1 Flowers zygomorphic (except O. curtiflora and O. glaucifolia), white to pink; petals distinctly clawed, blades elliptic to narrowly obovate, rounded at apex; fruits indehiscent; seeds 1-6 (-8) per capsule, 1.5-3.5 mm long; [section Gaura]

1 Flowers actinomorphic, yellow or white to pink; petals not notably clawed, elliptic to broadly obcordate; fruit dehiscent; seeds (10-) 50-many per capsule, 0.3-2 mm long.

  2 Capsules lanceoloid to cylindric (roughly isodiametric or thickest below middle), nearly terete except for shallow grooves along valves, often curved or contorted; ovary essentially terete (note: buds may be 4-angled).

    3 Flowers white, fading pink; buds nodding; stems white or pink; [western species; in our area only in OK, TX, or as rare waifs eastward].

      4 Seeds in 2 rows per locule, ellipsoid to subglobose, surface with regular longitudinal lines of pits; stems 0.5-3 dm tall or long, decumbent, from large, persistent basal rosettes, cauline leaves reduced; [sect. Kleinia]

      4 Seeds in 1 row per locule, obovoid, surface minutely honeycomb-reticulate, but appearing smooth at lower magnifications; stems 1-10 dm tall, erect and branched, rosettes poorly developed or absent at anthesis; [sect. Anogra].

        5 Plants glabrous proximally, sometimes glandular-puberulent or sparsely strigillose in inflorescence; leaves narrowly lanceolate or oblanceolate, 0.3-0.6 (-1) cm wide, usually entire; stems often single, branched above

        5 Plants densely strigillose; leaves oblong-lanceolate or lanceolate, (0.4-) 0.7-1.5 cm wide, sinuate-dentate or denticulate; stems cespitose, several-many, branched from base or unbranched

    3 Flowers bright yellow, rarely fading pinkish; buds erect; stems green, gray-green, brown, or sometimes flushed red; [collectively widespread].

          6 Stigmas peltate, disc-shaped or shallowly 4-lobed; leaves linear to narrowly lanceolate or oblanceolate, stiff, spinulose-serrate to subentire, less than 1 cm wide; plants perennials less than 6 dm tall, often suffrutescent, stems several to many, branching from near base (O. capillifolia var. capillifolia may be annual and single-stemmed); [MO, s. IL, TX, and OK westward, or disjunct in MS and AL in calcareous soils]; [section Calylophus]

          6 Stigmas deeply divided into 3-4 linear lobes; leaves narrowly to broadly lanceolate to oblanceolate, subentire to deeply sinuate-lobed, up to 6+ cm wide; plants annuals, biennials, or short-lived perennials, stems to 30 dm tall; [collectively widespread]; [section Oenothera]

             7 Capsules lanceoloid, thickest below the middle, tapering to the apex; inflorescence spicate or racemose, dense; plants 3-30 dm tall, erect or ascending

             7 Capsules linear, nearly isodiametric, sometimes very slightly thickened distally; inflorescence lax, appearing to consist of solitary axillary flowers; plants to 4 dm tall, often decumbent; [subsection Raimmania]

               8 Nonflowering portion of stems stiff, densely and uniformly strigillose, sometimes also sparsely villous; leaves thick, gray-green, densely strigillose, subentire, shallowly dentate, or occasionally lyrate; [coastal dunes and barrier islands].

                 9 Sepals 2.0-3.3 cm long; petals 2.5-4.5 cm long; stigma elevated above the anthers at anthesis; capsule 2.5-5.5 cm long; rosette leaves 5-14 cm long, 1-2 cm wide

                 9 Sepals 0.3-1.1 cm long; petals 0.45-1.6 cm long; stigma surrounded by the anthers at anthesis; capsule 1.5-4.5 cm long; rosette leaves 4-8 cm long, 0.7-1.0 cm wide

               8 Nonflowering portion of stem not stiff, sparsely to moderately strigillose, often villous, sometimes densely so; leaves green (sometimes gray-green in O. falfurriae and O. mexicana of inland s. TX), sparsely to moderately strigillose and usually villous, mostly deeply sinuate-lobed or dentate; [sandy soils inland, often disturbed]

                   10 Petals 1.3-4.5cm long, sepals 1-3cm long; styles 3.5-7.5cm long, the stigma lobes slightly to strongly elevated above anthers at anthesis.

                     11 Petals 1.3-2.5cm; styles 3.5-5cm, the stigma lobes only slightly elevated above the anthers at anthesis; [se. TX]

                     11 Petals 2.5-4.5cm; styles 4-7.5cm, the stigma lobes elevated well above the anthers at anthesis; [more widespread]

                   10 Petals 0.5-1.5cm long (sometimes -2.2cm in O. laciniata), sepals 0.5-1.5cm long; styles 2-5 cm long, the stigma lobes surrounded by the anthers at anthesis.

                       12 Bracts flat, spreading, light green; pollen ca. 50% fertile [widespread and weedy]

                       12 Bracts revolute, erect, grayish-green; pollen 85-100% fertile; [se. TX]

  2 Capsules clavate, ellipsoid, or oblong (thickest at or above the middle), strongly 4-8-ribbed or -winged (weakly in O. linifolia); ovary 4-angled or 4-winged.

                          13 Flowers pink or white.

                            14 Petals pink or rose-purple; capsules with rounded ridges, narrowly obovoid, apex narrowed to a sterile beak; [collectively widespread]; [section Hartmannia]

                              15 Flowers rose-purple, petals 4-12 mm long, floral tubes 4-8 mm long; buds erect; capsules narrowed to slender stipe 5-20 mm long; [s. TX]

                              15 Flowers pink, petals 15-45 mm long, floral tubes 12-25 mm long; buds nodding; capsules with thick cylindrical base; [widespread]

                            14 Petals white; capsules with wings 0.5-4mm wide, ellipsoid, broadly rounded to bluntly acuminate; [TX and OK westwards]; [section Leucocoryne]

                                16 Stigmas surrounded by anthers at anthesis; petals 8-25mm; pollen 35-65% fertile

                                16 Stigmas exserted beyond anthers at anthesis; petals 20-43mm; pollen 85-100% fertile

                          13 Flowers yellow, sometimes fading orange or reddish.

                                  17 Leaves all basal, pinnatifid; [section Lavauxia, subsection Lavauxia]

                                  17 Leaves all or partly cauline, entire or toothed.

                                    18 Petals 50-70 mm long; flowers opening in the evening; wings of the fruit 10-25 mm wide; [section Megapterium]

                                       19 Plants strigillose

                                       19 Plants glabrous

                                    18 Petals 3-30 mm long; flowers opening in the day; wings of the fruit <3 mm wide.

                                         20 Cauline leaves lanceolate to ovate, > 1 mm wide; petals 5-30 mm long; floral bracts longer than the ovaries they subtend; mature fruits clavate to oblong-elliptic, 3.5-20 mm long, mostly narrowly winged; perennial (O. spachiana annual); [section Kneiffia].