FEEMENTATION OP POLYSACCHAEIDES 221 



Tliis change is brought about by the activity of microorgan- 

 isms of many types. Some species apparently work best 

 under anaerobic conditions. Others are essentially aerobic. 



Several species of bacteria belonging to the genus Clos- 

 tridium have been described as capable of bringing about 

 anaerobic decomposition of cellulose. It is a matter of com- 

 mon observation that when dead plant tissues decompose 

 under anaerobic conditions, as in the mud at the bottom of 

 a pond, carbon dioxide, methane (CH^) and hydrogen gas 

 are evolved. It is found that during this process of decom- 

 position there are developed compounds having higher per- 

 centages of carbon until, under the right conditions, prac- 

 tically the entire organic remains consist of carbon and 

 hydrocarbon. This is the kind of change occurring in the 

 process of peat formation. The plant roots, mosses and 

 remains of stems buried under water gradually decompose, 

 but residues high in carbon are left. It is probable that 

 coal has originated in tue same manner. Probably a por- 

 tion at least of the humus in the soil is developed by the 

 same general process, although aerobic bacteria probably 

 play a considerable part in its manufacture. 



A distinct group of organisms, either anaerobic or facul- 

 tative forms, are of importance in bringing about cellulose 

 digestion in the alimentary tract of herbivorous animals. 

 The horse and the cow, for example, eat large quantities of 

 cellulose. A study of the digestion occurring in these ani- 

 mals reveals that a part, at least, of this cellulose is digested 

 and utilized. Careful search, however, has failed to show 

 that herbivorous animals in general are capable of pro- 

 ducing any enzyme which will digest cellulose. It is quite 

 probable that microorganisms present in the paunch of 

 ruminants and in the large intestine of the horse are 

 responsible for much, if not all, of this fermentation and 

 digestion. Many efforts have been made to isolate the 

 organisms responsible, and although some species capable of 



