Water-chestnut

Trapa natans, the water chestnut or water caltrop, is a floating-leaved aquatic annual rooted in the mud of still or slow-moving fresh water. From the submerged stem arise finely divided, feathery underwater leaves and, at the surface, a rosette of rhombic to fan-shaped floating leaves with toothed margins, borne on inflated, spongy stalks that act as floats. Small white four-petalled flowers emerge briefly above the water. After pollination the flower sinks and develops into the characteristic hard, woody nut armed with two to four sharp barbed horns or spines, enclosing a single large white starchy seed. Native to warm-temperate parts of Eurasia and Africa, it grows in ponds, lakes, and sluggish rivers, and the edible nuts have long been gathered and cultivated, though the plant is an aggressive invasive where introduced.