1. Pistia stratiotes Linnaeus, Sp. Pl. 2: 963. 1753.
Pistia spathulata Michaux
Roots to 50 cm, with short branches. Leaves light green to grayish green, 2--15(--20) cm, spongy, pubescence dense, white; major veins 5--13(--15), nearly parallel, abaxially prominent. Inflorescences: spathe white to pale green, convolute basally, slightly constricted above middle, spreading apically, pwhite to pale green, pubescent outside, glabrous inside; spadix adnate to spathe more than 1/2 its length, shorter than spathe; axis naked at base of staminate part and sometimes extending beyond staminate flowers. Flowers: staminate flowers (2--)6--8, in single whorl around central stalk, stamens 2, connate; pistillate flower solitary; ovariesy 1-locular, 4--5 mm; ovules 4--15(--20), orthotropous; styles ca. 3 mm; stigmas obtuse, with small hairs. ; staminate flowers (2--)6--8, in single whorl around central stalk; stamens 2, connate. Fruits with thin pericarp. Seeds light brown, cylindric, 2 ´ 1 mm. 2n = 28 (India, Borneo).
Flowering mainly late summer--early winter (although; plants have been collected in flower in almost all months). Slow-moving streams, canals, drainage ditches, ponds, lakes, and springs; 0--10 m; Fla., La., Tex.; Mexico; West Indies; Central America; South America; Asia; Africa; Pacific Islands; Australia.
Pistia stratiotes has been collected north and west of its U.S. range in Arizona, California, Georgia, Missouri, New Jersey, Ohio, and South Carolina. These are probably populations that do not persist. Although P. stratiotes is often reported as occurring from Florida to Texas, I saw no verifying herbarium specimens from Alabama or Mississippi, nor were any populations located during my fieldwork in those states.
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Center. 1989. Seed production by Pistia stratiotes L. (water lettuce) in the United States. Aquatic Bot. 33: 155--160. Duncan, W. H. and J. T. Kartesz. 1981. Vascular Flora of Georgia: An Annotated Checklist. Athens, Ga. Eastwood, A. 1900. Some plants of Mendocino County new to the flora of California. Zoëe 5: 58--60. Fernald, M. L., and A. C. Kinsey, and (revised by R. C. Rollins). 1958. Edible Wild Plants of Eastern North America, revised ed. New York. Greenwell, A. B. 1947. Taro---with special reference to its culture and uses in Hawaii. Econ. Bot. 1: 276--289. Grayum, M. H. 1987. A summary of the evidence and arguments supporting the removal of Acorus from the Araceae. Taxon 36: 723--729. Harley, K. L., R. C. Kassulke, D. P. Sands, and M. D. Day. 1990. Biological control of water lettuce, Pistia stratiotes (Araceae) by Neohydronomus affinis (Coleoptera: Curculionidae). Entomophaga 35: 363--374. Henkel, A. 1907. American root drugs. U.S.D.A. Bur. Pl. Industr. Bull. 107: 1--80. 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Taxon Will Key To Distinguishing CharacterISTICs Comments Aglaonema commutatum Schott Peltandra Leaves oblanceolate to oblong with thick midveins and parallel venation; creeping or erect above-ground stem; spathe not differentiated into tube and blade Dade Co., Florida; persisting after cultivation, rarerarely found Alocasia macrorrhizos (Linnaeus) Schott Peltandra Leaves net-veined between primary lateral veins; aboveground stem; spadix with sterile appendage Dade Co., Florida; persisting for short periods, rarerarely found Arum italicum Linnaeus Peltandra Leaves net-veined between primary lateral veins; long sterile appendage California, Louisiana, North Carolina, Oregon; escaping locally and forming colonies Caladium bicolor (Aiton) Ventenat Colocasia Leaves usually variegated with pink, red, or white; spadix without sterile appendage Florida, Louisiana; rarerarely found Dracunculus vulgaris Schott Arisaema dracontium Spadix about as long as spathe; spathe with large distinct blade California, Oregon; rarerarely found Epipremnum pinnatum (Linnaeus) Engler cv. 'Aureum' Will not key well to any genus Stem scandent; juvenile leaves entire, adult leaves irregularly pinnatifid, leaves variegated with yellow; rarely flowering Florida; escaping and persisting in shady hammocks and on roadside trees Pinellia ternata (Thunberg) Makino Arisaema triphyllum Petiole with bulbils basally and at apex; base of spadix adnate to spathe on one side California, District of Columbia, Maryland, New Jersey, New York, Ohio, Pennsylvania, West Virginia; very local, forming colonies usually in gardens and nurseries Syngonium podophyllum Schott Arisaema Stems scandent; milky sap; juvenile leaves entire, adult leaves pedatisect Florida; established on roadside trees and hammocks, local; other species of Syngonium may persist, but are rarerarely found Xanthosoma spp. Peltandra Leaves not peltate, net-veined between primary lateral veins; may have aboveground stem; spadix without sterile appendage Southern Florida, Louisiana, and Texas; persisting from cultivation, rarely spreading Zantedeschia aethiopica (Linnaeus) Sprengel Peltandra Spathe funnel-shaped, not constricted, white California; uncommon