Eupatorium
See list of 11 species in this genusThe traditionally defined genus Eupatorium has been dismantled into smaller, homogeneous, and monophyletic groups based on morphological and phylogenetic evidence (see also the genera Ageratina and Eutrochium). Most authors’ treatments of Eupatorium (including the one referenced here) suffer from poor taxonomy—they repeatedly treat hybrid-derived taxa as a variety of one of the putative parents, a system that creates obvious conflicts due to its non-monophyletic and arbitrary nature (i.e., which parent should the hybrid be allied to, and on what basis?). Hybrid-derived taxa are here treated as species because they are reproductively competent and can be found outside of the range of parental co-occurence. Reference: Siripun and Schilling (2006).
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1a. Capitula with 9–23 flowers; leaves with either evident petioles longer than 1 cm or the blades connate at the base
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2a. Leaf blades narrowly to broadly tapering to a distinct petiole
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2b. Leaf blades broad-based, usually connate and surrounding the stem
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1b. Capitula with (3–) 5 (–9) flowers; leaves sessile or subsessile, not connate at the base
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3a. Leaf blades linear to elliptic or oblanceolate, cuneate (i.e., tapering) at the base
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4a. Involucre 8–11 mm tall; involucral bracts glabrous abaxially or sparsely pubescent near the base
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4b. Involucre 4–7 mm tall; involucral bracts moderately to densely pubescent over the abaxial surface
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5a. Involucral bracts acute to acuminate and prominently white-scarious at the apex [Fig. 403]
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6a. Principal leaf blades 8–15 (–20) mm wide, usually ± evenly serrate with narrow, sharp teeth; capitula with 5–7 (–9) disk flowers; body of cypsela 2–3 mm long
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6b. Principal leaf blades (10–) 20–40 mm wide, coarsely and sometimes irregularly serrate to crenate-serrate with broad, pointed to blunt teeth [Fig. 404]; capitula with (4–) 5 (–6) disk flowers; body of cypsela 3–4 mm long (in part)
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5b. Involucral bracts rounded to obtuse (sometimes acute) at the apex, with only a thin, white-scarious border near apex [Fig. 405]
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7a. Leaf blades 8–30 mm wide, lanceolate to elliptic-oblong, mostly 2.5–7 times as long as wide, opposite; nodes without axillary fascicles or these poorly developed and consisting of few leaves
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7b. Leaf blades 0.5–10 (–17) mm wide, linear to lanceolate, mostly 6–40 times as long as wide, whorled in 3s or 4s at each node (sometimes opposite or alternate at the upper nodes); nodes often with well-developed axillary fascicles containing many small leaves
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8a. Leaves usually 4 per node, sometimes only 3 (rarely fewer), with blades 0.5–5 (–10) mm wide, entire or irregularly few-toothed, 10–40 times as long as wide, the principal leaves with blades 2–7 cm long [Fig. 402]; reproductive stems 30–100 cm tall
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8b. Leaves usually 3 per node, sometimes 2 or 4 per node (rarely only 1), with blades 5–10 (–17) mm wide, serrate to nearly lacerate-toothed, 6–15 times as long as wide, the principal leaves with blades 6–11 cm long; reproductive stems mostly 75–150 cm tall
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3b. Leaf blades lanceolate or oblong to suborbicular, broad-cuneate to cordate at the base [Figs. 404,406]
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9a. Leaf blades pinnately veined, lanceolate to oblong, acuminate to subacuminate at the apex; stem glabrous
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9b. Leaf blades triple-veined from near the base, oblong to suborbicular, rounded to acute at the apex; stem pubescent
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10a. Leaf blades oblong or oblong-lanceolate to narrow-ovate, each margin with 3–12 coarse teeth, sometimes the blades more incised with lobe-like teeth near the base [Fig. 404]; upper leaves and lower branches of capitulescence often alternate (in part)
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10b. Leaf blades oblong-ovate to suborbicular, each margin with 8–25 teeth [Fig. 406]; upper leaves and lower branches of capitulescence usually opposite
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11a. Leaf blades triangular-ovate to suborbicular, mostly 1–1.5 (–1.7) times as long as wide, rounded to obtuse at the apex, usually with crenate margins; principal pair of lateral veins originating above base of leaf blade
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11b. Leaf blades oblong-ovate to ovate, mostly 1.5–2 times as long as wide, obtuse to acute at apex, usually with serrate margins [Fig. 406]; principal pair of lateral veins originating at the base of leaf blade
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Each photo represents one species in this genus.