PROBLEM I. Why Plants and Aii'mials Continue to Exist 379 



excrete wastes containing nitrogen and It is said then to be a balanced aquarhnn. 



other mineral elements. Then the bac- In a balanced aquarium each important 



teria, which are always present, change element is kept in circulation. In the 



these wastes to nitrates and other mineral same way on our planet the important 



compounds. When just the proper num- elements are kept in circulation through 



ber of green plants, fish, snails, and bac- the activities of bacteria, green plants, 



teria are included, the plants and animals and animals. Thus the continued exist- 



\\\\\ live for a long period of time with- ence of all living things is made possible, 



out the addition or removal of anything. See Exercise 7. 



Questions 



1. Give three examples of food chains. How do the organisms at and 

 near the beginning of a food chain differ from those at or near the 

 end? 



2. Define parasite and saprophyte. Name some animals and some plants 

 that live as parasites in or on you. Give examples of saprophytes. 



3. How does symbiosis differ from parasitism? Give examples. 



4. Which are the only animals in a food chain that do not eat other 

 animals? Why have the earth materials used by plants not been ex- 

 hausted up to this time? What important elements are present in the 

 earth materials used by plants? 



5. Explain how the oxygen supply on the earth depends on photo- 

 synthesis and oxidation. Explain how the element hydrogen in water 

 may pass from plants to animals and back again to plants. 



6. Describe a carbon cycle. What group of organisms is active in pre- 

 venting carbon from being tied up indefinitely in the bodies of plants 

 and animals? In what form has some carbon been kept out of circula- 

 tion millions of years? 



7. How does the plant use nitrates taken from the soil? How do the 

 bacteria of decay enter into the nitrogen cycle? By what organisms 

 are the simple ammonium compounds built up into nitrates again? 



8. Contrast the work of denitrifying and nitrifying bacteria. In what 

 form must nitrogen be to be of use to plants? 



9. Describe the work of nitrogen-fixing bacteria. Where are they found? 

 Explain how thunderstorms affect the nitrogen cycle. 



10. Summarize briefly how the continued existence of plants and animals 

 is made possible. Explain how a balanced aquarium imitates these con- 

 ditions at least to a limited degree, 



