Agaricus bisporus
Cultivated Mushroom
Names
Scientific: Agaricus bisporus
English: Cultivated Mushroom
(other scientific names include Psalliota hortensis f. bispora, Psalliota bispora, Agaricus campestris var. hortensis, Psalliota hortensis, Agaricus hortensis, Agaricus campestris var. buchanani, Agaricus campestris var. elongatus and Agaricus campestris var. bisporus)
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.
Agaricus bisporus (Cultivated mushroom)
Fruiting:
Free growing records of this species in Scotland occur anytime between April and December (so far with the exception of July).
Habitat:
This species appears on soil, often amongst grassland in parks, pastures and gardens. It occasionally appears on compost heaps or manure and is of course extensively cultivated.
Distribution:
Astonishingly few records of this species in Scotland although the NBN map is rather misleading as very few of the records on the FRDBI have accompanying grid references – so don’t forget to add the grid reference with any record that you send in – otherwise it won’t be added to the mapping programme!
The Checklist gives the distribution as England ‘occasional’ and everywhere else ‘distribution unknown’, in general widespread. The total number of sites for this species on the Fungal Records Database of Britain and Ireland (FRDBI) is 270 with 28 of these from Scotland. The FRDBI suggests that most of the records are from the Edinburgh area with outliers from Mull, Shetland, Perthshire and Uist (Chris Johnson has found it at the latter site this spring). An anonymous recorder has added an Edinburgh record ‘on sale in Marks and Spencer’ ….!
The two-spored basidia described by the specific name ‘bisporus’ are clearly visible and two spore attachment points are indicate on this basidia with arrows. This photo was taken using 1000x magnification.
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.
Please remember to submit your records to your local recording group or via the Scottish Fungi online recording form.
Text by Liz Holden; Photos by Chris Johnson