Stonewort
Stonewort, common name for about 200 species of algae belonging to the phylum Charophyta of the plant kingdom. Stoneworts are so called because the plant surface is usually covered by a thick, brittle, limy crust of calcium carbonate. They are closely related to green algae and have sometimes been regarded as a class (Charophyceae) within that phylum. Stoneworts grow submerged in hard (alkaline) water and often form dense mats on the bottom of ponds and brackish waters. They are much larger than typical green algae and superficially resemble higher plants in their rootlike rhizoids, leaflike branches at regular intervals, and upright cylindrical axes surrounded by a sheath of cells. Asexual reproduction usually involves the shedding of reproductive branches. Sexual reproduction is by the formation, at leaf nodes, of a large egg in a female sexual structure called an oogonium, and of sperm in filaments constituting the male sexual organ, or antheridium.