Description from Flora of China
Plants 2-4 cm tall. Branches ascending. Leaves opposite; stipules small, slightly conspicuous; petioles on upper part of stem shorter, those on lower part longer; leaf blade oblong, oblong-elliptic, or subspatulate, 2-5 × ca. 0.5 mm, base attenuate into petiole, margin entire, apex obtuse. Flowers solitary in leaf axils, subsessile or very shortly pedicellate. Sepals 4, oblong, 0.6-7(-0.9) × ca. 0.3 mm, apex rounded. Petals 4, obovate or broadly elliptic, 0.8-1 × ca. 0.4 mm, slightly longer and wider than sepals, apex rounded. Stamens 8, shorter than petals. Ovary 4-loculed; styles 4. Capsule compressed globose, ca. 2 mm in diam., 4-septicidal. Seeds horseshoe-shaped, 0.5-0.7 mm, finely and densely hexagonal reticulate-striate.
Pools, water on river banks, marshy places. Heilongjiang, Jilin, Liaoning [Russia (Siberia); Europe].