Description from Flora of China
Ammannia densiflora Roth in Roemer & Schultes, Syst. Veg. 3: 304. 1818; Rotala densiflora subsp. uliginosa (Roth) Koehne; Sellowia uliginosa Roth.
Herbs, annual, terrestrial or amphibious, 7-10[-40] cm. Stem usually creeping, sparsely to densely branched, 4-winged, wings not running into leaves. Leaves decussate, often alternate toward stem apex, narrowly elliptic, linear-lanceolate, oblong, or ovate, [2-]12-30 × 1.5-5 mm, base cordate to obtuse, apex obtuse, acute, or acuminate. Bracts of stem and lower branches like foliage leaves; bracts of axillary branchlets much smaller and broadly ovate, clasping. Flowers solitary in axils on main stem or in axillary spikes; bracteoles pink, lanceolate, equaling to surpassing floral tube. Floral tube (4 or)5-merous, campanulate, 1-2 mm; sepals (3-)5; epicalyx segments present between sepals, setiform, ca. 1/2 as long as to longer than sepals. Petals (4 or)5, bright pink or white, equal to or surpassing sepals, persistent. Stamens (3-)5; anthers reaching margin of floral tube. Ovary globose; style shorter than ovary, slightly exserted. Capsules subglobose, ca. 1.5 mm in diam., included, 3-valved. Seeds ca. 0.5 mm. Fl. and fr. Aug.
The habit varies considerably depending on whether the flowers are sessile at nodes of the main stem or develop in the axils of lateral branchlets. If branchlets occur, the stem leaves are much larger than the bracts of the branchlets.
Wet places. Guangdong, Jiangsu [India, Indonesia (Kalimantan), Nepal, Pakistan, Sri Lanka; C Asia, Australia].