Preliminaries (Installment Five)

A mushroom is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground on soil or on its food source. 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.
The word "mushroom" can also be used for a wide 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 placement in the order 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.

Though mushroom fruiting bodies are short-lived, the underlying mycelium can itself be long-lived and massive. A colony of Armillaria ostoyae in Malheur National Forest in the United States is estimated to be 2,400 years old, possibly older, and spans an estimated 2,200 acres. Most of the fungus is underground and in decaying wood or dying tree roots in the form of white mycelia combined with black shoelace-like rhizomorphs that bridge colonized separated woody substrates.

From Wikipedia’s “Mushrooms”

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People have known about the "honey mushroom" for some time, but were not aware of how large and invasive this species of fungus could be. The fungus was investigated more closely by researchers when they realized that it was responsible for killing large groves of evergreen trees. When foresters cut into an infected tree they would find spreading white filaments, mycelia, which draw water and carbohydrates from the tree to feed the fungus. Researchers collected samples of the fungus from a widespread area and analyzed the DNA. A large sample of the specimens they collected turned out to be from a single organism.

Humongous Fungus

Until August of 2000 it was thought that the largest living organism was a fungus of the same species (Armillaria ostoyae) that covered 1,500 acres (600 hectares) found living in the state of Washington. But then mycology experts surmised that if an Armillaria that large could be found in Washington, then perhaps one just as large could be responsible for the trees dying in the Malheur National Forest in the Blue Mountains of eastern Oregon. Researchers were astonished at the sheer magnitude of the find. This most recent find was estimated to cover over 2,200 acres (890 hectares) and be at least 2,400 years old, possibly older.

To go into the forest where this giant makes its home you would not look at it and see a huge, looming mushroom. Armillaria grows and spreads primarily underground and the sheer bulk of this organism lies in the earth, out of sight. Occasionally, during the fall season, this specimen will send up golden-colored "honey mushrooms" (see photo, above left) that are the visible evidence of its hulking mass beneath. Scientists have not yet begun to attempt to estimate the weight of this specimen of Armillaria.
How is it possible for a single fungus to get so big? Scientists who study this species of funges have postulated that the huge size may be a function of the dry climate in eastern Oregon. Spores have a hard time establishing new organisms, making room for the old-timers to spread. Without competition from other specimens this enormous Armillaria has been able to grow and spread unchecked

From extremescience.com/biggestlivingthing.htm