5. Tribe Eupatorieae Cass.

(King and Robinson, 1987)

Plants annual (in Ageratum) or perennial herbs (shrubs elsewhere), sometimes from a woody rootstock or corm, occasionally twining climbers, the sap not milky. Stems not spiny or prickly. Leaves alternate, opposite, or whorled, sometimes also in a basal rosette, sessile to long-petiolate, not spiny or prickly. Leaf blades entire to pinnately dissected, the venation mostly pinnate, with 1 or 3(–7) main veins. Inflorescences terminal or less commonly axillary panicles, spikes, or racemes, rarely appearing as solitary or clustered axillary heads. Heads discoid. Involucre of 2 to several series of bracts (1 series in Mikania), these overlapping, of more or less equal to strongly unequal lengths, not spiny or tuberculate. Receptacle flat to slightly convex, less commonly conical, naked (chaffy elsewhere). Disc florets all perfect, the corolla white, pink, or purple to nearly blue, the 5 short or less commonly long lobes spreading to ascending. Pappus most commonly of usually numerous capillary bristles, less commonly of relatively few scales or awns, more or less persistent at fruiting. Stamens with the filaments not fused together or fused into a short tube toward the base, the anthers fused into a tube, each tip with a flattened appendage, each base truncate or broadly rounded. Style branches usually not flattened, each with a short stigmatic line along each inner margin, the sterile tip elongate, usually with dense, minute papillae. Fruits monomorphic, mostly several-angled or several-ribbed in cross-section, oblong to slightly wedge-shaped in profile, not beaked. About 170 genera, about 2,400 species, nearly worldwide, but most diverse in the New World.

The modern classification of genera of Eupatorieae largely has been the work of Robert Merrill King and Harold Robinson (both then at the Smithsonian Institution). Starting with a lengthy series of morphological taxonomic studies published over more than two decades and culminating in a comprehensive book-length monograph of the tribe (King and Robinson, 1987), the generic classification of the tribe is among the most thoroughly researched projects in the family. Subsequent molecular studies (Schilling et al., 1999; Schmidt and Schilling, 2000; Ito et al., 2000a, b) have largely supported King and Robinson’s insightful taxonomic observations of morphological and anatomical details. As has been the trend in other tribes of Asteraceae, these studies have resulted in the dissection of the large, polymorphic traditional genus Eupatorium into a series of smaller, more homogeneous genera, four of which occur in Missouri.