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.
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 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.
Fig. 118. Forms of Bacteria
a, grippe;
b, bubonic plague;
c, diphtheria;
d, tuberculosis;
e, typhoid fever