The fungi kingdom sits on the border between plants and animals and between micro- and macro-biology. The mycelium, plural mycelia, exemplifies how the microscopic elements of fungi can combine to form a larger whole. Mycelia are the diffuse vegetative parts of multicellular filamentous fungi. Filamentous fungi can be divided into microfungi and macrofungi, but the mycelia of both groups have similar form and function. They are made up of a network of threads that are often too fine to be seen by the naked eye, known as hyphae.
Hyphae are compartmentalized tubes that grow out into food sources in order to digest and absorb nutrients. Fungi are heterotrophs, they need to digest other organisms to get their energy, and they can digest tough foods, such as dead trees and insect carapaces. Hyphae grow out from the end of the tube and can branch, forming networks of threads, each not more than one hundredth of a millimeter (0.0004 inches) in diameter. In total, this network is known as the mycelium. Hyphae are why the mold on your bread looks fuzzy.
As a mycelium grows out into a substrate it excretes enzymes at the tips of its hyphae that digest the substrate into a form that can be absorbed by the fungus. The more nutrients are in the substrate, the more branches the mycelia forms to take advantage of the food source. Mycellia move out from the location of the original fungal spore, but since it uses up all of the nutrients in the center, the middle of the circle gets cannibalized, causing a ring-like pattern recognizable in fairy rings and ringworm infections.
The ability of the mycelium to spread through a substrate as they digest it makes filamentous microfungi both important decomposers and parasites. There are over 13,000 species that have been identified in the United States, but that likely represents only a tiny fraction of the species out there. The mycelia of Phytophthora infestans spreads through potato tubers, rotting them, the cause of the Irish potato famine. The mycelia of Trichoderma reesei, a fungus that breaks down dead plant matter, excretes three different types of cellulase to fully digest the cellulose in its food supply.
The mycelia of most fungi are microscopic, but there are times when mycelia form larger conglomerate structures. The structure that is most familiar is the fruiting body, or mushroom, a reproductive structure used to disseminate spores to new environments. Fungal mycelia can also form rhizomorphs, or cords of bundled hyphae, and sclerotia, or structures that anchor the fungus and store nutrients to use during adverse conditions. While the individual hyphae are microscopic, a single honey mushroom is actually the largest and oldest known living organism, spread over 890 hectares (2,200 acres) of ground and dubbed the humongous fungus.
