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FNA | Family List | FNA Vol. 21 | Asteraceae | Eupatorium

21. Eupatorium rotundifolium Linnaeus, Sp. Pl. 2: 837. 1753.

Roundleaf thoroughwort

Perennials, 40–100+ cm. Stems (from short rhizomes) single, sparsely branched distally, puber-ulent. Leaves usually opposite (distal sometimes alternate, lateral buds dormant or producing 1 pair of leaves); sessile or subsessile; blades ± 3-nerved, usually ± deltate to orbiculate, sometimes ovate, 15–50(–70) × 15–40(–60) mm (lengths mostly 1–2 times widths), bases broadly rounded to truncate, margins crenate to serrate, apices acute, faces puberulent to villous, gland-dotted. Heads in corymbiform arrays. Phyllaries 8–10 in 2–3 series, elliptic to oblanceolate, 2–6 × 0.5–1.5 mm, apices acute, abaxial faces puberulent, gland-dotted. Florets 5; corollas 3–4 mm. Cypselae 2–3 mm; pappi of 30–40 bristles 3.5–4 mm.

Varieties 3 (3 in the flora): e North America.

Eupatorium rotundifolium is one of the more variable species of the genus; its apparent propensity to hybridize with other species has led it to be described as a compilospecies and has complicated its delimitation. Hybrids with E. perfoliatum and E. sessilifolium are sufficiently distinctive to be treated here as distinctive entities (E. ×cordigerum and E. godfreyanum, respectively), and the varieties ovatum and scabridum have also been proposed to be of hybrid origin. The boundaries between varieties are indistinct and individual specimens may be difficult to place with confidence.

1 Leaf blades 3-nerved from bases, bases subtruncate to broadly cuneate, margins crenate   21b Eupatorium rotundifolium var. rotundifolium
+ Leaf blades 3-nerved distal to bases, bases broadly cuneate to cuneate, margins serrate   (2)
2 (1) Leaf blades 30–70 × 30–60 mm, broadest nearmiddles   21a Eupatorium rotundifolium var. ovatum
+ Leaf blades 20–50 × 15–30 mm, broadest nearbases   21c Eupatorium rotundifolium var. scabridum


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