Thacla natans (Pallas) Deyl & Sojak.

Plants submerged or creeping. Stems 20--50 cm tall, 2--4 mm in diam., branched. Cauline leaves long petiolate; petiole 2.5--7 cm, sheathed at base; leaf blade cordate-reniform or cordate, 1--2 × 1.5--2.4 cm, base deeply cordate-reniform or cordate, margin entire or repand or crenulate below middle, apex rounded. Monochasium terminal, (2- or)3--5-flowered. Pedicels 2--4 cm. Flowers small, ca. 5 mm in diam. Sepals 5, white or pinkish, obovate, ca. 3 × 2 mm, apex rounded. Stamens ca. 2 mm; anthers elliptic, ca. 0.5 mm. Follicles (10--)20--30, ca. 5 mm, sessile; stigmas nearly sessile. Seeds black, ellipsoid-globose, less than 1 mm. Fl. Jul. 2n = 16, 32.

Wet meadows, marshes, in water. Heilongjiang, NE Nei Mongol [Mongolia, Russia (Siberia); North America].