130 THE RELATIONS OF PLANTS TO ANIMALS 



Method. Make a hay infusion by placing a wisp of hay in a 

 jar of warm water. Let it stand a few days. 



Observations. What has happened to the hay? Any change 

 in color? Appearance? Odor? What do you know has hap- 

 pened to materials within the hay infusion? 



With a bulb pipette take a drop of water from the edge of the 

 jar near the surface of the water. Place it on a glass slide. 

 Examine with the low power of the compound microscope. The 

 tiny structures moving about are one-celled animals. 



Grass for hay is often cut near pools that dry up at haying time. 

 These pools contain millions of one-celled animals (Protozoa) which, 

 as the pond dries up, proceed to form a heavy wall about each 

 tiny body. In this form (like spores of mold) they may be 

 blown about in dust and still retain their vitality. 



Conclusion. 1. What does the presence of decay in the hay 

 infusion indicate? 



2. How do the Protozoa get in the infusion? 



3. On what might the Protozoa feed ? 



4. Why is the hay infusion unbalanced? 



5. How long might life exist in it? 



PROBLEM QUESTIONS 



1. Why is an aquarium called balanced? 



2. What factors are necessary for the balance? 



3. What are the food relations existing between plants and 

 animals in an aquarium? 



4. Compare life on the earth to a balanced aquarium. 



5. What kinds of bacteria are necessary to life on the earth? 

 Why? 



6. What substances are formed through the influence of the 

 bacteria of decay? 



7. What is meant by the carbon cycle? 



8. What do you understand by the oxygen cycle? 



9. Explain the nitrogen cycle in an aquarium; on the earth. 



10. What are the indispensable bacteria ? Why ? 



11. In what stage must the one-celled animals have been when 

 they were attached to the hay ? Why ? 



