2. (Mil.) An instrument with four iron points, so disposed that, any three of them being on the ground, the other projects upward. They are scattered on the ground where an enemy's cavalry are to pass, to impede their progress by endangering the horses' fee
From Waynes world noteworthy plants.
"This strange, horny pod that was presented to Professor Armstrong by a summer biology student. This bizarre horny fruit (not the student) has two prominent, downcurved horns and superficially resembles the head of a bull. The fruit body has a woody, sculptured surface that resembles a face. To some people the entire structure resembles a bat, especially if they are slightly delirious. It comes from an oriental aquatic plant often called "water chestnut" (Trapa bicornis). [Note: This plant is not to be confused with the crunchy, tuberous roots of a vegetable called water chestnut (Eleocharis dulcis) commonly served in Chinese restaurants.] Other common names for this plant include "water caltrop" and the Chinese name of "ling kio" or "ling chio." It belongs to the water-caltrop family (Trapaceae) and includes the single genus (Trapa) with several closely-related species. The generic name Trapa is derived from calcitrappa, Latin name of the caltrop, in reference to the peculiar, horned fruits. During medieval times, a vicious weapon called a caltrop was used in European warfare. This was an iron device with four points so designed that one was always facing upward, whichever way it landed, to impale the hooves of enemy cavalry horses. A similar device was also used in World War II to destroy truck tires on enemy supply convoys. Actually, the widespread and more commonly-known water caltrop (Trapa natans) has a four-pronged fruit that more closely resembles the caltrop. The fruits of puncture vine also resemble a caltrop, especially when they impale your bicycle tires. In fact, this ubiquitous weed belongs to another unrelated plant family called the caltrop family (Zygophyllaceae).
Water caltrop plants are anchored to the bottom of lakes and ponds, and send to the surface a slender stem with a rosette of floating leaves. Like water hyacinths, the leaf stalks are inflated with air and are very buoyant. Solitary flowers are produced in the leaf axils followed by the strange horny fruits. Under the tough outer wall, the fruit contains a pulpy seed rich in starch. The fruits can be roasted or boiled like true chestnuts of the eastern United States. They are delicious sauteed with rice and vegetables. According to Joy Larkcom ("Oriental Vegetables," 1991), they contain toxins and should not be eaten raw. Seeds of some species are preserved in honey and sugar, candied, or ground into flour for making bread. In Italy Trapa natans is the main ingredient of a famous risotto (rice cooked in meat stock with shallots and butter). The unusual, bullhead pods of Trapa bicornis sometimes float down rivers and into the ocean where they occasionally drift ashore on Asian beaches. Local people make ingenious necklaces from the pods and sell them to tourists. With a backing attached to your lapel, they actually make a nice little emblem to wear on special occasions, or an attractive slide for a bolo tie."