The little plants which make up these molds are called _fungi_. Some
fungi, such as the toadstools, puffballs, and devil's snuff-box, are
quite large; others, namely the molds, are very small; and others are
even smaller than the molds. Fungi never have the green color of
ordinary plants, always reproduce by spores, and feed on living matter
or matter that was once alive. Puffballs, for example, are found on
rotting wood or dead twigs or roots. Some fungi grow on living plants,
and these produce plant disease by taking their nourishment from the
plant on which they grow; the latter plant is called the _host_.

The same blue mold that grows on bread often attacks apples that have
been slightly bruised; it cannot pierce healthy apple skin. You can
plant the mold in the bruised apple just as you did on bread and watch
its rapid spread through the apple. You learn from this the need of
preventing bruised or decayed apples from coming in contact with healthy
fruit.

[Illustration: FIG. 116. SPORES OF THE PEAR SCAB
The spores are borne on stalks]

Just as the fungus studied above lives in the apple or bread, so other
varieties live on leaves, bark, etc. Fig. 113 represents the surface of
a mildewed rose leaf greatly magnified. This mildew is a fungus. You can
see its creeping stems, its upright stalk, and numerous spores ready to
fall off and spread the disease with the first breath of wind. You must
remember that this figure is greatly magnified, and that the whole
portion shown in the figure is only about one tenth of an inch across.
Fig. 114 shows the general appearance of a twig affected by this
disease.

Mildew on the rose or on any other plant may be killed by spraying the
leaves with a solution of liver of sulphur; to make this solution, use
one ounce of the liver of sulphur to two gallons of water.

The fungus that causes the pear-leaf spots has its spores in little pits
(Fig. 115). The spores of some fungi also grow on stalks, as shown in
Fig. 116. This figure represents an enlarged view of the pear scab,
which causes so much destruction.

You see, then, that fungi are living plants that grow at the expense of
other plants and cause disease. Now if you can cover the leaf with a
poison that will kill the spore when it comes, you can prevent the
disease. One such poison is the Bordeaux (_bor-do_') mixture, which
has proved of great value to farmers.

Since the fungus in most cases lives within the leaves, the