The nonmotile female gamete in plants and some algae. In angiosperms (flowering plants) it is a cell in the embryo sac of the ovule. In other plants it is situated in an archegonium. In algae, such as Fucus, the oosphere is protected by an oogonium until it is shed into the water prior to fertilization. Many oospheres store food in the form of starch or oil droplets.