Realignment of the Species Placed in Exogonium (Convolvulaceae)

Abstract

Exogonium has never been widely accepted as a genus although there have been proponents of this rank since Choisy (1834, 1838, 1845) first described the taxon. House (1908), Matuda (1963) and Standley & Williams (1970) have been among the recent authors keeping the species as a separate genus. Others have suggested that the species could better be ranked at some infrageneric level. Grisebach (1864) reduced it to a section of Ipomoea, while Meisner (1869) considered the plants a subgenus. Since the origin of the name Exogonium by Choisy (1834) 31 species have been placed in the taxon, many authors varying the definition of the group slightly. Although usually unstated, the major criteria for inclusion in the taxon were red flowers, salverform corollas, and exserted stamens and stigmas. Floral morphology suggests that the species included in Exogonium are mostly adapted for bird pollination; the species exhibit the characters classically associated with this syndrome (van der Pijl, 1960, 1961; Meeuse, 1961; Percival, 1965; Faegri & van der Pijl, 1971). However, a polyphyletic taxon has been created because species from several lines have been lumped solely on the basis of a common pollination system. The following treatment is a revision of the binomials placed in Exogonium. Several other related species are also included. Some of the nomenclatural and biological problems within Ipomoea have been discussed elsewhere (Verdcourt, 1957, 1963; Austin, 1975a, 1975b). Consideration of all morphological criteria indicates that the species proposed for inclusion in Exogonium should be placed in the following taxonomic groups.