XIX 



BACTERIA AND DISEASES 



281 



peas, beans, lentils, vetches, lupines, etc., have the 

 power to use the free nitrogen of the air because in the 

 little knots, balls, or tubercles found 

 in their roots there are countless 

 bacteria living as parasites and pro- 

 ducing important nitrate compounds 

 which the plants use for food. No 

 plant can grow in a soil that does not 

 contain nitrates unless it can produce 

 them by the aid of bacteria. All 

 leguminous plants are important as 

 fertilizers because, when they decay, 

 they give to the soil nitrates produced 

 by bacteria. So useful are these tiny 

 organisms that neither plants nor ani- 

 mals could live without them. 



192. Bacteria as Enemies. The temperature, mois- 

 ture, and food supply in certain parts of the human 

 body furnish a favorable soil for some kinds of bacteria. 

 They are very fond of albumins, such as occur in the 

 blood and lymph. The mouth, throat, and intestines 

 are always swarming with these little organisms, where 

 they cause decay or decomposition of foods, or attack 

 the blood directly when the capillaries become inflamed, 

 as in colds. 



The conditions under which the different kinds of 

 bacteria grow vary greatly. The vast majority cannot 

 grow in the human body at all, a few grow nowhere else, 

 some grow only in certain of the lower animals, others 

 in all alike, some will grow only in a single organ or 



FIG. 153. Tubercles 

 on root of common 

 bean (Johnstone). 



