THE DISTRIBUTION OF MICRO-ORGANISMS 175 



The environment and the surfaces of growing plants and animals 

 are rich in micro-organisms, especially bacteria, but the interior 

 of the living tissues is generally germ-free in health. To this 

 statement there are certain exceptions, namely, the occurrence 

 of a few bacteria in the liver, the thoracic duct and the blood of 

 animals during active digestion, which are, however, soon de- 

 stroyed by the healthy tissue; and the invasion of the root tissues 

 of leguminous plants byPs.radicicola and the growth of the bac- 

 terium within the plant tissues, which results not in injury to 

 the host but in a definite improvement of its nutrition by enabling 

 it to utilize atmospheric nitrogen. 



Micro-organisms of the Soil. The germ-content of soil depends 

 chiefly upon the amount of organic matter present. They may 

 be present in millions per gram of soil. Bacteria, molds and pro- 

 tozoa are the most numerous. Their relation to soil fertility 

 seem to be important, and they probably play a large part in 

 preparing the organic matter of the soil for use as food by plants. 

 A great many soil bacteria decompose protein and set free am- 

 monia, and the urea bacteria are especially important in the 

 transformation of urea and of animal manures into ammoniacal 

 compounds. The transformation of ammoniacal compounds 

 into nitrates, so-called nitrification, is accomplished by the nitri- 

 fying bacteria, of which a few species have been obtained in pure 

 culture, Nitrosomonas of Winogradski which produces nitrite 

 from ammonia, and his genus Nitrobacter which oxidizes nitrites 

 to nitrates. Very many species of soil bacteria are able to change 

 nitrogen in the opposite direction, reducing nitrates to nitrites 

 and further to ammonia or to free nitrogen gas. Of special interest 

 are the soil bacteria which are able to fix atmospheric nitrogen, 

 that is, absorb nitrogen from the air and combine it so as to make 

 it available for plant food. The various species of the genus 

 Azotobacter (A. chroococcum, A. beyerincki) accomplish this as 

 they grow in the presence of dextrose, and the organism of the 

 root tubercles, Pseudomonas radicicola, fixes nitrogen as it grows 

 within the tissues of the legume roots. Numerous soil bacteria 



