any freshwater perennial herb of the genus Alisma, commonly found in lakes, ponds, and ditches. The three or four species are widely distributed throughout the North Temperate Zone and Australia. Water-plantain leaves float or extend out of the water. They are sometimes ribbonlike or grasslike, are without lobes, and are often heart-shaped or tapered at the base. Flowers have three green sepals and three white or pinkish petals.