Water caltrop

The water caltrop or water chestnut is either of two species of the genus ''Trapa''--''Trapa natans'' and ''Trapa bicornis''. Both species are floating aquatic plants, growing in slow-moving water up to 5 meters deep, native to warm temperate parts of Eurasia and Africa. They bear ornately shaped fruits that resemble the head of a bull, each containing a single very large starchy seed. It has been cultivated in China for at least 3,000 years for these seeds, which are boiled and sold as an occasional streetside snack in the south of that country.

Etymology

The generic name ''Trapa'' is derived from the word for "thistle," calcitrappa, as is the name caltrop. Caltrop also refers to a small iron weapon used in medieval times, with four points, designed to pierce the of enemy cavalry horses. A similar device was used during World War II to destroy the truck tires of enemy convoys.

The plant's Chinese name is ''língjiǎo'' , ''líng'' meaning "water caltrop" and ''jiǎo'' meaning "horn."

This plant should not be confused with the unrelated ''Eleocharis dulcis'', also called water chestnut, an aquatic plant raised for food since ancient times in China. ''Eleocharis dulcis'' is a whose round, crisp-fleshed corms are common in Western-style Chinese food.

Biology

The water caltrop's submerged reaches 12 to 15 ft in length, anchored into the mud by very fine roots. It has two types of , finely divided feather-like submerged leaves born along the length of the stem, and undivided floating leaves born in a rosette at the water's surface. The floating leaves have saw-tooth edges and are ovoid or triangular in shape, 2–3 cm long, on inflated petioles 5–9 cm long which provide added buoyancy for the leafy portion. Four-petaled white flowers form in early summer and are insect-pollinated. The fruit is a nut with four 0.5 in , barbed spines. Seeds can remain viable for up to 12 years, although most will germinate within the first two years.

The plant spreads by the rosette and fruits detaching from the stem and floating to another area on currents or by fruits clinging to objects, birds and other animals.

History

Investigations of archaeological material from southern Germany indicate that the prehistoric population of that region may well have relied significantly upon wild water chestnuts to supplement their normal diet and, in times of cultivated cereal crop failure, water chestnuts may even have been the main dietary component.

In the Zhou Dynasty, water caltrop was an important food for worship as prayer offerings. The Rites of Zhou mentioned that a worshipper "should use a bamboo basket containing dried water caltrops, the seeds of and chestnuts" . The ''Chinese Herbal Medicine Summary'' indicates that water caltrop can help fever and drunkenness.

It was possible to buy water chestnuts in markets all over Europe until 1880. In northern Italy the nuts were offered roasted, much as sweet chestnuts are still sold today. At many places in Europe water chestnuts were known and used for human food until the beginning of the 20th century. Today, however, it is a rare plant. There may be several reasons for its near extinction, such as climate fluctuations, changes in the nutrient content of water bodies, and the drainage of many wetlands, ponds and oxbow lakes.

In Australia, and its state of water caltrop is declared as a noxious weed.

Disease

Fasciolopsiasis can be transmitted by the surface of the plants.