CARBON AND OXYGEN CYCLES 285 



Self-Testing Exercise 



(1) and (2) live together in communities. Such a 



place is called a (3). A balanced aquarium shows the 



(4) between animals and plants, the former give (5) 



(6), and (7) wastes to the plants which in turn 



(8) organic food which the (9) (10). 



This illustrates the give and take between plants and animals in the 

 (11). 



PROBLEM II. WHAT DO WE MEAN BY THE NITROGEN, 

 OXYGEN, AND CARBON CYCLES IN NATURE? 



Nitrogen cycle. The animals supply much of the carbon dioxide 

 that the plant uses in carbohydrate making. They supply some of 

 the nitrogenous matter used by the plants, another part being 

 given the plants from the dead bodies of other plants, and still 

 another part being prepared from the nitrogen of the air through 

 the agency of bacteria which live upon the roots of certain plants. 

 These bacteria are the only organisms that can take nitrogen from 

 the air. Thus, in spite of all the nitrogen in the atmosphere, 

 plants and animals are limited in the amount available. Eaten 

 in protein food by an animal, nitrogen may be given off as nitroge- 

 nous waste, get into the soil, and be taken up by a plant through 

 the roots. Eventually the nitrogen forms part of the food supply 

 in the body of the plant, and then may become part of its living 

 matter. When the plant dies, the nitrogen is returned to the soil. 

 Thus the usable nitrogen is kept in circulation. 1 



Practical Exercise 2. Illustrate what is meant by the nitrogen cycle with 

 reference to your own environment. 



Make a diagram to show the way the nitrogen cycle works out in life on 

 the earth. 



Carbon and oxygen cycles. There are also two other cycles 

 in nature that are easily seen. Oxygen dissolved in the water is 

 taken up by the fish in the aquarium, and is released in the form of 

 an oxide of carbon or carbon dioxide. In this form it is taken 



*A small amount of nitrogen gas is returned to the atmosphere by the action of 

 the decomposing bacteria on the ammonia compounds in the soil. (See figure of 

 nitrogen cycle.) 



