514 TEXTBOOK OF BOTANY 



time. Moreover, without a comparatively rapid digestion of cellulose and 

 the subsequent oxidation of the products of digestion, the supply of 

 carbon dioxide in the air would gradually become depleted ( Fig. 228 ) . 



During decay not only is carbon dioxide formed and liberated to the 

 air, but inorganic ions are liberated from their organic union and are 

 again available to green plants. 



Some saprophytic plants are valued in industry because they do not 

 completely oxidize certain compounds, but leave a residue which man 

 values for certain purposes. These residues are often referred to as prod- 

 ucts of fermentation. The complete oxidation of a substance in sapro- 

 phytic plants usually consists of a series of steps, each of which is the 

 result of the activity of successively different organisms. Sugar is oxidized 

 to alcohol by yeast. Acetic acid bacteria oxidize the alcohol to acetic 

 acid. Other organisms then oxidize the acetic acid to simpler compounds 

 and finally all are oxidized to carbon dioxide and water. After insoluble 

 substances are digested, many intemiediate and successively simpler 

 compounds may be formed before oxidation is complete. Moreover, the 

 kinds of intermediate products formed vary with the species of organism 

 that oxidizes each of them. The industries producing beer, wine, vinegar, 

 glycerin, alcohol, and cheese all depend upon the fermentation products 

 of carefully cultivated species of saprophytes. The quantity and quality 

 of the products vary with the kinds of fungi and bacteria present. 



The unpleasant odors of decaying matter likewise are due to the in- 

 termediate products of oxidation-reduction which are formed during the 

 growth of certain bacteria and fungi. 



Saprophvtes are ever-present agents of destruction, and are the or- 

 ganisms that make cold storage and refrigeration necessary in our modern 

 civilization. The freezing, canning, drying, and preserving industries are 

 based on methods of eliminating saprophytes. Buildings and other wood 

 structures are protected from them by means of metal, paint, tar, and 

 creosote. On the other hand their very destructiveness is valued as a 

 means of removing sewage and other undesired organic products. Sapro- 

 phytes, then, are as intimately associated with our daily lives as are the 

 green plants and parasites. 



