BACTERIA 27 



For this reason, anthrax sometimes appears suddenly and 

 without any apparent cause when cattle are turned into a 

 pasture where diseased animals had fed years before. The 

 disease is largely due to the rapid multiplication of the 

 bacteria in the blood vessels so that the circulation is stopped. 

 A toxin is also produced. Animals are made immune to 

 anthrax for about a year by vaccinating them with a culture 

 of anthrax bacilli that have been weakened by exposure to 

 air at a rather high temperature. 



41. Nitrogen-fixing Bacteria (Fig. n). Disease-produc- 

 ing bacteria live within the tissues of the host and are 



FIG. ii. Nitrogen-fixing bacteria. A, roots of a leguminous plant 

 (such as clover), bearing many small swellings. B, the method of entrance 

 of the bacteria through a root-hair, and their progress through the tissues 

 of the clover. C, a single cell of a clover root containing many bacteria, 

 which appear as small dark dots. D, single bacteria as they appear in' an 

 artificial culture. E, as they appear in a cell of the clover. 



injurious to it. A quite different relation exists between 

 clover, alfalfa, beans, peas, and other plants of the same 

 family, and the bacteria which live within their roots. If 

 we pull up a clover plant we find many small swellings on all 

 parts of the roots. The swellings are outgrowths of the root 



