mycelium, the mass of branched, tubular filaments (hyphae) of fungi. The mycelium makes up the thallus, or undifferentiated body, of a typical fungus. It may be microscopic in size or developed into visible structures, such as brackets, mushrooms, puffballs, rhizomorphs (long strands of hyphae cemented together), sclerotia (hard, compact masses), stinkhorns, toadstools, and truffles. At a certain stage it produces spores, directly or through special fruiting bodies.