A large nonmotile gamete that is rich in nutrients and is normally designated as female. One or several oospheres may be formed within an oogonium. They may be released unfertilized from the plant, as in the brown alga Fucus, or they may be retained within the oogonium and there form a zygote or *oospore
following fertilization. The oosphere is equivalent to the egg cells formed in the archegonia or embryo sacs of land plants.