6 AGRICULTURAL BACTERIOLOGY 



plant, and is built into some one of its compounds. These 

 are used by some member of the destructive group, or more 

 commonly by a series of members of this group, with the 

 result that the element becomes again available to the green 

 plant. 



This passage of the elements from one form of life to 

 another is called the cycle of the elements. An atom of 

 carbon may be in the air to-day in the form of carbon- 

 dioxide; to-morrow it may be in a sugar molecule of 

 a plant; the next day in the tissues of an animal; and the 

 succeeding day it may be again present in the air in a mole- 

 cule of carbon-dioxide; ready for another of its ceaseless 

 passages, carrying with it a supply of energy for the animal 

 and the fungus plant. 



The chief agents in the decomposition of organic matter 

 are the protozoa; or simple animal forms, and the simple 

 plant forms, which include the bacteria, the yeasts, and the 

 molds. The term microorganism is often applied to these 

 various types, and microbiology to their study. It is with 

 these forms that this volume treats, and especially with 

 the ways in which they influence the life of man. He meets 

 them in the soil he tills ; he makes use of them in the prep- 

 aration of foods and products of industrial value ; he is con- 

 stantly striving to protect his food supplies from their ac- 

 tion, and to protect himself and his animals from the 

 diseases that they cause. They present themselves to him 

 at every moment of his life, to his benefit or his injury. 

 He must employ them, and fight them, either conscious or 

 unconscious of the nature of his acts ; and he who has intel- 

 ligent acquaintance with them will certainly fare far bet- 

 ter than one ignorant of the part they play. A knowledge 

 of the role of microorganisms in nature is as essential as 

 knowledge concerning the higher plants and animals. 



