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.
_A_, a single plant; _B_, group of two budding cells; _C_, group of
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 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
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
_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
Next: Prevention Of Plant Disease