A foundation monograph of Ipomoea (Convolvulaceae) in the New World

Infrageneric classifications
An inevitable result of the situation described in the previous paragraphs is that all existing infrageneric classifications of Ipomoea are to a degree unnatural and many subgroups are neither monophyletic nor well-defined, something that goes far towards explaining the instability of all previous infrageneric classifications.
Choisy (1845) divided Ipomoea into groups based on habit but, while superficially practical, this is clearly artificial and so has only been occasionally and partially adopted by subsequent authors, such as Meisner (1869) and Matuda (1964, 1965, 1966a,b). Grisebach (1862b) reincorporated Quamoclit, Calonyction and Pharbitis into Ipomoea as sections and this treatment was followed by Bentham in Bentham and Hooker (1876), although these authors also incorporated other elements such as Aniseia Choisy within Ipomoea. Clarke (1883) followed Grisebach but treated the sections as subgenera. Hallier (1893b) began the introduction of a hierarchy of infrageneric taxa by recognizing subsections as well as sections and this process was continued by House (1908b), who multiplied the number of subsections. There was then a lull in attempts at an infrageneric classification of American species (O’Donell (1941 and passim) appears to have had no interest) until a major reformulation was made by Austin (1979, 1980), who recognized an even more extensive hierarchy with sections, subsections and series. Austin’s work culminated in a detailed and very complex hierarchical classification published 16 years later (Austin and Huáman 1996).
Apart from the repeated changes of status that these infrageneric taxa have undergone resulting in many groupings being re-graded from subgenus to section to series, the increasing multiplication of infrageneric taxa illustrates the difficulties of achieving a satisfactory classification. Apart from Quamoclit, Pharbitis, Calonyction, Old World Astripomoea and eventually Batatas (Austin 1975a, 1978b), none of these groupings are entirely natural, not even Stictocardia or Arborescens (McPherson 1981), both of which comprise a diagnostic and readily identifiable core of species but with a varying number of other heterogeneous elements. The essential instability of these classifications is well illustrated by the history of series Anisomeres. This was recognized by Austin and Huáman (1996) as a distinct series but was discarded a year later in a paper dissolving the Anisomeres series (Austin 1997). We believe that any attempt to provide a subgeneric classification following a traditional Linnaean model is bound to be artificial, impractical and doomed to failure (Carine and Scotland 2002).