MICROORGANISMS 381 



familiar with the result of the action of these organisms on 

 our food and other materials, but we are likely to overlook 

 the agents of this destruction and to say that the food decays 

 or spoils. The reason that we do not recognize these plant 

 competitors as well as our animal competitors lies in the fact 

 that most of the former are so small as to escape our notice 

 unless we make a special study of them. Most of these non- 

 green plants are so small that they can be seen only with the 

 aid of a microscope. For this reason, they are conveniently 

 grouped together with a host of microscopic animals under the 

 general name, MICROORGANISMS. 



In Sec. I of the present chapter, we shall make a study of 

 these plant microorganisms and of the methods which have 

 been devised to enable man to ward off the attacks which they 

 make on the food and other materials which he wishes to 

 preserve. We shall learn also that, aside from the consump- 

 tion of materials which man wishes to preserve, these organisms 

 render man a very valuable service by consuming materials 

 which are worthless to him and changing them into forms which 

 are available again as raw materials for the green plants. We 

 shall see that the fertility of the soil which means so much to 

 man and to all the rest of the living world is largely dependent 

 on the action of these organisms. 



433. Saprophytes and Parasites. In Sec. II of the present 

 chapter, we shall see that some of these microorganisms find 

 their food in the living bodies of man and other animals and of 

 the higher plants. Organisms which thus live and find their 

 nourishment in the living bodies of other organisms are known 

 as PARASITES. Many animals can live only within or on the 

 bodies of other living animals and are therefore ANIMAL 

 PARASITES. We shall be concerned here, however, mainly 

 with the PLANT PARASITES. Fortunately, the vast majority 

 of the plant microorganisms can not live in the bodies of other 

 organisms and are therefore detrimental to man's interests, if 

 at all, only because they consume food and other materials 

 which he wishes to preserve. We say "other materials" here 



