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Key to Oenothera, Key A: section (or genus) Gaura

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1 Pedicels (0.5-) 1-4 mm long; fruit with a slender stipe (0.5-) 2-10 mm long at maturity; clumped or mat-forming perennials from woody rhizomes or rootstocks; [subsection Stipogaura].
  2 Stems and leaves densely soft-villous, hairs mostly appressed; plants suffrutescent, grayish-green; [c. TX and OK westwards].
    3 Stems strigillose (in addition to the longer villous pubescence); plants often stipitate-glandular
    3 Stems hispidulous (in addition to the longer villous pubescence); stipitate glands absent
  2 Stems and leaves glabrous, strigillose, or spreading-villous; plants not suffrutescent.
      4 Stems conspicuously spreading-villous with hairs 2-4 mm, also ± strigillose; cauline leaves 1-3 (-7) cm long; stems ascending, 3-7 (-12) dm; [s. TX]
      4 Stems glabrous or strigillose, rarely sparsely villous proximally; cauline leaves (1-) 2.5-12 cm long; stems erect, 4-25 (-30) dm.
        5 Plants aggressively rhizomatous (also with a taproot), mat-forming; stems, leaves, and inflorescences glabrous or inconspicuously strigillose
        5 Plants single-stemmed or cespitose, taprooted; plants glabrate to densely strigillose, sometimes also villous or glandular-puberulent.
          6 Stamens presented in lower 1/2 of flower at anthesis; mature capsules narrowly ovoid or lanceoloid; stems sparsely strigillose to glabrate; [gypsum, caliche, and other arid calcareous substrates; c. and w. TX]
          6 Stamens evenly distributed at anthesis; mature capsules ovoid; stems sparsely to densely strigillose [various habitats; LA eastward]
1 Pedicels 0-1 mm long; fruit sessile or with thick cylindric base 0.2-2.2 mm long; annuals or biennials (O. hispida, O. suffrutescens, and O. lindheimeri perennial).
             7 Flowers small, with sepals 2-3.5 mm long, petals 1.5-3 mm long; anthers ellipsoid, ca. 1 mm long; inflorescences long, densely spicate, flexuous and conspicuously nodding apically (stiffening in fruit); [subsection Schizocarya]
             7 Flowers larger, with sepals 2.5-12 mm long, petals 2.5-15 mm long; anthers linear, 1.5-6 mm long; inflorescences erect, not as above.
               8 Flowers essentially actinomorphic, the petals abruptly clawed, blades broadly rhombic-ovate, bright white; fruits 3-4 mm long; leaves narrowly oblong-lanceolate, auriculate at base, entire; inflorescences densely spicate, long and wandlike, stiff; [section Stenosiphon]
               8 Flowers zygomorphic, petals more gradually narrowed to clawed base, white or pinkish; fruits 4-13 mm long; leaves mostly broader or toothed; inflorescences mostly shorter, looser.
                 9 Fruit pyramidal, abruptly constricted below to cylindric base about 1/2 the length and 1/4-1/2 the width of the body; plants perennial, with creeping rhizomes or with woody caudices; [west of the Mississippi River].
                   10 Fruits 7-13 mm long, 3-5mm wide, glabrous or nearly so, base of pyramidal portion bulging; inflorescences loose; leaves usually sinuate-lobed, green; pubescence of stems and leaves relatively sparse, stiffly erect or incurved; plants rhizomatous, stems irregularly branched or single; [subsection Xenogaura]
                   10 Fruits 4-9 mm long, 1-3 mm wide, densely strigillose, base of pyramidal portion not bulging; leaves entire or remotely serrate, bluish-green; pubescence of stems and leaves very dense, whitish, appressed; plants from woody caudices, branched belowground or near soil surface; [subsection Campogaura]
                 9 Fruit fusiform to ellipsoid, ovoid, or obovoid, not abruptly constricted; plants annuals or biennials, rarely monocarpic perennials (except O. lindheimeri); [collectively widespread].
                     11 Plants perennial, branched from base; sepals spreading-villous, stems spreading-villous at least proximally, usually stipitate-glandular in inflorescence, not strigillose; flowers large, opening at sunrise, with petals 10-15 mm long, rhombic-obovate or broadly elliptic, sepals 9-17 mm long; [native in coastal prairies of sw. LA and e. TX, widely cultivated, sometimes naturalizing]
                     11 Plants annual or biennial (rarely monocarpic perennial); sepals glabrous to strigillose (rarely short hirtellous), stem pubescence various, but often strigillose; flowers opening at sunset (except O. demareei), petals 3.5-15 mm long, often narrower, sepals 2.5-18 mm long.
                       12 Capsules narrowly (3-) 4-winged, furrowed between the wings; stems 1.5-6.5 (-12) dm, spreading-villous proximally, glabrate distally (rarely somewhat strigillose); [west of the Mississippi River, or disjunct in sw. MS]
                          13 Flowers with sepals 10-15 mm long, petals 8-13 mm long; leaf margins subentire to shallowly sinuate-dentate; fruits ovoid, broadest near base
                          13 Flowers with sepals 11-21 mm long, petals 10-15 mm long; leaf margins mostly distinctly sinuate-dentate; fruits ellipsoid, broadest near midlength
                       12 Capsules 3-4-angled; stems 5-40dm (shorter in G. triangulata), villous, hirtellous, glandular-puberulent, and/or strigillose (see below); [collectively widespread].
                            14 Flowers small, often 3-merous or a mixture of 3-4-merous, with sepals 2.5-8 mm long, petals 3.5-8 mm long; fruits 3-4-angled; stems mostly branching from near the base, often from persistent rosettes, the inflorescence of relatively few, narrow, ±spicate branches; [acid sandy soils; TX and OK, or FL and outer coastal plain from NC to se. MS].
                              15 Capsules 3-4-angled, ovoid to ellipsoid, broadest at or below the middle; plants robust, stems 6-18 dm tall; [MS eastwards]
                              15 Capsules usually 3-angled, narrowly obovoid, broadest above the middle; plants short, stems 1.5-6.5 (-8.5) dm tall; [TX and OK]
                            14 Flowers larger, 4-merous, with sepals 5-20 mm long, petals 6-17 mm long; fruits 4-angled; plants usually copiously branched at and above the middle, the inflorescence open and diffuse; [of a variety of substrates, often calcareous; widespread inland]
                                16 Stems and leaves conspicuously spreading-villous at least proximally, lacking strigillose hairs, and also glandular-puberulent in inflorescence (rarely glabrate); pollen 35-65% fertile; [Northeast and Upper Midwest south to n. AL and GA, absent from the Coastal Plain southward]
                                16 Stems and leaves densely and finely appressed-strigillose nearly throughout, sometimes also sparsely villous or short-hirtellous, glandular-puberulent or not in inflorescence; pollen 90-100% fertile; [mostly west of the MS river, eastward to AL, TN, KY].
                                  17 Plants exclusively appressed-strigillose; flowers opening at sunrise; [endemic to w. AK and adjacent se. OK]
                                  17 Plants usually hirtellous or villous on lower stems or leaf midribs, or glandular-puberulent in the inflorescence; flowers opening at sunset; [more widespread]