Herbs perennial, acaulescent or caulescent; from a stout, woody taproot, sometimes (O. brachycarpa, O. howardii) lateral roots producing adventitious shoots. Stems ascending or becoming decumbent, usually unbranched. Leaves in a basal rosette, often also cauline, (2.8–) 5–21 (–34) cm; blade margins entire, dentate, or pinnatifid. Inflorescences solitary flowers in axils of distal leaves. Flowers opening near sunset, with sweet scent or nearly unscented; buds erect, quadrangular, with free tips; floral-tube (21–) 35–210 (–220) mm; sepals splitting along one suture, remaining coherent and reflexed as a unit at anthesis; petals yellow, fading yellow, orange, pink, or deep red, obovate to rhombic-obovate; stigma deeply divided into 4 linear lobes. Capsules papery, leathery, or corky in age, ovoid, narrowly lanceoloid to broadly ellipsoid, or globose, winged, wings 10–32 mm wide throughout, or capsule walls with corky thickening and wings not developed (sometimes in O. brachycarpa), then capsule appearing only 4-angled, apex truncate to cuneate, dehiscent 1/4–1/3 their length; pedicellate, sometimes disarticulating from plant at maturity. Seeds numerous, in 1 or 2 rows per locule, grayish to yellowish-brown, brown, or dark purplish brown, obovoid or subcuboid, angled or rounded, usually with an erose wing distally, surface coarsely rugose and reticulate, thickened, especially at distal end, this area with an internal cavity adjacent to embryo. 2n = 14, 28, 42, 56.

Distribution

w, c United States, n Mexico

Discussion

Species 4 (4 in the flora).

Section Megapterium consists of four species (eight taxa); two (Oenothera brachycarpa,O. macrocarpa) are diploid (2n = 14), one (O. coryi) is hexaploid (2n = 42), and one (O. howardii) has tetraploid, hexaploid, and octoploid populations (2n = 28, 42, 56) (W. L. Wagner et al. 2007). The species usually occur on xeric rocky sites of limestone, sandstone, shale, or gypsum, rarely (O. brachycarpa) on volcanic soil, from eastern Nevada, Utah, and eastern Colorado east to the Mississippi River in Missouri, and south through northern Arkansas and Texas, to Coahuila, Durango, and Nuevo León, Mexico; there are only two isolated records (O. macrocarpa subsp. macrocarpa from St. Clair County, Illinois, and Rutherford County, Tennessee) from east of the Mississippi River, at 130–3000 m elevation. All species are self-incompatible and vespertine, the flowers fading the following morning, or sometimes remaining open for a second day in O. macrocarpa, pollinated by hawkmoths including Hyles, Manduca, and Sphinx (see Wagner et al. for summary).

Selected References

None.

Lower Taxa

Oenothera brachycarpa, Oenothera coryi, Oenothera howardii, Oenothera macrocarpa

Key

1	Leaves (2.8–)3.7–12.5(–17) cm, blade margins entire or denticulate or serrulate; plants caulescent; stems (1–)4–40(–60) cm.	Oenothera macrocarpa
1	Leaves (3.1–)5–21(–34) cm, blade margins usually pinnately lobed, sometimes lobed only proximally; plants acaulescent or sometimes caulescent; stems 0–20(–36) cm.	> 2
2	Petals pale yellow to yellow, drying lavender to purple, usually broadly rhombic-obovate, rarely obovate, usually appearing crumpled; floral tubes (90–)120–210(–220) mm; leaf blade usually with a large terminal lobe, margins usually irregularly pinnatifid with some of the sinuses extending nearly to midrib, margins usually erose; capsule wings 0–3(–5) mm wide.	Oenothera brachycarpa
2	Petals lemon-yellow to brilliant yellow, drying purple to reddish brown, rarely lavender, broadly obovate, not crumpled; floral tubes (43–)60–110(–125) mm; leaf blade without a large terminal lobe, margins entire or irregularly pinnately lobed, sinuses usually extending less than 1/2 to midrib, margins not erose; capsule wings (2–)4–7(–11) mm wide.	> 3
3	Petals brilliant yellow, (30–)40–60(–73) mm, drying deep reddish purple to reddish brown; flowers with strong, sweet scent; buds with free tips 1–3(–4) mm; leaves (0.5–)1–2(–3) cm wide, blades usually oblanceolate, elliptic to narrowly elliptic, rarely lanceolate.	Oenothera howardii
3	Petals lemon-yellow, 35–43 mm, drying lavender to purple; flowers with weak scent; buds with free tips 0.7–1.2 mm; leaves (0.2–)0.3–0.5(–0.7) cm wide, blades linear to narrowly lanceolate.	Oenothera coryi

"/4" is not declared as a valid unit of measurement for this property.