Oenothera sect. Gaura
Syst. Bot. Monogr. 83: 165. 2007.
Herbs annual, biennial, or perennial, caulescent; from a taproot, sometimes woody or producing rhizomes. Stems usually erect or ascending, sometimes decumbent, branched or unbranched. Leaves in a basal rosette and cauline (sometimes not present at flowering), (0.5–)2–8(–13) cm; blade margins sinuate-dentate to denticulate, serrate, lobed, or entire. Inflorescences solitary flowers in axils of distal leaves, forming a spike, erect or nodding. Flowers opening near sunset or sunrise; buds erect, terete, without free tips; floral tube 1.5–20[–42] mm, usually lanate in distal 1/2 within; sepals splitting along one suture, remaining coherent and reflexed as a unit at anthesis, or separating in pairs or sometimes individually; petals white [rarely yellow], fading pink to red, purple, or off-white, spatulate to elliptic, rhombic, or, sometimes, oblanceolate, usually clawed; filaments with basal scale 0.3–0.5 mm, these nearly closing mouth of floral tube, or sometimes reduced or absent; stigma deeply divided into (3 or) 4 linear lobes. Capsules woody and nutlike, ovoid, fusiform, lanceoloid, ellipsoid, obovoid, or pyramidal, (3- or)4-angled, sometimes weakly so, or (3- or)4-winged, apex acute to attenuate or, sometimes, rounded, indehiscent, septa fragile, not evident at maturity; sessile, sometimes disarticulating from plant at maturity. Seeds reduced to 1–4(–8), usually ovoid, rarely oblanceoloid (O. glaucifolia), surface smooth. 2n = 14, 28, 42, 56.
North America, Mexico, Central America (Guatemala), introduced in South America, Europe, Asia, s Africa, Australia.
Species 25 (23 in the flora).
Section Gaura consists of 25 species (26 taxa) that are subdivided into eight subsections [seven in the flora area; subsect. Gauridium (Spach) W. L. Wagner & Hoch is is found only in Mexico]. All species have indehiscent capsules, a feature otherwise found in Oenothera only in O. canescens (sect. Gauropsis). Oenothera havardii (sect. Paradoxus) and O. linifolia (sect. Peniophyllum) have tardily and only partially dehiscent capsules. Twenty-one species in four subsections (Campogaura, Gaura, Stipogaura, and Xenogaura) have zygomorphic flowers; the other four subsections (Gauridium, Schizocarya, Stenosiphon, and Xerogaura), each with a single species, have actinomorphic, or nearly actinomorphic, flowers. Since Linnaeus described Gaura, it has been maintained as distinct at the generic level and, at various times, even at the tribal level. Its distinct status rested on several characteristic features including a scale at the base of the filaments; a peltate indusium at the base of the stigma; indehiscent, nutlike capsules; and seeds reduced to 1–4(–8) (P. H. Raven and D. P. Gregory 1972; W. L. Wagner et al. 2007). The most recent molecular studies (G. D. Hoggard et al. 2004; R. A. Levin et al. 2004) place Gaura strongly within the Oenothera clade and equally strongly in a clade with other sections possessing winged/angled capsules that are sometimes indehiscent or nearly so. Hoggard et al. found strong support for the inclusion of the monotypic Stenosiphon within Gaura; Levin et al. concurred, finding strong support for the monophyly of the Gaura lineage, but placed it unequivocally within Oenothera. Reconsidering the distinctive features of Gaura, Wagner et al. found that the indusium characterizes the whole genus Oenothera, and the indehiscent fruits seem to characterize a larger clade in the genus. The reduction in seed number appears to be a strong synapomorphy for the reconstituted Oenothera sect. Gaura. Wagner et al. recognized eight subsections within sect. Gaura that also includes Stenosiphon. The subsections are arranged according to the synthesis of morphological characters, crossing analyses, and molecular data (Raven and Gregory; Wagner et al.). Oenothera anomala Curtis and O. hexandra (Ortega) W. L. Wagner & Hoch are Mexican species that occur well south of the flora area.
|1||Filaments without basal scales or with minute scale; flowers nearly actinomorphic.||> 2|
|1||Filaments with basal scales; flowers zygomorphic or sometimes nearly actinomorphic.||> 3|
|2||Floral tubes 6–17 mm; herbs probably biennial, glaucous at least in proximal part.||Oenothera subsect. Stenosiphon|
|2||Floral tubes 1.5–5 mm; herbs annual, not glaucous.||Oenothera subsect. Schizocarya|
|3||Capsules with a slender stipe (0.5–)2–10 mm||Oenothera subsect. Stipogaura|
|3||Capsules usually with a stipe 0.2–2.2 mm.||> 4|
|4||Capsules pyramidal in distal 1/2, abruptly constricted to a cylindrical proximal part.||> 5|
|4||Capsules fusiform, ellipsoid, ovoid, or obovoid and then abruptly constricted or cuneate to base.||> 6|
|5||Capsules not conspicuously bulging at base of distal pyramidal 1/2; plants not rhizomatous.||Oenothera subsect. Campogaura|
|5||Capsule conspicuously bulging at base of the distal pyramidal 1/2; plants rhizomatous.||Oenothera subsect. Xenogaura|
|6||Capsules ellipsoid, ovoid, or obovoid.||Oenothera subsect. Gaura|
|6||Capsules fusiform.||> 7|
|7||Floral tubes 9–13 mm; flowers nearly actinomorphic.||Oenothera subsect. Xerogaura|
|7||Floral tubes 3–11(–13) mm; flowers zygomorphic.||Oenothera subsect. Campogaura|