36 AGRICULTURAL BACTERIOLOGY. 



bacteria. These do represent, however, the chief changes pro- 

 duced by microorganisms which are well enough understood 

 by our chemists to warrant a summary at the present day. 



These chemical changes may be grouped under two quite 

 distinct heads. 



Synthetical Processes. Anabolism. These consist in the 

 building of complex bodies out of simpler ones. The funda- 

 mental importance of synthetical processes to the continuation 

 of life is evident enough. The animal kingdom, in general, de- 

 mands complex compounds as foods and cannot live upon such 

 substances as are found in nature, like carbonic dioxide, nitro- 

 gen, ammonia, etc. (CO,, N, NH 3 ). To make use of the ele- 

 ments existing in nature some process must build these ele- 

 ments and simple compounds into complex bodies which may 

 serve as foods. This is largely accomplished by the green 

 plants that furnish animals with food. But even these plants 

 demand some of their food in a complex form, not being able, 

 for example, to use nature's nitrogen store in the atmosphere 

 until it has been built up into some compound like nitric acid. 

 The constructive processes are thus of the most fundamental im- 

 portance to the problem of life processes. Among the chem- 

 ical changes which are brought about by bacteria some are of 

 this synthetic character and, indeed, appear to form the con- 

 necting link between the mineral and organic world. The 

 synthetic changes are, however, not possessed by most species 

 of bacteria, to any considerable extent. 



Analytical Processes. Decomposition; Katabolism. The 

 most common action of bacteria is that of decomposition. The 

 great majority of species, like animals, live upon complex 

 chemical foods, as already noticed, and these compounds are 

 broken to pieces by their action and reduced to simpler mole- 

 cules. Acting in this way, the bacteria arc the most important 

 agents in nature for reducing to a simpler condition the great 

 quantity of organic matter which would otherwise accumulate 



