NUTRITION OF PARASITES AND SAPROPHYTES 133 



served. In some very dry climates, trees when they fall are 

 preserved in a sound condition for a long time because the mois- 

 ture is insufficient to favor a rapid growth of the mycelium of the 

 wood-destroying fungi. The processes of disintegration of wood 

 and leaves described in paragraph 215 are processes of decay. 

 If it were not for such organisms as the fungi and bacteria, our 

 forests would soon become choked up with dead trees, and nitrog- 

 enous foods in the soil would in time become used up so that all 

 life would disappear from the earth. The fungi are chiefly con- 

 cerned in the disintegration and decay of carbohydrates, as starch, 

 sugar, cellulose and woody structures, while bacteria are chiefly 

 concerned in the decay of nitrogenous substances. The decay of 

 nitrogenous bodies, especially if they are in bulk, is usually called 

 putrefaction because much of the process of decay is carried on in 

 the absence of air by anaerobic bacteria, and among other products 

 foul smelling gases are evolved. These processes of decay so 

 often destructive of economic products, and which modern indus- 

 trial development has done much to prevent as applied to food 

 products for man, are really of the greatest importance and value 

 viewed from the standpoint of economy in nature. All dead 

 plants and animals left to the operation of nature's laws, by the 

 action of a long series of organisms, are finally reduced to a condi- 

 tion in which they can be used as food by the higher plants again, 

 thus perpetuating life, through the decay of the dead, and the 

 endless circulation of food substances. Some of these processes 

 cf decay are useful in preparing certain foods. Cream is " rip- 

 ened " for butter by the action of special bacteria. The great 

 variety of cheeses with their distinct flavors and odors is made 

 possible by the action of specific kinds of fungi. Fresh meats are 

 made more tender and to some more palatable after a process of 

 " ripening," which is in reality an incipient decay. Very poison- 

 ous products called ptomaines are sometimes formed in fish and 

 other meats as a result of incipient decay by bacteria which cause 

 serious illness and sometimes death to those who eat them. 



218. Fermentation. Fermentation is really a process of 

 decay. Starch can be fermented into sugar by certain fungi. 



