XII. THE RELATIONS OF PLANTS TO ANIMALS 



Problems. To determine tlw general biological relations ex- 

 isting between plants and animals. 



(a) As shown in a balanced aquarium. 

 (&) 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. 



Problem 1O8 : To study some biological relations of plants 

 and animals in a balanced aquarium. 



Materials. A balanced aquarium containing living green 

 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 opening their mouths as if biting. What might the 

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

 in the sunlight, what gas is given off from the green plants? 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 ani- 



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