Class of algae , certain members of which are commonly known as stoneworts.
Stoneworts contain calcium carbonate deposits so extensive that they may form the major part of lake bottoms. Superficially resembling higher plants, stoneworts have rootlike and stemlike structures, as well as whorls of branches at regular intervals. They grow underwater, attached to the muddy bottoms of fresh or brackish rivers and lakes. Except for one form that is sometimes destructive in fish hatcheries, they are of little importance to humans.