NUTRITION OF PARASITES AND SAPROPHYTES 141 



heated to kill the germs of fungi and bacteria and then sealed in 

 "cans" to shut out the air and the entrance of germs, are pre- 

 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 217, 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 anareobic bacteria, and among other prod- 

 ucts foul smelling gases are evolved. These processes of decay 

 so often destructive of economic products, and which modern 

 industrial 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 

 condition 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 pro- 

 cesses of decay are useful in preparing certain foods. Cream is 

 " ripened " 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 which cause serious illness and 

 sometimes death to those who eat them, are sometimes formed 

 in fish and other meats as a result of .incipient decay by bacteria. 



