166 PRACTICAL BOTANY 



worth of cattle and sheep. The spores form only when the 

 bacteria are exposed to the air. When an animal dies of an- 

 thrax, if its body decays while exposed to the air, millions of 

 anthrax spores are formed. These spores can lie in the field 

 for very long periods, probably several seasons, and withstand 

 the variations in temperature, moisture, and light. Under ordi- 

 nary conditions these will not germinate until they are intro- 

 duced into the body of an animal, when they again begin their 

 growth. 



153. Nutrition. Bacteria absorb their food material directly 

 tlirough the walls of their cells. Living, as they do, within 

 or upon their food supply, direct contact with it is secured. 

 Most kinds of bacteria live upon organic foods. The sources 

 of this food are as numerous as are the kinds of organic sub- 

 stances in the universe, — living and dead bodies of plants 

 and animals, plant and animal products, materials in solution 

 in water, materials in the air and the soil. Some bacteria'' 

 thrive without free oxygen and thus can live upon food ma- 

 terial in places where other organisms cannot live. A few 

 bacteria can construct food somewhat as the green plants do. 



It must also be noted that bacteria, like other living things, 

 produce and excrete substances that, if retained, would be in- 

 jurious to them. If excreted and accumulated about the bac- 

 teria in great quantity, these substances would soon kill them. 

 If a jar of beef broth is carefully sealed after any ordinary 

 bacteria have been introduced into it, there will at first be a 

 rapid increase in their number and the liquid will become 

 clouded with the organisms and their products. But the 

 excretions soon accumulate to such an extent that the bac- 

 teria can no longer grow. They become dormant or die and 

 settle to the bottom of the jar or collect in a jelly-like mass 

 at the surface. 



154. Relation to decay. The bacteria and certain other de- 

 pendent organisms (as molds, yeasts, many animals), while 

 living upon the bodies or the products of plants and animals, 



1 Known as anaerobic species. 



