The genus Platanthera (Orchidaceae) in New Guinea with description of Platanthera valkenburgii sp. nov.

Platanthera Rich. is a genus of ca. 125 species with a predominantly Northern Hemisphere distribution. A recent monograph of the genus did not cite important material from New Guinea. On the island the genus is represented by species from the tropical section Mecosa (Blume) Kraenzl. of subgenus Platanthera, which are not widely represented in herbaria and each specimen has a high value for taxonomic research. Studied materials not cited in the revision of the genus are cited as additional examined specimens here. A list of species with taxonomic and nomenclatural decisions is provided as a summary of knowledge about the genus on the island. Five taxa of Platanthera are recorded in this work for the region of New Guinea. One new taxon, Platanthera valkenburgii S.Nowak, Efimov, Szlach. & Kolan., is described. illustrated, and a photograph of the holotype is provided. The new species is readily distinguished by its strongly thickened to keel-like mid vein of the floral bract and dorsal sepal, petals at least as wide or wider than they are long, and its distinctly dilated lip. A key to the New Guinea taxa is provided. Two lectotypes are designated and taxonomic problems within Platanthera klossii (Ridl.) Efimov are discussed; one new combination is made.