30 BACTERIOLOGY. 



these are known as the chromogenic^ forms. Just what 

 their exact role in Nature is, it is difficult to say ; but it 

 is probable that in addition to their most conspicuous 

 function of color production, they are also in some 

 way concerned in the great process of disintegration 

 which is constantly going on in all dead organic sub- 

 stances. 



Others, the so-called photogenic, or phosphorescent 

 bacteria, possess the property of producing light or of 

 illuminating the medium on which they grow by a 

 peculiar phosphorescence. These are found in sea-water 

 and in decomposing phosphorescent fish and meat. 



Still others, the so-called zymogenic bacteria, are con- 

 cerned in the various fermentations, while the putrefac- 

 tive or saprogenic bacteria are those that produce the 

 particular fermentation that we know as putrefaction. 

 Another very important saprophytic group comprises 

 the so-called nitrifying and denitrifying bacteria, whose 

 activities result in specific forms of fermentation — the 

 former oxidizing ammonia to nitrous and nitric acid, 

 the latter reducing nitric acid to nitrous acid and am- 

 monia. The so-called thiogenic bacteria convert sul- 

 phuretted hydrogen into higher sulphur compounds. 



We have said that through the agency of chlorophyll, 

 in the presence of sunlight, the green plants are enabled to. 

 obtain the amount of nitrogen and carbon which is neces- 

 sary to their growth from such simple bodies as carbon 

 dioxide and ammonia, which they decompose into their 

 elementary constituents. The bacteria, on the other 

 hand, owing to the absence of chlorophyll from their 

 tissues, do not possess this power. They must, there- 



' Chromogenlo, possessing the property of generating color. 



