CHAPTER IX 



BACTERIA 



Bacteria are the smallest of known organisms, but 

 they are of quite extraordinary importance in the 

 economy of nature, an importance only exceeded by 

 the green plants. While the latter form the basis of 

 all Ufe by building up living substance from inorganic 

 materials, the former destroy dead organic matter, 

 breaking it down into simpler forms and eventually 

 into inorganic substances which are available for the 

 food of green plants. Bacteria are also of direct prac- 

 tical importan.ce to men, not only because some of them 

 cause various deadly diseases, but because others are 

 used by him, as yeast is used, to carry out fermentations 

 — not alcoholic fermentations, but processes of the same 

 general nature, such as the " ripening " of cheese, the 

 " curing " of tobacco, and so on. There are a large 

 number of different species of bacteria known, and 

 certainly far more exist than have yet been recognised. 

 Size, Shape and Structure. — The diameter of an 

 average bacterial cell is only about i /x, far less than 

 that of other unicellular organisms. There certainly 

 exist, however, Uving organisms which may probably 

 belong to the Bacteria and which are too small to recog- 

 nise under the highest powers of the microscope. ^ Thus 

 we know that yellow fever, and also swine fever, are 



' It will be remembered that an object less than about -15^ In 

 diameter cannot be clearly defined under the highest powers of the 



microscope (p. 50). 



138 



