1 68 GENERAL BIOLOGY OF MICRO-ORGANISMS 



variation slightly removed in respect to certain characters may 

 flourish better and become the mean type about which the 

 fluctuating variants group themselves. Thus the pure culture 

 seems to respond to environmental change. Whether the 

 fluctuating variations are due to small differences in the imme- 

 diate surroundings of the individual microbes, or whether they 

 arise as a result of a property of variability inherent in protoplasm, 

 may be disputed, but the latter view is more commonly held by 

 biologists. 



THE PRODUCTS OF MICROBIC GROWTH. 



The effects resulting from the growth of a micro-organism 

 depend on the one hand upon the nature of the organism and on 

 the other upon the environment, more especially the medium in 

 which it grows and the conditions of temperature and oxygen 

 supply. Apparently slight variations in the latter may influence 

 the results to a marked degree. 



Physical Effects. Heat is evolved by many actively growing 

 bacterial cultures and is especially evident in the fermentation 

 of such substances as ensilage and manure. Perhaps some of 

 the heat may result directly from microbic activity, but the most 

 of it appears to arise from secondary chemical reactions in which 

 the microbic products sometimes play a part. Microbes which 

 produce heat are designated as thermogenic. Light is also emitted 

 by some microbic cultures. Here it seems certain that the light 

 is produced by the oxidation of a bacterial product and not emitted 

 directly by the micro-organisms. These phosphorescent or 

 photogenic organisms occur in salt water and on fish and they 

 have rarely been found in other places. 



Chemical Effects. These are the most important results of 

 microbic growth. As we have just seen, the production cf 

 heat and light is probably due to a secondary reaction entered 

 into by some of the chemical products of growth. Almost all 

 the other important practical effects of the growth of micro- 



