When we can see fungi growing on a tree we may safely assume that they are already in an advanced state of development. We generally discover their presence when their fruiting bodies appear on the surface of the tree as shown in Fig 109. These fruiting bodies are the familiar mushrooms, puffballs, toadstools or shelf-like brackets that one often sees on trees. In some cases they spread over the surface of the wood in thin patches. They vary in size from large bodies to mere pustules barely visible to the naked eye. Their variation in color is also significant, ranging from colorless to black and red but never green. They often emulate the color of the bark, Fig. 110. Radiating from these fruiting bodies into the tissues of the tree are a large number of minute fibers, comprising the _mycelium_ of the fungus. These fibers penetrate the body of the tree in all directions and absorb its food. The mycelium is the most important part of the fungous growth. If the fruiting body is removed, another soon takes its place, but if the entire mycelium is cut out, the fungus will never come back. The fruiting body of the fungus bears the seed or _spores_. These spores are carried by the wind or insects to other trees where they take root in some wound or crevice of the bark and start a new infestation. [Illustration: FIG. 109.--The Fruiting Body of a Fungus.] The infestation will be favored in its growth if the spore can find plenty of food, water, warmth and darkness. As these conditions generally exist in wounds and cavities of trees, it is wise to keep all wounds well covered with coal tar and to so drain the cavities that moisture cannot lodge in them. This subject will be gone into more fully in the following two studies on "Pruning Trees" and "Tree Repair." [Illustration: FIG. 110.--The Birch-fungus rot. (_Polyponis betulinus_ Fr.) Note the similarity in the color of the fruiting body and bark of the tree.] While the majority of the fungi grow on the trunks and limbs of trees, some attack the leaves, some the twigs and others the roots. Some fungi grow on living wood some on dead wood and some on both. Those that attack the living trees are the most dangerous from the standpoint of disease.