Ruehssia sumiderensis (Apocynaceae), a new species from Chiapas state, Mexico

It is herein presented Ruehssia sumiderensis as a new species known only in the Callon del Sumidero National Park, in Chiapas. Mexico. It is illustrated with a line drawing, live images and SEM photographs of the gynostegium and pollinarium. Morphological and molecular evidence with plastid data (trnL intron and trnL-F intergenic spacer) confirms its position in Ruehssia, a recently proposed genus that includes American Marsdenieae species. It is similar to Marsdenia laxiflora and M pinetorum due to the basally rounded or truncate leaves, rotate corolla without callous cushions in the sinuses. Two new combinations are also made in Ruehssia, for M. laxiflora and M pinetorum.