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 



Balaiv&ed A<pigriun». aquarium. How 



could you destroy 

 this balance? 



3. FiU out the ac- 

 companying balance 

 sheet. 



Problem 109 : To learn what we mean hy 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? 



