Agaricus bisporus

Agaricus bisporus is an edible basidiomycete mushroom native to grasslands in Europe and North America. It has two color states while immature—white and brown—both of which have various names. In recent years, growers found that if they allowed the fruiting body to grow far beyond the normal size with the cap spreading to expose all the gills, it became the size of a hamburger bun, thus spurring the veggie burger craze. The name marketers agreed on was portobello mushroom (also portabella or portobella). When immature and white, this mushroom may be known as common mushroom, button mushroom, cultivated mushroom, table mushroom, and champignon mushroom. When immature and brown, it may be known variously as Swiss brown mushroom, Roman brown mushroom, Italian brown mushroom, cremini/crimini mushroom, or chestnut mushroom. A. bisporus is cultivated in more than seventy countries, and is one of the most commonly and widely consumed mushrooms in the world. The common mushroom has a complicated taxonomic history. It was first described by English botanist Mordecai Cubitt Cooke in his 1871 Handbook of British Fungi, as a variety (var. hortensis) of Agaricus campestris. Danish mycologist Jakob Emanuel Lange later reviewed a cultivar specimen, and dubbed it Psalliota hortensis var. bispora in 1926. In 1938, it was promoted to species status and renamed Psalliota bispora. Emil Imbach (1897–1970) imparted the current scientific name of the species, Agaricus bisporus, after the genus Psalliota was renamed to Agaricus in 1946. The specific epithet bispora distinguishes the two-spored basidia from four-spored varieties. The pileus or cap of the original wild species is a pale grey-brown in color, with broad, flat scales on a paler background and fading toward the margins. It is first hemispherical in shape before flattening out with maturity, and 5–10 centimetres (2–4 inches) in diameter. The narrow, crowded gills are free and initially, pink, then red-brown and finally a dark brown with a whitish edge from the cheilocystidia. The cylindrical stipe is up to 6 cm (2 1⁄3 in) tall by 1–2 cm wide and bears a thick and narrow ring, which may be streaked on the upper side. The firm flesh is white, although stains a pale pinkish-red on bruising. The spore print is dark brown. The spores are oval to round and measure approximately 4.5–5.5 μm × 5–7.5 μm, and the basidia usually two-spored, although two-tetrasporic varieties have been described from the Mojave Desert and the Mediterranean, with predominantly heterothallic and homothallic lifestyles, respectively. This mushroom is commonly found worldwide in fields and grassy areas following rain, from late spring through to autumn, especially in association with manure. It is widely collected and eaten, even by those who would not normally experiment with mushroom hunting. The common mushroom could be confused with young specimens of the deadly poisonous destroying angel (Amanita sp.), but the latter may be distinguished by their volva or cup at the base of the mushroom and pure white gills (as opposed to pinkish or brown of A. bisporus). Thus it is always important to clear away debris and examine the base of such similar mushrooms, as well as cutting open young specimens to check the gills. Furthermore, the destroying angel grows in mossy woods and lives symbiotically with spruce. A more common and less dangerous mistake is to confuse A. bisporus with Agaricus xanthodermus, an inedible mushroom found worldwide in grassy areas. A. xanthodermus has an odor reminiscent of phenol; its flesh turns yellow when bruised. This fungus causes nausea and vomiting in some people.