1. Spirodela polyrhiza (Linnaeus) Schleiden, Linnaea. 13: 392. 1839.
紫萍 zi ping
Lemna polyrhiza Linnaeus, Sp. Pl. 2: 970. 1753.
Fronds obovate to orbicular, 3-10 mm, 1-1.5 × as long as wide, flat, rarely gibbous, sometimes with indistinct papillae on upper surface along veins. Roots (5-)7-21, 1(or 2) perforating scale, 0.5-3 cm. Turions sometimes present, without roots, brownish to olive, orbicular to reniform, 1-2 mm. Ovary with 1 or 2 ovules. Fruit laterally winged toward apex. Seed with 12-20 ribs. Fl. (very rare) Jun-Sep. 2n = 30, 32, 38, 40*, 50, 80.
Ponds, lakesides, rice fields, pools, ditches; not seen in regions with cool summers; sea level to 2900 m. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Qinghai, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Yunnan, Zhejiang [worldwide].
A variety, Spirodela polyrhiza var. masonii Daubs was proposed for the ecotype, which has gibbous fronds. This variety, however, occurs only rarely throughout the whole distribution range, so the use of this name is not systematically necessary.
Spirodela polyrhiza is used as a Chinese medicinal herb for diuretic effect, for reducing swelling, and for reducing perspiration.