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Water Chestnut (Trapa natans) is an annual, floating-leaved aquatic plant of temperate and tropical freshwater wetlands, rivers, lakes, ponds, and slightly brackish estuaries. It is native to Eurasia and Africa, where it has been widely gathered for its large nutritious seeds since the Neolithic. It is now widely distributed in Eurasia, Africa, and the northeastern United States.
Water Chestnut is viewed by humans quite differently in different parts of the world. It is cultivated for food in Asia. In Europe and Russia, it is now a species of conservation concern. However, in the northeastern United States, where it was introduced in the mid-1800s, it has spread widely and is viewed as a nuisance weed. Water Chestnut is considered a pest in the U.S. because it forms extensive, dense beds in lakes, rivers, and freshwater-tidal habitats. This results in displacement of aquatic plants; interference with boating, fishing, and swimming; and depletion of dissolved oxygen, which adversely affects fish communities. A range of control methods have been explored, including treatment with ultrasound (Wu and Wu 2006).
Two varieties of Water Chestnut are now generally recognized. The widespread variety of Water Chestnut, which produces a 4-horned fruit, is often known as Water Caltrop (T. natans var. natans) and is now found in Eurasia, Africa, and the northeastern United States. The other variety, often known as Singhara Nut (T. natans var. bispinosa) grows in China, Japan, India, and Southeast Asia and produces a fruit with two stout curved horns (according to Hummel and Kiviat , this variety has at times been referred to as T. bicornis, T. bicornuta, and T. japonica). Although formerly placed in the family Trapaceae, modern treatments generally place Trapa in the Lythraceae (Graham et al. 2005). Water Chestnut should not be confused with the unrelated Chinese Water Chestnut (Eleocharis dulcis) (family Cyperaceae), a spikesedge with an edible tuber that is commonly used in Chinese cuisine.
Water Chestnut has floating leathery leaves, up to 5 cm wide, that are broadly triangular or ovate in outline with a toothed margin. The petiole (stalk) of each floating leaf has a spongy, swollen float that allows the foliage to form a rosette, up to 30 cm in diameter, on the surface of the water. Beneath the surface of the water is a flexible stem, 1 to 5 m long, that bears submersed, linear or spatulate leaves These submersed leaves drop early and are replaced by pairs of fine, pinnate structures up to 8 cm long. These plume-like structures have been variously considered to be stipules, leaves, or adventitious roots (i.e., roots originating from the stem, branches, leaves, or old woody roots rather than from primary roots). The small white 4-petaled flowers, which are borne singly in the axils of the floating leaves, yield dark brown woody fruits, 2.5 to 5 cm across, the outer portions of which quickly disintegrate to reveal the "nuts", which sink rapidly to the bottom where they overwinter in sediment.
Water Chestnut kernels contain 16% starch and 2% protein; because of possible toxicity, it is recommended to boil the kernels for an hour before consumption (Vaughan and Geissler 1997).
(Vaughan and Geissler 1997; Hummel and Kiviat 2004 and references therein) .