XII. THE RELATIONS OF PLANTS TO ANIMALS 



Problems. — To determine the general biological relations ex- 

 isting between plants and animals. 



(a) As shown in a balanced aquarium. 



(b) As shown in hay infusion. 



Suggestions for Laboratory Work 



Demonstration of life in a ^^ balanced^' and ^' unbalanced ^^ aquarium. — 

 Determination of factors causing balance. 



Demonstration of hay infusion. — Examination to show forms of animal 

 and plant life. 



Tabular comparison between balanced aquarium and hay infusion. 



Some Ways in which Plants affect Animals. — We have been 

 studying the life of plants in order better to understand the life 

 of animals and men. We have seen first that green plants play 

 indirectly a tremendous part in man's welfare by supplying him 

 with food. We have found that the colorless plants directly 

 affected his welfare by causing disease, and by causing decay, 

 thus making usable the nitrogen locked up in dead bodies of plants 

 and animals, and by some even supplying nitrogen from the at- . 

 mosphere. The dependence of animals upon plants has been 

 shown and the interdependence of plants on animals has also been 

 seen in cross-pollination and in the supply of raw food materials 

 to plants by animals. 



Study of a Balanced Aquarium. -7- 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. 



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