genus of submerged aquatic plants useful in aquariums and in laboratory demonstrations of cellular activities. Elodea comprises 12 species in the frog's-bit family (Hydrocharitaceae), native to the New World. The common names waterweed and ditch moss reflect their weedy character in ponds and quiet waterways.

Several species are grown in schools as an experimental plant for demonstrating chloroplasts (green bodies in cells in which photosynthesis takes place), cell nuclei, cyclosis (movement of cellular contents), and oxygen production during photosynthesis. They are also important occasionally outside their natural range (North America) as an obstacle to lake navigation. In Europe, for example, the Canadian waterweed (Elodea canadensis) exists as an escaped population of female plants only, which reproduce vegetatively by breaking up.