Microorganisms in Relation to Man 557 



Air is the fourth necessary condition. Molds require air, 

 as may be seen by their growth only where air is available. 

 But moving air is unfavorable, not only disturbing the molds, 

 but hastening the evaporation of moisture. Air that is stag- 

 nant or static, as in food containers, such as closed dishes, is 

 by far the most favorable. 



SUMMARY 



Nitrogen and oxygen are the chief constituents of the atmosphere. 

 Small amounts ofother gases are found, also water vapor and dust. 



The constant accumulation of dust necessitates unceasing vigilance 

 to prevent its becoming a source of discomfort and possible menace to 

 health. 



Many devices are employed in the warfare against dust, but water 

 is the universal and most effective cleanser. 



Hard water, which is not as useful as soft, may be softened by heat 

 or by the addition of chemicals, such as washing soda. 



Street dust is made up of many different materials and may carry 

 disease germs which endanger health. 



Molds belong to the group of fungi, plants which derive their food 

 materials from other plants or animals. 



Fungi and molds differ from the green plants in being destructive, 

 promoting decomposition and decay. 



Saprophytes are plants that get their food from organic matter that 

 is dead or decaying. 



Parasites are such plants (or animals) as derive their food from 

 living plants or animals. 



Some of the saprophytic molds are useful in promoting the decay of 

 organic matter. 



Many molds are industrially useful, as in the production of certain 

 foods and beverages. 



Certain parasitic molds cause diseases of plants, men, and other 

 animals. 



Molds and their spores resist extremely high and low tempera- 

 tures, as well as moisture and drought. 



Conditions favorable to growth of molds are moisture, warmth, dim 

 light, and still air. 



REVIEW QUESTIONS 



1. What are the four principal constituents of the air? 2. What 

 other substances are present in small amounts? 3. How does the 

 dust of the atmosphere affect our health ? Our homes ? Our clothing ? 



