372 GENERAL SCIENCE 



Food which we want to keep for some time must be 

 protected from bacteria or they will cause it to decay. 

 This is done by heating the food and then sealing it, 

 or by using a preservative in which bacteria cannot 

 grow. Many of these preservatives are harmful to man 

 as well as to the bacteria, and should not be used. 



QUESTIONS AND EXERCISES 



1. Examine old stone fences and the bark of large trees to see if 

 they are covered with some greenish substance. The green material 

 consists of a one-celled alga called pleurococcus. 



2. Visit a pond or swamp and examine some pond scum. In 

 what way is it useful? 



3. What use is made of mushrooms, mold, and yeast? 



4. Explain in what way certain bacteria are more useful than 

 harmful. 



