A mushroom is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground on soil or on its food source. Mushrooms are not plants, they are a fungus. They eat organic matter, they do not photosynthesize like plants do. The standard for the name “mushroom” is the cultivated white button mushroom, Agaricus bisporus, hence the word mushroom is most often applied to those fungi (Basidiomycota, Agaricomycetes) that have a stem (stipe), a cap (pileus), and gills (lamellae, sing. lamella) on the underside of the cap, just as do store-bought white mushrooms.
“Mushroom” describes a variety of gilled fungi, with or without stems, and the term is used even more generally, to describe both the fleshy fruiting bodies of some Ascomycota and the woody or leathery fruiting bodies of some Basidiomycota, depending upon the context of the word.
Forms deviating from the standard morphology usually have more specific names, such as “puffball”, “stinkhorn”, and “morel”, and gilled mushrooms themselves are often called “agarics” in reference to their similarity to Agaricus or their place Agaricales. By extension, the term “mushroom” can also designate the entire fungus when in culture or the thallus (called a mycelium) of species forming the fruiting bodies called mushrooms, or the species itself.
Edible mushrooms are used extensively in cooking, in many cuisines (notably Chinese, European, and Japanese). Many species are high in dietary fiber, protein, and vitamins such as thiamine, riboflavin, niacin, biotin, cobalamins, and ascorbic acid. Though not normally a significant source of vitamin D, some mushrooms can become significant sources after exposure to ultraviolet light; this also darkens their skin. Mushrooms are also a source of some minerals, including selenium, potassium and phosphorus. Most mushrooms that are sold in supermarkets have been commercially grown on mushroom farms. The most popular of these, Agaricus bisporus, is generally considered safe for most people to eat because it is grown in controlled, sterilized environments, though some individuals do not tolerate it well. Several varieties of A. bisporus are grown commercially, including whites, crimini, and portobello. Other cultivated species now available at many grocers include shiitake, maitake or hen-of-the-woods, oyster, and enoki. In recent years increasing affluence in developing countries has led to a considerable growth in interest in mushroom cultivation, which is now seen as a potentially important economic activity for small farmers.
There are a number of species of mushroom that are poisonous, and although some resemble certain edible species, consuming them could be fatal. Eating mushrooms gathered in the wild is risky and should not be undertaken by individuals not knowledgeable in mushroom identification, unless the individuals limit themselves to a relatively small number of good edible species that are visually distinctive. A. bisporus contains carcinogens called hydrazines, the most abundant of which is agaritine. However, the carcinogens are destroyed by moderate heat when cooking.
More generally, and particularly with gilled mushrooms, separating edible from poisonous species requires meticulous attention to detail; there is no single trait by which all toxic mushrooms can be identified, nor one by which all edible mushrooms can be identified. Additionally, even edible mushrooms may produce an allergic reaction in susceptible individuals, from a mild asthmatic response to severe anaphylactic shock.
People who collect mushrooms for consumption are known as mycophagists, and the act of collecting them for such is known as mushroom hunting, or simply “mushrooming”.
China is the world’s largest edible mushroom producer. The country produces about half of all cultivated mushrooms, and around 2.7 kilograms (6.0 lb) of mushrooms is consumed per year by over a billion people.