326 ANNUAL REPORT OF THE Off. Doc. 



dilute acids. It consists in the conversion of the cellulose into 

 some form of sugar, which differs with the forms of cellulose acted 

 upon. These different forms of sugar are glucose, mannose, galac- 

 tose, zylcse and arabinose. 



The pectoses, which have been found also to be important constitu- 

 ents of the cell wall, are under the action of cj-tase converted into 

 reducing sugars. The different forms of sugar are then acted upon 

 by other ferments and converted into organic acids. 



This explains the common tendency of soils rich in vegetable mat- 

 ter to become acid, unless continued cultivation stimulates bacterial 

 growth sufficient to decompose these less readily decomposable or- 

 ganic acids into their final gaseous products, carbon dioxide and 

 marsh gas. 



It has already been noted that all of the constituents of cell walls 

 do not undergo dissolution equally. Hence when vegetable tibre 

 undergoes fermentation in the soil there remains a residue which 

 for a longer time withstands the action of these ferments. This lat- 

 ter constitutes the great bulk of that heterogeneous material which 

 is called humus. Humus is, therefore, in the main, the product of the 

 incomplete decomposition of vegetable fibre. 



4. The Fermentation of Carbohydrates. 



The carbohydrates in vegetable tissues exist mainly in the form of 

 starch and sugar. In crops ordinarily used for green manuring they 

 constitute between 40 and 50 per cent, of the dry weight of the plant. 

 Starch is the most abundant carbohydrate of green crops, but sugar 

 exists in small amounts, usually a fraction of a per cent. However, 

 in fleshy root®, fruits, and in special cases, it may run much higher, 

 reaching a maximum in the beet and sugar-cane of 15 per cent. 



Sugar exists in different vegetable tissues in three forms: as cane 

 sugar or saccharose, grape sugar or dextrose and ■ fruit sugar or 

 levulose. In animal tissues and fluids the carbohydrates exist: as 

 glycogen, a modified starch, as dextrose, and in milk as milk sugar 

 or lactose. 



Bacteria play an important role in the fermentation of carbohy- 

 drate, and those concerned in these processes are abundantly 

 present in all soils. The great majority of bacteria when growing in 

 media containing grape sugar, milk sugar or cane sugar produce 

 therein greater or less quantities of organic acids accompanied in 

 some cases by the evolution of gas. 



These acids are lactic, acetic, butyric, formic, propionic^ valerianic 

 and succinic. When milk sours, lactic acid is produced at the ex- 

 pense of the milk sugar by certain bacteria normally present in the 



