genus of yellow-green algae characterized by multinucleate tubular branches lacking cross walls except in association with reproductive organs or an injury. Food is stored as oil globules. Asexual reproduction is by motile multiflagellate zoospores and nonmotile aplanospores; sexual reproduction also occurs. The spherical female sex organ (oogonium) and the slender hook-shaped male sex organ (antheridium) are usually produced on branches close to each other. After the nonmotile egg is fertilized by a biflagellate sperm, the zygote may enter a resting phase for several weeks before germinating into a new plant. Most species occur in fresh water, and some are marine.