34 BACTERIOLOGY. 



functions oxygen is essential. In addition to these 

 there is a third group, for the maintenance of whose 

 existence the absence or presence of oxygen is appar- 

 ently of no moment — development progresses as well 

 with as without it; the members of this group comprise 

 the class known as facultative in their relation to this 

 gas. It is to this third group, the facultative, that the 

 majority of bacteria belong. Though the multiplication 

 of the facultative varieties is not interfered with by 

 either the presence or absence of oxygen, yet experi- 

 ments 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 chromo- 

 genic 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 tlie surface of the medium and 

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

 the bacillus prodigiosus and of the spirillum rubrum. 

 With the former the red color is apparently a product 

 dependent upon the presence of oxygen, while in the 

 latter tlie greatest intensity of color occurs at the point 

 farthest removed from the action of oxygen. 



Another factor which plays a highly important part 

 in the biological functions of these organisms is the 

 temperature under wliidi they exist. The extremes of 

 temperature between which the majority of bacteria are 

 known to grow range from 5.5° to 43° C. At the 

 former temperature development is hardly appreciable; 

 it becomes more and more active until 38° C. is reached, 



