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Annual or perennial, aquatic or marsh herbs. Stem erect, prostrate or creeping, rooting at nodes. Submerged leaves verticillate, pinnatifid to capillary-multifid, aerial leaves opposite to verticillate, lanceolate or pinnatifid. Flowers axillary and solitary or in lax or compact, terminal or axillary spikes or racemes. Calyx tubular, lobes subequal. Corolla tubular or funnel-shaped, 2-lipped. Stamens 4, included, didynamous, anthers free. Ovary glabrous, stigmas 2-lamellate. Capsules ellipsoid to globose, septicidally 4-valved. Seeds minute, angular or ribbed.

Key to the species

Serial Key Reference
1a. Leaves with pinnate nerves, arching from midrib; no whorls of pinnatifid multifid leaves 2
1b. Leaves with 3-5 nerves running from base to apex, or lower leaves whorled, pinnatifid 7
2a. Calyx lobes smooth or with 5 raised nerves in fruit. Bracteoles absent L. rugosa
2b. Calyx lobes striate in fruit 3
3a. Leaves petioled; elliptic, ovate or oblong-lanceolate, subacute, all opposite, calyx hirsute L. diffusa
3b. Leaves sessile or rarely subpetioled 4
4a. Flowers sessile 5
4b. Flowers pedicelled. Leaves linear-oblong, often 3-4-nately whorled 6
5a. Leaves oblong or elliptic-oblong, obtuse, crenate-serrate, sessile or sometimes subpetioled; erect or diffuse herbs; flowers c 6 mm long; calyx lanceolate-acuminate L. repens
5b. Leaves linear, entire or subserrate, always sessile; small herbs; flowers c 3 mm long; calyx lobes subulate L. micrantha
6a. Calyx glabrate; capsule oblong L. aromatica
6b. Calyx hirsute, rarely glabrous; capsule suborbicular L. chinensis
7a. Flowers sessile or very shortly pedicelled 8
7b. Flowers pedicelled, pedicels as long as or longer than calyx. Stems 10-20 cm high, diffsely branched, flowers axillary, solitary, very rarely subracemose L. indica
8a. Stems usually hirsute, rarely glabrate; leaves all 3-6- nately whorled; flowers all axillary, solitary L. sessiliflora
8b. Stems always glabrous; leaves the 2 uppermost usually opposite, the next below 4-6- nately whorled pinnate, the lowest submerged multifid; occasionally the uppermost flowers in leafy spike L. heterophylla