thumb|right|500px|Diagram showing a basidiomycete mushroom, gill structure, and spore-bearing basidia on the gill margins.

A basidium (: basidia) is a microscopic spore-producing structure found on the hymenophore of reproductive bodies of basidiomycete fungi. The presence of basidia is one of the main characteristic features of the group. These bodies are also called tertiary mycelia, which are highly coiled versions of secondary mycelia. A basidium usually bears four sexual spores called basidiospores. Occasionally the number may be two or even eight. Each reproductive spore is produced at the tip of a narrow prong or horn called a sterigma (), and is forcefully expelled at full growth.

The word basidium literally means "little pedestal". This is the way the basidium supports the spores. However, some biologists suggest that the structure looks more like a club. A partially grown basidium is known as a basidiole.

Structure

Most basidiomycota have single celled basidia (holobasidia), but some have ones with many cells (a phragmobasidia). For instance, rust fungi in the order Puccinales have phragmobasidia with four cells that are separated by walls along their cross section. Some jelly fungi in the order Tremellales also have phragmobasidia with four cells that are separated by walls and are shaped like a cross. Sometimes the basidium develops from a probasidium, which is not elongated like a typical hypha. The basidium may be stalked or attached directly to the hyphae.

The basidium is normally club-shaped: narrow at the stem and wide near its outer end. It is widest in the middle hemispherical dome at its apex, and its base is about half the size of the widest diameter at the highest point. Basidia with a short and narrow base are shaped like an inverted egg, and occur in genera such as Paullicorticium, Oliveonia, and Tulasnella. Basidia with a wide base are often shaped like a barrel.

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  • AmericanMushrooms.com: How do fungi reproduce?
  • APSnet Illustrated Glossary of Plant Pathology: Basidum
  • Demonstrating basidiospore discharge by John Webster. Mycological Society of America Lab Manual
  • IMA Mycological Glossary: Basidum
  • Spore discharge and dispersal in mushrooms by Heino Lepp, Australian National Botanic Gardens.
  • "Using a Microscope: Basidia and Cystidia" by Michael Kuo, MushroomExpert.com