the best procedure and may save many neighboring trees from
contagious infection. For this, however, no rules can be laid down
and much will depend on the local conditions and the judgment and
knowledge of the person concerned.

[Illustration: FIG. 108.--A Bracket Fungus (_Elfvingia megaloma_) on a
Tulip Tree.]

Fungi as factors of disease: The trees, the shrubs and the flowers with
which we are familiar are rooted in the ground and derive their food
both from the soil and from the air. There is, however, another
group of plants,--_the fungi_,--the roots 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.