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the sexual generation in ferns, horsetails, club mosses, and selaginellas, which develops from spores and forms sexual organs (the male antheridium and the female archegonium).
After fertilization the prothallium sprouts sporophytes (the asexual generation), on which spores—cells of asexual reproduction—form. In this manner, the developmental cycle of each such organism follows the orderly alternation of the sexual (prothallium) and asexual (sporophyte) generations. The prothallium can be from several millimeters to 5 cm long, in the shape of whole or segmented plates, threads, or glomerules of simple anatomical structure. The prothallium is usually very short-lived, but the prothallia of club mosses can live as long as 15–20 years.
In isosporous higher plants (ferns, horsetails, and club mosses), the cells of the prothallium grow in the soil or on its surface. Prothallia feed autotrophically from the soil and air (as in ferns and horsetails) or heterotrophically from the decaying remains of other plants (as in the saprophytic prothallia or club mosses). The prothallia of these plants are bisexual or unisexual.
In heterosporous higher plants (aquatic ferns, selaginellas) the prothallia are diclinous. The male prothallia are greatly reduced and are formed from the nutritive substances of the spore; the female prothallia look like small green plates, and their cells are capable of carrying on photosynthesis.
In seed plants the prothallia are even more reduced, because of their adaptation to a terrestrial way of life and their transition to fertilization outside of an aquatic environment. They have formations homologous to the prothallium— endosperms in gymnosperms and a mature germinal sac in angiosperms.