Oldenlandia salzmannii (DC.) Benth. & Hook.f. ex B.D. Jacks.
Oldenlandia lancifolia (Schumach.) DC. is introduced into South America, Central America, West Indies and Mexico.
O. salzmannii is introduced into the southeastern United States (AL, FL).
Stem creeping to slightly ascending, filiform, faintly squarish, sparsely branched usually from the base, glabrous; rooting at lower nodes. Leaves opposite, decussate; petiole flattened, lower half united to stipules; stipules minute, thin; leaf blade narrowly to broadly elliptic; margin entire or usually with several trichome-like setae of varied lengths, giving the margin a lobed appearance if short and blunt; venation obscure, midrib only. Flowers in leaf axils, solitary or 2 per node; pedicels straight or somewhat curved; sepals 4, fused, lobes ovate, apex acute; corolla campanulate, 4-lobed, lobes ovate, apex ± acute, externally glabrous, internally hairy; white to pinkish.
Within this genus of nearly 250 species, here we focus solely on the aquatic species Oldenlandia salzmannii. It is often referred to and offered under the synonym Hedyotis salzmannii.
Oldenlandia was often treated as a subgenus of Hedyotis, later being recognized as a distinct genus due to chromosome and pollen data (Lewis, 1965). Although acknowledged as separate genera, an unequivocal phylogeny has yet to be published. A recent phylogenetic tree produced from limited molecular data still offers insight to the most probable placement of O. salzmannii. This poorly supported phylogeny demonstrates that O. salzmannii, although not nested within Oldenlandia, is more closely related to this genus than Hedyotis (Neupane et al, 2015). Further studies must be done but for now the accepted name in most flora, and the one we recognize here, is O. salzmannii.