THE NATURE OF BACTERIA. 35 



Although bacteria are plants, they do not possess chloro- 

 phyll, and are not able, like green plants, to live upon mineral 

 matter. It is true that certain species can live upon food which 

 is much closer to pure mineral matter than that upon which 

 any animals can feed ; true also that some of them, under some 

 circumstances, may live upon compounds simpler even than 

 the minerals which form the food of green plants, and which 

 are in no sense organic. This fact is a matter of the greatest 

 significance, as we shall see later. Nevertheless, the great 

 majority of bacteria require organic food of high complexity, 

 in this respect resembling animals. All types of organic ma- 

 terial may serve as food for them. Proteids, fats, starches, 

 wood, cellulose, bone, etc., are all used by them under proper 

 conditions. This group of organisms may be looked upon as 

 the greatest agent in nature for destroying the organic ma- 

 terial produced by both animal and vegetable life, either using 

 it as food, or causing its chemical destruction and disintegra- 

 tion in some other way. As they feed upon this organic ma- 

 terial, they produce within it profound chemical changes. The 

 chemical changes thus brought about are very numerous. 

 The chemist of to-day has hardly begun to study them and his 

 knowledge is as yet veiy fragmentary. Only a veiy few of 

 them are even in the slightest degree understood, and in re- 

 gard to the simplest of these it must be recognized that as yet 

 our knowledge of the phenomena is lacking in many important 

 respects. 



The great host of chemical changes which occur in organic 

 material under the action of bacteria must be left for the future 

 to describe and explain. At the present time we have a gen- 

 eral knowledge of a few of them, and such of these as bear 

 particularly upon the subject of agriculture will be explained 

 in the following pages. We must remember, however, that 

 those referred to are only a few of the numerous and profound 

 chemical decompositions which occur under the agency of 



