Megapterium missourense (Sims) Spach; Oenothera macrocarpa var. missourensis (Sims) Carrière; O. missourensis Sims; O. missourensis var. latifolia A. Gray

Herbs strigillose and glandular puberulent distally. Stems several, unbranched, sometimes with shorter secondary branches, 10–40(–60) cm. Leaves green, younger ones grayish green, (6–)8–12(–14.5) × 0.4–2.3(–3) cm; blade often lanceolate-elliptic to broadly elliptic, sometimes linear or lanceolate, margins usually flat, sometimes undulate, entire or inconspicuously denticulate, apex acute. Flowers: buds with unequal free tips (4–)8–10(–12) mm; floral tube (78–)95–115(–140) mm; sepals (45–)50–65(–75) mm; petals (40–)50–65(–68) mm; filaments (25–)30–40(–44) mm, anthers (15–)17–24 mm; style (120–)135–160(–190) mm. Capsules ovoid, narrowly ovoid, narrowly lanceoloid to broadly ellipsoid, or subglobose, not twisted, wings (14–)18–34 mm wide, body 52–70(–115) × 7–8 mm. 2n = 14.

Flowering Apr–Jun(Sep). Rocky, clay, alkaline soil, unglaciated prairies, glades, bluffs, open prairie hillsides, disturbed sites, limestone or dolomite; 100–500 m; Ark., Ill., Kans., Mo., Nebr., Okla., Tenn., Tex.

Subspecies macrocarpa is known from three disjunct areas: southeastern Nebraska, eastern half of Kansas, Craig and Washington counties, Oklahoma, east to Missouri south of the Missouri River and St. Clair County, Illinois, and to the northern tier of counties in Arkansas; glades near Murfreesboro, Rutherford County, Tennessee; and Blackland Prairies, Cross Timbers and eastern Edwards Plateau, from Bryan, Johnston, and Pontotoc counties, Oklahoma, southwest to Bexar, Coke, Kerr, and McCulloch counties, Texas.