poison on
the outside does no good after the fungus is established. The treatment
can be used only to _prevent_ attack, not to cure, except in the case of
a few mildews that live on the outside of the leaf, as does the rose
mildew.

=EXERCISE=

Why do things mold more readily in damp places? Do you now
understand why fruit is heated before it is canned? Try to grow
several kinds of mold. Do you know any fungi which may be eaten?

Transfer disease from a rotten apple to a healthy one and note the
rapidity of decay. How many really healthy leaves can you find on a
strawberry plant? Do you find any spots with reddish borders and
white centers? Do you know that this is a serious disease of the
strawberry? What damage does fruit mold do to peaches, plums, or
strawberries?

Write to your experiment station for bulletins on plant diseases
and methods for making and using spraying mixtures.

SECTION XXVIII. YEAST AND BACTERIA

Can you imagine a plant so small that it would take one hundred plants
lying side by side to equal the thickness of a sheet of writing-paper?
There are plants that are so small. Moreover, these same plants are of
the utmost importance to man. Some of them do him great injury, while
others aid him very much.

You will see their importance when you are told that certain of them in
their habits of life cause great change in the substances in which they
live. For example, when living in a sugary substance they change the
sugar into a gas and an alcohol. Do you remember the bright bubbles of
gas you have seen rising in sweet cider or in wine as it soured? These
bubbles are caused by one of these small plants--the yeast plant. As the
yeast plant grows in the sweet fruit juice, alcohol is made and a gas is
given off at the same time, and this gas makes the bubbles.

[Illustration: FIG. 117. YEAST PLANTS
_A_, a single plant; _B_, group of two budding cells; _C_, group of
several cells]

Later, other kinds of plants equally small will grow and change the
alcohol into an acid which you will recognize as vinegar by its sour
taste and peculiar odor. Thus vinegar is made by the action of two
different kinds of little living plants in the cider. That these are
living beings you can prove by heating the cider and keeping it tightly
sealed so that nothing can enter it. You will find that because the
living germs have been killed by the heat, the