Oenothera sect. Kneiffia
Oenothera riparia
usually erect or ascending, sometimes decumbent, branched or unbranched.
erect or ascending, usually many-branched throughout, proximal branches often somewhat spongy, 50–120 cm.
in a basal rosette and cauline, cauline 3–10(–13) cm;
blade margins entire, subentire, denticulate, or coarsely dentate.
in a basal rosette and cauline, softly succulent, 4–13.5 × 0.8–2.1 cm, petiole 0–1.5 cm, blade lanceolate to elliptic-lanceolate, margins remotely denticulate.
solitary flowers in axils of distal leaves.
erect, flowers in axils of distalmost few nodes.
opening near sunrise, faintly scented;
buds erect, terete, with free tips;
floral tube 3–25 mm;
sepals splitting along one suture, remaining coherent and reflexed as a unit at anthesis, or separating in pairs, or all separating individually;
petals yellow, fading pale pink or lavender, or orangish pink or yellow, or not changing color, obcordate to obovate;
stigma deeply divided into 4 linear lobes.
opening near sunrise;
buds with free tips 0.5–2(–5) mm, erect to spreading;
floral tube 14–20 mm;
sepals 20–30 mm;
petals bright yellow, fading pale pink or lavender, 16–27 mm;
filaments 10–15 mm, anthers 7–8 mm, pollen 85–100% fertile;
style 15–30 mm, stigma exserted beyond anthers at anthesis.
leathery, straight, usually clavate or oblong, sometimes ellipsoid, angled or winged, apex rounded to truncate or weakly emarginate, tapering to a sterile, pedicel-like base (stipe), valve midrib raised, initially apically dehiscent, eventually dehiscent nearly throughout;
sessile.
oblong-clavate to oblong-ellipsoid, 4-angled or 4-winged, and wings 0.1–0.2 mm, 7–15 × 4–6 mm, stipe 2–5 mm;
sessile.
usually numerous, clustered in each locule, ovoid, surface minutely papillose.
0.8 × 0.4 mm.
= 14, 28, 42, 56.
= 56.
Oenothera sect. Kneiffia
Oenothera riparia
Species 6 (6 in the flora).
Section Kneiffia consists of six species (seven taxa) widely distributed in the eastern half of the United States and adjacent Canada, at 0–1900 m elevation (G. B. Straley 1977). Oenothera spachiana is the only annual species and occurs in open fields, prairies, rocky or sandy sites, and along roadsides; the remaining species are all perennial and occupy diverse habitats, including fresh and brackish swampy areas, wood margins, meadows, prairies, and sandy sites. Three species are self-incompatible (O. fruticosa, O. pilosella, and O. riparia) and the other three are self-compatible (O. perennis, O. sessilis, and O. spachiana). The flowers are diurnal, opening near sunrise and closing near sunset; in some populations of outcrossing species they may reopen for several days. In the outcrossing taxa, the flowers are pollinated by bees (Halictidae and Bombus). Oenothera spachiana and O. perennis are autogamous, the former often cleistogamous and the latter a PTH species. Taxonomy of the section here differs somewhat from Straley in that molecular data (K. N. Krakos et al. 2014) indicate that O. sessilis should be recognized at the species level since the closest relative is O. fruticosa, not O. pilosella as previously thought. Similarly, O. riparia is separated from O. fruticosa since it apparently is most closely related to O. perennis despite the morphological resemblance to O. fruticosa.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Oenothera riparia has not been accepted as a distinct species for a long time. The conservative approach used by G. B. Straley (1977) is here followed in recognizing a broadly delimited O. fruticosa, except that the very distinctive coastal tidal-freshwater semi-aquatic octoploid populations (Straley 1982) are recognized as O. riparia. Plants of these coastal populations, which occur from southern Virginia to North Carolina, are more robust, more branched, and less pubescent than those of the two subspecies of O. fruticosa, and have slightly succulent leaves and more prominent adventitious roots (Straley 1982; D. Boufford, pers. comm.). Further field studies and cytological analyses are needed to fully understand the geographical and ecological limits of O. riparia and confirm that it is strictly an octoploid species. K. N. Krakos et al. (2014) determined that Oenothera riparia is self-incompatible and is pollinated by bees (Bombus, Lassioglossum, Megachile, Parallelia, Xylocopa, and Zale).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Herbs annual, from a taproot; flowers in leaf axils in distal 1/2 of plant. | O. spachiana |
1. Herbs perennial, from fibrous roots, sometimes producing rhizomes; flowers in axils of distalmost few nodes of plant. | → 2 |
2. Stigmas surrounded by anthers at anthesis; petals 5–10 mm; inflorescences nodding; pollen 40–70% fertile. | O. perennis |
2. Stigmas exserted beyond anthers at anthesis; petals (8–)15–30 mm; inflorescences usually erect, sometimes nodding; pollen 85–100% fertile. | → 3 |
3. Plants usually exclusively hirsute, rarely glabrous, from fibrous roots and producing rhizomes. | O. pilosella |
3. Plants usually strigillose, glandular puberulent, glabrate, or glabrous, sometimes villous, usually with fibrous or fleshy roots, not or rarely producing rhizomes. | → 4 |
4. Capsules ellipsoid, 8–10 mm; plants densely strigillose, glabrate proximally. | O. sessilis |
4. Capsules clavate to oblong-clavate or oblong-ellipsoid, (5–)10–17(–20) mm; plants usually strigillose, and/or glandular puberulent, or glabrous, sometimes villous. | → 5 |
5. Plants usually moderately to densely strigillose throughout, sometimes and/or glandular puberulent or villous; cauline leaf blades 0.1–1(–1.7) cm wide, margins entire or weakly and remotely denticulate. | O. fruticosa |
5. Plants glabrous or sparsely strigillose, sometimes also glandular puberulent distally; cauline leaf blades (0.2–)0.8–3(–5) cm wide, margins dentate to remotely denticulate. | → 6 |
6. Roots fibrous; leaf blade margins dentate to remotely denticulate; petioles 0.1–2(–6) cm; inland upland meadows, stream margins, edges of woods. | O. fruticosa |
6. Roots somewhat fleshy, with adventitious roots where submerged; petioles 0–1.5 cm; leaf blade margins remotely denticulate; coastal marshes, slow-running rivers. | O. riparia |
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