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