In the closed system the earth represents, the dead organic matter built up by plants, animals, and the fungi themselves, must be broken down to continue the cycle of life.
Most people think of fungi in terms of mushrooms and the other obvious aspects of the fungi.
The majority of the earth's fungal biomass is hidden from human eyes and consists of vast mats of fungal hyphae buried in the soil, humus, dead leaves, and dead logs. Although fungi have a fantastically diverse series of life histories, ecological strategies, and reproductive structures, most of a fungi's life is spent in a relativaely simple form. Basically fungi grow as a series of thin, filamentous tubes called hyphae. The intermingled hyphae are called a mycelium. The hyphae may or may not have cross walls, i.e. the hyphae may not be composed of separate cells.