THE GERM DISEASES 235 



the air and pass it through complicated chemical changes to 

 the clover and alfalfa. Consequently these crops can be grown 

 on worn-out soil or in waste land that is deficient in nitrates. 

 Indeed, soils may now be inoculated with fluid cultures of these 

 " nitrogen-fixing bacteria," so that the organisms will immedi- 

 ately establish root tubercles on the seedlings of these legumes, 

 when sown, or the seeds themselves may be soaked in cul- 

 tures insuring the application of the bacteria. 1 Therefore, when 

 a soil becomes barren of nitrogen through successive crops of 

 wheat, for example, the nitrogen may be largely restored by 

 planting clover or alfalfa and plowing the crops under. Barren 

 soil may also be inoculated more certainly by distributing over 

 it earth from an old clover field. 



The " nitrogen-fixing bacteria " make available the almost in- 

 exhaustible supply of free nitrogen in the air which cannot be 

 absorbed by green plants and which consequently has been of no 

 service to agriculture. As indicated in the diagram (Fig. 207), 

 free nitrogen is constantly being brought into the nitrogen 

 circle through the bacteria which form root tubercles (symbiotic 

 bacteria), and this helps to make up the loss of nitrogen from 

 the nitrogen circle, which comes in various ways, as by fire or 

 the escape of ammonia into the air. 



257. The germ diseases. There is a class of contagious, and 

 in some cases very dangerous, diseases caused by certain bacte- 

 ria which are frequently called microbes, or germs. The most 

 serious are diphtheria, typhoid fever, tuberculosis (consumption), 

 cholera, leprosy, bubonic plague, pneumonia, influenza or grippe, 

 and whooping cough. Some other germ diseases, such as malaria, 

 tropical dysentery, and possibly smallpox, are caused by lowly 

 organisms which are not, however, bacteria. The germ diseases 

 are due to the parasitic development of the organism within the 



1 See Moore, " Soil Inoculation for Legumes," United States Department 

 of Agriculture, Bureau of Plant Industry, Bulletin 71, 1905, and Wood, 

 " Inoculation of Soil with Nitrogen-Fixing Bacteria," Bulletin 72, Part IV, 

 1906. 



