160 THE RELATIONS OF PLANTS TO ANIMALS 



We have seen that green plants under favorable conditions of 

 sunlight, heat, moisture, and with a supply of raw food materials, 

 give oft oxygen as a by-product while manufacturing food in their 

 green cells. We know the necessary raw materials for starch 

 manufacture are carbon dioxide and water, while nitrogenous 

 material is necessary for the making of proteins within the plant. 



A balanced aquarium. Explain the term " balanced." 



In previous experiments we have proved that carbon dioxide is 

 given off by any living thing when oxidation occurs in the body. 

 The crawling 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 car- 

 bon dioxide. The green plants in the daytime use up the carbon 

 dioxide obtained from the various sources and, with the water 



