Isnardia Linnaeus, Sp. Pl. 1: 120. 1753; Gen. Pl. ed. 5, 56. 1754; Ludwigia [unranked] Isnardia (Linnaeus) Torrey & A. Gray; Ludwigia [unranked] Ludwigiantha Torrey & A. Gray; Ludwigia sect. Microcarpium Munz; Ludwigiantha (Torrey & A. Gray) Small

Herbs, perennial, with stolons or rhizomes, or creeping and rooting at nodes, rarely floating. Stems usually erect or ascending, sometimes prostrate, decumbent, or sprawling, terete, subterete, or slightly ridged, rarely winged. Leaves alternate or opposite. Flowers 4-merous; petals absent or present, yellow; stamens as many as sepals; pollen shed in tetrads or as monads. Capsules 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. Seeds in several rows per locule, free, raphe inconspicuous. 2n = 16, 32, 48, 64.

Species 19 (18 in the flora): North America, Mexico, West Indies, Central America, n South America, e Asia; introduced in Eurasia, Africa, Pacific Islands, Australasia.

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).

SELECTED REFERENCES Peng, C. I. 1988. The biosystematics of Ludwigia sect. Microcarpium (Onagraceae). Ann. Missouri Bot. Gard. 75: 970–1009. Peng, C. I. 1989. The systematics and evolution of Ludwigia sect. Microcarpium (Onagraceae). Ann. Missouri Bot. Gard. 76: 221–302. Peng, C. I. et al. 2005. Systematics and evolution of Ludwigia section Dantia (Onagraceae). Ann. Missouri Bot. Gard. 92: 307–359. Peng, C. I. and H. Tobe. 1987. Capsule wall anatomy in relation to capsular dehiscence in Ludwigia sect. Microcarpium (Onagraceae). Amer. J. Bot. 74: 1102–1110. Tobe, H., P. H. Raven, and C. I. Peng. 1988. Seed coat anatomy and relationships of Ludwigia sects. Microcarpium, Dantia, and Miquelia (Onagraceae) and notes on fossil seeds of Ludwigia from Europe. Bot. Gaz. 149: 450–457.