The outer layer of a spore wall ( Jackson, 1928). Comment: A perispore, if present, lies outside the exospore, and is not part of it. Probably homologous with exine in pollen. Mycologists use the term for spores formed on the outside of a sporangium. See also: endospore, perispore. Exosporium ( Erdtman, 1943) Synonym of exine.