Agaricus bisporus

Description

Like many of the fungi that are associated with the dung of grazing animals, Agaricus species have dark, thick walled spores. It seems likely that this is to enable the spores to pass through the gut of the herbivore undamaged and pop out already immersed in the desired growing micro-habitat! When it fruits, the spores land on nearby plants, get eaten and the cycle starts again.

The cap usually starts out hemispherical but flattens as it grows, to a gently convex shape, sometimes reaching as much as 12cm in diameter. The cap is usually covered with brown flattened scales on a lighter background. The stipe has a relatively thick floccose (downy woolly with small tufts) ring and the flesh of a stipe will redden when cut, particularly in young collections. The feature that gives the species its name ‘bisporus’ is microscopic – the basidia have only two sterigmata (attachment points) supporting two spores (in general basidia have 4 spores, although 2 spored collections do occur in other genera and some genera eg Sistotrema, will have 6 – 8).

Spooner and Roberts in their ‘New Naturalist’ book ‘Fungi’, suggest that this species and the related Agaricus bitorquis make up around 40% of the worldwide business of growing fungi for food with a total estimated value of around £2800,000,000! The first record of growing fungi rather than just collecting them from the fields is from France in 1650; by 1707 the first scientific account of growing mushrooms was published based on a cultivation system near Paris and by1883 there are accounts of the ‘great mushroom caves’ outside Paris. Today most Agaricus species are grown in trays or shelves in sheds. The original brown coloured Agaricus bisporus is still grown and marketed as the ‘chestnut’ or ‘portabella’ mushroom. A strain of white, scaleless fruitbodies found growing amongst the brown mushrooms in 1927, is said to be the origin of the familiar white mushrooms on sale in Britain today.

The thick walled spores of Agaricus bisporus are oval and 4.5–5.5 by 5–7.5 μm (1μm is 1/1000th of a mm). Mounted in Congo Red hence the reddish tones.