cider will not ferment or
sour as it did before. The germs could of course be killed by poisons,
but then the cider would be unfit for use. It is this same little yeast
plant that causes bread to rise.

When you see any decaying matter you may know that in it minute plants
much like the yeast plant are at work. Since decay is due to them, we
take advantage of the fact that they cannot grow in strong brine or
smoke; and we prepare meat for keeping by salting it or by smoking it or
by both of these methods.

You see that some of the yeast plants and _bacteria_, as many of these
forms are called, are very friendly to us, while others do us great
harm.

Some bacteria grow within the bodies of men and other animals or in
plants. When they do so they may produce disease. Typhoid fever,
diphtheria, consumption, and many other serious diseases are caused by
bacteria. Fig. 118, _e_, shows the bacterium that causes typhoid fever.
In the picture, of course, it is very greatly magnified. In reality
these bacteria are so small that about twenty-five thousand of them side
by side would extend only one inch. These small beings produce their
great effects by very rapid multiplication and by giving off powerful
poisons.

[Illustration: FIG. 118. FORMS OF BACTERIA
_a_, grippe; _b_, bubonic plague; _c_, diphtheria; _d_, tuberculosis;
_e_, typhoid fever]

Bacteria are so small that they are readily borne on the dust particles
of the air and are often taken into the body through the breath and also
through water or milk. You can therefore see how careful you should be
to prevent germs from getting into the air or into water or milk when
there is disease about your home. You should heed carefully all
instructions of your physician on this point, so that you may not spread
disease.

SECTION XXIX. PREVENTION OF PLANT DISEASE

In the last two sections you have learned something of the nature of
those fungi and bacteria that cause disease in animals and plants. Now
let us see how we can use this knowledge to lessen the diseases of our
crops. Farmers lose through plant diseases much that could be saved by
proper precaution.

First, you must remember that every diseased fruit, twig, or leaf bears
millions of spores. These must be destroyed by burning. They must not be
allowed to lie about and spread the disease in the spring. See that
decayed fruit in the bin or on the trees is destroyed in the same
manner. Never throw decayed fruit