Herbs annual or perennial, acaulescent or caulescent; from a usually stout taproot, sometimes lateral roots producing adventitious shoots. Stems (when present) usually ascending, sometimes erect or decumbent, branched or unbranched. Leaves in a basal rosette, sometimes also cauline, (0.5–)1.7–26(–36) cm; blade margins usually coarsely dentate to pinnatifid, sometimes serrate or subentire. Inflorescences solitary flowers from rosette or in axils of distal leaves. Flowers opening near sunset with a sweet scent or nearly unscented; buds erect or nodding by recurved floral tube, quadrangular, without free tips; floral tube (2–)3–140(–165) mm; sepals separating individually or in pairs; petals white, fading rose purple to pink, obovate or obcordate; pollen 90–100% fertile; stigma deeply divided into 4 linear lobes. Capsules thick-walled and woody, straight to falcate or sigmoid, lanceoloid or ellipsoid-ovoid to cylindrical, or sometimes obtusely 4-angled, tapering to a sterile beak, valve margins with a row of tubercles or a thickened, undulate ridge, dehiscent 1/3–7/8 their length; sessile or pedicellate. Seeds usually numerous, in (1 or) 2 rows per locule, obovoid to oblong, sometimes suborbicular or triangular, adaxial face with hollow chamber (seed collar) or, rarely (in O. brandegeei), filled with large, spongy cells, area above raphe a translucent membrane, surface papillose, reticulate, or irregularly roughened. 2n = 14, 28.

Distribution

w North America, nw Mexico.

Discussion

Species 5 (4 in the flora).

Members of sect. Pachylophus occur from southern Canadian prairies through the western United States and northern Mexico (northern Chihuahua and Sonora); O. brandegeei (Munz) P. H. Raven is disjunct in central Baja California. The center of diversity of sect. Pachylophus is in the Great Basin region, especially in Colorado (five taxa) and Utah (six taxa). The section is characterized by white petals, capsule valve margins tuberculate or ridged, and seeds with an unusual hollow seed collar, and rarely (only O. psammophila) a stem epidermis that produces viscid exudates. Two species of the section were included in a molecular analysis showing 100% strong support for the section (R. A. Levin et al. 2004). The position of sect. Pachylophus was not supported as a member of the two main lineages within the genus; it was sister to sect. Calylophus at the base of the phylogenetic tree. Reproductive features include: self-incompatible (3 spp.) or self-compatible (2 spp.); flowers vespertine, fragrance sweet or like rubber; large-flowered species outcrossing and pollinated by hawkmoths (Hyles, Manduca, and Sphinx) or Noctuidae (noctuids), with pollen-gathering bees sometimes effecting pollination (E. G. Linsley et al. 1963b; D. P. Gregory 1964; W. L. Wagner et al. 1985; D. Artz et al. 2010), and small-flowered species (O. brandegeei and O. cavernae) largely autogamous. Wagner et al. reported that for O. psammophila noctuids were the primary pollinators and hawkmoths secondary; recent study of populations by R. Raguso (unpubl.) indicates predominant hawkmoth pollination.

Selected References

None.

Lower Taxa

Oenothera cavernae, Oenothera cespitosa, Oenothera harringtonii, Oenothera psammophila

Key

1	Petals (6.5–)8–20(–25) mm; herbs winter or spring annuals, stems 2–4 cm; stigmas surrounded by anthers at anthesis.	Oenothera cavernae
1	Petals (16–)20–50(–60) mm; herbs perennial or, sometimes, annual; stems 10–40 cm; stigmas exserted beyond anthers at anthesis.	> 2
2	Plants glabrous, with resinous exudate, especially on younger leaves; capsules some-what curved and often somewhat twisted, valve margins with irregular, wavy ridges.	Oenothera psammophila
2	Plants usually pubescent, sometimes glabrous, without resinous exudate; capsules not twisted, valve margins tuberculate or ridged.	> 3
3	Herbs perennial, acaulescent or caulescent; stems, when present, usually ascending, sometimes decumbent; capsule valve margins with tubercles or ridges; flowers: 1–4(–6) per stem opening per day.	Oenothera cespitosa
3	Herbs robust spring annuals, rarely overwintering for a 2nd year, caulescent; stems densely leafy, ascending to erect; capsule valve margins with conspicuous tubercles; flowers: usually 5–10 per stem opening per day.	Oenothera harringtonii