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 alge, 
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,, NH,, and H,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, 
3 
