XII. THE RELATIONS OF PLANTS TO ANIMALS 



Problems. — To determine the general biological relations tw- 

 isting between plants and animals. 



{a) As shown in a balanced aquarium. 

 ib) 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 ani- 

 mal and plant Life. 



Tabular comparison between balanced aquarium and hay infusion. 



To THE Teacher. — The gap between plants and animals is not a wide one. 

 The bridging of the gap is undertaken by means of the exercises which follow. 

 First the pupil is led to see the interdependence of organisms on the earth ; then 

 the dependence of one kind of organism upon another ; and then he is brought face 

 to face with the fact that there are two kinds of organisms, one constructive, the 

 other destructive. These, he learns, may both live in a small aquarium jar and 

 they may both be single cells. 



JProhlem 108: To study some biological relations of plants 

 and animals in a balanced aquarium. 



Materials. — A balanced aquarium containing; livin«i; ^iccn 

 plants, fish, tadpoles, snails, and other forms of animal life. 



Observations. — Watch the animals within the aquarium to see 

 if any are feeding. Note what they eat, also that the fish are 

 continually openinp; their mouths as if hitinp;. What mij^ht the 

 fish be taking from the water (not food) ? In an a(iuarium placed 

 in the sunlight, what gas is given off from the green j^lants? How 

 might this gas be useful to animals? 



Does this explain the action of the fish mentioned above ? What 

 gas is given off by animals that plants would use under certain 

 conditions? Are these conditions present? Fish and other aiii- 



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