136 INTERRELATIONS OF PLANTS AND ANIMALS 



.Animal Life 



sing Bacteria 



Nitrites 

 "Nitric Bacteria 



c. The Nitrogen Cycle (The Nitrogen Relation) 



Method. Begin at the statement "Free N." and trace 

 Figure 40 in the direction of the arrows. 



1. Where is free 

 nitrogen to be found ? 



2. What is the only 

 way that free nitro- 

 gen may be obtained 

 from plants? 



3. What do plants 

 manufacture in addi- 

 tion to starch that 

 becomes available for 

 animals ? 



4. What waste sub- 

 stance containing nitrogen is being constantly set free from 

 animals? 



5. What causes dead animal tissues to break up into their 

 parts ? 



6. What are some of these parts ? What becomes of 

 them ? 



Conclusions. 1. What are the two most important links 

 or factors in the nitrogen cycle ? 



2. Write a paragraph explaining what is meant by the 

 1 nitrogen cycle.' 



3. Try filling out a final summary in tabular form, some- 

 what as follows : 



FIG. 40. The nitrogen cycle. 



