150 



PLANTS WITHOUT CHLOROPHYLL 



whatever form, is sooner or later decomposed by the action of 

 untold millions of bacteria which live in the air, water, and soil. 

 These soil bacteria are most numerous in rich damp soils contain- 

 ing large amounts of organic material. They are very numerous 

 around and in the dead bodies of plants and animals. To a con- 

 siderable ilegree, then, these bacteria are useful in feeding upon 

 these dead bodies, which otherwise would soon cover the surface 

 of the earth to the exclusion of everything else. Bacteria may 

 thus be scavengers. They oxidize organic materials, changing 

 them to ('()m])ounds that can be absorbed by plants and used 



in building protoplasm. With- 

 out bacteria and fungi it would 

 be impossible for life to exist 

 on the earth, for green plants 

 would be unable to get the 

 raw food materials in forms 

 that could be used in making 

 food and living matter. In 

 this respect bacteria are of the 

 greatest service to mankind. 



Relation to Fermentation.--- 

 They may incidentally, as a 

 result of this process of decay, 



Microscopic appearance of ordinary milk, Continue the proceSS of fer- 



v^hTch"^.,^' th^^"^'' ^""f ^-^v""'^ mentation begun by the yeasts. 



v/nich cause the souring of milk. ^ o j j 



In making vinegar the yeasts 

 first make alcohol (see page 135) which the bacteria change to 

 acetic acid. The lactic acid bacteria, which sour milk, changing 

 the milk sugar to an acid, grow very rapidly in a warm tempera- 

 ture ; hence milk which is cooled immediately and kept cool or 

 which is pasteurized and kept in a cool place will not sour readily. 

 Why? These same lactic acid bacteria may be useful when they 

 sour the milk for the cheese maker. 



Other Useful Bacteria. — Certain bacteria give flavor to cheese 

 and butter, while still other bacteria aid in the '' curing " of 

 tobacco, in the production of the dye indigo, in the preparation of 

 certain fibers of plants for the market, as hemp, flax, etc., in the 



