genus of filamentous green algae, commonly found in quiet waters, either attached to other plants or as a free-floating mass. Each cylindrical cell, with the exception of the basal cell that serves as a holdfast, contains a netlike chloroplast and a large central vacuole. Vegetative reproduction results in a ringlike scar at the top of the cylindrical cell (apical cap). Asexual reproduction is by motile and nonmotile spores, formed only in cells with an apical cap. The motile reproductive bodies are characterized by a ring of cilia at the anterior end. Sexual reproduction is oogamous; the male filament, producing sperm cells that resemble small spores with long flagella, is either the same size as the female filament or only a few cells long, in which case it becomes attached to a female filament. After fertilization, the resting zygote eventually divides into four motile spores (zoospores) that germinate to form new filaments.