1 8 TEXTBOOK OP BOTANY 



bodies of plants and animals and returning the elements of 

 which they are made to the soil, water, and air to be used 

 again in building up the bodies of living animals and plants. 

 If it were not for decay, the surface of the earth would long 

 ago have been covered with dead organic matter, and the life 

 of the higher plants and animals, including man, would now 

 be impossible. 



Most bacteria, including many that cause decay, like 

 Bacillus subtilis, can obtain the oxygen that they need for 

 respiration only from the air. For this reason, the decay of a 

 substance can often be stopped or prevented if air is entirely 

 excluded from it. This is one reason why fruits and vege- 

 tables are preserved in air-tight cans and jars. On the other 

 hand, some bacteria cannot grow in the presence of air ; so 

 they thrive only in places, for example deep in the soil, to 

 which air does not reach. Such bacteria need oxygen for 

 respiration just as all living organisms do ; but they obtain 

 it from the organic substances that they decompose, and not 

 directly from the air. An example of this class is Bacillus 

 tetanus (Fig. 9, D), which causes lockjaw. Since this bacillus 

 cannot multiply in the presence of air, it is important not to 

 allow wounds into which the lockjaw germ may have entered 

 to heal over too soon upon the surface, and not to keep them 

 covered so tightly as to keep out the air. There is a third 

 group of bacteria that can take their oxygen from the air, 

 but which, if they are shut away from the air, can obtain it 

 by the breaking down of organic materials. 



27. Soil Bacteria. Great numbers of bacteria are found 

 in the soil, especially in the upper layers to a depth of six 

 inches or a foot. Various countings of bacteria in the surface 

 layers of different kinds of soil have shown from 15,000,000 to 

 300,000,000 bacteria per ounce of soil. At greater depths the 

 numbers are smaller, and relatively few are found more than 

 five or six feet below the surface. Among those in the deeper 

 soil layers are many kinds that cause decay and putrefaction. 



