the formation in plants of spores of one size.

Isospory is characteristic of ferns (excluding the aquatic varieties—the salvinias, pepperworts, and azollas), horsetails, and club mosses (excluding the selaginellas). On some horsetails, smaller spores (male prothallia with antheridia) or larger spores (female, with archegonia) develop out of externally identical spores, depending on the conditions of nourishment and light. Thus, by regulating the conditions for spore germination, it is possible to alter the sex of the prothallus (physiological heterospory).