Oenothera sect. Pachydium Fischer & C. A. Meyer, Index Seminum (St. Petersburg) 2: 45. 1836; Boisduvalia Spach, Hist. Nat. Vég. 4: 383. 1835; Boisduvalia [unranked] Dictyopetalum (Fischer & C. A. Meyer) Endlicher; Boisduvalia [unranked] Pachydium (Fischer & C. A. Meyer) Endlicher; Boisduvalia subg. Pachydium (Fischer & C. A. Meyer) Reichenbach; Cratericarpium Spach; Epilobium sect. Boisduvalia (Spach) Hoch & P. H. Raven; Dictyopetalum (Fischer & C. A. Meyer) Baillon; Oenothera [unranked] Boisduvalia (Spach) Torrey & A. Gray; Oenothera sect. Dictyopetalum Fischer & C. A. Meyer

Herbs annual, with taproot. Stems: epidermis peeling proximally. Leaves opposite and early deciduous in proximal pairs, alternate distally. Flowers actinomorphic, chasmogamous or rarely cleistogamous; floral tube not bulbous, without scales inside; petals rose-purple to white; pollen in tetrads; stigma clavate, entire or sometimes obscurely 4-lobed. Capsules fusiform-clavate to lanceolate-linear, sometimes torulose, splitting to base, central column persistent or not, sessile. Seeds 2–8 per locule in 1 row, or 8–12 per capsule, pushed into a single row by disintegration of central column and septa, irregularly angular-fusiform; coma absent.

Species 4 (3 in the flora): w North America, nw Mexico, w South America.

Section Pachydium consists of four self-compatible, often cleistogamous, annual species, two of which are endemic to and widespread in western North America, and a third that also reaches northern Mexico. Epilobium subdentatum (Meyen) Lievens & Hoch, occurs only in Chile (widespread) and Argentina (only in Chubut Province), and has larger, often outcrossing flowers pollinated by bees (P. H. Raven and D. M. Moore 1965). The species in sect. Pachydium are diverse cytologically: E. torreyi has the lowest chromosome number in Epilobieae (n = 9), E. densiflorum and E. pallidum have n = 10, and E. subdentatum has n = 19. Contrary to suggestions by G. L. Stebbins (1971) and Raven (1976), the numbers n = 9 and 10 apparently were derived as direct or stepwise aneuploid reductions from the paleotetraploid x = 18 base number for Epilobieae (D. A. Baum et al. 1994). Experimental hybrids were formed in all combinations among the four species, although most had notably aberrant chromosome pairing and very low pollen fertility (S. R. Seavey 1992). The so-called tetraploid E. subdentatum (n = 19) apparently arose following hybridization between E. torreyi (n = 9) and E. densiflorum or a close relative with n = 10 (Seavey), followed by long-distance dispersal to South America (Raven and Moore). The monophyly of the section is scarcely supported by analysis of ITS sequence (Baum et al.), and the exact relationship of these species is not well-resolved.