A basidium () is a microscopic sporangium (a spore-producing structure) found on the hymenophore of

fruiting bodies The sporocarp (also known as fruiting body, fruit body or fruitbody) of fungi is a multicellular structure on which spore-producing structures, such as basidia or asci, are borne. The fruitbody is part of the sexual phase of a fungal life cyc ...

of basidiomycete fungi which are also called tertiary mycelium, developed from secondary mycelium. Tertiary mycelium is highly-coiled secondary myceliuma

dikaryon The dikaryon is a nuclear feature which is unique to certain fungi. (The green alga ''Derbesia'' had been long considered an exception, until the heterokaryotic hypothesis was challenged by later studies.) Compatible cell-types can fuse cytoplasms ( ...

. 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 In biology, a sterigma (pl. sterigmata) is a small supporting structure. It commonly refers to an extension of the basidium (the spore-bearing cells) consisting of a basal filamentous part and a slender projection which carries a spore at the ti ...

(), 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

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. An immature basidium is known as a basidiole.

Structure

Most basidiomycota have single celled basidia (holobasidia), but in some groups basidia can be multicellular (a phragmobasidia). For instance,

rust fungi Rusts are plant diseases caused by pathogenic fungi of the order Pucciniales (previously known as Uredinales). An estimated 168 rust genera and approximately 7,000 species, more than half of which belong to the genus ''Puccinia'', are currently a ...

in the order ''Puccinales'' have four-celled phragmobasidia that are transversely septate; some jelly fungi in the order

Tremellales The Tremellales are an order of fungi in the class Tremellomycetes. The order contains both teleomorphic and anamorphic species, most of the latter being yeasts. All teleomorphic species in the Tremellales are parasites of other fungi, though th ...

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

: Narrow at the stem and wide near its outer end. It is widest at the waist of the hemispherical dome at its apex, and its base is about half the size of the widest apical diameter. Versions where the basidium is shorter and narrower at the base are called "obovoid", and occur in genera such as '' Paullicorticium'', ''

Oliveonia ''Oliveonia'' is a genus of fungi in the order Auriculariales. Species form thin, effused, corticioid basidiocarps (fruit bodies) with microscopically prominent cystidia and aseptate basidia producing basidiospores that give rise to secondary s ...

'', and ''

Tulasnella ''Tulasnella'' is a genus of effused (patch-forming) fungi in the order Cantharellales. Basidiocarps (fruit bodies), when visible, are typically smooth, ceraceous (waxy) to subgelatinous, frequently lilaceous to violet-grey, and formed on the und ...

''. Basidia with a wide base are often described as "barrel-shaped".

Mechanism of basidiospore discharge

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 In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ...

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 Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substan ...

regions on the basidiospore surface. Basidiospore discharge can only succeed after sufficient water vapor has condensed on the spore. Upon maturity of a basidiospore, sugars present in the cell wall begin to serve as condensation loci for water vapor in the air. Two separate regions of condensation are critical. At the pointed tip of the spore (the hilum) closest to the supporting basidium, Buller's drop accumulates as a large, almost spherical water droplet. At the same time, condensation occurs in thin film on the adaxial face of the spore. When these two bodies of water coalesce, the release of surface tension and the sudden change in the center of mass leads to sudden discharge of the basidiospore. Remarkably, the initial acceleration of the spore is estimated to be about 10,000 .

Evolutionary loss of forcible discharge

Some basidiomycetes do not have a means to forcibly expel their basidiospores, although they still form them. In each of these groups, spore dispersal occurs through other discharge mechanisms. For example: * Members of the order Phallales (

stinkhorns Phallaceae is a family of fungi, commonly known as stinkhorns, within the order Phallales. Stinkhorns have a worldwide distribution, but are especially prevalent in tropical regions. They are known for their foul-smelling, sticky spore masses, ...

) rely on insect vectors for dispersal. * The dry spores of the

Lycoperdales The Lycoperdales are a now outdated order of fungi. The order included some well-known types such as the giant puffball, the earthstars, and other tuberous fungi. They were defined as having epigeous basidiomes, a hymenium present, one to three ...

(

puffball Puffballs are a type of fungus featuring a ball-shaped fruit body that bursts on impact, releasing a cloud of dust-like spores when mature. Puffballs belong to the division Basidiomycota and encompass several genera, including '' Calvatia'', '' ...

s) and

Sclerodermataceae The Sclerodermataceae are a family of fungi in the order Boletales, containing several genera of unusual fungi that little resemble boletes. Taxa, which include species commonly known as the ‘hard-skinned puffballs’, ‘ earthballs’, or 'ea ...

(earth balls and kin) are dispersed when the basidiocarps are disturbed. * Species of the

Nidulariales The Nidulariaceae ('nidulus' - small nest) are a family of fungi in the order Agaricales. Commonly known as the bird's nest fungi, their fruiting bodies resemble tiny egg-filled birds' nests. As they are saprobic, feeding on decomposing organic ...

(bird's nest fungi) use a splash cup mechanism. In these cases the basidiospore typically lacks a hilar appendage, and no forcible discharge occurs. Each example is thought to represent an independent evolutionary loss of the forcible discharge mechanism ancestral to all basidiomycetes.

References

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External links

AmericanMushrooms.com: How do fungi reproduce?

by John Webster. Mycological Society of America Lab Manual

by Heino Lepp, Australian National Botanic Gardens.

by Michael Kuo, ''MushroomExpert.com'' {{Fungus structure Basidiomycota Fungal morphology and anatomy