56 BACTERIOLOGY 



to grow and perform all its important functions in ordinary 

 atmospheric air. 



Since all growing bacteria, anaerobic as well as aerobic, 

 generate carbonic acid in the course of their development, 

 it is evident that oxygen must in reality be obtained by 

 them from some source, and must be regarded as essential 

 to their life processes; but the manner in which it is 

 appropriated by them varies, the aerobic species taking 

 it from the air as free oxygen, while the anaerobic species, 

 not possessed of this ability, obtain it through the decom- 

 position of more or less stable oxygen-containing com- 

 pounds. 



Though the multiplication of the facultative varieties is 

 not interfered with by either the presence or absence of free 

 oxygen, yet experiments demonstrate that the products of 

 their growth are different under the varying conditions of 

 absence or presence of this gas. For example: in the case' 

 of certain of the chromogenic forms the presence or absence 

 of oxygen has a very decided effect upon the production of 

 the pigments by which they are characterized. 



NOTE. Observe the difference between the intensity of 

 color produced upon the surface of the medium and that 

 along the track of the needle in stab-cultures of bacillus 

 prodigiosus and of spirillum rubrum. In the former the red 

 color is apparently a product dependent upon the presence 

 of oxygen, while in the latter the greatest intensity of color 

 occurs at the point furthest removed from the action of 

 oxygen. 



Influence of Temperature upon the Growth. Another 

 factor which plays a highly important part in the biological 



