Oenothera sect. Kneiffia
Oenothera sessilis
usually erect or ascending, sometimes decumbent, branched or unbranched.
ascending, unbranched to few-branched, 30–65 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, basal 2.5–7 ×0.7–2.3 cm, petiole 1–1.5 cm, blade oblanceolate, margins subentire, undulate;
cauline (3–)6–7(–9) ×(0.3–)0.6–0.8(–1.1) cm, sessile, blade lanceolate to narrowly lanceolate, margins subentire.
solitary flowers in axils of distal leaves.
nodding, 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 1–2 mm, connivent to spreading;
floral tube 10–15(–20) mm;
sepals 10–18 mm;
petals bright yellow, fading pale pink, 15–25 mm;
filaments 7–9 mm, anthers 5–8 mm, pollen 85–100% fertile;
style 10–12 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.
ellipsoid, 4-angled, 8–10 × 3–4 mm, stipe 0–2 mm;
sessile.
usually numerous, clustered in each locule, ovoid, surface minutely papillose.
1 × 0.5 mm.
= 14, 28, 42, 56.
= 56.
Oenothera sect. Kneiffia
Oenothera sessilis
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 sessilis is relatively rare within its range and has a narrow overall distribution, occurring in Ashley, Phillips, Prairie, and St. Francis counties in Arkansas, Allen, Claiborne, and Tensas parishes in Louisiana, and Galveston County in Texas. Oenothera sessilis appears to be relatively rare and may no longer occur in Texas; it was last collected there in the 1840s by Lindheimer on Galveston Island. It is also rare in Louisiana but has been collected in recent decades. It is currently most common in Arkansas. P. A. Munz (1937, 1965) treated this taxon as O. sessilis, but G. B. Straley (1977) in his revision of sect. Kneiffia placed it as a subspecies of O. pilosella based on a common octoploid (2n = 56) chromosome number, morphology, and field studies. K. N. Krakos et al. (2014), based on new field studies, controlled greenhouse breeding experiments, and phylogenetic data found that this taxon differs morphologically from O. pilosella by having consistently shorter stature and smaller flowers, is self-compatible, and does not form a monophyletic group with O. pilosella in molecular analyses, and is here reinstated as a distinct species.
Oenothera sessilis is in the Center for Plant Conservation’s National Collection of Endangered Plants as O. pilosella subsp. sessilis.
(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 |
LA Plants
WildflowerSearch
iNaturalist
USDA Plants Database
