Trapa incisa Sieb. & Zucc.

Stem 1-2.5 mm in diam. Petiole 5-15 cm, slender, slightly swollen distally or not; leaf blade glossy and dark green adaxially, green or sometimes purplish abaxially, often black-brown or with 2 dark spots basally, rhombic-triangular, 1.5-3 × 2-4 cm, glabrous or sparsely pubescent on veins, adaxially glabrous, base broadly cuneate, margin coarsely and sharply incised-dentate distally. Petals pink to pale purplish or white, 5-7 mm. Fruit narrowly rhombic, 0.8-1.5 × 1.2-2 × 0.7-1 cm, 4-horned, surface variously ribbed to smooth, crest absent, crown dome-shaped to inconspicuous, 1-3 mm, beak finely conic; horns conic, 1-1.5 cm, unequal, lower horns descending, upper horns horizontal to ascending, apex barbellate. Fl. May-Oct, fr. Jul-Nov. 2n = 48, 88*, 90*, 92*, ?96.

Swamps, ponds; near sea level to 1000 m, to 2000 m in SW China. Anhui, Fujian, Guangdong, Guizhou, Hainan, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Shaanxi, Sichuan, Taiwan, Yunnan, Zhejiang [India, Indonesia, Japan, Korea, Laos, Malaysia, Russia (Far East), Thailand, Vietnam].

Trapa bispinosa Roxburgh var. incisa (Siebold & Zuccarini) Franchet & Savatier; T. incisa var. quadricaudata Glück; T. maximowiczii Korshinsky; T. maximowiczii var. tonkinensis Gagnepain; T. natans Linnaeus var. incisa (Siebold & Zuccarini) Makino.

Flowering from May to October; fruiting from July to November.

Trapa incisa is close relative of Trapa natans, but differs from the latter in its plants small (vs. stout), stem 1-2.5 mm in diameter (vs. 2.5-6 mm), leaf blade rhombic-triangular (vs. deltoid-rhombic), 1.5-3 × 2-4 cm (vs. 4-6 × 4-8 cm), fruit narrowly rhombic (vs. turbinate to shortly rhombic), 4-horned (vs. 2-4-horned), horns conic (vs. triangular to conic), tapering to a sharp point (vs. apex blunt to sharp).

Trapa incisa is occurring in Anhui, Fujian, Guangdong, Guizhou, Hainan, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Shaanxi, Sichuan, Taiwan, Yunnan, Zhejiang of China, India, Indonesia, Japan, Korea, Laos, Malaysia, Russia (Far East), Thailand, Vietnam.

Stem 1-2.5 mm in diameter. Petiole 5-15 cm, slender, slightly swollen distally or not; leaf blade glossy and dark green adaxially, green or sometimes purplish abaxially, often black-brown or with 2 dark spots basally, rhombic-triangular, 1.5-3 cm long, 2-4 cm wide, glabrous or sparsely pubescent on veins, adaxially glabrous, base broadly cuneate, margin coarsely and sharply incised-dentate distally. Petals pink to pale purplish or white, 5-7 mm. Fruit narrowly rhombic, 0.8-1.5 × 1.2-2 × 0.7-1 cm, 4-horned, surface variously ribbed to smooth, crest absent, crown dome-shaped to inconspicuous, 1-3 mm, beak finely conic; horns conic, 1-1.5 cm, unequal, lower horns descending, upper horns horizontal to ascending, apex barbellate.

The chromosomal number of Trapa incisa is 2n = 48, 88, 90, 92 (Huang et al., 1996; Oginuma et al., 1996).

Growing in swamps, ponds; near sea level to 2000 m.