128 GENERAL PHYSIOLOGY 



External Conditions. The external conditions may be 

 without effect on the bacteria, or they may stimulate activity, 

 that is, they may have a tonic effect ; or, on the other hand, they 

 may have a repressive effect, and it is the purpose in this con- 

 nection to consider the most important of the external con- 

 ditions and determine whether their action is stimulative or 

 repressive, and find out if possible the conditions which are best 

 adapted for the growth of the bacteria or conditions which will 

 lead to their destruction most quickly. 



Food. All forms of life must have food, and the bacteria 

 are no exceptions. Like other forms of life, also, they may be 

 able to get along with a very small amount, that is, on a 

 maintenance ration, or they may do a great deal better on a 

 different or larger amount, that is, on a growing ration. As 

 to the actual amount which is needed by the bacteria it is to 

 be noted that the bacteria are divided into two classes. One 

 class are accustomed to live upon the body of animals or plants, 

 and are known as parasites. These require a considerable amount 

 of food. The other class live on dead organic matter, and are 

 known as the saprophytes. Some of these may subsist on a 

 surprisingly small amount of organic food. Meade Bolton 

 some years ago called attention to the fact that certain water 

 bacteria were able to live and grow with great rapidity in 

 double-distilled water where the only source of organic matter 

 was the dirt on the sides of the vessels, which he was unable to 

 remove by the most careful methods of cleaning. This is one 

 extreme. Many of the bacteria, however, require a consider- 



