552 Introduction to the Study of Science 



and others may be so minute that they can be seen only with 

 the aid of a microscope. In color they vary widely. They 

 may be blue, black or brown, pink, red, yellow or green, though 

 the green is not like that of ordinary green plants. 



Classification on the basis of food source. All fungi are alike 

 in respect to the source of the food material. They require 

 the same chemical substances for food that ordinary green 

 plants use and derive directly from the air and the soil. But 

 the fungi can utilize only such materials as are already organized. 

 The fungi are thus dependent upon organic matter for their 

 food supply. They have therefore very few of the features of 

 green plants and differ from them fundamentally in their 

 products. Green plants in their nutrition and growth are 

 peculiarly constructive or creative of organic compounds such 

 as starch, sugar, and proteins. But the fungi in general and 

 the molds in particular are destructive, bringing about decom- 

 position and decay. 



Saprophytes. The fungi derive the materials they need 

 for food either from organic substances that were once alive 

 and are now decaying, or from living plants and animals. Those 

 that derive their food from dead or decaying organic matter 

 are called saprophytes. Common examples of such are abun- 

 dant, as the slime molds on damp rotting branches, trunks, and 

 leaves of trees, the toadstools, and the molds often found on 

 bread, fruit, and vegetables, and on milk and cheese. 



Some of the common saprophytic molds are important just 

 because of the work they do in initiating the decay and de- 

 struction of dead organic materials, as trees, leaves, vegetation, 

 and animal matter. They aid in restoring to the soil substances 

 which are necessary to the growth of living things, and at the 

 same time in ridding the earth of vast amounts of useless matter. 



Certain saprophytic molds are utilized in the production of 

 certain foods and drinks. Some species convert sugar properly 

 aerated and warmed into citric acid. Other molds together 

 with bacteria are extensively employed in the manufacture of 



