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An expanded circumscription of the genus Podostemum Michx. to include Crenias Spreng.and Devillea Tul. & Wedd. is presented. Justification for the expanded genus concept is based on phylogeneticanalyses of morphological characters. Three synapomorphies support the monophyly of Podostemum:anisolobous ovary, capsule at an angle on the pedicel, and one capsule valve caducous. Eleven species (includingtwo subspecies) are recognized in Podostemum. One new name (Podostemum ovatum) and five new combinations(Podostemum flagelliforme, P. rutifolium subsp. ricciiforme, P. saldanhanum, P. scaturiginum, P. weddellianum)are proposed. Ten New World species of Podostemum that were accepted by earlier authors areinterpreted as environmental forms of widespread species and their names reduced to synonymy. The monographincludes sections on morphology, ecology, key to species, descriptions, illustrations, distribution maps,and lists of specimens examined. Podostemum is restricted to the New World. The region of greatest species diversityis southeastern Brazil and Argentina, where all but three species occur. Podostemum rutifolium subsp.ricciiforme occurs primarily in Mexico, with disjunct populations in Belize, Costa Rica, and Colombia. Podostemumceratophyllum ranges from southeastern Canada to the south-central U.S.A., with disjunct populationsin the Dominican Republic and Honduras.
