File:Rhizomorphs (thick fungal threads) of Armillaria mellea - geograph.org.uk - 933530.jpg

English: Rhizomorphs (thick fungal threads) of Armillaria mellea. These tough black threads are covering a piece of fallen wood; they belong to Honey Fungus, Armillaria mellea. This particular example was photographed alongside the cycle path between Dumbarton and Bowling, where the path is confined between steep stone walls (the course of an old railway line); the photo was taken not far to the west of the tunnel shown in 712912.

Though we tend to notice the larger structures (mushrooms and brackets), a fungus largely consists of cottonwool-like threads (called hyphae) that spread inconspicuously through the ground, wood, or whatever the fungus is growing on. This photo shows a different kind of structure, rhizomorphs (Honey Fungus is also sometimes known as Bootlace Fungus; the picture shows why this is quite apt). The rhizomorphs not only allow the fungus to physically spread, but they also serve as supply lines (in the following quotation, "basidiocarps" are mushrooms):

"Clumps of basidiocarps of Armillaria mellea, the honey fungus, are common on and around dead tree stumps in the autumn. The stump or the dead trunk and roots are the operational saprotrophic base for the parasite to act. Black, water-proof, root-like rhizomorphs grow out, ten metres or more, from the base until they make contact with the roots or trunk base of another living tree. These rhizomorphs are aggregates of several thousand hyphae. Specialized hyphae within conduct nutrients from the colonized food base to the new victim. These nutrients provide a massive resource to enable the fungus to overcome the physical and chemical barrier of the bark and become established within" [Ingold & Hudson, "The Biology of Fungi", p162-4]

After penetrating the bark of their new victim, the threads spread beneath the bark, eventually merging into a solid black mat that encircles the inner wood of the tree (in places where bark has fallen from a tree, this mat can sometimes be seen); the rhizomorphs then send out hyphae (much smaller threads) to degrade the wood of the tree. Armillaria mellea causes an intensive white rot, and is one of the most dangerous parasites of trees, causing the loss of a great deal of timber.