Basidium

A basidium (pl., basidia) is a microscopic sporangium (or spore-producing structure) found on the hymenophore of fruiting bodies of basidiomycete fungi which are also called tertiary mycellium, developed from secondary mycelium. Tertiary mycelium is highly coiled secondary mycelium, a dikaryon. The presence of basidia is one of the main characteristic features of the Basidiomycota. A basidium usually bears four sexual spores called basidiospores; occasionally the number may be two or even eight. In a typical basidium, each basidiospore is borne at the tip of a narrow prong or horn called a sterigma (pl. sterigmata), and is forcibly discharged upon maturity. A basidium (pl., basidia) is a microscopic sporangium (or spore-producing structure) found on the hymenophore of fruiting bodies of basidiomycete fungi which are also called tertiary mycellium, developed from secondary mycelium. Tertiary mycelium is highly coiled secondary mycelium, a dikaryon. The presence of basidia is one of the main characteristic features of the Basidiomycota. A basidium usually bears four sexual spores called basidiospores; occasionally the number may be two or even eight. In a typical basidium, each basidiospore is borne at the tip of a narrow prong or horn called a sterigma (pl. sterigmata), and is forcibly discharged upon maturity. The word basidium literally means little pedestal, from the way in which the basidium supports the spores. However, some biologists suggest that the structure more closely resembles a club. An immature basidium is known as a basidiole. Most basidiomycota have single celled basidia (holobasidia), but in some groups basidia can be multicellular (a phragmobasidia). For instance, rust fungi in the order Puccinales have four-celled phragmobasidia that are transversely septate; some jelly fungi in the order Tremellales have four-celled phragmobasidia that are cruciately septate. Sometimes the basidium (metabasidium) develops from a probasidium, which is a specialized cell which is not elongated like a typical hypha. The basidium may be stalked or sessile. The basidium typically has the shape of a club, where it is widest at the base of the hemispherical dome at its apex, and its base is about half the width of the greatest apical diameter. Versions where the basidium is shorter and narrower at the base are called 'obovoid', and occur in genera such as Paullicorticium, Oliveonia, and Tulasnella. Basidia with a broad base are often described as 'barrel-shaped'. In most basidiomycota, the basidiospores are ballistospores—they are forcibly discharged. The propulsive force is derived from a sudden change in the center of gravity of the discharged spore. Important factors in forcible discharge include Buller's drop, a droplet of fluid that can be observed to accumulate at the proximal tip (hilar appendage) of each basidiospore; the offset attachment of the spore to the subtending sterigma, and the presence of hygroscopic regions on the basidiospore surface.