PLANTS IN THEIR RELATION TO HUMAN WELFARE 145 



action of bacteria these meats become tender and acquire 

 their distinctive flavors. A similar action takes place when 

 butter or cheese ripens, and the dairy industry has been per- 

 fected to such a degree that bacteria of certain kinds have 

 been proved to give rise to definite flavors, and these bacteria 

 can be produced in pure cultures for the dairymen. 



152. Bacteria in the industries. Without the help of 

 bacteria the preparation of linen, jute, and hemp would be 

 impossible. All these valuable products are plant fibers 

 which are connected with woody materials so closely that they 

 cannot be separated without first subjecting the stems of 

 flax, hemp, and jute to a process of decay in large tanks of 

 water. Moisture and warmth induce the rapid growth of 

 germs, and this process loosens the tough fibers so they can 

 be separated from the useless parts of the plant. 



The change of alcohol into vinegar is caused by bacteria. 

 Likewise in the preparation of indigo other forms of bacteria 

 are all important. 



153. Bacteria as the foes of man. Unfortunately, how- 

 ever, there are certain germs that find favorable conditions 

 for growth only in living animal tissue. Thus the bacterium 

 of consumption grows in the lungs, the germ of diphtheria in 

 the throat, and the bacteria that cause typhoid fever in the 

 intestines. These disease-producing germs are called by Dr. 

 Prudden " man's invisible foes." Yet wonderful progress is 

 being made in the fight against them. We have learned how 

 to check the ravages of cholera, typhoid, and diphtheria, and 

 even consumption is found to be a preventable disease. 

 Further discussion of bacteria will be found in several of the 

 subsequent chapters. 1 



1 For laboratory work on bacteria, see Chapter II, "Human Bi- 

 ology." Also consult Peabody's " Laboratory Exercises in Anatomy 

 and Physiology" (1902), pp. 100-107. 



