ots of which grow in trees and
other plants and which obtain their food entirely from the trees or
plants upon which they grow. The fungi cannot manufacture their own
food as other plants do and consequently absorb the food of their
host, eventually reducing it to dust. The fungi are thus
disease-producing factors and the source of most of the diseases of
trees.

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