Description from Flora of China
Ammannia mexicana (Chamisso & Schlechtendal) Baillon; Hypobrichia spruceana Bentham; Rotala mexicana var. spruceana (Bentham) Koehne; R. pusilla Tulasne; R. verticillaris Linnaeus var. spruceana (Bentham) Hiern.
Herbs, annual, aquatic, amphibious, or terrestrial. Stem 3-10 cm, creeping, floating, or erect and ascending, branching. Leaves 3-5(-8)-whorled; aerial leaves narrowly lanceolate to broadly linear [or ovate], 5-10 × 1.5-2 mm, base narrow, apex truncate and often 2-cleft; submerged leaves linear, to 15 × 0.5 mm. Flowers solitary, sessile, axillary; bracteoles linear, ca. as long as floral tube. Floral tube (3 or)4(or 5)-merous, campanulate, translucent, ca. 0.5 mm; sepals (3 or)4(or 5); epicalyx absent. Petals absent. Stamens (1 or)2 or 3(or 4), included. Ovary subglobose; style included, much shorter than ovary. Capsule ca. 1 mm in diam., (2 or)3-valved. Seeds ca. 0.3 mm. Fl. Sep-Dec.
Common in paddy fields and wet places. Henan, Jiangsu, Shanxi, Taiwan (Xinzhu), Zhejiang [tropics and warm-temperate regions of the world].