The segregate genera of Eupatorium (Asteraceae)
Eupatorium, as traditionally (i.e., broadly) defined, is a large and variable genus. It incorporates plants with tremendous differences in morphology under its generic umbrella (i.e., more variability than many other genera in the Asteraceae, making it an inconsistent ranking system). Robert King and Harold Robinson wrote a series of papers that provided arguments for the dismantling of Eupatorium into smaller, more homogenous splinter genera. Their work was supported by detailed morphological descriptions after world-wide review of species (King and Robinson 1970a, 1970b). Furthermore, King and Robinson used chromosome numbers and biogeography to provide additional data for recognition of taxa. Unfortunately, most floristic manuals chose not to recognize their work, delaying the acceptance of a more appropriate taxonomic system. This was likely due to the fact that many of the characters used to delimit genera were relatively microscopic (similar to the situation with Aster). Schmidt and Schilling (2000) used nuclear ribosomal DNA to test the theories of King and Robinson. The results of that molecular study showed that a broadly defined Eupatorium was polyphyletic (i.e., artificial). If Eupatorium was to be maintained as treated in most floristic manuals, it would need to also include many other genera, one of these being Liatris (blazing stars). Combined data sets show strong support for the need to redefine Eupatorium. Figures 1–3 show some of the micromorphological characters used to define segregate genera.
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| Figures 1 (left), 2 (middle), and 3 (right). Corollas of Eupatorium and related genera. 1—Eupatorium; note relatively lack of specialized features found in segregate genera. 2—Eutrochium; note anthocyanic color (i.e., pink to purple) and setae along connate (i.e., fused) portion. 3—Ageratina; note non-anthocyanic color and papillae on adaxial surface of corolla lobes. | ||
Following are keys to New England segregate genera of Eupatorium.
Key 1 (based primarily on macromorphological characters)
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1a. Leaves both opposite and borne on distinct petioles |
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2a. Involucral bracts all of similar length (1 or 2 outer, very small bracts may also be present); pappus bristles tapering to a fine point ............................. Ageratina |
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2b. Involucral bracts of (2–)3 or more conspicuously different lengths; pappus bristles blunt at the apex ............ (E. serotinum, a rare introduction) Eupatorium |
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1b. Leaves opposite and +/- sessile (i.e., the petioles, if present, ill-defined), or whorled |
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2a. Leaves in whorls of 3–7; corollas, and often also the involucral bracts, pink to purple; involucres usually cylindric in flower, the margins parallel or slightly upwardly flared ............................................................................................. Eutrochium |
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2b. Corollas white (pink to purple in rare forms of E. perfoliatum); phyllaries variously colored, usually with green and white, but not pink to purple; leaves usually opposite (whorled in E. hyssopifolium); involucres usually with a distinct upward flare in flower, obviously narrower near base compared with apex .......................................................................................... (in part) Eupatorium |
Key 2 (based on micromorphological characters—view at 40× or higher)
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1a. Adaxial (i.e., inner) surface of corolla lobes papillose (i.e., with a layer of projecting cells); style base enlarged, glabrous ...................................... Ageratina |
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1b. Adaxial surface of corolla lobes smooth; style base minutely pubescent, often not enlarged |
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2a. Corollas pink to purple, with stomates on the abaxial (i.e., outer) surface of the lobes; connate portion of corolla (i.e., the tube) and/or body of cypsela with setae ........................................................................................... Eutrochium |
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2b. Corollas without stomates on the abaxial surface; corolla and body of cypsela glabrous, lacking setae ..................................................... Eupatorium |
Literature Cited
King, R.M. and H. Robinson. 1970. Studies in the Eupatorieae (Compositae). XIX. New combinations in Ageratina. Phytologia 19: 208–229.
King, R.M. and H. Robinson. 1970. Studies in the Eupatorieae (Compositae). XXV. A new genus Eupatoriadelphus. Phytologia 19: 431–432.
Schmidt, G.J. and E.E. Schilling. 2000. Phylogeny and biogeography of Eupatorium (Asteraceae: Eupatorieae) based on nuclear ITS sequence data. American Journal of Botany 87: 716–726.