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 animals. The green plants and the animals now upon the earth have proved their fitness to survive by successfully resisting these one-celled vegetable germs, or bacteria. Microbe diseases attack only the weaker individuals of the human species, or those who have gone to regions where there are microbes which their bodies have not yet acquired the power of resisting.

Usefulness of Bacteria.—Their chief work is to destroy dead tissue and return it to the soil and air for the use of green plants again, otherwise the earth would be filled with carcasses, etc. They are indispensable in soil formation. They give the agreeable flavors to butter and cheese, and cause milk to sour. A rod-shaped bacterium is called a bacillus (Fig. 130); a spherical one is a coccus.

Multiplication of Bacteria.—This is by division or fission. Sometimes, instead of dividing, a little rounded mass known as a spore appears. The spore breaks out and the bacterium itself perishes. Species which do not produce spores are readily destroyed, but spores have a hard, tough shell, and they may be dried or heated even to boiling without being killed. Spores float through the air and start new colonies. Most common bacteria grow best between 70° and 95° F. They render it difficult to preserve foods, especially proteid foods (cheese, lean meat, eggs, etc.). Food decays slowly if at all below 70° and above 125°. Direct sunlight, or the temperature of boiling water (212° F.) kills bacteria but not spores. Pantries, kitchen, and sick-*rooms should have bright walls and all the light possible. Boiling water should be poured into the sink, and dish cloths should be thoroughly washed in boiling water.

Diseases due to Bacteria.—A germ disease is usually due partly or wholly to substances called toxins produced by the bacteria. Most disease germs attack a single organ