376 THE NON-VASCULAR PLANTS 



of lack of water, and even of lack of oxygen, which would 

 promptly kill all other organisms. Typhoid fever, cholera, 

 tuberculosis, and diphtheria are among the human diseases 

 caused by bacteria. 



Bacteria are of great importance in agriculture. Though 

 they cause some diseases which are injurious to crops, this 

 injury is more than offset by other ways in which they are 

 helpful to crops. There are bacteria which inhabit the soil 

 and make it fertile. There are certain substances, common 

 in soil, which contain elements that plants need, but which 

 might as well not be there so far as being of direct use to 

 green plants is concerned. The plant cannot use them any 

 more than we can use sea water for drinking purposes. 

 Certain bacteria, however, are able to act upon these sub- 

 stances and so to alter them that they do become available 

 for plant use. Salt water must be distilled before it be- 

 comes available for human use as drinking water. The 

 action of bacteria is nothing like the distilHng process, but 

 this comparison may help you understand what is meant by 

 making certain substances available for plant nutrition. 



Nitrogen is very important in the nutrition of green 

 plants. These soil bacteria, in their own nutrition, act 

 upon certain compounds which contain nitrogen. As a 

 result of this action, certain other compounds which con- 

 tain nitrogen are produced, and these other compounds 

 may be readily absorbed and used by the plant, whereas 

 the original nitrogen-containing compounds could not be 

 used. Since the fertility of the soil depends largely upon 

 the amount of nitrogen in it which is available for plant 

 use, the benefit of these bacteria is obvious. 



It has been demonstrated that soil probably never con- 

 tains at any one time enough of available nitrogen com- 



