All land plants have evolved from aquatic ancestors. Species from nearly one hundred flowering plant families, along with some ferns, mosses, and liver-worts, have reinvaded the water. Many land plants can tolerate flooding for some time. A good definition of aquatic plants is therefore difficult. The term is normally used for plants that grow completely underwater or with leaves floating on the surface. Parts of the shoot, particularly flowering stems, will often grow up above the water. Most aquatic plants live in freshwater—in lakes, ponds, reservoirs, canals, or rivers and streams. These habitats are very different in water depth, flow rates, temperature, acidity and alkalinity (pH), and mineral content. Some aquatic plants live in river mouths, with an ever-changing mixture of fresh- and saltwater, and a few (such as sea grasses) live completely submerged in the sea. Aquatic plants can be free-floating (e.g., water...