The class Potametea includes vegetation of aquatic macrophytes which are rooted in the bottom of water bodies or streams. Most of these plants are perennial. They are either completely submerged, or have natant leaves. Some vegetation types within this class require high water levels throughout the year, while some others tolerate or even require short-term exposure of the bottom, but not its complete desiccation. This class is cosmopolitan, but it is most common and diverse in precipitation-rich areas of the temperate zone.