Revision of Cuphea sect. Melvilla provides the first study of the section since the monograph by Emil Koehne in 1903 and is part of an on-going revision of the approximately 250 species constituting this New World genus. The section, with six subsections, is one of 13 in Cuphea. It consists of perennial herbs to small shrubs with large, intensely coloured floral tubes mostly more than 20 mm long. The species are distributed from northwestern Mexico and the Caribbean to northern Argentina in locally moist or wet habitats. Forty-two species and four varieties are treated, an increase from the 27 species originally recognized in the section.
The species are restricted either to North America (including Central America and the Caribbean): 23 spp. in two subsections; or to South America: 19 spp. in three subsections. Separate keys are presented for the species of each continent. Species descriptions, accounts of pollen and seed morphology, illustrations, and distribution maps are provided. Chromosome numbers and seed oil composition data are given for many species. Cuphea setifera S.A.Graham from Mexico is described as new. Lectotypes are designated for three subsections of sect. Melvilla and for C. bracteolosa, C. caeciliae, C. cuiabensis, C. grandiflora, C. heydei, C. hybogyna, C. intermedia, C. micropetale var. hirtella, C. niederleinii, C. paradoxa, C. subuligera, and C. watsoniana. Neotypes are designated for C. ignea, C. melvilla, and C. schumannii. Relationships suggested by morphology are compared to results from recent molecular-based phylogenetic studies of Cuphea.
Twenty-seven of the 42 species of sect. Melvilla compose six informal groups of species based on unique suites of morphological characters. They correspond in part to two of the original six subsections. The remaining 15 species combine character states in diverse combinations that do not sort into groups or lineages. The molecular studies indicate that sect. Melvilla is polyphyletic and characterized by extensive homoplasy. Members of sect. Melvilla are distributed in three of the five major clades of the genus. Within the section, increases in flower size, intensity of floral tube and petal colour, and pollinator reward, appear to have evolved together with attraction of large bee and hummingbird pollinators. The present infrasectional classification of sect. Melvilla is not accepted. A new classification of species currently in the section is expected upon completion of the revision of the genus.