). It is widely distributed through the world but is most diverse and species- rich in tropical and subtropical regions, where an estimated 600–750 species occur (Jermy, 1990; Valdespino, 1993a ; Mickel et al., 2004 ; Valdespino et al., 2015). The genus grows under a wide range of climate, soil, and light regimes (Korall & Kenrick, 2002). For South America, the only treatment of Selaginella is that of Alston et al. (1981), where 133 species, including five subspecies and one variety, are recognized. That treatment contains a key to the taxa, species synonymy, a list of collectors and collectors ’ numbers, and descriptions for a few newly described taxa there- in. However, the previously published S. carinata R.M. Tryon (Tryon, 1955), which is only the third known isophyllous (subg. Tetragonostachys Jermy) species in South America, is overlooked. Subsequently, new species of Selaginella were described for the continent, including one by Steyermark and Smith (1986), thirteen by Smith (1990), three by Valdespino (1992), four by Kessler et al. (2006), two by Valdespino (see Cremers & Boudrie, 2007, and Gibby, 2007), two by Valdespino ( 2015 ), and seven by Valdespino et al. (2015). Nine of the taxa treated by Alston et al. (1981) and one of the species described by Kessler et al. (2006) from South America, as well as one included in Alston (1934) for Trinidad and Tobago, are now known Brittonia, DOI 10.1007/s12228-015-9398-9 ISSN: 0007-196X (print) ISSN: 1938-436X (electronic) © 2015, by The New York Botanical Garden Press, Bronx, NY 10458-5126 U.S.A.