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Leaves, when submerged in truly aquatic species, laciniate, pinnatifid to capillary-multifid, glabrous; aerial leaves opposite to verticillate, sessile or petiolate, entire to serrate, laciniate or pinnately divided, where undivided, pinnately or parallel nerved; punctate.
Calyx tubular, 5-lobed; lobes sub-equal or with adaxial lobe enlarged; tube terete with 0–5 prominent nerves or striate with more than 10 prominent nerves present at maturity.
Corolla tubular or infundibuliform, 5-lobed, bilabiate; adaxial lip outside in bud, entire or bilobed; abaxial lip three-lobed, erect or spreading.
Flowers sessile or pedicellate, solitary-axillary or in lax or compact, terminal or axillary spikes or racemes; bracteoles 0 or 2.
Annual or perennial, aquatic or marsh herbs, frequently aromatic when bruised, glabrous, pubescent or glandular.
Stamens 4, included, didynamous, posterior pair shorter; anthers free, loculi stipitate.
Stems erect, prostrate or creeping, rooting at nodes, simple or more usually, branched.
Capsule ellipsoid to globose, septicidally 4-valved; valves bifid.
Style filiform, deflexed at apex.
Seeds small, numerous.
Stigma bilamellate.
Ovary glabrous.
