THE AGENTS OF DECOMPOSITION 261 



the course of their basic metabolism into simpler com- 

 pounds such as urea and uric acid, excreted by animals in 

 the urine, but with no corresponding excretion by plants. 

 This difference between the behavior of plants and the be- 

 havior of animals may be more apparent than real, but 

 present knowledge throws no light on the matter. The 

 nitrogenous compounds produced in the metabolic activi- 

 ties of plant cells seem to be used again by them in the 

 reconstruction of proteins. (See Chap. V.) 



(e) Carbohydrate Fermentation. Sugar, starch, cellu- 

 lose and its derivatives (wood, etc.), and similar com- 

 pounds, are broken down in the process called fermenta- 

 tion, with the formation of alcohols, acids, and other 

 substances, and finally CO2 and H2O. This process is 

 always due to the activities of microorganisms and is 

 brought about by means of enzyms which they manu- 

 facture. In the various stages of the oxidations which 

 constitute essential parts of fermentation, corresponding 

 amounts of energy are liberated which commonly show 

 as heat. 



(/) Putrefaction. In putrefaction, organic compounds 

 containing nitrogen — many of them with sulphur or 

 phosphorus in addition — are broken down, commonly 

 with the production of offensive odors, ultimately to am- 

 monia and carbon dioxid. 



The Agents of Decomposition. — Animals, bacteria, 

 and fungi are ordinarily the immediate agents of decom- 

 position. In many cases the enzyms produced by the 

 living organisms are known to facilitate or accelerate if 

 not to accomplish the decomposition. Hence the infer- 

 ence is easy that all decompositions, whether autolytic or 

 carried on by invading organisms, are accomplished by 

 enzyms. 



The Principles of Decomposition. — The principles 

 controlling decomposition will become clearer if we study 

 certain examples, bearing in mind that we must examine 

 (a) the substance to be decomposed, (t>) the organism 



