30 BACTERIOLOGY. 



exact role in nature is it is difficult to say ;* but it is prob- 

 able that, in addition to their most conspicuous function 

 of color-production, they are also in some way concerned 

 in the omnipresent process of disintegration which is 

 constantly going on in all dead organic substances. 



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 pecu- 

 liar 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 activi- 

 ties are concerned in specific forms of fermentation the 

 former oxidizing ammonia to nitrous and nitric acids, the 

 latter reducing nitric acid to nitrous acid and ammonia. 

 It is through their association (symbiosis) with the nitri- 

 fying bacteria that certain plants, the leguminous, are 

 enabled to make up their nitrogen deficit in part from 

 the free nitrogen of the air. The discovery of this 

 phenomenon gave to free atmospheric nitrogen a biolog- 

 ical significance that had hitherto been denied it. The 

 so-called thiogenic bacteria convert sulphuretted hydro- 

 gen 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 

 necessary to their growth from such simple bodies as 

 carbon dioxide and ammonia, which they decompose 

 into their elementary constituents. The bacteria, on 



