Description from Flora of China
Annual, biennial or perennial herbs, caulescent or acaulescent, with a taproot or fibrous roots, occasionally with rhizomes or shoots arising from spreading lateral roots. Leaves alternate or in a basal rosette that often is absent in mature plants, entire, toothed to pinnatifid; stipules absent. Flowers perfect, actinomorphic, in axils of upper leaves, when numerous forming terminal leafy spikes, racemes, or corymbs, opening near sunset or near sunrise. Floral tube usually well developed, cylindric and somewhat flared near mouth, deciduous soon after anthesis. Sepals 4, green or yellowish, often tinged or striped red or purple. Petals 4, yellow, purple, pink, or white. Stamens 8; anthers versatile; pollen shed singly. Ovary with 4 locules; ovules numerous; stigma divided into 4 linear lobes, receptive all around, and subtended by a ± conspicuous ringlike indusium in early development, but often obscured when receptive. Fruit a dehiscent capsule [rarely indehiscent outside of China], straight or curved, terete to 4-angled or winged, sessile, occasionally pedicellate, or basal portion sterile and stipelike. Seeds numerous, in 1 or 2(or 3) rows or in clusters in each of 4 locules. 2n = 14, 28, 42, 56.
Oenothera is currently divided into 15 sections, only three of which are represented in China. An evolutionary phenomenon that has occurred repeatedly in Oenothera (52 species) and several other genera of tribe Onagreae is permanent translocation heterozygosity, a peculiar, specialized genetic system based on heterozygosity for successive chromosomal translocations and manifested by autogamy and formation of a ring of 14 chromosomes at meiotic metaphase I (for reviews see Cleland, Oenothera Cytogenetics and Evolution. 1972; Holsinger and Ellstrand, Amer. Naturalist 124: 48-71. 1984). Permanent translocation heterozygote individuals breed true for their series of reciprocal translocations and are maintained by either balanced lethals or selective fertilization. These plants are essentially clonal. Many species of Oenothera that have become naturalized outside their natural range are permanent translocation heterozygotes, as noted in their descriptions.
Several ornamental species of Oenothera are known only from cultivation in China, often in Beijing, Kunming, or other botanical gardens. For example, O. macrocarpa Nuttall subsp. macrocarpa (O. sect. Megapterium (Spach) Endlicher) is native to the Great Plains region of C North America but has never become naturalized outside of its indigenous distribution because it is a self-incompatible outcrosser with rather specific habitat requirements. It can be distinguished by its large, yellow corollas (up to 14 cm in diam. at anthesis), 4-winged capsules (wings up to 3.4 cm wide), floral tube (7.8-)9.5-11.5(-14) cm, and coarsely rugose, distally winged seeds. A second species, O. acaulis Cavanilles (O. sect. Lavauxia (Spach) Endlicher, O. subsect. Australis W. L. Wagner & Dietrich), likewise known only from cultivation in China, is native to S South America and is characterized by white petals and capsules winged in the distal half.
One hundred and twenty-one species: open, often disturbed habitats in temperate to subtropical areas of North, Central, and South America, with the center of diversity in SW North America; ten species (all naturalized within the past 200 years) in China.
(Authors: Chen Jiarui (陈家瑞 Chen Chia-jui); Peter C. Hoch, Warren L. Wagner)