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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:

“Is this the largest organism in the world? This 2,400-acre (9.7 km2) site in eastern Oregon had a contiguous growth of mycelium before logging roads cut through it. Estimated at 1,665 football fields in size and 2,200 years old, this one fungus has killed the forest above it several times over, and in so doing has built deeper soil layers that allow the growth of ever-larger stands of trees. Mushroom-forming forest fungi are unique in that their mycelial mats can achieve such massive proportions.”
—Paul Stamets, Mycelium Running

Many species of fungus also form symbiotic relationships with plants in an ecosystem. These relationships are usually characterized by mycelium forming what is called a mycorrhizal relationship with the plant. Mycelium grow close to, on, or around the roots of plants. Sometimes they even encapsulate, or sheath, the root tips of the plants. They participate in an exchange of  nutrients between themselves and the plants close by.  As described in Luoma's Hidden Forest, these relationships are integral to the plant's health, and the health of the forest in general. Many times, specific fungus species will only form mycorrhizal relationships with one specific tree or plant species. Tuber Gibossum, for instance, only forms relationships with Douglas Firs. The fruiting bodies of Gibossum are truffles. This helps to explain why truffles are frequently found only near or in Douglas Fir stands.

It is through the mycelium that a fungus absorbs nutrients from its environment. It does this in a two-stage process. 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.