CLASSIFICATION OF MICROORGANISMS 29 



absolutely without color, for they may be bluish, reddish, 

 gray, or white, or, indeed, they may show other colors; 

 but it means that they do not have the green color char- 

 acteristic of the majority of plants in nature. The absence 

 of this green coloring makes them unable to live upon the 

 food in the soil, and forces them to live upon a kind of 

 food different from that of ordinary plants. Ordinary 

 green plants can live upon minerals which they obtain 

 from the soil, and upon gases which they obtain from the 

 air, but the colorless plants cannot use such materials 

 at all. They need a more complex type of food. 



The materials in nature are frequently divided into 

 mineral and organic substances. Mineral, or inorganic, sub- 

 stances are such materials as rocks, sand, earth, etc. 

 Organic substances (wood, bones, fruit, muscle, etc.) 

 are those which have been produced by animals or by 

 plants, i.e. by organisms. Evidently the foods we eat — 

 meats, fruits, vegetables, etc. — are organic, since they all 

 come from plants or animals. Colorless plants, such as fungi 

 and bacteria, are, like animals, obliged to have organic sub- 

 stances for foods, and therefore feed upon materials es- 

 sentially similar to those which form the food of animals, 

 i.e. meats, fats, sugars, etc. Since the colorless plants and 

 the animals are in need of the same kinds of food they 

 become rivals in nature. The green plants, on the other 

 hand, living upon totally different foods, are in no sense 

 the rivals of animals, but their allies. It is this fact, their 

 living upon organic foods, that makes the colorless plants 

 of so much importance for good or ill, and explains their 

 close relation to the problems of the household with which 

 we are concerned. 



