Water lilies are aquatic rhizomatous perennial herbs, sometimes with stolons, as well. The leaves grow from the rhizome on long petioles. Most of them float on the surface of the water. The blades have smooth or spine-toothed edges, and they can be rounded or pointed. The flowers rise out of the water or float on the surface, opening during the day or at night. Water lilies are not only decorative, but also provide useful shade which helps reduce the growth of algae in ponds and lakes.