XIV. THE RELATIONS OF PLANTS^ TO ANIMALS 



Froblem XXIV. Tlie general hiological relations existing he- 

 tweenplants and animals. {Laboratory Manual, Prob. XXZ"F.) 



(a) vl balanced aquarium. 



(b) Relations between green plants and animals, 

 ic) The nitrogen cycle. 



id) A hay infusion. 



Study of a Balanced Aquarium. — Perhaps the best way for us 

 to understand the interrelation between plants and animals is to 

 study an aquarium in which plants and animals live and in which 

 a balance has been established between the plant life on one side 

 and animal life on the other. Aquaria containing green pond 

 weeds, either floating or rooted, a few snails, some tiny animals 

 known as water fleas, and a fish or two will, if kept near a light 

 window, show this relation. 



We have seen that green plants under favorable conditions of 

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

 give off oxygen as a by-product while manufacturing food in the 

 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 proteids within the plant. 

 In previous experiments we have proved that carbon dioxide is 

 given off by any living thing Avhen 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, night and 

 day, 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 taken in, manufacture 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. 



But the plants are continually growing larger. The snails and 

 fish, too, eat parts of the plants. Thus the plant life gives food 



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