170 



BOTANY 



alcohol in cider, and by a chemical process change the alcohol to 

 vinegar. Others, called the lactic acid bacteria, bring about the 

 souring of milk by acting on the sugar found in the milk. Thus 

 they aid in butter and cheese making. Others give the flavor to 

 cheese or butter. Some are of great economic importance, as we 

 have already seen, in their relation to the roots of some plants, 

 where they fix nitrogen in such a form that it can be used by 

 the plants as food. Bacteria seem to prefer to feed upon sub- 

 stances that contain nitrogen, this element being necessary to 



Microscopic appearance of ordinary milk showing fat globules and bacteria. The cluster of 

 bacteria on left side are lactioracid-forming germs. (H. L. Russell, Wis. Bui. No. 62.) 



form protoplasm. They are found in great numbers upon all 

 nitrogenous foods, as milk, meats, fish, etc. Feeding upon such 

 substances, they decompose them and cause decay to take place. 

 Typhoid. — Such bacteria as are parasitic in the human body 

 may cause disease. Sometimes the harm is done by certain poisons 

 called ptomaines, which are formed by the bacteria during the 

 process of growth. The disease we call typhoid fever is thus 

 caused. The bacterium causing the fever is taken into the body 

 through the mouth in water, milk, or other food. Once in the 

 intestine the bacteria multiply very rapidly, producing a ptomaine. 

 This poison gets into the blood and is passed to all parts of the 



