Description from Flora of China
Peplis indica Willdenow, Sp. Pl. 2: 244. 1799; Ameletia indica (Willdenow) Candolle; A. uliginosa Miquel; Rotala densiflora (Roth) Koehne var. formosana Hayata; R. elatinomorpha Makino; R. indica var. koreana Nakai; R. indica var. uliginosa (Miquel) Koehne; R. koreana (Nakai) Mori; R. uliginosa (Miquel) Nakai.
Herbs, annual, terrestrial or amphibious, to ca. 40 cm. Stem creeping and branched at base, ascending, or erect, terete to weakly 4-angled. Leaves decussate, obovate-elliptic or obovate-oblong, 5-17 × 3-8 mm, base cuneate, margin translucent to opaque, white cartilaginous, apex obtuse. Bracts like foliage leaves or distinctly smaller on axillary spikes. Flowers in axillary spikes or sessile in bracts on main stem; bracteoles linear, reaching sepals or longer. Floral tube 4-merous, pink-red at anthesis, narrowly to broadly campanulate, 1.5-2.5 mm, 4-angled; sepals 4, lanceolate-deltate; epicalyx absent. Petals 4, pink, minute to ca. 1/2 as long as sepals. Stamens 4; anthers reaching sinus of sepals. Ovary ellipsoidal; style ca. 1/2 as long as ovary, slightly exserted. Capsule ellipsoidal, ca. 1 mm in diam., slightly exserted, 2-valved. Seeds ca. 0.4 mm. Fl. Sep-Oct, fr. Oct-Apr. 2n = 32*.
The species is closely associated with rice cultivation. There are unbranched plants with flowers sessile at nodes of the main stem that do not form lateral spikelike inflorescences. Cook (1979) found that although the unbranched form was sometimes genetically fixed, in many geographically scattered populations it was not fixed but ecologically induced as a result of crowded growing conditions.
Wet places, paddy fields. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hubei, Hunan, Jiangsu, Jiangxi, Shanxi, Sichuan, Taiwan, Yunnan, Zhejiang [Bhutan, Cambodia, India, Indonesia, Japan, Korea, Laos, Malaysia, Myanmar, Nepal, Philippines, Sri Lanka, Thailand, Vietnam; C Asia; introduced in rice fields in Africa (Congo), Europe (Italy, Portugal), and North America (United States)].