☆ any of a genus (Podostemon, family Podostemaceae) of small, many-branched, aquatic dicotyledonous plants (order Podostemales) adhering to stones by means of suckerlike roots
See riverweed in American Heritage Dictionary 4
(rĭvˈər-wēdˌ)
noun
An eastern North American plant (Podostemum ceratophyllum) having olive-green foliage resembling seaweed and growing on rocks in rapidly flowing streams.