Herbs usually facultatively biennial or short-lived perennial, rarely annual; from taproot. Stems usually erect or ascending, rarely decumbent. Inflorescences forming elongate, dense, erect or somewhat curved spike. Flowers: buds erect, weakly quadrangular, with free tips terminal or subterminal, erect or spreading; floral tube straight, (15–)22–135(–160) mm; petals obovate or obcordate. Capsules usually straight, rarely curved (O. argillicola), narrowly lanceoloid or ovoid, bluntly 4-angled, (3–)4–8[–12] mm diam. Seeds in 2 rows per locule, dark brown to almost black, prismatic and angled, surface reticulate and irregularly pitted. 2n = 14.

Distribution

North America, Mexico, Central America, introduced in South America, Europe, Asia, Africa, Atlantic Islands, Pacific Islands (New Zealand), Australia.

Discussion

Species 13 (12 in the flora).

Subsection Oenothera consists of 13 species (16 taxa) characterized by a weedy biennial, short-lived perennial, or rarely annual habit, stems mostly erect with many-flowered spikes, capsules lanceoloid or ovoid and bluntly 4-angled, and seeds black, prismatic, and angled, with the surface minutely pitted (W. Dietrich et al. 1997). All species are self-compatible, except rarely O. grandiflora is self-incompatible. The flowers are vespertine, with five species outcrossing and pollinated by hawkmoths (D. P. Gregory 1963, 1964), seven autogamous PTH species, or one regularly outcrossing PTH species (O. glazioviana) pollinated by hawkmoths (Dietrich et al.). The eight PTH species form a ring of 14 chromosomes or a ring of 12 and 1 bivalent in meiosis. This group of closely related species (in literature also as Euoenothera) has had a long history of scientific study resulting in many hundreds of research papers and several books, including the excellent summary by R. E. Cleland (1972) and more recent ones by W. Stubbe and P. H. Raven (1979), and C. Harte (1994), which recount nearly a century of experimental studies of the group. Among the first investigations of Oenothera were those of Hugo de Vries, which opened the modern era of study of mutation and its effect on evolution and speciation. Based on crossing studies of numerous individual types across the group, Stubbe (1964) formed a system of classification of plastome and genome types. His characterizations grouped the plastome types from hundreds of individual lines of species of subsect. Oenothera resulting in the recognition of five principal types, designated plastome I to V. These plastome types are genetically discernible by their compatibility or incompatibility with different nuclear genotypes that occur in homozygous (AA, BB, CC) and heterozygous (AB, BA, AC, BC) combinations (summarized by Stubbe and Raven; Dietrich et al.). Application of various molecular techniques has confirmed the genetic distinctness of each of the plastome types and has provided data relevant for the assessment of evolutionary relationships.

Species of subsect. Oenothera occur in open, often disturbed sites, in the drier parts of their range often in wet habitats. The indigenous range of the subsection extends in North America from southern Canada at sea level on both the Pacific and Atlantic coasts to elevations up to 3200 m in the Rocky Mountains southward through central Mexico, Guatemala, El Salvador, Costa Rica, and Panama. The range has been greatly extended by several of the PTH species (O. biennis, O. oakesiana, O. parviflora, and O. villosa subsp. villosa), which are widely naturalized in many parts of the world. One other species, the mostly outcrossing bivalent-forming O. jamesii, is sparingly naturalized in South Africa, the Canary Islands, and Japan. Two additional species, O. glazioviana and O. stucchii Soldano, apparently have arisen recently by stabilized hybridization and PTH formation; the former is now widely distributed around the world, and the latter occurs in Italy and Bouches-du-Rhône, France.

The revision presented by P. H. Raven et al. (1979) and detailed by W. Dietrich et al. (1997) accepts 13 species, eight of which are PTH. In that taxonomy, we recognize five mostly outcrossing, bivalent-forming, genomically homozygous species, and eight PTH species based on the combination of genomes and plastomes. Species were delimited in a broad sense, based on recognition of the three genomic types and the five plastome types, in conjunction with associated morphological characters, to provide a comprehensive taxonomic system that reflects the knowledge of the evolutionary history of the group and provides a reliable means for identification and for information synthesis and retrieval.

Several superfluous names, or names based on them, exist for this group: Brunyera Bubani; Oenothera sect. Onagra Seringe; Oenothera subg. Onagra (Seringe) Jepson; Onagra Miller; and Usoricum Lunell.

Selected References

None.

Lower Taxa

Oenothera argillicola, Oenothera biennis, Oenothera elata, Oenothera glazioviana, Oenothera grandiflora, Oenothera jamesii, Oenothera longissima, Oenothera nutans, Oenothera oakesiana, Oenothera parviflora, Oenothera villosa, Oenothera wolfii

Key

1	Stigmas exserted beyond anthers at anthesis; petals (25–)30–65 mm; pollen 90–100% fertile (except 50% fertile in O. glazioviana).	> 2
2	Floral tubes 60–135(–160) mm.	> 3
3	Flower buds 5–9 mm diam.; capsules 4–9 mm diam., free tips of valves 1–2(–3) mm.	Oenothera longissima
3	Flower buds 7–12 mm diam.; capsules 6–12 mm diam., free tips of valves 2.5–5 mm.	Oenothera jamesii
2	Floral tubes (20–)30–55 mm.	> 4
4	Inflorescences curved with ascending tip; flower buds with subterminal free tips, these divergent; cauline leaf blades 0.4–1 cm wide; capsules spreading at nearly a right angle to stem, apex long-attenuate, usually conspicuously curved.	Oenothera argillicola
4	Inflorescences erect; flower buds with terminal free tips, these usually erect; cauline leaf blades (1–)1.5–6.5 cm wide; capsules erect or slightly spreading, apex gradually attenuate, straight.	> 5
5	Cauline leaf blades 2.5–4 cm wide, narrowly elliptic to lanceolate, margins usually strongly crinkled; pollen ca. 50% fertile; to 50% seeds abortive.	Oenothera glazioviana
5	Cauline leaf blades 1–4.5(–6.5) cm wide, narrowly oblanceolate, oblanceolate to narrowly obovate, or sometimes narrowly elliptic or elliptic, margins not strongly crinkled; flower pollen 90–100% fertile; few seeds abortive.	> 6
6	Plant densely strigillose and either sparsely or moderately villous, with appressed or spreading hairs (some with red-pustulate bases), distally sometimes also glandular puberulent; bracts persistent.	Oenothera elata
6	Plant often appearing glabrous to naked eye, usually strigillose and sparsely to moderately villous with pustulate, translucent hairs proximal to inflorescence, pustules not red (in fresh material), inflorescence glabrous, glandular puberulent, or strigillose and glandular puberulent; bracts caducous.	Oenothera grandiflora
1	Stigmas surrounded by anthers, sometimes (in O. wolfii) exserted beyond anthers and then petals conspicuously shorter than sepals; petals 7–25(–30) mm; pollen ca. 50% fertile.	> 7
7	Plants predominately strigillose.	> 8
8	Inflorescences with curved apices; flower buds with subterminal free tips, spreading to erect; dry capsules usually rusty brown.	Oenothera oakesiana
8	Inflorescences with erect apices; flower buds with terminal free tips, erect; dry capsules grayish green or dull green.	> 9
9	Leaf blades dull green to grayish green; plants densely strigillose, sometimes also sparsely villous with appressed or subappressed hairs, these without, or sometimes with, red or green pustulate bases, rarely sparsely glandular puberulent distally.	Oenothera villosa
9	Leaf blades green to pale green; plants sparsely to moderately strigillose and glandular puberulent, sometimes also scattered villous, hairs erect to appressed, and with pustulate bases.	Oenothera biennis
7	Plants conspicuously strigillose and villous with pustulate hairs, or appearing glabrate.	> 10
10	Inflorescences with curved apices; flower buds with subterminal free tips.	> 11
11	Plants predominantly villous or appearing glabrous to naked eye; leaf blades usually bright green; capsules usually greenish black when dry.	Oenothera parviflora
11	Plants, at least proximally, predominantly strigillose; leaf blades grayish green to dull green; capsules usually rusty brown when dry.	Oenothera oakesiana
10	Inflorescences with erect apices; flower buds with subterminal or terminal free tips.	> 12
12	Inflorescences glabrous or appearing so to naked eye.	> 13
13	Flower buds with terminal free tips; bracts caducous; petals 14–25(–30) mm, fading pale yellowish white; capsules dull green when dry.	Oenothera nutans
13	Flower buds with subterminal free tips; bracts persistent; petals 8–15(–20) mm, fading pale yellow to pale yellowish orange; capsules usually greenish black when dry.	Oenothera parviflora
12	Inflorescences conspicuously pubescent.	> 14
14	Sepals 17–28 mm.	> 15
15	Anthers 7–12 mm; petals conspicuously shorter than sepals.	Oenothera wolfii
15	Anthers 3–6(–9) mm; petals nearly equaling sepals.	Oenothera biennis
14	Sepals 8–18 mm.	> 16
16	Ovary variously pubescent, never villous with pustulate-based hairs.	Oenothera biennis
16	Ovary villous with pustulate-based hairs, often also with other hair types.	> 17
17	Inflorescences usually open (internodes in fruit usually as long as or longer than capsule), villous with appressed to erect hairs, with pustulate bases, also glandular puberulent; petals 7–20 mm; sepals 9–18 mm, usually red-striped or flushed red.	Oenothera villosa
17	Inflorescences dense (internodes in fruit usually shorter than capsule), sparsely to moderately strigillose and glandular puberulent, sometimes also scattered villous, the hairs erect to appressed, with or without pustulate bases; petals 10–25(–30) mm; sepals 12–22(–28) mm, usually green or yellowish, rarely red-striped or flushed red.	Oenothera biennis