Agaricus bisporus - Commercial (Button) Mushroom

Taxonomy

Phylum: Basidiomycota

Class: Agaricomycetes

Order: Agaricales

Agaricus bisporus isnot particularly common in the wild, where like the Field Mushroom and the Horse Mushroom it springs up in fields and appears after rain. The specific name bisporus refers to the fact that each of the basidia bear just two spores - most Agaricus species have four-spored basidia. (You will need a powerful microscope if you want to see these features on the gill faces of agaricoid fungi.)

The very pale-capped mushrooms sold in most supermarkets are a derived form of Agaricus bisporus, which in the wild has a brown cap as pictured above. When fully expanded, the caps are sometimes referred to as Portobello Mushrooms (sometimes spelt Portobella).

Identification guide

Cap	Initially hemispherical, becoming convex and eventually flattening. At maturity, the cap diameter is usually between 5 and 10cm.
Gills	The crowded gills are narrow and free; initially pink they turn reddish and then chocolate brown as the fruitbody matures.
Stem	At 3 to 6cm tall and 1.5 to 2cm. Below the insubstantial membranous double ring, the surface of the stem is flaky.
Spore print	Deep chocolate brown.
Odour/taste	They have a 'mushroomy' odour, of course...
Habitat	In permanent pastures and other grassy places.
Season	Late spring to autumn.
Occurrence	Fairly rare.
Similar species	Agaricus campestris, the Field Mushroom, is very similar but has four-spored basidia.