30. Epilobium hornemannii Reichenbach, Iconogr. Bot. Pl. Crit. 2: 73. 1824. (as hornemanni).
Hornemann’s willowherb, épilobe de Hornemann
Epilobium nutans Hornemann in G. C. Oeder et al., Fl. Dan. 8: plate 1387. 1810, not F. W. Schmidt 1794
Herbs with short, scaly hypogeous or leafy epigeous soboles. Stems ascending to erect, clumped, terete, 10–45 cm, usually simple, rarely branched proximally, subglabrous proximal to inflorescence with sparsely strigillose lines decurrent from margins of petioles, ± sparsely mixed strigillose and glandular puberulent distally. Leaves opposite proximal to inflorescence, usually alternate distally, petioles 3–9 mm proximally to subsessile distally; blade broadly elliptic to spatulate proximally, ovate to lanceolate distally, ± coriaceous or not, 1.5–6.2 × 0.7–2.9 cm, base attenuate to cuneate or rounded, margins subentire proximally, denticulate distally with 10–25 teeth per side, veins often inconspicuous, 4–7 per side, apex obtuse to subacute, surfaces glabrous or, sometimes, strigillose along margins; bracts reduced. Inflorescences erect or nodding, open racemes, mixed strigillose and glandular puberulent. Flowers erect; buds 2–5.5 × 2–4 mm; pedicel 2–5 mm; floral tube 1–2.2 × 1.3–2.8 mm, sparse ring of hairs at mouth inside or ring absent; sepals sometimes red-tipped or bright red, 2–7 × 1–2.2 mm, abaxial surface sparsely strigillose and glandular puberulent; petals usually rose-purple or magenta to light pink, rarely white, 3–10(–11) × 2–6 mm, apical notch 0.7–2.4 mm; filaments cream to light pink, those of longer stamens 1.4–5(–6) mm, those of shorter ones 1.2–4 mm; anthers light yellow, 0.4–1.2 × 0.3–0.6 mm; ovary 15–25 mm, glandular puberulent, sometimes mixed strigillose; style white or cream, 2–8 mm, stigma cream, clavate or cylindrical, entire, 1.2–3 × 0.5–1 mm, usually surrounded by, rarely exserted beyond, anthers. Capsules 35–65 mm, surfaces glandular puberulent, sometimes mixed strigillose; pedicel 5–15(–25) mm. Seeds narrowly fusiform or oblanceoloid, 0.9–1.6 × 0.3–0.5 mm, chalazal collar short, 0.05–0.1 mm, blond to brown, surface distinctly papillose or reticulate/smooth; coma readily detached, dingy white, 6–11 mm.
Subspecies 2 (2 in the flora): North America, Eurasia.
Epilobium hornemannii occurs widely in montane and boreal regions in North America and western Eurasia, and also in Japan and the Russian Far East. It is characterized by having the CC chromosome arrangement and is included in the Alpinae alliance with E. anagallidifolium, E. lactiflorum, and others (I. Kytövuori 1972).
W. Trelease (1891) discussed eastern and western forms of Epilobium hornemannii, the latter divided into two variations; however, he did not formally recognize any of these variants.
P. A. Munz (1965) included the Eurasian Epilobium alsinifolium Villars in his North American treatment, noting that it occurred in Greenland. However, B. Fredskild (1984) suggested that, for the most part, these determinations represent misidentifications of E. hornemannii.
The two subspecies recognized here intergrade throughout much of their shared range, but whereas subsp. Hornemannii is commonly found in high montane to alpine regions, in the northern part of its range it grows at much lower elevations, and in maritime areas is replaced by coriaceous-leaved forms here designated as subsp. Behringianum. The situation is rather analogous to the pattern seen in E. ciliatum in which subsp. Ciliatum is wide-ranging and variable, but replaced in Pacific maritime areas by subsp. Watsonii, from which it differs consistently in most specimens, but sometimes intergrades.