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 characteristic in the microscopic identification of mushrooms and other basidiomycetes. Cystidia are sometimes referred to as sterile cells because they do not produce spores, however this is not correct. If one were to call them "sterile cells" you would have to call all cells in the fungus except the basidia "sterile cells".

Cystidia may occur on the gill edge (cheilocystidia), on the face of the gill (pleurocystidia), on the surface of the cap (dermatocystidia or pileocystidia) or on the stipe (caulocystidia). Especially the pleurocystidia and cheilocystidia are important for identification within many genera. Sometimes the cheilocystidia give the gill edge a distinct colour which is visible to the naked eye or with a hand lens. Boletes also have cystidia; here, of course, the definitions should refer to pores, not gills.

Chrysocystidia are cystidia whose contents contain a distinct refractive yellow body, that becomes more deeply yellow when exposed to ammonia or other alkaline compounds. Chrysocystidia are characteristic of many (though not all) members of the agaric family Strophariaceae.

Due to their high refractive index, Gloeocystidia have an oily or granular appearance under the microscope. Like gloeohyphae, they may be yellowish or clear (hyaline) and can sometimes selectively be coloured by sulphovanillin or other reagents.

In general the purpose of the cystidia from the point of view of the fungus is a mystery. Cheilocystidia, which occur at the edge of the gills, have an excretory function. The margin of gills with abundant cheilocystidia are often beaded with tiny droplets of liquid (see photo, which can be expanded by clicking), so one function of cystidia may be to exude substances which in this case are visible as droplets.

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