CHAPTER XI 



THE RELATIONS OF PLANTS TO ANIMALS 



Problems: To determine the general biological relations existing 

 between plants and animals, as shown in a balanced aquarium. 

 To understand the meaning of symbiosis. 



Suggestions for Laboratory Work 



Demonstration of life in a "balanced" and in an "unbalanced" aquarium. 

 Determination of factors causing balance. 



Demonstration of some examples of symbiosis. 



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 es- 

 tablished between the plant 

 life on one side and animal 

 life on the other. Aquariums 

 containing green pond weeds, 

 either floating or rooted, a 

 few snails, some tiny animals 

 known as water fleas, and a fish 

 or two, if kept near a light win- 

 dow, will show this relation. 



We have seen that green 

 plants, in favorable conditions 

 of sunlight, heat, moisture, 

 and with a supply of raw food 

 materials, give off oxygen as a 

 by-product whfle 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 



102 



A balanced aquarium. Explain the term 

 "balanced." 



