Daily Fungi Fact 166: A cystidium is a large cell found on the fruiting body of a basidiomycete(fungi in the Basidiomycota phylum), often found between groups of basidia(microscopic structure that makes spores in basidiomycetes). Cystidia(plural of cystidium) are distinct and unique to each species or genus, making them useful for identification. The purpose of cystidia is still unknown.