THE FUNGI 255 



Another common method of sterilization is to subject the 

 material to be sterilized to steam at a very much higher temper- 

 ature (120 C.) at the outset than that indicated above. Then 

 a single heating is usually sufficient to kill the spores. 



In pasteurization no attempt is made to kill the more resistant 

 spores, but simply to kill the vegetative cells of certain disease 

 germs. For this purpose milk or other substances to be pasteur- 

 ized are heated to about 65 or 75 C. By this means fer- 

 mentation or any other change induced by active bacteria in the 

 pasteurized material is temporarily checked, and the pasteurized 

 milk or food will thus remain unchanged for a longer period of 

 time. The ordinary disease germs which occur in milk, as well 

 as the bacteria which sour milk, are killed by pasteurization, and 

 the deleterious effects which result from the high .temperatures 

 necessary for sterilization are also avoided. 



Decay. The process commonly known as decay is very largely 

 due to fermentation or to digestion by bacteria and other 

 saprophytic fungi that live and feed on the organic material 

 which exhibits the appearance and phenomena of decay. The 

 bacteria which cause decay are saprophytes, and like the yeasts 

 they secrete digestive and energy-forming ferments which at- 

 tack the cell walls and other lifeless parts of animal and plant 

 remains upon which they live. Parts of these organic remains 

 are thus fermented and digested, with the result that the entire 

 form and structure of the part attacked is often destroyed and 

 crumbles away. Accompanying this fermentation and digestion 

 of organic matter, gases, such as ammonia and sulphureted hy- 

 drogen, are produced. In the case of the flesh of animals or of 

 material containing a large percentage of nitrogenous com- 

 pounds the gases liberated give rise to odors which we associate 

 with putrefaction. Nitrates and sulphates are also formed which 

 are carried back into the soil, and these compounds ultimately 

 serve as raw food elements absorbed by the roots of green plants. 

 The decay of leaves and tree trunks in a forest, already mentioned, 

 is a good instance of decay produced by saprophytic bacteria 

 and other fungi, and similar activities of such bacteria in liquids 

 are familiar phenomena. The souring of milk is caused by a 



