34 BACTERIOLOGY. 



for the maintenance of whose existence the absence or 

 presence of oxygen is apparently of no moment their 

 development progresses as well with as without it ; these 

 constitute the class known as facultative in their relation 

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

 that the majority of bacteria belong. Though the mul- 

 tiplication of the facultative varieties is not interfered 

 with by either the presence or absence of 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 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 the 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 the greatest intensity of color occurs at the point 

 farthest removed from the action of oxygen. 



Another element which plays a highly important part 

 in the biological functions of these organisms is the 

 temperature under which they exist. The extremes of 

 temperature under which the majority of bacteria are 

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

 former temperature development is hardly appreciable ; 

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

 when it is at its optimum, and, as a rule, ceases with 43 



