Diffuse to erect perennials, often aquatic or hygrophilous herbs. Stems quadrangular and articulated, decumbent and rooting at nodes. Leaves opposite, sessile to petiolate; blades usually narrow, cystoliths present. Inflorescences of dichasia in leaf axils, often appearing as verticels, dichasia of 1-many sessile or subsessile flowers; bracts and bracteoles linear, usually much shorter than calyx. Flowers weakly bilipped, homostylous; calyx 5-lobed, lobes subequal, linear, mature calyx tube often rupturing in weak hyaline regions between lobes; corolla tube cylindrical, upper half funnel-shaped, slightly curved limb ± personate, upper lip 2-dentate to 2-lobed, lower lip 3-lobed, corolla lobes contorted in bud; stamens 4, didynamous, or 2 fertile stamens and 2 staminodes, filaments of each pair usually basally connate by a membrane, anthers 2-thecous, thecae equal in length, parallel to sagittate, equally inserted, basally muticous or mucronulate; ovary with several ovules in each locule, style ending in filiform stigmatic lobes, posterior stigma lobe abortive. Capsules estipitate, linear-ellipsoid, retinacula present, short and obtuse, septae with retinacula remaining attached to inner wall of mature capsule; seeds 4-18, lenticular, unequal at base, covered with hygroscopic trichomes.