This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Erect, mostly branching, perennial herbs, with opposite or verticillate, or sometimes alternate, often punctate leaves, and in our species cymose-paniculate discoid heads of white, blue or purple flowers. Involucre oblong, ovoid, campanulate, or hemispheric, the bracts imbricated in 2-several series. Receptacle flat, convex, or conic, naked. Corolla regular, its tube slender, its limb 5-lobed or 5-toothed. Anthers obtuse and entire at the base, appendiculate at the apex. Style-branches elongated, flattened, or thickened above, stigmatic at the base. Achenes 5-angled, truncate. Pappus of numerous capillary usually scabrous bristles arranged in 1 row. [Named for Mithridates Eupator, i. e., of a noble father.]
Over 500 species, mostly of warm or tropical regions. Besides the following, some 35 others occur in the southern and western parts of North America. Type species: Eupatorium canna~ binum L.
* Leaves alternate, pinnatifid into filiform segments.
1. E. capillifolium.
** Leaves petioled, verticillate in 3's-6's,or the upper opposite; involucral bracts in several series. Leaves thick, rugose, pubescent; inflorescence depressed. Leaves ovate, acute.
2. E. maculatum.
Leaves lanceolate to ovate-lanceolate, acuminate.
3. E. Bruneri.
Leaves thin, nearly glabrous; inflorescence pyramidal.
4. E. purpureum.
*** Leaves opposite (rarely in 3's), or the uppermost alternate.
† Involucral bracts imbricated in 2 or more series, the outer shorter. 1. Leaves, at least the lower, slender-petioled.
5. E. serotinum.
2. Leaves all sessile, short-petioled or connate-perfoliate. a. Leaves not clasping nor connate-perfoliate. § Leaves narrowed at the base. Bracts of the involucre acute or cuspidate, scarious-tipped, white. Leaves linear-lanceolate, sparingly toothed, 2"-6" wide.
6. E. leucolepis.
Leaves oblong or lanceolate, coarsely toothed, 1/2 '-1 1/2' wide.
7. E. album.
Bracts of the involucre obtuse, not scarious, or but slightly so. Leaves linear, crowded, usually entire, obtuse.
8. E. hyssopifolium.
Leaves lanceolate, oval, or oblong, usually dentate. Leaves linear-lanceolate, acute, dentate.
9. E. Torreyanum.
Leaves oblong to oval, sharply dentate, obtusish or acute.
10. E. semiserratum.
Leaves lanceolate, sparingly dentate, long-acuminate.
11. E. altissimum.
§§ Leaves rounded, obtuse or truncate at the base. Plant glabrous; leaves lanceolate, long-acuminate.
12. E. sessilifolium.
Plants pubescent; leaves ovate or oblong, acute or obtuse.
Leaves ovate-oblone. rounded or narrowed at the base, usually obtuse.
Leaves broadly ovate, crenate-dentate, mostly truncate at the base, obtusish.
Leaves ovate, dentate, acute.
15. E. pubescens.
b. Leaves clasping or connate-perfoliate at the base. Leaves connate-perfoliate; involucral bracts acute.
16. E. perfoliatum.
Leaves merely clasping; involucral bracts obtuse.
†† Involucral bracts in 1 or 2 series, all equal or nearly so.
17. E. resinosum.
Flowers white; leaves ovate.
Leaves thin. 2'-s' long, sharply dentate, acuminate.
Leaves firm, 1'-2' long, obtusely dentate, acute or obtusish.
19. E. aromaticum.
Flowers pink to purple; leaves deltoid-ovate.
20. E. incarnatum.
Receptacle conic; flowers blue or violet; leaves petioled.
21. E. coelestinum.