4. Callitriche palustris Linnaeus, Sp. Pl. 2: 969. 1753.
水马齿 shui ma chi
Plants submerged, with floating rosettes or on wet mud. Blade of lingulate leaves linear to narrowly expanded, (4-)5.6-9.7 × 0.5-1.2 mm, emarginate, with single vein; blade of expanded leaves elliptic to ± orbicular, 3.6-9.9(-10.3) × 1.2-4.3(-4.5) mm; most leaves with tertiary vein, generally also with additional veins arising from secondary veins, either as loops rejoining adjacent secondary veins or as short slightly curved veins pointing ± directly to leaf blade margin. Flowers usually represented by a carpellate flower in each axil or a staminate and carpellate flower in an axil opposed by a single carpellate flower, or more occasionally 1 staminate and up to 3 carpellate flowers opposite a single carpellate flower in an axil; solitary flowers of both sexes may occur with either a shoot or nothing in opposite axil; bracts translucent, whitish, persistent, caducous or lacking; pollen 6-24 × 5-23 µm, probably inaperturate, yellow, ± spherical, exine strongly ornamented, sculpture type a reticulate pattern or muri, lacking supratectal elements. Fruits 0.9-1.4 × 0.8-1.1 mm, sessile, higher than wide; wing 0.05-0.16 mm wide or lacking, when present wider toward apex of fruits.
Lakes, ponds, swamps, marshes, streams, ditches, rice fields; near sea level to 9500 m. Anhui, Fujian, Guangdong, Guizhou, N Heilongjiang, Hubei, Jiangsu, Jiangxi, E Jilin, E Liaoning, E Nei Mongol, Qinghai, Sichuan, Taiwan, SE Xizang, Yunnan, Zhejiang [Bhutan, India, Japan, Kashmir, Korea, Nepal, Russia; Europe, North America].