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FNA | Family List | FNA Vol. 10 | Onagraceae

5. Epilobium Linnaeus, Sp. Pl. 1: 347. 1753; Gen. Pl. ed. 5, 164. 1754.

Willowherb [Greek epi, on, lobos, pod or capsule, and iov (ion), violet, alluding to violet flower at apex of fruit]

Peter C. Hoch

Herbs, annual or perennial, sometimes suffrutescent, caulescent, often with basal rosettes, fleshy decussate turions, soboles, stolons, which may be tipped with turions, or rarely buds (gemmae) in leaf axils; with woody base or caudex, or with taproots. Stems erect to ascending or decumbent, simple to well-branched distally or sometimes from base, in some species proximal epi­dermis exfoliating, strigillose, glandular-puberulent, villous, often mixed, or glabrous, often with raised hairy lines decurrent from leaf axils. Leaves cauline, sometimes also basal, opposite and decussate proximal to inflorescence or only in proximal pairs, or rarely throughout, alter­nate or, sometimes, fasciculate distally; stipules absent; subsessile to petiolate; blade margins entire or toothed; bracts usually reduced distally. Inflorescences terminal, usually racemes or spikes, rarely panicles, flowers solitary in leaf axils, erect or ascending, sometimes also flowering from proximal nodes; bracteoles absent. Flowers bisexual, usually actinomorphic, rarely zygo­morphic, buds often erect, sometimes recurved; floral tube deciduous (with sepals, petals, and stamens) after anthesis, obconic or cylindrical, sometimes with bulbous base, with scales or ring of hairs or raised ring of tissue near mouth inside, nectary at base of tube; sepals 4, spreading individually, usually green or flushed with red or cream, rarely same color as petals, lanceolate; petals 4, usually rose-purple to white, rarely cream-yellow or orange-red, usually obcordate or broadly obovate, emarginate; stamens 8, in 2 unequal series with episepalous longer or rarely subequal, erect, anthers versatile, on smallest flowers appearing basifixed, pollen usually shed in tetrads, rarely singly; ovary 4-locular, style erect, stigma entire and clavate to capitate, or deeply 4-lobed, commissural, receptive only on inner surfaces, surface dry with multicellular papillae. Fruit a capsule, straight or slightly curved, narrowly cylindrical to fusiform or rarely narrowly ellipsoidal, usually terete, rarely ± 4-angled, loculicidally dehiscent, usually splitting to base with intact central column, rarely splitting only on distal 1/3 with central column disintegrating; pedicellate or sessile. Seeds usually numerous (1–100+ per locule), usually in 1, rarely 2, rows per locule, very rarely 1 row per capsule by dissolution of septa and central column, surface papillose to finely reticulate or longitudinally ridged, sometimes constricted near micropylar end, rarely with inflated rim around body of seed on adaxial side, with persistent coma at chalazal end or coma absent. x = 9, 10, 12, 13, 15, 16, 18, 19.

Species ca. 165 (41 in the flora): North America, Mexico, West Indies, Central America, South America, Eurasia, Africa, Pacific Islands (New Zealand), Australasia.

Epilobium is the largest genus in Onagraceae, distributed mainly in cool temperate, montane, boreal, and arctic areas on all continents except Antarctica. P. H. Raven (1976) proposed many elements of the current classification of Epilobieae, recognizing six sections in Epilobium, including for the first time the genus Zauschneria as a separate section, based on patterns of variation in chromosome number (H. Lewis and Raven 1961; M. Kurabayashi et al. 1962), wood anatomy (S. Carlquist 1975), seed morphology (S. R. Seavey et al. 1977), and pollen wall (J. J. Skvarla et al. 1976) and viscin thread morphology (Skvarla et al. 1978). Raven excluded Boisduvalia from Epilobium, though it later was included (P. C. Hoch and Raven 1992) as two distinct sections, and Raven included Chamaenerion as a section, which was later excluded (Hoch and Raven 1999; W. L. Wagner et al. 2007). Both of these changes from the 1976 classification were based in large part on relationships supported by molecular data (D. A. Baum et al. 1994). The monophyly of this re-defined Epilobium was very strongly supported in analyses of ITS alone (Baum et al.) and ITS plus trnL-F (R. A. Levin et al. 2004), as was the segregation of Chamaenerion from Epilobium. In addition, both analyses strongly supported a clade that includes the former segregated genera Boisduvalia and Zauschneria as well as the sections of Epilobium with chromosome numbers other than n = 18. As treated here, Epilobium is divided into eight sections, one of which has two subsections (Wagner et al.).

The phylogeny of Epilobium as elucidated by D. A. Baum et al. (1994) and later R. A. Levin et al. (2004) suggests complex chromosomal evolution in the genus. Since the outgroup (Chamaenerion) and one major clade (sect. Epilobium) share the base number x = 18, the derivation of the diverse other gametic numbers (n = 9, 10, 12, 13, 15, 16, and 19) appears to have involved aneuploid reduction from x = 18. One explanation of the relation­ships proposed by the molecular data suggests the early evolution of a lineage with n = 15 (sects. Cordylophorum, Epilobiopsis, and Zauschneria), from which arose a branch with n = 12 (sect. Xerolobium), a separate branch with n = 13 and n = 16 (sect. Crossostigma), and a third branch with n = 9, 10, and 19 (sect. Pachydium). This hypothesis requires the fewest aneu­ploidy changes, but confirming it will require additional molecular and cytological analysis.

Almost all Epilobium species tested are self-compatible, but at least E. obcordatum is apparently self-incompatible (S. R. Seavey and S. K. Carter 1994, 1996). All species have hermaphroditic, diurnal flowers that usually remain open for more than one day. Many species are modally autogamous, a few primarily cleistogamous, but others, including 15 species in North America, are partly or wholly outcrossing. In the latter group, flowers are often markedly protandrous, with a 4-lobed stigma exserted beyond anthers. Primary pollinators include bees, flower flies, butterflies, moths, and sometimes hummingbirds (sect. Zauschneria). In general, flowering commences in the first year of growth.

Only the nine species (eight in the flora area) of sects. Pachydium, Crossostigma, Epilobiopsis, and Xerolobium are annuals; all other taxa of Epilobium (sects. Cordylophorum, Epilobium, Macrocarpa, and Zauschneria) are perennials. Annual species have taproots and proximal stems usually with peeling epidermis. Perennial species of Epilobium, which have more fibrous root systems and rarely peeling epidermis, vary greatly in the habit and stature, degree of clumping, degree of branching, and mode of perennation.

The various types of perennating structures in Epilobium (R. C. Keating et al. 1982; Chen C. J. et al. 1992) are extremely useful for identification of many taxa; unfortunately, many plants are collected without the structures. Unless care is taken to dig up the bases of the plants, some of the most useful diagnostic features such as turions are lost, making identification more difficult. Perennating structures are less useful for phylogenetic analysis due to the difficulty in determining whether the common possession of a particular type represents an analogous or homologous pattern. However, one type of structure (soboles) is found in at least some taxa of all perennial sections and may be the most generalized type in the genus.

Soboles are here defined as simple, scaly, underground shoots with somewhat elongated inter­nodes and arising from the caudex. Soboles generally give rise to more or less clumped plants with ascending stems. In certain species, including Epilobium rigidum (sect. Macrocarpa), all taxa of sects. Cordylophorum and Zauschneria, and some in sect. Epilobium, soboles arise from more or less woody caudices, a character state that may be plesiomorphic in the genus; all other types of perennating structures are found only in sect. Epilobium and are discussed there.

In addition to habit and perennating structures, certain other morphological features particularly useful for diagnosing plants include: leaf size, shape, attachment, and arrangement on stem; seed surface (papillose, reticulate, or ridged), size, and shape (S. R. Seavey et al. 1977); pubescence type (mainly strigillose: short incurved hairs; villous: long, thin, spreading hairs; and/or glandular-puberulent) and pattern on stem (all around or restricted to raised lines decurrent from base of petioles) and leaves; and some floral features including size, shape, and color. In fact, floral features are diagnostic for only certain sections or species; most species have very similar flowers.

SELECTED REFERENCES Baum, D. A., K. J. Sytsma, and P. C. Hoch. 1994. The phylogeny of Epilobium (Onagraceae) based on nuclear ribosomal DNA sequences. Syst. Bot. 19: 363–388. Keating, R. C., K. J. Sytsma, and P. C. Hoch. 1982. Perennation in Epilobium (Onagraceae) and its relation to classification and ecology. Syst. Bot. 7: 379–404. Munz, P. A. 1941. A revision of the genus Boisduvalia (Onagraceae). Darwiniana 5: 124–153. Raven, P. H. and D. M. Moore. 1965. A revision of Boisduvalia (Onagraceae). Brittonia 17: 238–254. Trelease, W. 1891. A revision of the American species of Epilobium occurring north of Mexico. Rep. (Annual) Missouri Bot. Gard. 2: 69–117.

1 Herbs annual, with taproot, stem epidermis peeling proximally; leaves opposite in proximal pairs, alternate or fasciculate distally.   (2)
+ Herbs perennial, sometimes woody, ± without taproot, stem epidermis usually not peeling, sometimes peeling proximally; leaves usually opposite proximal to inflorescences, alternate distally, not fasciculate.   (9)
2 (1) Seed coma usually present, very rarely absent, often easily or readily detached.   (3)
+ Seed coma absent.   (5)
3 (2) Herbs 15–200 cm; floral tubes 1–16 mm, sepals 1–8.5 mm, petals 1.5–15(–20) mm; seeds 1.5–2.7 mm; pollen usually shed singly, rarely in tetrads [5b. Epilobium sect. Xerolobium]   2 Epilobium brachycarpum
+ Herbs 5–45 cm; floral tubes 0.4–1.5 mm, sepals 0.5–2.5 mm, petals 1.4–5 mm; seeds 0.6–1.2 mm; pollen shed in tetrads [5c. Epilobium sect. Crossostigma].   (4)
4 (3) Leaves usually folded along midrib, often fasciculate distally; inflorescences congested; petals 1.4–2.5 mm; seeds 0.6–0.9 mm, surface papillose   3 Epilobium ravenii
+ Leaves flat, not fasciculate; inflorescences not congested; petals 2–5 mm; seeds 0.9–1.2 mm, surface reticulate   4 Epilobium minutum
5 (2) Capsule walls tough, usually splitting only in distal 1/3; seeds in 2 rows per locule; often with decumbent proximal branches; flowers usually cleistogamous [5e. Epilobium sect. Epilobiopsis].   (6)
+ Capsule walls flexible, splitting to base; seeds in 1 row per locule or compressed to 1 row per capsule; proximal branches ascending, erect, or absent; flowers chasmogamous or cleistogamous [5f. Epilobium sect. Pachydium].   (7)
6 (5) Capsules 8–12 mm, sharply 4-angled; sepals 1.5–3 mm; seeds 1.2–1.5 mm   8 Epilobium cleistogamum
+ Capsules 4.5–8 mm, subterete; sepals 0.7–1.9 mm; seeds 1–1.3 mm   9 Epilobium campestre
7 (5) Capsules with beak 0–0.5 mm, septifragal, septa adherent to intact central axis; bracts wider than cauline leaves   10 Epilobium densiflorum
+ Capsules with beak 2–5 mm, loculicidal, septa adhering to valves, central axis disintegrating; bracts not wider than cauline leaves.   (8)
8 (7) Floral tubes 1.5–3 mm, sepals (1.5–)2–6 mm, petals (2.8–)3.5–10 mm; capsules (10–)14–21 mm; seeds in 1 staggered row distally, 1.8–2.3 mm; flowers ± chasmogamous   11 Epilobium pallidum
+ Floral tubes 0.4–1 mm, sepals 0.7–2 mm, petals 1.2–3.2 mm; capsules (6–)8–14 mm; seeds in 4 rows (1 row per locule), 0.9–1.6 mm; flowers ± cleistogamous   12 Epilobium torreyi
9 (1) Floral tubes 16–32 mm, bulbous near base, irregular scales at mouth inside; flowers: floral tube, sepals, and petals usually orange-red, very rarely white, slightly zygomorphic, upper petals ± flared at right angle to calyx tube [5g. Epilobium sect. Zauschneria].   (10)
+ Floral tubes 0.3–9.5 mm, not or very rarely bulbous near base, scales absent; flowers: floral tube and sepals green or flushed reddish green, petals usually pink, white, or rose-purple, rarely cream, usually actinomorphic, very rarely slightly zygomorphic.   (11)
10 (9) Herbs clumped perennials to subshrubs, 10–120 cm; leaves not dimorphic, usually mixed pubescent, rarely glabrate, not white-canescent   13 Epilobium canum
+ Herbs matted perennials, 5–25 cm; leaves dimorphic, proximal ones densely white-canescent and eglandular, distal ones glandular puberulent and scattered villous, not canescent   14 Epilobium septentrionale
11 (9) Herbs suffrutescent or rhizomatous, rarely herbaceous; proximal stem epidermis usually peeling; stigmas 4-lobed; seeds 1.5–3.4 mm, ± prominently constricted near micropylar end.   (12)
+ Herbs herbaceous or, sometimes, ± suffrutescent; proximal stem epidermis not peeling; stigmas entire or 4-lobed; seeds 0.8–2.2 mm, not constricted near micropylar end [5h. Epilobium sect. Epilobium].   (15)
12 (11) Leaf blades 20–45 mm, not folded on midrib; sepals 9.5–14.5 mm; petals 16–20 mm; floral tubes 1–1.8 mm [5a. Epilobium sect. Macrocarpa]   1 Epilobium rigidum
+ Leaf blades 9–25 mm, folded on midrib or not; sepals 2.6–6.5 mm; petals 5–9.5 mm; floral tubes 1.8–9.5 mm [5d. Epilobium sect. Cordylophorum].   (13)
13 (12) Leaf blades not folded on midrib; petals cream to light yellow; flowers slightly zygomorphic (upper petals slightly longer, styles declined); sepals 4–6.5 mm; capsules 10–30 mm; seeds 4–8+ seeds per locule; ovaries eglandular   7 Epilobium suffruticosum
+ Leaf blades ± folded on midrib; petals rose-purple; flowers not zygomorphic; sepals 2.6–4.4 mm; capsules 8–16 mm; seeds 1–3 per locule; ovaries glandular pubescent.   (14)
14 (13) Leaf blade margins denticulate, surfaces usually glabrescent, rarely strigillose-villous; floral tubes 2.7–3.2(–5) mm; seeds 2.1–2.9 mm   5 Epilobium nevadense
+ Leaf blade margins subentire or low-denticulate, surfaces densely spreading-hairy; floral tubes 5.2–9.5 mm; seeds 1.5–2.4 mm   6 Epilobium nivium
15 (11) Stigmas 4-lobed, exserted beyond or rarely surrounded by anthers; floral tubes 1–5.5 mm; sepals (2.5–)5–14 mm; petals (4–)8–26 mm.   (16)
+ Stigmas entire, clavate or capitate, surrounded by or, rarely, exserted beyond anthers; floral tubes 0.3–2.6 mm; sepals 1.1–7.5 mm; petals 1.6–10(–14) mm.   (22)
16 (15) Petals cream-yellow; seed coma tawny, persistent   29 Epilobium luteum
+ Petals pink to rose-purple; seed coma white, dull white, or dingy, sometimes tawny, ± easily or readily detached.   (17)
17 (16) Herbs ± suffrutescent, forming shoots from woody caudex; stems 5–25 cm, clumped to cespitose; floral tubes 2.1–5.5 mm.   (18)
+ Herbs herbaceous, forming basal rosettes, stolons, turions, or soboles; stems (5–)15–120(–250) cm, erect, sometimes clumped; floral tubes 1–3 mm.   (19)
18 (17) Herbs ± glabrous and glaucous proximal to inflorescences; leaf blades 6–18(–24) mm; floral tubes 3.2–5.5 mm, slightly raised ring of tissue inside   15 Epilobium obcordatum
+ Herbs pubescent throughout, not glaucous; leaf blades 13–26 mm; floral tubes 2.1–4 mm, prominent raised ring of tissue inside   16 Epilobium siskiyouense
19 (17) Stems densely villous, often mixed glandular puberulent proximal to inflorescences, 18–120(–250) cm.   (20)
+ Stems densely strigillose to ± glabrous proximal to inflorescences, not villous, (5–)15–75 cm.   (21)
20 (19) Leaves sessile, clasping; sepals 6–12 mm; petals 9–20 mm; herbs with thick stolons sometimes terminating in rosette   26 Epilobium hirsutum
+ Leaves with petioles 1–3 mm proximally, subsessile distally, not clasping; sepals 2.5–6 mm; petals 4–8.5 mm; herbs with short-stalked leafy rosettes   27 Epilobium parviflorum
21 (19) Stems densely strigillose proximal to inflorescences; seeds densely papillose; sepals 5–6.5 mm; petals 7.5–10 mm   28 Epilobium montanum
+ Stems ± glabrous and glaucous proximal to inflorescences; seeds ridged; sepals 6–10 mm; petals (6–)10–15 mm   38 Epilobium oreganum
22 (15) Herbs with ± threadlike stolons, with or without terminal turions; stems usually erect from base, rarely ascending.   (23)
+ Herbs with rosettes, turions, fleshy soboles or shoots; stem bases erect or ascending.   (28)
23 (22) Stolons terminating in fleshy turions; stems (5–)15–95 cm, simple to well branched.   (24)
+ Stolons not terminating in turions; stems 5–30 (–40) cm, usually simple, rarely branched, rarely stems 20–80 cm, well branched.   (26)
24 (23) Stems densely villous, without raised lines from margins of petioles   19 Epilobium densum
+ Stems strigillose to subglabrous, not villous, sometimes with faint raised strigillose lines from margins of petioles.   (25)
25 (24) Leaf blade surfaces strigillose abaxially, subglabrous adaxially, with strigillose margins and veins; inflorescences nodding in bud   17 Epilobium palustre
+ Leaf blade surfaces densely strigillose on both sides; inflorescences erect in bud   18 Epilobium leptophyllum
26 (23) Leaf blades 1.5–10 cm; stems 20–80 cm, well branched, not clumped or matted   25 Epilobium obscurum
+ Leaf blades 0.4–2.1 cm; stems 5–30(–40) cm, not or rarely branched, ± loosely clumped or matted.   (27)
27 (26) Stems subglabrous, often matted, 8–30(–40) cm; leaf blades broadly elliptic proximally, narrowly elliptic or lanceolate to sublinear distally   22 Epilobium oregonense
+ Stems densely glandular puberulent, loosely clumped, 5–20 cm; leaf blades broadly obovate to orbiculate proximally to ovate or lanceolate distally   37 Epilobium howellii
28 (22) Herbs with small-leafed epigeous or scaly hypogeous soboles; stems usually ± ascending, rarely erect, ± clumped or matted, rarely with basal scales.   (29)
+ Herbs usually with leafy rosettes or fleshy hypogeous turions, rarely axillary bulblets; stems ± erect, usually not clumped, usually with dark basal scales.   (34)
29 (28) Stems 8–75(–85) cm, glabrous and ± glaucous proximal to inflorescences, often without raised lines; leaves subsessile, ± clasping; seeds 0.7–1(–1.3) mm, surfaces densely papillose   40 Epilobium glaberrimum
+ Stems 3–50 cm, subglabrous or strigillose, not glaucous, proximal to inflorescences, with distinct raised strigillose lines decurrent from margins of petioles; leaves subsessile or with petiole 1–12 mm, not clasping; seeds 0.7–2.1 mm, surfaces reticulate to densely papillose.   (30)
30 (29) Stems strigillose proximal to inflorescences, densely glandular puberulent distally   34 Epilobium smithii
+ Stems subglabrous with raised strigillose lines proximal to inflorescences, mixed strigillose and glandular puberulent or subglabrous distally.   (31)
31 (30) Stems 10–50 cm; leaf blades 1.5–6.2 × 0.7–2.9 cm, margins denticulate; capsules 35–100 mm.   (32)
+ Stems 3–20(–25) cm; leaf blades (0.5–)0.7–2.8 × 0.3–1.6 cm, margins subentire to sparsely denticulate; capsules 17–40(–55) mm.   (33)
32 (31) Petals usually pink to rose-purple, rarely white; pedicels 5–15(–25) mm in fruit; capsules 35–65 mm; petioles 3–9 mm proximally, to 0 mm distally, not winged; seed surfaces usually papillose, sometimes reticulate   30 Epilobium hornemannii
+ Petals white, rarely red-veined or fading pink; pedicels 15–45 mm in fruit; capsules 50–100 mm; petioles 3–12 mm, often winged; seed surfaces reticulate or, sometimes, barely rugose   31 Epilobium lactiflorum
33 (31) Herbs with thin leafy soboles, no woody rootstock, densely clumped or matted, stems nodding in bud; pedicels 5–35(–68) mm in fruit; seeds 0.7–1.4 mm; stems subglabrous, rarely mixed strigillose and sparsely glandular puberulent distally   32 Epilobium anagallidifolium
+ Herbs with wiry, scaly soboles and extended semi-woody rootstock, clumped, stems ± erect; pedicels 2–21 mm in fruit; seeds (1.3–)1.5–2.1 mm; stems subglabrous proximally, ± densely strigillose and mixed glandular- puberulent distally   33 Epilobium clavatum
34 (28) Herbs with basal sessile or, rarely, short-stalked, leafy epigeous rosettes.   (35)
+ Herbs with fleshy underground turions, rarely also with bulblets in proximal or mid-cauline nodes.   (38)
35 (34) Stems (2–)5–40(–45) cm, simple; leaf blades 0.8-4.5 cm, margins usually subentire, rarely denticulate, subsessile; inflorescences racemes, nodding in bud.   (36)
+ Stems (3–)10–85(–190) cm, ± branched; leaf blades (1–)3–10(–16) cm, margins denticulate to densely serrulate, subsessile or petiole to 10 mm; inflorescences erect racemes, panicles, or corymbs.   (37)
36 (35) Stems 10–40(–45) cm; leaf blades (1.2–)2–4.5 cm, narrowly oblong to narrowly lanceolate proximally; seed surfaces papillate   20 Epilobium davuricum
+ Stems (2–)5–18 cm; leaf blades 0.8–2.1 cm, obovate to narrowly elliptic proximally; seed surfaces reticulate   21 Epilobium arcticum
37 (35) Seed coma tawny; seed surfaces papillose; leaf blade margins sharply serrulate, 30–75 teeth per side; petioles 4–10 mm; inflorescences eglandular; petals 2.5–5.5 mm, white   24 Epilobium coloratum
+ Seed coma white or dingy; seed surfaces ridged; leaf blade margins serrulate, (8–)15–40 teeth per side; petioles 0–5(–10) mm; inflorescences usually mixed glandular pubescent; petals 2–14 mm, pink to rose-purple or white   39 Epilobium ciliatum (in part)
38 (34) Herbs ± eglandular; stems 7–30 cm.   (39)
+ Herbs mixed glandular puberulent, at least on inflorescences; stems 2–120(–190) cm, usually more than 30 cm.   (40)
39 (38) Pedicels 5–16 mm; seeds 1.7–2.2 mm, surfaces reticulate to low papillose; stems only with hypogeous turions   35 Epilobium mirabile
+ Pedicels 15–38 mm; seeds 0.8–1.2 mm, surfaces papillose; stems with turions and, usually, bulblets in proximal or mid-cauline nodes   36 Epilobium leptocarpum
40 (38) Seed surfaces ridged; stems (3–)10–120(–190) cm; leaf blades (1–)3–12(–16) cm; petals 2–14 mm, white, pink, or rose purple   39 Epilobium ciliatum (in part)
+ Seed surfaces papillose or reticulate; stems 2–55(–60) cm; leaf blades 0.5–5.5(–6.5) cm; petals 1.6–5.5(–7) mm, white, fading or rarely pink.   (41)
41 (40) Pedicels 8–40 mm; capsules ascending, spreading; leaves not clasping, veins inconspicuous; inflorescences ± nodding in bud   23 Epilobium hallianum
+ Pedicels 0–5 mm; capsules erect, appressed to stem; leaves clasping, veins conspicuous; inflorescences ± erect   41 Epilobium saximontanum

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