Ludwigia lanceolata
Ludwigia sect. Isnardia
lanceleaf primrose-willow
erect, subterete or slightly ridged, well branched distally, 45–100 cm, glabrous, with raisedlines decurrent from leaf axils.
usually erect or ascending, sometimes prostrate, decumbent, or sprawling, terete, subterete, or slightly ridged, rarely winged.
alternate;
stipules ovate to very broadly ovate, 0.2–0.5 ×0.1–0.3 mm, succulent;
stolons: petiole winged, 0.2–1 cm, blade orbiculate or elliptic to broadly elliptic, 0.5–2.7 × 0.6–1.2 cm, base attenuate, apex rounded to acute;
main stem: petiole winged, 0.1–0.5 cm, blade elliptic, oblanceolate, or narrowly oblanceolate to sublinear, 2–7.5 × 0.2–0.8(–1.4) cm, base narrowly cuneate, sometimes attenuate, margins entire with minute hydathodal glands, apex acute to narrowly acute, leaves on side branches much reduced;
bracts sublinear, moderately reduced.
alternate or opposite.
open spikes, flowers solitary in distal leaf axils;
bracteoles attached in opposite pairs at or slightly distal to base of ovary, ovate-elliptic to very narrowly elliptic, 1.5–4.3 × 0.4–1.4 mm, often with a swollen base, margins minutely papillose, apex subacute.
sepals ascending, pale green, broadly ovate-deltate, 1.5–2.5 × 1.8–3.3 mm, margins minutely papillose, apex usually acute, rarely acuminate, surfaces glabrous;
petals 0;
filaments nearly translucent, 1–1.4 mm, base dilated, anthers 0.4–0.6(–0.8) × 0.5–0.6 mm;
pollen shed in tetrads;
ovary broadly obovoid or cup-shaped, 2.5–3.5 × 2.2–3.2 mm;
nectary disc elevated 0.4–0.6 mm on ovary apex, yellowish green, 1.8–2.6 mm diam., 4-lobed, glabrous;
style yellowish green, 0.5–0.7 mm, glabrous, stigma yellowish green, broadly capitate to subglobose, 0.3–0.5 × 0.7–1 mm, not exserted beyond anthers.
4-merous;
petals absent or present, yellow;
stamens as many as sepals;
pollen shed in tetrads or as monads.
obpyramidal, sharply 4-angled and 4-winged, wings 0.3–0.7mm wide, 3.5–5 × 2.5–4.5 mm, hard-walled, dehiscent by apical ring, pedicel 0.1–0.5 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.
light brown, narrowly oblong with constricted ends, 0.6–0.8 ×0.2–0.3 mm, surface cells nearly isodiametric.
in several rows per locule, free, raphe inconspicuous.
= 32.
= 16, 32, 48, 64.
Ludwigia lanceolata
Ludwigia sect. Isnardia
Ludwigia lanceolata is fairly uncommon, with scattered populations occurring along the Atlantic coast of southern North Carolina, South Carolina, eastern and southern Georgia, and peninsular Florida. It reaches its western limit in the central panhandle of Florida. C. I. Peng (1988, 1989) found this tetraploid species to be interfertile with other tetraploid species in the section, but few natural hybrids are found, perhaps due to persistent autogamy and habitat specialization.
(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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