whether the tree is receiving plenty of light or is too much exposed; and whether it is free from insects and fungi.

If, after a thorough examination, it is found that the ailment has gone too far, it may not be wise to try to save the tree.  A timely removal of a tree badly infested with insects or fungi may often be 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.

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.]