Mutual Relations of Different Species 165 



combination of species designated the "bacterial flora." 

 adapted in each case to any particular set of soil and 

 climatic conditions. In the com- 

 petition for food, the various 

 species in this combination hold 

 one another in check and influ- 

 ence, thereby, the degree and 

 kind of the chemical changes 

 produced. For in- 

 stance, any one 

 kind of anaerobic 

 bacteria living by it 

 self will not develop 

 under conditions 

 where air is freely 

 admitted, because 

 they cannot stand the 

 presence of free oxy- 

 gen. When, however, 

 aerobic species are 

 living with them, they 

 begin to grow and 

 multiply as if no oxy- 

 gen were present. 

 This phenomenon is 

 explained by the fact 

 that the aerobic or- 

 ganisms use up the 

 oxygen in the im- 

 mediate vicinity of 

 their anaerobic neigh- 



X 



I 



/ 



Fig. 25. Ammonifying bacteria. — 1. Pro- 

 teM vulgaris: X 2,600. (Rodella.) 2. 

 Bacillus megatherium; X 2,600. 

 (Hinterberger.) 3. Bacillus mycoides; 

 X 2,600. (Emmerling.) 4. Bacillus 

 cereus; X 2,600. (Wilhelmy.) 



