Exostema (Rubiaceae): Taxonomic history, nomenclature, position and subgeneric classification

The taxonomic history of the genus Exostema (Rubiaceae) is reviewed The authority Exostema (Pers.) Bonpl. is accepted here, and reasons for rejecting alternatives are given. A lectotype is designated for E. caribaeum (Jacq.) Roem. & Schult., the here accepted lectotype species for the genus. Tribal affinity of Exostema remains uncertain, however the genus is clearly excluded from the Cinchoneae, and appears closely related to Coutarea; associations between these two genera and the Portlandia group or members of the Chiococceae suggested by various authors are considered plausible. Infrageneric classifications of Exostema are reviewed and a new sectional subdivision of the genus is presented. Three sections are identified on the basis of inflorescence position and flower length, and are supported as natural groups by additional morphological and anatomical characters. Descriptions for the genus and sections and a key to the sections are provided.