4. Oenothera Linn., Sp. Pl. 346. 1753; Gen. Pl, ed. 5. 163. 1754; C. B. Clarke in Hook. f., Fl. Brit. Ind. 2: 582. 1879; Shteinberg in Schischkin & Bobrov, Fl. URSS. 15: 628, 630. 1949; Raven in Rech. f., Fl. Iran. 7: 1. 1964; Raven in Tutin et al., Fl. Eur. 2: 306. 1968; Chamberlain in Davis, Fl. Turk. 4: 181. 1972; R.R. Stewart, Ann.. Cat. Vasc. Pl. W. Pak. & Kashm. 508. 1972; W. Dietrich, Ann. Missouri Bot. Gard. 64: 425. 1977 (1978); Raven et al., Syst. Bot. 4: 242. 1979.
Peter C. Hoch and Peter H. Raven
Onagra P. Miller, Gard. Dict. Abr, ed. 4: 1754; Spach, Nouv. Ann. Mus. Paris III. 4: 351. 1835; Shteinberg in Schischkin & Bobrov, Fl. URSS. 15: 628. 1949; Hartmannia Spach, Hist. VJg. Phan. 4: 378. 1835.
Annual, biennial, or perennial herbs, caulescent or acaulescent, with a taproot or spreading underground parts. Basal rosette present or absent; leaves alternate, stipules absent. Flowers 4-merous, actinomorphic, in axils of upper leaves, opening near sunset or near sunrise; floral tube well-developed and prolonged beyond the ovary, deciduous after anthesis. Petals yellow, purplish, or white; stamens 8, anthers versatile, the sporogenous tissue in each locale undivided; pollen shed singly; stigma deeply 4-lobed. Capsule many-seeded, sessile or pedicellate, mostly straight, terete or somewhat 4-angled, loculicidally dehiscent or indehiscent. Seeds naked, in 1 to several rows in each of the 4 locules. Basic chromosome number, x = 7.
A genus of about 125 species native to North and South America, with some species widely introduced as escapes from cultivation throughout the world. Several species are now extensively naturalized throughout many temperate regions of the world. Although the group is native only in the New World, several genetically distinctive strains have originated and subsequently become established in the Old World, following hybridization between introduced species. Many species of Oenothera, nearly all of which are weedy, are complex heterozygotes and form a ring of 14 or other large rings of chromosomes at meiotic metaphase I. Most complex heterozygotes have originated within the limits of a taxonomic species, while others have originated after hybridization between species. Genetic recombination is strongly restricted in these species on account of lethal genes or selective fertilization, predominant self-pollination, metacentric chromosomes which are involved in reciprocal translocations, and alternate disjunction at meiotic anaphase I. The two sets of 7 chromosomes behave then as if they are only a single set of chromosomes when segregating at anaphase. Therefore, any new hybridization event can, in effect, lead directly to the formation of a new 'species', whose progeny will breed true. This situation is not comparable with species recognized in most other groups of flowering plants. Represented in Pakistan by 4 naturalized or cultivated species.