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the microscopic fruiting body of fungi of the group Pyrenomycetes. The perithecium opens by a crack or fissure. It may be globose, hemispheric, urceolate, bottle-shaped, or pear-shaped. Perithecia may form on the mycelium, on the stroma (the supporting framework of the mycelium), or within the stroma. Pouches filled with spores develop in the peritheci-um’s cavity; in some Pyrenomycetes, unicellular or multicellular filaments, or paraphyses, form as well. Cleistothecia, or cleis-tocarps, formerly were included among perithecia. Cleistothecia are the completely closed fruiting bodies of the fungi Plectascales and Erysiphales.