Rhizomorphs

A fungus most often grows by the extension of the hypha, in a one-dimensional manner. However, multicellular structures can also be formed by aggregations of hyphae and the formation of extracellular polysaccharide matrices in many "higher" fungi (basidiomycetes and ascomycetes). These are analogous to the tissues in animals and plants. Fruiting bodies (i.e. "mushrooms") are the most obvious examples of these - often with the function of producing and releasing sexual spores.
Rhizomorphs are another example. These are bundles of differentiated hyphae, up to 4mm in diameter and sometimes composed of up to 1000 hyphae per millimetre width.

Armillaria spp. such as the honey fungus [rhizomorphs pictured] produce these structures to enable rapid spread through forest environments, where they are serious pathogens of trees. These rhizomorphs are pigmented with melanin (hence the black colour) and are protected by mucilage and external, thick-walled hyphae.

Rhizomorphs can also play a positive role, for example in tropical forests. Aerial canopy hyphae [example pictured, likely Marasmius sp.] tether and trap falling leaf litter. These leaves are exploited for nutrition by the fungi, but also support invertebrate and microbe communities in the canopy.