This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
Many of the mushrooms, and their kind, grow on wood. A visit to the damp forest during the summer months, or during the autumn, will reveal large numbers of these plants growing on logs, stumps, from buried roots or rotten wood, on standing dead trunks, or even on living trees. In the latter case the mushroom usually grows from some knothole or wound in the tree (Fig. 9). Many of the forms which appear on the trunks of dead or living trees are plants of tough or woody consistency. They are known as shelving or bracket fungi, or popularly as "fungoids" or "fungos." Both these latter words are very unfortunate and inappropriate. Many of these shelving or bracket fungi are perennial and live from year to year. They may therefore be found during the winter as well as in the summer. The writer has found specimens over eighty years old. The shelves or brackets are the fruit bodies, and consist of the pileus with the fruiting surface below. The fruiting surface is either in the form of gills like Agaricus, or it is honeycombed, or spinous, or entirely smooth.
Polyporus borealis, showing wound at base of hemlock spruce caused by falling tree. Bracket fruit form of Polyporus borealis growing from wound. (1/15 natural size.)
While the fruit bodies are on the outside of the trunk, the mycelium, or vegetative part of the fungus, is within the wood or bark. By stripping off the bark from decaying logs where these fungi are growing, the mycelium is often found in great abundance. By tearing open the rotting wood it can be traced all through the decaying parts. In fact, the mycelium is largely if not wholly responsible for the rapid disintegration of the wood. In living trees the mycelium of certain bracket fungi enters through a wound and grows into the heart wood. Now the heart wood is dead and cannot long resist the entrance and destructive action of the mycelium. The mycelium spreads through the heart of the tree, causing it to rot (Fig. 10). When it has spread over a large feeding area it can then grow out through a wound or old knothole and form the bracket fruit body, in case the knothole or wound has not completely healed over so as to imprison the fungus mycelium.
Polyporus borealis. Strands of mycelium extending radially in the wood of the same living hemlock spruce shown in Fig. 9. (Natural size.)
Plate 2, Figure II
Mycelium of Agaricus melleus on large door in passage coal mine, Wilkebarre, Pa, (1/20 natural size.)