STEliS BREviloBA (ORCHIDACEAE, PlEUROTHAllIDINAE), A NEW sPECIEs FROM MEXICO

Background: Stelis subgen. Physosiphon is basically a Mesoamerican group, and its best-known species is Stelis emarginata, characterized by its reddish orange flowers and distribution along the Gulf of Mexico slope in Mexico and Guatemala. Morphologically similar specimens from the Sierra Madre del Sur, on the Pacific slope of Guerrero and Oaxaca could represent a different and undescribed taxon.Question: Are the Sierra Madre del Sur populations similar to Stelis emarginata a different species?Study site: Sierra Madre del Sur in Guerrero and Oaxaca, Mexico. Methods: The description of the new species was based on live and pressed specimens; a line drawing and photographs were made from fresh material; its known localities were georeferenced and displayed on a map of Mexico. A morphological comparison of the new taxon with similar species was done based on specialized literature.Results: Stelis breviloba is described and illustrated as a new species from southern Guerrero and Oaxaca. It differs from S. emarginata by its shorter plants, less-floriferous racemes, smaller flowers, oblong petals, and its allopatric distribution, restricted to the Pacific slope of the Sierra Madre del Sur. A key for the identification of the Mesoamerican species of Stelis subgen. Physosiphon is provided.Conclusion: Stelis breviloba is an endemic of the Sierra Madre del Sur and probable vicariant species of S. emarginata. Most species of Stelis subgen. Physosiphon (six out of eight) are present in the Mexican flora.