1. Hydrilla verticillata (Linnaeus f.) Royle, Ill. Bot. Himal. Mts. 1: t. 376. 1839.
黑藻 hei zao
Leaves in whorls of 3-8, linear or narrowly strap-shaped, 0.7-1.7 cm × 1-4 mm, midvein distinct, margin conspicuously denticulate, apex acute. Male spathe green; sepals white, ca. 2.3 × 0.7 mm; petals reflexed, white or reddish, ca. 2 × 0.5 mm. Female spathe green; flowers similar to male ones. Fruit with 2-9 spinelike projections or smooth and without projections. Seeds (1 or)2-6. Fl. and fr. May-Oct. 2n = 16, 24.
Ponds, running water. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Liaoning, Shaanxi, Shandong, Sichuan, Taiwan, Xizang, Yunnan, Zhejiang [Afghanistan, Bangladesh, Bhutan, India, Indonesia, Japan, Kazakhstan, Korea, Malaysia, Myanmar, Nepal, New Guinea, Pakistan, Philippines, Russia, Sri Lanka, Thailand, Vietnam; Africa, SW Asia, Australia, Europe; introduced in North America].