Ludwigia linifolia
Ludwigia sect. Isnardia
flaxleaf seedbox, southeastern primrose-willow
erect or ascending, slightly ridged, usually well branched, 12–55(–62) cm, glabrous.
usually erect or ascending, sometimes prostrate, decumbent, or sprawling, terete, subterete, or slightly ridged, rarely winged.
alternate;
stipules narrowly ovate to narrowly lanceolate, 0.2–0.3 × 0.1–0.2 mm;
stolons: petiole narrowly attenuate, 0.05–0.5 cm, blade narrowly obovate or oblanceolate to spatulate, 0.5–2 × 0.1–0.6 cm;
stems: sessile, blade linear to linear-oblanceolate, 1.5–4 × 0.1–0.4(–0.6) cm, base very narrowly cuneate, margins entire with obscure hydathodal glands, apex acuminate to acute;
bracts linear, reduced.
alternate or opposite.
leafy spikes, flowers solitary in leaf axils;
bracteoles attached 0–1.5 mm distal to base of ovary, very narrowly oblanceolate to linear, (1.5–)2.5–9(–13) × 0.2–0.8 mm, margins entire, apex acute.
sepals ascending, green, narrowly lanceolate-deltate, (3–)4–7 × 1.1–1.7 mm, margins entire, apex narrowly acute, surfaces glabrous or minutely papillose;
petals narrowly obovate-elliptic, 4–6 × 2–4 mm, base obtuse, apex obtuse or rounded;
filaments pale yellow, (1.3–)1.5–2.5(–3) mm, anthers oblong, 0.6–1.1 × 0.5–0.8 mm;
pollen shed in tetrads;
ovary subcylindric, 3–4 × 1–1.5 mm;
nectary disc elevated 0.3–0.7 mm on ovary apex, bright yellow, 0.8–1.5 mm diam., prominently 4-lobed, minutely papillose;
style yellow, 1.25–2.5 mm, glabrous, stigma subcapitate, 0.3–0.6 × 0.6–0.8 mm, shallowly 4-lobed, not exserted beyond anthers.
4-merous;
petals absent or present, yellow;
stamens as many as sepals;
pollen shed in tetrads or as monads.
subcylindric, terete or slightly angled, 5–10(–12) ×1.3–2(–2.2) mm, hard-walled, irregularly dehiscent, pedicel 0 mm.
subcylindric to clavate, oblong-obovoid, obconic, broadly obpyramidal or subglobose, terete to sharply 4-angled, sometimes 4-winged, with hard or thin walls, irregularly dehiscent or dehiscent by an apical ring or lenticular slits along locule edges, rarely dispersing as unit.
reddish brown, oblong-elliptic, 0.6–0.7 × 0.2–0.3 mm, surface cells nearly isodiametric.
in several rows per locule, free, raphe inconspicuous.
= 16.
= 16, 32, 48, 64.
Ludwigia linifolia
Ludwigia sect. Isnardia
Ludwigia linifolia is primarily a coastal species that extends farther inland in Georgia and the Carolinas. Being one of five diploids in sect. Isnardia, it has particularly prominent nectary lobes and appears to be modally outcrossing (C. I. Peng 1989). It also is known from a disjunct population on the Yucatán Peninsula in Tabasco, Mexico.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Species 19 (18 in the flora).
Based on recent molecular and genomic analyses of the monophyletic group of North Temperate haplostemonous species (Liu S. H. et al. 2017), sect. Isnardia (previously referred to as sect. Dantia Baillon; C. I. Peng et al. 2005) has been expanded to include the species treated previously as sect. Microcarpium (Peng 1988, 1989). Section Isnardia now comprises a polyploid complex of 19 species with a center of distribution in the southeastern United States. Most species are restricted to the Gulf and/or southeastern Coastal Plains, usually from Florida to Texas and to New Jersey. Several species extend farther north (L. glandulosa and L. sphaerocarpa) or only occur farther north (L. polycarpa); L. palustris occurs widely across eastern North America from Newfoundland, Quebec, Minnesota, and Kansas to the Gulf and east coasts, is well established on the west coast from California to British Columbia, ranges south to the West Indies, Mexico, Central America, and northern South America, and also occurs in Europe. Ludwigia repens also extends to the West Indies and Mexico, with disjunct occurrences in Arizona, California, and Oregon; several other species also extend to the West Indies, and L. stricta (C. Wright ex Grisebach) C. Wright is endemic to Cuba (Peng and H. Tobe 1987; Peng 1988, 1989).
Section Isnardia now consists of five diploids (2n = 16), nine tetraploids (2n = 32), four hexaploids (2n = 48), and one octoploid (2n = 64); one diploid, Ludwigia stricta, does not occur in North America. Species in this treatment are arranged by ploidy level, then by presumed genomic and phylogenetic order (H. Tobe et al. 1988; C. I. Peng 1988, 1989). Despite the different ploidy levels, natural hybridization among the species of this section is relatively common, and all species above the diploid level are allopolyploid (Peng 1988, 1989; Peng et al. 2005). Most hybrids are sterile, but may persist vegetatively. One sterile hybrid, L. ×lacustris Eames (L. brevipes × L. palustris), has persisted in the same area in Connecticut and Rhode Island for at least 70 years through vegetative reproduction; one of its parents, L. brevipes, no longer occurs in this region (Peng et al. 2005).
Most species in sect. Isnardia have stolons and fibrous roots, and the more aquatic species have aerenchyma near the base of their stems. Five species (the former sect. Dantia, in the strict sense) differ from all others in Ludwigia by having opposite rather than alternate leaves. Most species (14 of 19 in the section) have no petals (some occasionally have one to four vestigial petals). Eight species shed their pollen as monads, most others as tetrads. Some of the apetalous species have showy sepals that are creamy white, light yellow, or light green, and insect visitors have been observed visiting them (C. I. Peng 1989; Peng et al. 2005); all species in the section that have been tested are self-compatible, and only Ludwigia arcuata has large enough flowers, with the stigma regularly exserted beyond the anthers, that it regularly outcrosses.
Several species in sect. Isnardia are popular aquarium plants due to their aquatic nature and small stature.
Several superfluous generic names have been proposed for this section, including Dantia Boehmer (and several combinations based on the genus at the sectional level).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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