Description of Oenothera sect. Oenothera:
Distribution: This very large section consists of 68 species (79 taxa) divided into six subsections, one of which is further divided into three series. The section has a wide geographic distribution from Canada south to Panama and throughout temperate South America, essentially encompassing the full natural distribution of the genus, although there is very sparse representation (only O. elata) from central Mexico south to Panama. Species of this section occur in a variety of habitats, often disturbed ones, from sea level to 5,000 elevation. In sect. Oenothera, as in several other sections of the genus, the diploid, bivalent-forming, usually outcrossing species have relatively narrow geographic and ecological ranges, whereas closely related polyploid or PTH species derived from them are often autogamous and have much wider ranges.
Habit: Annual, biennial, or sometimes perennial herbs, from a usually large taproot, rarely with fibrous roots or with shoots arising from spreading lateral roots; stems present, sometimes short, erect to ascending or decumbent, and then sometimes rooting at the nodes.
Leaves: Leaves forming a basal rosette and also cauline, rosette rarely few-leaved and fugacious; the cauline leaves (1-) 3-25 cm long, pinnately lobed to sinuate-dentate, serrate to dentate or subentire, gradually tapering to the base, sessile or with a winged petiole.
Flowers: Buds erect or recurved, terete or weakly quadrangular, with free sepal tips, floral tube 10-190 mm long, sepals separating in pairs and reflexed or sometimes splitting along only one suture and reflexed to one side as a unit, or separate and reflexed individually, petals yellow, rarely with a red basal spot or entirely red, usually fading orange, yellow, or sometimes red.
Fruit: Capsule cylindrical to narrowly lanceoloid or ovoid, straight, curved or somewhat sigmoid, bluntly quadrangular or terete, straight or curved, sessile, rarely (O. stubbei) basally with a short, sterile and stipelike portion, dehiscent nearly throughout capsule length.
Seeds: Seeds numerous, in (1) 2 rows per locule, prismatic and angled, ellipsoid to subglobose, rarely obovoid and obtusely angled, the testa reticulate and regularly or irregularly pitted, rarely flat.
Chromosomes: Chromosome number: n = 7 (in 37 spp. 1 bivalent + ring of 12, or ring of 14).