Our Balanced World 537 



form of nitrogenous waste it is once more taken up by the plants 

 and made into protein plant tissue that animals again eat. This 

 process is sometimes called the nitrogen cycle. 



A Balanced Terrarium. A balanced terrarium is a box con- 

 taining earth in which land animals and land plants supply each 

 other with the materials needed for continued life. The animals 

 provide substances which the plants are able to use as raw mate- 

 rial in making food, and the plants, in turn, provide food and 

 oxygen for the animals. In a balanced terrarium containing 

 insects and earthworms and growing plants, the animals feed on 

 plant tissue and breathe air containing oxygen given off by the 

 plants. The plants use the carbon dioxide and nitrogenous matter 

 excreted by the animals, together with the nitrogenous matter 

 resulting from the death of organisms in the terrarium, as raw 

 material for the manufacture of food, the same as in a balanced 

 aquarium. Life will continue in a balanced terrarium even if it is 

 sealed air-tight. 



A Balanced World. In the balanced aquarium and the 

 balanced terrarium life is maintained because the plants and the 

 animals mutually supply each other with the necessities of life. 

 Our world is really a giant balanced aquarium and terrarium com- 

 bined. The green plants make and store food. Animals and cer- 

 tain non-green plants that cannot make their own food live on 

 this stored food. In the bodies of these animals and plants the 

 food is broken up into new compounds a part of which is returned 

 to the soil to be recombined into food by the green plants. These 

 processes are continuous. 



Starch and sugar, the carbohydrate foods, are broken up in 

 the bodies of man and animals into carbon dioxide and water. 

 The carbon dioxide is returned to the air, and the water to the 

 soil. Both are taken up again and remade into food by the green 

 plants. Protein foods are broken up into compounds containing 

 nitrogen. These are returned to the soil, and are taken up by 

 plants to make more protein food. 



These processes have been going on for ages and will continue 

 as long as climatic conditions permit. They are never-ending 

 cycles and insure the retention of a balanced world. 



