CHAPTER I. 



THE GENERAL CHARACTERISTICS OF BACTERIA 

 THEIR MORPHOLOGY AND CHEMICAL COMPOSITION. 



BACTERIA are among the smallest of all known liv- 

 ing organisms, the largest of them having a diameter 

 of only a few micromillimetres, while the smallest do 

 not measure more than a fraction of a micromillimetre. 

 Structurally and morphologically they are extremely 

 simple, though biologically very variable. Through 

 their ability to derive their carbon from tartrates and 

 their nitrogen from ammonia or its salts, they are 

 ranked in the vegetable kingdom. They obtain their 

 food entirely through the surface absorption of soluble 

 nutritious substances. They are reproduced by trans- 

 verse division, and in some respects resemble the fungi ; 

 hence called by Nageli fission-fungi, or schizomycetes. 

 They are also closely allied to certain kinds of algae, 

 though they must receive their nourishment from living 

 or dead organic material, since they are without chloro- 

 phyll, the green coloring matter possessed by the higher 

 plants, by means of which they are enabled, in the 

 presence of sunlight, to decompose CO 2 , NH 3 , and H 2 S 

 into their elementary constituents. A few varieties of 

 unicellular organisms resemble bacteria in all their 

 known characteristics, except that they possess chloro- 

 phyll or substances similar to it. Others, still, which 

 have no chlorophyll, are able in the absence of light to 

 build up organic substances synthetically. Bacteria, 



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