Hygrophila costata Nees & T.Nees

Erect to spreading, aquatic to subaquatic herb, 0.5-2 m tall. Younger stem quadrate-sulcate to hexagonal-ridged, reddish, glabrous to sparingly hirtellous at nodes, older stem subquadrangular, glabrescent, inconspicuously lenticellate. Petiole 0.5-3 cm long, glabrate or sparingly pilose; blade lanceolate to narrowly elliptic, 5-25 x 0.5-4 cm, apex acuminate, base attenuate and decurrent on petiole, both surfaces sparsely pubescent to glabrate, margin entire to subcrenate, cystoliths usually difficult to detect; secondary veins 10-15 pairs. Inflorescence of opposite or alternate dichasia, 5-or 7-flowered; peduncle to 1 mm long; bracts and bracteoles narrowly elliptic to linear, 4-7 x 1-1.25 mm, glabrous or with a few scattered trichomes, margin ciliate. Flowers sessile to subsessile; calyx 6-12 mm long, 5 equal lobes often basally fused for about one-third, lobes narrowly lanceolate, apically long-attenuate, glabrous or with few scattered trichomes, margin whitish or hyaline; corolla white, 6-9 mm long, pubescent, tube 3.5-4 mm long, upper lip 2-lobed, 2.8-3 mm long, lobes rounded, 0.8 x 0.8 mm, lower lip 3-lobed, 3 mm long, lobes elliptic, 1.5-2 x 0.8-1.2 mm; stamens inserted near base of throat, longer pair 3 mm long, shorter pair 2.2 mm long, anthers 0.8 mm, without basal appendages; ovary glabrous, 1.8 mm high, each locule with about 7 ovules, style 3 mm long, glabrous. Capsule narrowly oblong, 9-14 x 2 x 1.5-1.6 mm, glabrous, glossy, obtuse and apiculate, retinacula 0.5-0.8 mm long; seeds 12-18, flattened, brown, irregularly orbicular, 1.2 x 1 mm.

It is a subtropical plant. In Argentina it grows from sea level to 1,000 m above sea level.