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FNA | Family List | FNA Vol. 10 | Onagraceae | Epilobium

29. Epilobium luteum Pursh, Fl. Amer. Sept. 1: 259. 1813.

Yellow willowherb

Herbs usually with scaly, sub­terranean soboles, rarely con­densed basal turions. Stems several, ascending to suberect, loosely clumped, subterete, 15–75 cm, simple or sparsely branched distally, subglabrous proximal to inflorescence with densely strigillose lines decur­rent from margins of petioles, mixed strigillose and glandular puberulent distally. Leaves opposite prox­imal to inflorescence, alternate distally, petioles 1–3 mm proximally, subsessile distally; blade ovate or elliptic, 2.5–7.8 × 1.2–3.5 cm, base attenuate, margins den­ticulate with 8–20 low, ± pellucid teeth per side, 4–7 prominent veins per side, apex acute to acuminate, surfaces subglabrous with scattered strigillose hairs on margin and abaxial midrib; bracts much reduced, narrower, more acuminate and denticulate. Inflores­cences nodding, later erect, racemes, congested, simple to sparsely branched, densely mixed strigillose and glandular puberulent. Flowers erect; buds 6–10 × 4–5 mm; pedicel 5–8 mm; floral tube 1.2–3 × 3–4.6 mm, ring of spreading hairs at mouth inside; sepals greenish cream, lanceolate, 10–12 × 3–3.5 mm, abaxial surface densely glandular puberulent; petals cream to pale yellow, 12–22 × 9–13 mm, broad apical notch 1.2–2.4 mm; filaments cream, those of longer stamens 13–16.5 mm, those of shorter ones 10–15 mm; anthers yellow, 2.2–3 × 0.7–1.3 mm; ovary 20–35 mm, densely glan­dular puberulent; style cream, 15–22 mm, stigma 4-lobed, 1–2 × 2.8–4.5 mm, lobes spreading to recurved, strongly exserted beyond anthers. Capsules erect, 35–75 mm, surfaces sparsely glandular puberulent; pedicel 10–22 mm. Seeds narrowly fusiform or oblanceoloid, 1–1.2 × 0.4–0.5 mm, chalazal collar 0.06–0.1 mm, surface reticulate; coma persistent, tawny, 6.5–8 mm. 2n = 36.

Flowering Jul–Sep. Moist rocky slopes, seeps, banks of lakes, streams, springs, and gravel bars along coastal (boreal) to montane or subalpine areas near snowfields; 0–2200 m; Alta., B.C.; Alaska, Calif., Oreg., Wash.

Epilobium luteum is one of the most distinctive species in the genus due to its large creamy flowers, a color otherwise known only in the distantly related E. suffruticosum. It appears that E. luteum is most closely related to species of the Alpinae group, sharing not only similarities in perennating habit and structures, but also the derived CC chromosome arrangement (S. R. Seavey and P. H. Raven 1977, 1978).

Because Epilobium luteum is modally outcrossing and commonly pollinated by bees, it sometimes hybri­dizes with several other species of Epilobium when they grow sympatrically. One recurrent hybrid combination is E. luteum × E. ciliatum subsp. glandulosum, first de­scribed and named E. ×treleasianum H. Léveillé, and later E. luteum var. lilacinum L. F. Henderson. Following a suggestion by P. A. Munz (1965) that E. ×treleasianum was a hybrid of that particular parentage, S. R. Seavey (1993) verified the relationship in a series of crossing experiments. Epilobium ×treleasianum occurs over a wide geographical range in Alaska, British Columbia, and Washington, forming repeatedly when the parental species co-occur, and often backcrossing with one or both parents, forming hybrid swarms with varying morphological combinations. It also grows vigorously vegetatively.

Another less common hybrid of Epilobium luteum was described as E. ×pulchrum Suksdorf, and a minor variant as E. ×pulchrum var. albiflorum Suksdorf (S. R. Seavey 1993). The second parent of these hybrids is less obvious, but based on morphological features, it is most likely E. hornemannii, which also grows frequently in sympatry with E. luteum.


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