DUSTS, MOLDS, AND BACTERIA OF THE AIR 83 



these are so numerous that they are likely to fall upon every 

 favorable place, and of course upon unfavorable places as well. 

 91. Dependent plants. In a preceding study we saw that 

 green plants make their food from carbon 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 therefore unable to make their food and are obliged to 

 secure it from other plants or animals. Since these plants de- 

 pend 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. Others live upon 



FIG. 44. Saprophytes 

 These plants secure their nourishment from decayed wood and similar substances 



the material secured from dead bodies of plants and animals, 

 or materials formed by plants and animals. These are called 

 saprophytes. The common mushrooms live and grow in soil 

 in which pieces of dead leaves, decaying wood, and bark are 

 mingled, and it is from this material that they secure their 

 food. They are therefore saprophytes (fig. 44). 



A great many kinds of parasites (fig. 45) and saprophytes 

 exist, and some of them, particularly the parasites, cause great 

 damage to crops, but we shall not study many of them at 

 this time. Some of the bacteria which are under discussion 

 in this chapter are parasites while others are saprophytes. 

 Either plants or animals may be hosts for parasitic bacteria. 



