BACTERIA 13 



the material of the hay into a soluble form before it cari use 

 it. This is one of the things that bacteria do on a large 

 scale. They are able to change many organic materials 

 with which they come in contact into a soluble form — or, 

 as we commonly say, the materials are digested — and then 

 the bacteria absorb such parts of the solution as they 

 can use. 



If we keep the hay infusion supplied with water so that 

 it does not dry out, the solid substance of the hay will gradu- 

 ally disappear, and finally there will remain only a slimy 

 solution with a disagreeable smell. The same thing would 

 happen to bread or meat or to a potato that was left in water 

 under similar conditions. The organisms that would be at 

 work in the digestion of meat would not be altogether the 

 same as those we find in the hay infusion ; but the most 

 numerous and active among them would be bacteria, although 

 Bacillus subtilis might ndt be one of them. Bacteria digest 

 solid foods like hay or meat by means of special substances 

 which they form within their cells and pass out through the 

 walls, just as the cells that line the human stomach form the 

 gastric juice which digests meat and other foods within the 

 stomach. Such substances, formed inside living cells, which 

 can produce changes in the chemical nature of other sub- 

 stances, are called enzyms. We shall learn that very many 

 different enzyms are produced by plant and animal cells, 

 and that each kind of enzym can cause one particular 

 kind of 'chemical change. 



21. Respiration. — Like ourselves, the bacillus must 

 respire; that is, it must have oxygen if it is to remain active, 

 and this oxygen it takes in from the air. The oxygen is used, 

 not in building up, but in tearing down the substances 

 within the cell wall. This tearing down is just as necessary 

 as the building up ; it supplies the energy that the cell must 

 have in order to do work. One form of the energy so obtained 

 is heat. 



