Editing Basidium (section)

==How basidiospores are expelled==
In most [[basidiomycota]], the basidiospores are forcibly expelled. 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 drop of fluid that builds up at the nearer tip ('''hilar appendage''') of each basidiospore; the offset attachment of the spore to the extending narrow prong, and the presence of [[hygroscopic]] regions on the basidiospore surface. Basidiospore discharge can only succeed after sufficient water vapor has condensed on the spore.

When a basidiospore matures, sugars present in the cell wall begin to serve as condensation loci for water vapour 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 builds up as a large, almost spherical water droplet.

[[File:03 01 04 discharge of ballistospore (M. Piepenbring).png|thumb|upright=1.2|right]]

At the same time, condensation occurs in a thin film on the stalk-facing part of the spore. When these two bodies of water combine, the release of surface tension and the sudden change in the center of gravity suddenly expels the basidiospore. Remarkably, the initial acceleration of the spore is estimated to be about 10,000&nbsp;[[Standard gravity|{{mvar|g}}]].<ref name=Money-1998/>

===Evolutionary loss of expulsion by force===
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 means of expulsion.{{cn|date=February 2025}}

For example:
* Members of the order Phallales ([[stinkhorns]]) rely on insect [[dispersal vector|vectors for dispersal]].{{cn|date=February 2025}}
* The dry spores of the [[Lycoperdales]] ([[puffball]]s) and [[Sclerodermataceae]] (earth balls and kin) are dispersed when the basidiocarps are disturbed.{{cn|date=February 2025}}
* Species of the [[Nidulariales]] (bird's nest fungi) use a splash cup mechanism.{{cn|date=February 2025}}

In these cases the basidiospore typically lacks a hilar appendage, and expulsion by force does not occur. Each example is thought to represent an independent evolutionary loss of the forcible discharge that comes before all basidiomycetes.{{cn|date=February 2025}}