FUNCTIONS OF SOIL BACTERIA 405 



containing no nitrogen. The non-nitrogenous sub- 

 stances decompose quite rapidly, and the organic 

 nitrogen disappears less rapidly than the carbon, 

 hydrogen and oxygen of organic bodies. 



Humus always contains a higher percentage of 

 nitrogen than do the plants from which it is formed 

 (page 123). 



The non-nitrogenous substances consist of cellulose 

 and allied compounds forming the cell-walls of plants, 

 and the carbohydrates, organic acids, fats, etc., con- 

 tained in them. The dissolution of cellulose is brought 

 about by the action of the enzyme cytase secreted 

 by a number of fungi, and is also probably accomplished 

 by the Bacillus amylobacter, but whether through the 

 secretion of an enzyme is not known. ' Other bacteria 

 have been reported to secrete a cytase that acts on 

 certain constituents of the cell-wall. It is probable 

 that numerous organisms capable of fermenting cellu- 

 lose and allied substances exist in the soil, which 

 decomposition they accomplished through the pro- 

 duction of cytase. 



The effect of cytase upon cellulose and other fiber 

 is to hydrolyse it with the formation of sugar, as glu- 

 cose, mannose, zylose, aribinose, etc. 



Starch is converted into glucose by a ferment 

 (diastase) either present in the plant itself or possibly 

 secreted by fungi or bacteria. All the sugars are finally 

 converted into organic acids which may combine with 

 mineral bases. Distinct organisms have been isolated 

 that can utilize for their development formates, acetates 

 propionates, butyrates, etc., the final product being 



