STUDY OF A BALANCED AQUARIUM 



103 



for the making of proteins within the plant. In previous experi- 

 ments we have proved that carbon dioxide is given off by hving 

 things when oxidation occurs in the body. The crawHng snails 

 and the swimming fish give off carbon dioxide, which is dissolved 

 in the water ; the plants themselves, at all times, oxidize food within 

 their bodies, and so must pass off some carbon dioxide. The green 

 plants in the daytime use up the carbon dioxide obtained from the 

 various sources and, with the water which they take in, manu- 

 facture starch. While this process is going on, oxygen is given off 

 to the water of the aquarium, and this free oxygen is used by the 

 animals there. 



The plants are continually growing; but the snails and fish 

 eat parts of the plants. Thus the plant life gives food to the 

 animals within the aquarium. 

 The animals give off certain 

 nitrogenous wastes. These ma- 

 terials, with other nitrogenous 

 matter from dead animals and 

 parts of the plants, form part of 

 the raw material used for protein 

 manufacture in the plant. This 

 nitrogenous matter is prepared 

 for use by several different kinds 

 of bacteria which first break the 

 dead bodies down and then give 

 the material to the plants in the 

 form of soluble nitrates. The 

 green plants manufacture food, 

 the animals eat the plants and give off carbon dioxide and nitro- 

 genous waste, from which the plants in turn make food and living 

 matter. The plants give oxygen to the animals, and the animals 

 give carbon dioxide to the plants. Thus a balance exists between 

 the plants and animals in the aquarium. Make a table to show 

 this balance. 



Relations between Green Plants and Animals. — What goes on 

 in the aquarium is an example of the relation existing between 

 green plants and animals. Everywhere in the world green plants 

 are making food which becomes, sooner or later, the food of animals. 



The carbon and oxygen cycle in the 

 balanced aquarium. Trace by means 

 of the arrows the carbon from the time 

 plants take it in as CO2 until animals 

 give it off. Show what happens to the 

 oxygen. 



