Erect to spreading, aquatic to subaquatic herb, 20-30 cm. Stem obtusely quadrangular and deeply sulcate, reddish, sparingly hirsute, older stem glabrescent, inconspicuously lenticellate. Leaf sessile; blade (lower) lanceolate, 4.5 x 1 cm, apex subacute, base attenuate and decurrent on petiole, rather thin and glabrous, upper blades oblong, thicker than lower ones, 3 x 0.9 cm, apex obtuse, base attenuate and decurrent on petiole, hirsute and becoming glabrescent with age below, except for primary and secondary veins, secondary veins 6-7 pairs, cystoliths conspicuous on upper surface. Inflorescence of opposite or alternate dichasia, 1-3 flowered; bracts subtending lateral flowers spathulate, 6 x 1.8-2 mm, glabrescent, margin ciliolate; bracteoles 4 mm long, inserted at middle of 1 mm long pedicel. Flowers sessile to subsessile; calyx about 6 mm long, accrescent to 9.5 mm, 5 equal lobes often basally fused for about one-half, lobes narrowly lanceolate, apically long-attenuate, hirtellous to glabrescent; corolla light violet, 9 mm long, pubescent without, tube slightly longer than limb, upper lip 2-lobed, lobes retuse, 0.6 mm long, lower lip 3-lobed, lobes elliptic, 2 x 0.8-1 mm; filaments united at base, 2 mm and 2.7 mm long respectively, glabrous, anthers 1-1.2 mm long, basally apiculate, staminodes absent; ovary glabrous, 1.8 mm high, each locule with about 7 ovules, style 3 mm long, glabrous. Capsule narrowly oblong, 9-10 x 1.6 x 1-1.2 mm, glabrous, glossy, obtuse and apiculate, retinacula 0.8 mm long; seeds 14, flattened, brown, irregularly orbicular, 0.7 x 0.5 mm.