STUDIES IN CRYPTOGAMS 183 



formation of gas) are due to these organisms. Other bacteria 

 oxidize alcohol to acetic acid, and produce lactic acid in milk and 

 butyric acid in butter. Bacteria live in the mouth, stomach, in- 

 testines, and on the surface of the skin of animals. Some secrete 

 gelatinous sheaths around themselves ; others secrete sulfur or 

 iron, giving the substratum a vivid color. 



Were it not for bacteria, man could not live on the earth, for 

 not only are they agents in the process of decay, but they are 

 concerned in certain healthful processes of plants and animals. 

 We have learned in Chap. VIII how bacteria are related to nitro- 

 gen-gathering. 



Bacteria are of economic importance not alone because of their 

 effect on materials used by man, but also because of the disease- 

 producing power of certain species. Pus is caused by a spherical 

 form, tetanus or lock-jaw by a rod-shaped form, diphtheria by 

 short oblong chains, tuberculosis or " consumption " by more slen- 

 der oblong chains, and typhoid fever, cholera, and other diseases 

 by other forms. Many diseases of animals and plants are 

 caused by bacteria. Disease-producing bacteria are said to be 

 pathogenic. 



The ability to grow in other nutrient substances than the natu- 

 ral one has greatly facilitated the study of these minute forms 

 of life. By the use of suitable culture media and proper precau- 

 tions, pure cultures of a particular disease-producing bacterium 

 may be obtained with which further experiments may be con- 

 ducted. 



Milk provides an excellent collecting place for bacteria coming 

 from the air, from the coat of the cow and from the milker. Dis- 

 ease germs are sometimes carried in milk. If a drop of milk is 

 spread on a culture medium (as agar), and provided with proper 

 temperature, the bacteria will multiply, each one forming a colony 

 visible to the naked eye. In this way, the number of bacteria 

 originally contained in the milk may be counted. 



Bacteria are disseminated in water, as the germ of typhoid fever 

 and cholera ; in milk and other fluids ; in the air ; and on the 

 bodies of flies, feet of birds, and otherwise. 



Bacteria are thought by many to have descended from algae by 

 the loss of chlorophyll and decrease in size due to the more 

 specialized acquired saprophytic and parasitic habit. 



The alga; comprise most of the green floating " scum " which 

 covers the surfaces of ponds and other quiet waters. The masses 

 of plants are often called " frog spittle." Others are attached to 

 stones, pieces of wood, and other objects submerged in streams 



