106 NUTRITION AND METABOLISM. 



known to favor denitrification. The best studied decompositions of 

 cellulose are the two anaerobic fermentations, generally distinguished as 

 methane and hydrogen fermentation. The "methane" and the "hydrogen" 

 bacillus usually are found together; pure cultures have not been obtained 

 as yet though the two organisms can be separated from each other. The 

 so-called hydrogen fermentation of cellulose is caused by a thin, slender 

 rod, producing spores at the end of the cell, some of which have drum- 

 stick forms. The products of this destruction of cellulose are acetic and 

 butyric acid, probably a little valeric acid, carbon dioxide, and hy- 

 drogen. The mixture of gases contains at first more than 80 per cent of 

 hydrogen, at a later stage it contains only 4.5 per cent hydrogen, and the 

 remainder carbon dioxide. 



The " methane" bacillus, which looks very much like the "hydrogen" 

 bacillus, gives mainly acetic acid with a little butyric acid, while carbon 

 dioxide and methane escape. The amount of methane is at first about 

 75 per cent of the total gas, but soon drops to about 30 per cent. The 

 marsh gas in the mud of marshes, lakes, and other waters is formed by 

 this organism. 



STARCH is decomposed in many different ways. It may be oxidized 

 completely like cellulose to carbon dioxide and water. The same chem- 

 ical equation applies to this process. Some molds and some bacteria 

 will change the starch to various acids (lactic, acetic, oxalic, etc.) or to 

 alcohol and carbon dioxide. In every instance, the starch is converted 

 first into sugar, and then the sugar is fermented. The starch is not 

 fermented directly, 'since it is insoluble. (The fermentations of sugar 

 are dealt with in the next paragraph.) Dextrins are in all respects 

 similar to starch and are also converted into soluble sugars before being 

 fermented. Those organisms which cannot break up the starch or dextrin 

 molecule to sugar cannot use it as food. 



SUGARS can be decomposed in several different ways. Some fermenta- 

 tions have been studied much in detail, especially those of commercial 

 importance; others are known to some extent, but no special effort has 

 been made to determine all products quantitatively, consequently the 

 equations of these fermentations cannot be exactly given. 



The alcoholic fermentation by yeasts with its by-products has been 

 discussed so extensively in the paragraph above that there is little to be 

 added. Among the molds, the species of Mucor are the best-known 

 alcohol producers; nearly all mucorshave this quality and some of them, 



