^88 How Living Things Affect One Another unit vn 



4. List the different ways in w^hich other animals are useful to us. 



5. Name various kinds of complex plants that are useful to us and state 

 in which ways they are used. 



6. Explain the use of yeast to us. Explain how bacteria and molds are 

 used in flavoring foods and in other industries. 



7. Besides causing disease, how are microorganisms harmful to us? List 

 the ways we use to preserve foods and other materials against bacte- 

 ria of decay. 



8. What is a weed? Explain how weeds may cause a serious financial loss. 



9. What kinds of organisms cause plant diseases? Describe each of three 

 plant diseases, naming the plants affected. Why do imported organ- 

 isms often spread rapidly? 



10. Give examples of changes in our relationships which result from our 

 civilization. 



Exercises 



1. Make your own list of the ways in which we use animals. Make the 

 list specific; it can easily be made much longer than the list in the book. 



2. Prepare a list of plants used by man and compare it with the list in 

 the text. Perhaps members of the class can pool their lists. 



3. If you have not already observed the process of fermentation add a 

 pinch of yeast cake (from the grocery store) to a dilute solution of mo- 

 lasses or sugar in water. Do not cork. Why? After 12 or 24 hours taste 

 and smell the mixture. What do you notice? Explain, 



4. What is the gas produced in the mixture of sugar and yeast? A flask 

 containing a yeast mixture four or five hours old should be closed with a 

 one-hole rubber stopper. Lead a delivery tube through the hole in the 

 stopper into a vial of limewater, making sure that the lower end of the 

 tube is below the surface of the limewater. What do you see after about 

 an hour? Explain. 



5. Carefully study the labels on a variety of cans, jars, and bottles of 

 food. State what you find. 



6. Study five common weeds. What is there about the roots, the stems, 

 the leaves, the fruits, or the seeds that makes them so hard to get rid of? 



Further Activities in Biology 



1. If you have a flower or a vegetable garden identify the worst weed 

 offenders. Do your observations agree with those of your classmates? 



2. Look through your list of titles of Farmer's Bulletins published by 

 the United States Department of Agriculture and other bulletins and 

 write for those that apply to this problem. Report to the class. 



3. Posters giving information on plant diseases are sometimes displayed 

 in post offices or other public places. Study these and report to the class. 

 If you can draw well you might make some posters of your own. 



