What is a Mushroom Mycelium?

Mycelium (plural mycelia) is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or within many other substrates. A typical single spore germinates into a homokaryotic mycelium, which cannot reproduce sexually; when two compatible homokaryotic mycelia join and form a dikaryotic mycelium, that mycelium may form fruiting bodies such as mushrooms. A mycelium may be minute, forming a colony that is too small to see, or it may be extensive:

It is through the mycelium that a fungus absorbs nutrients from its environment. It does this in a two-stage process. First, the hyphae secrete enzymes onto or into the food source, which break down biological polymers into smaller units such as monomers. These monomers are then absorbed into the mycelium by facilitated diffusion and active transport.
Mycelium is vital in terrestrial and aquatic ecosystems for its role in the decomposition of plant material. It contributes to the organic fraction of soil, and its growth releases carbon dioxide back into the atmosphere. The mycelium of mycorrhizal fungi increases the efficiency of water and nutrient absorption of most plants and confers resistance to some plant pathogens. Mycelium is an important food source for many soil invertebrates.

Mycelium is sometimes used in farming or landscaping. In addition to its ability to replenish nutrients, the mycelium of some types of fungus forms a symbiotic relationship, called a mycorrhiza, with the roots of a plant. The plant and the fungus in a mycorrhiza help each other receive nutrients that they could not obtain on their own. Mycelia can also be used as an organic filter for soil or water in a process called mycofiltration, in which a mycelial mat keeps out harmful chemicals and microorganisms. Mycelia are also sometimes used to hold new soil in place on unpaved roads.
Some kinds of fungi use sexual reproduction as well as asexual reproduction. The sexually reproducing portion of a fungus arises from the mycelium and is called the sporocarp or fruiting body. The fruiting bodies of many different types of fungus are known as mushrooms or truffles, though there are some other types.