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Section through Gill of a Mushroom

The spore mother cell in the Basidiomycota is called a basidium. Usually four basidiospores are produced on each basidium (right centre). Each basidiospore is produced at the apex of a pointed prolongation called a sterigma. In mushrooms, the basidia are packed into a layer that lines the face of the gill called a hymenium (purplish layer).  In the photomicrograph, there are two mature basidia in the hymenium with the spores on the prongs ready to be discharged.   When it is ripe, each spore is shot off a very small distance (0.2 mm) between adjacent gills. It then falls by gravity below the bottom level of the gills and is carried off by air currents for dispersal.