The mycelium is the vegetative portion of a fungus, meaning that it is the portion that propagates through asexual reproduction. It consists of a web of hyphae, branching or forking thread-like structures. Depending upon the type of fungus, and how well developed it is, mycelia may be visible or microscopic. The mold that grows on decaying food, for example, is a visible form.
In addition to asexual reproduction, the mycelium is responsible for absorbing nutrients from the environment. It releases enzymes into the surrounding environment to break down the food source into a digestible form, then absorbs it. This process also helps dead plant material and other organic material to decompose. The material can also help renew the soil through this process, for example by decomposing contaminants such as pesticides. Some mycelia help plants absorb water and nutrients more efficiently, and others are also important food sources for invertebrates living in the soil.
Mycelia can also be extremely extensive. One notorious fungal colony in the forest of eastern Oregon once covered 2,400 acres (9.7 square km). It grew to such a size over about 2,200 years and killed the plant growth in the forest many times, each time leading to deeper layers of soil that could withstand the growth of larger and larger trees.
Sometimes, mycelium is used in farming or landscaping. In addition to its ability to replenish nutrients, some types of fungus form a symbiotic relationship, called a mycorrhiza, with the roots of a plant. The plant and the fungus 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. They 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.