ause trees have wants analogous to those of human beings, they also
have diseases similar to those which afflict human beings. In many cases
these diseases act like cancerous growths upon the human body; in some
instances the ailment may be a general failing due to improper feeding,
and in other cases it may be due to interference with the life processes
of the tree.

How to tell an ailing tree: Whatever the cause, an ailing tree will
manifest its ailment by one or more symptoms.

A change of color in the leaves at a time when they should be
perfectly green indicates that the tree is not growing under normal
conditions, possibly because of an insufficiency of moisture or
light or an overdose of foreign gases or salts. Withering of the
leaves is another sign of irregularity in water supply. Dead tops
point to some difficulty in the soil conditions or to some disease
of the roots or branches. Spotted leaves and mushroom-like growths
or brackets protruding from the bark as in Fig. 108, are sure signs
of disease.

In attempting to find out whether a tree is healthy or not, one
would therefore do well to consider whether the conditions under
which it is growing are normal or not; whether the tree is suitable
for the location; whether the soil is too dry or too wet; whether
the roots are deprived of their necessary water and air by an
impenetrable cover of concrete or soil; whether the soil is well
drained and free from foreign gases and salts; 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 ro