Color enhanced scanning electron micrograph of basidia of Basidiomycota. A basidium, (pl., basidia), is a microscopic, spore-producing structure found on the hymenophore of fruiting bodies of basidiomycete fungi. The presence of basidia is one of the main characteristic features of the Basidiomycota. Cross-fracture through gill of basidiomycete fungus Fairies' Bonnets; cells of the subhymenium give rise to basidia which each bear four sterigmata on which the basidospores are formed. These tiny ink cap fungi rarely exceed 1.5cm in cap diameter. They form dense masses on rotting tree stumps and roots, and are to be found from early spring until the onset of winter. The bell-shaped caps are grooved and the gills are grey, darkening with age.