DUST, MOLDS, AND BACTERIA OF THE AIR 91 



things represented in the air bacteria, yeasts, and mold 

 spores ; (3) that each of these is able to grow vigorously if 

 only it falls on the right kind of substance ; and (4) that all 

 of them are so numerous that they are likely to fall upon 

 every favorable place 

 and, of course, upon 

 unfavorable places as 

 well. 



103. The dependent 

 plants. In a preced- 

 ing study we saw 

 that green plants take 

 their food from car- 

 bon dioxide and water. 

 There are some plants 

 that are not green ; 

 that is, they have no 

 chlorophyll. Such are 

 the mushrooms and 

 molds. They are there- 

 fore unable to make 

 their food and are 

 obliged to secure it 

 from other plants or 

 animals. Since these 



FIG. 46. Corn smut a parasitic plant 



This parasite grows in the corn plant and produces 



masses of dark spores which may appear on the 



ear, stalk, or loaves of the corn 



plants depend upon 

 other living things 

 for their food, they 



are called dependent plants. Some of them secure food from 

 living plant or animal bodies and are called parasites (fig. 46). 

 Others live upon the material secured from the dead bodies 

 of plants and animals or upon materials formed by plants and 

 animals. These are called saprophytes. The common mush- 

 rooms live and grow in soil in which pieces of dead leaves, 

 decaying wood, and bark are mingled, and it is from this 



