Oedogonium, genus of filamentous green algae (family Oedogoniaceae), commonly found in quiet bodies of fresh water. They often are attached to other plants or exist as a free-floating mass. Oedogonium filaments are typically unbranched and only one cell thick. Each cylindrical cell of the filament, with the exception of the basal cell that serves as a rootlike holdfast, contains a netlike chloroplast and a large central vacuole.
Vegetative reproduction, or fragmentation, 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 the oogonia of a female filament. After fertilization, the resting zygote eventually divides into four motile spores (zoospores) that germinate to form new filaments.