Microorganisms in Relation to Man 



573 



absorbed by another species of bacteria growing in the roots, 

 the nodules, of leguminous plants (Fig. 170), and is "fixed" 

 or combined with other elements in compounds that can be 

 assimilated directly by the plants. 



Nitrogen compounds, produced by bacteria (Fig. 169) are 

 absorbed by chlorophyll-bearing plants and changed into or- 

 ganic compounds of which nitrogen is 

 the principal constituent. These com- 

 pounds are called proteins (page 517). 

 Animals and men live on plants and 

 plant products. The edible parts are 

 changed in the course of nutrition into 

 tissue for the growth and repair of the 

 body, and partly into waste products 

 that are excreted. Of the waste prod- 

 ucts urea, for example, is converted by 

 a species of bacteria into ammonia, 

 which in turn is oxidized by another 

 species and made into a nitrate. The 

 dead remains of plants and animals are 

 subject to decay, the work of sapro- 

 phytic molds and bacteria, in which 

 process nitrogen, with other elements, 

 is restored to the soil and started upon 

 a new cycle. A curious fact to be 

 noted is that nitrogen compounds are in certain conditions 

 attacked and broken up by denitrifying bacteria (page 472). 

 But, on the whole, the cycle of nitrogen in its different com- 

 binations indicates the complete interdependence of molds, 

 bacteria, green plants, animals, and men. 



286. Classification. On the basis of the sources of the 

 nutritive materials upon which they depend, bacteria may be 

 roughly grouped into three classes: 



(1) The saprophytes, the bacteria which derive their food 

 materials from dead and decaying organic matter; 



FIG. 170. A legume root 

 showing nodules formed by 

 nitrogen-fixing bacteria. 



