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Key to Oenothera, Key B: section (or genus) Calylophus

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1 Buds terete, the sepals without keeled midribs; stamens subequal; petals acute to obtuse; [sw. NE and c. OK and TX westward]; [subsection Salpingia]
  2 Flowers opening in morning; floral tubes 5-25 (-33) mm long, the upper half or more funnelform; plants glandular-puberulent.
  2 Flowers opening in late afternoon or evening; floral tubes 15-60 (-70) mm long, the upper half or less funnelform; plants glabrous, strigillose, hirtellous, or pilose, sometimes also with glandular pubescence in inflorescence; [Oenothera hartwegii complex]
    3 Leaf base clasping, truncate or subcordate; plants densely pubescent with a mixture of hairs, short-pilose and often hirtellous, sometimes also strigillose and glandular-puberulent in the inflorescence
    3 Leaf base attenuate to obtuse; plants glabrous or sparsely strigillose, sometimes glandular-puberulent in inflorescence.
      4 Plants glabrous throughout, or sometimes glandular-puberulent in inflorescence; blades linear to oblanceolate or lanceolate, margins entire to subentire, rarely undulate
      4 Plants sparsely strigillose, sometimes also glandular-puberulent in inflorescence; blades mostly oblanceolate or lanceolate, margins serrulate to subentire, undulate
1 Buds 4-angled, sepals with conspicuously keeled midribs; stamens of 2 lengths, antisepalous filaments about 2× as long as antipetalous filaments; petals truncate to emarginate; [more widespread]; [subsection Calylophus].
        5 Stigmas surrounded by anthers; petals 5-12 (-20) mm long; pollen 30-60 (-80)% fertile.
          6 Pubescence densely and finely strigillose; leaves linear to narrowly lanceolate or oblanceolate, 1-9 cm long, margins subentire to spinulose-serrate (if coarsely spinulose-serrate, longer than 3.5 cm); [widespread in the Great Plains]
          6 Pubescence rather sparse, hairs stout; leaves mostly oblanceolate, 1.5-3.5cm long, margins coarsely spinulose-serrate; [dunes, sandy soil; endemic to se. TX, mostly along the coast]
        5 Stigmas exserted beyond anthers; petals (6-) 9-25 mm long; pollen 90-100% fertile.
             7 Plants perennial from a woody rootstock, stems several to many, erect to decumbent, 1-4 dm tall or long; leaves 1-4 cm long; [TX westward]
             7 Plants annuals or short-lived perennials, stems one to several, erect, 3-8 dm tall; leaves 2.5-9 cm long; [disjunct eastward as far as AL in calcareous substrates].