128 THE RELATIONS OF PLANTS TO ANIMALS 



mals give off nitrogenous wastes. How might these be used by 

 the plants in the aquarium? 



Conclusion. — 1. What might the plants within the aquarium 

 furnish the animals? What might the animals furnish the 

 plants? 



2. Remembering that the sun furnishes energy, tell what makes 



the balance within the 

 Balaivoed A<jx* cerium. aquarium. How 



could you destroy 

 this balance? 



3. Fill out the ac- 

 companying balance 

 sheet. 



Problem 109 : To learn what we mean by the carbon and 

 the oxygen cycles. 



Method. — Carefully study the following figures. 



a. Carbon Cycle 



Observations. — In the plant world where does the carbon come 

 from ? Trace it to the animals. In what form do they take it in ? 

 In what forms do they release it ? How does carbon get back to 

 the plants? 



b. Oxygen Cycle 



Observations. — Begin with animals. What happens to oxygen 

 within their bodies ? In what form does it leave the animal body ? 



