64 BIOLOGY AND TECHNIQUE 



A. Fischer, regard the plant as a parasite upon the bacteria, in that 

 it derives the sole benefit from the relationship and eventually bodily 

 consumes its host. 



Nitrifying Bacteria. A process diametrically opposed in its 

 chemistry to denitrification and reduction is that which brings about 

 an oxidation of ammonia to nitrites and nitrates. The actual in- 

 crease of nitrates in soil allowed to stand for any length of time 

 and examined from time to time has been a well-established fact 

 for many years ; but it was believed until a comparatively short time 

 ago that this increase was due to a simple chemical oxidation of 

 ammonia by atmospheric oxygen. The dependence of nitrification 

 upon the presence of living organisms was finally proved by Muntz 

 and Schlossing 27 in 1887, who demonstrated that nitrification was 

 abruptly stopped when the soil was sterilized by heat or antiseptics. 

 It remained, however, to isolate and identify the organisms which 

 brought about this ammonia oxidation. This last step in our knowl- 

 edge of nitrification was taken in 1890, by Winogradsky. Wino- 

 gradsky 28 found that the failures experienced by others who had 

 attempted to isolate nitrifying bacteria, were due to the fact that 

 they had used the common culture media largely made up of organic 

 substances. By using culture media containing no organic matter, 

 Winogradsky succeeded in isolating free from the soil, bacteria 

 which have since that time been confirmed as being the causative 

 factors in nitrification. During his first experiments this author 

 observed that in some of his cultures the oxidation of ammonia went 

 only as far as the stage of nitrite formation, while in others complete 

 oxidation to nitrates took place. Following the clews indicated by 

 this discrepancy, he finally succeeded in demonstrating that nitrifica- 

 tion is a double process in which two entirely different varieties of 

 microorganisms take part, the one capable of oxidizing ammonia to 

 nitrites, the other continuing the process and converting the nitrites 

 to nitrates. The nitrite-forming bacteria discovered by Winograd- 

 sky, and named Nitromonas or Nitrosomonas, are easily cultivated 

 upon aqueous solutions containing ammonia, potassium sulphate, 

 and magnesium carbonate. According to their discoverer they 

 develop in this medium within a week as a gelatinous sediment, 

 After further growth this sediment seems to break up and the 



27 Munis und Schlossing, Cornpt. rend, de 1'acad. des sciences, 1887. 



28 Winogradsky, Ann. Past. Inst., iv and v, 1890, ]891. 



