230 THE FUNGI 



below freezing, and are able to live for very long periods. But 

 other forms, as the Bacillus of typhoid fever (Fig. 206, B), may 

 be certainly killed within a few minutes by boiling the water 

 in which they live. Certain bacteria, as the species which pro- 

 duce lockjaw and cause butter to become rancid, will live with- 

 out air, and are even injured by contact with free oxygen. They 

 obtain the oxygen necessary for respiration from compounds, 

 such as the carbohydrates, which contain it. 



The bacteria are present almost everywhere, floating in the 

 air on particles of dust, in the water, in the soil, and always 

 living within and upon the bodies of animals. Thus the bacte- 

 ria are ready to grow and multiply wherever they find favorable 

 conditions, but these are exceedingly various for the different 

 species. Some forms are restricted to a parasitic life on particu- 

 lar hosts, as certain animals or plants, or man. Other types are 

 connected with special chemical reactions, as in the processes of 

 decay, fermentation, nitrification, etc. Many bacteria are indis- 

 pensable to life on the earth, and of the greatest service to man. 

 Many forms are harmless, but of no special value to man. 

 Some cause dangerous contagious diseases. 



252. Decay. Decay is the destruction or decomposition of 

 highly complex organic compounds, such as the proteids, fats, 

 sugars, and cell walls of plants, by which they are broken down 

 into successively simpler substances, and finally into fluids and 

 gases, some of which are very ill smelling. The products of 

 decomposition form various chemical combinations, and are 

 finally used again in the constructive processes of life. The bac- 

 teria and other fungi are the chief agents of decay, and if it 

 were not for them the world would soon be filled with organic 

 waste products, together with the dead bodies of animals and 

 plants of no value as food. Thus all the chemical elements 

 capable of sustaining life would long ago have been used up and 

 life on the earth would have ceased. The bacteria are there- 

 fore chiefly responsible for a circulation of elements (see dia- 

 gram, Fig. 207), from the highly complex organic compounds of 



