A
cystidium (plural
cystidia) is a relatively large cell found on the
hymenium of a
basidiomycete (for example, on the surface of a
mushroom gill), often between clusters of
basidia. Since cystidia have highly varied and distinct shapes that are often unique to a particular species or genus, they are a useful micromorphological characteristic in the identification of basidiomycetes. In general, the adaptive significance of cystidia is not well understood.