Pistia stratioteswater cabbage,water lettuce,Nile cabbage,shellfloweris a free-floating plant with many spongy,dusty green simple leaves.The leaves are covered in very fine hairs and arranged in a spiral pattern from the center of the plant.It is a very aggressive invader and can form thick floating mats covering the entire surface of the pond. Its native distribution is uncertain, but probably pantropical.Duckweed water lens are flowering aquatic plants which float on or just beneath the surface of still or slow-moving bodies of fresh water and wetlands.They are very simple,lacking an obvious stem or leaves. The greater part of each plant is a small organized `thallus` or `frond` structure only a few cells thick, often with air pockets aerenchyma that allow it to float on or just under the water surface.