FUNCTIONS OF SAPROPHYTIC BACTERIA 37 



to provide energy. In so far as life upon the earth's surface 

 is concerned the nitrification going on in the soil as a result 

 of the activities of this group is one of the most important 

 phenomena in operation. It is to a large extent responsible 

 for supplying higher vegetation with nitrogen in a form 

 available for food. 



Denitrification, i. e., the reverse of nitrifaction, the reduc- 

 tion of nitrates and nitrites to ammonia is a function peculiar 

 to many bacteria, particularly many species found in the 

 soil. Often it does not appear to be a specific function and 

 is frequently accomplished under conditions where organic 

 matter is present and is utilized. In many cases the denitrifi- 

 cation seems to be less a phenomenon due to the specific 

 activities of the bacteria themselves than to the reducing 

 action of the products of their growth. In the case of the 

 few species that have been called "true denitrifiers," the 

 reduction appears to be due to the respiratory demands of 

 those species for oxygen; this robbing of the oxides of 

 nitrogen of their oxygen by the bacteria resulting, mani- 

 festly, in reduction. 



Nitrogen Fixation. — Another phenomenon having to do 

 with nitrogen, and resulting from the activity of saprophytes, 

 is the so-called "nitrogen fixation" by bacteria. For many 

 years we were taught that the nitrogen of the air, constitut- 

 ing about 80 per cent, of the entire atmosphere, was of no 

 biological significance and was put there by nature merely to 

 dilute the excessively active oxygen to a point compatible 

 with respiration by man and animals. This extraordinary 

 conception was always looked upon with suspicion by thought- 

 ful students. It was not, however, until about 1886 that 

 the real significance of atmospheric nitrogen was made 

 clear. Hellrigel and Wilfarth at that time demonstrated 



