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SCHOOL AND HOME GARDENING 



turn is used by animals in breathing ; it also helps to produce 

 combustion wherever fires are started. 



During darkness plants do not break up carbon dioxide, 

 but actually produce it, using some oxygen as animals do. 



10. Balance Between Plants and Animals. — In a large can or, 

 glass jar make an aquarium as shown in figure 41. Several kinds 

 of water plants secured from ponds or lakes are planted in clear sand 

 in the bottom of the jar. Pollywogs, water snails, small fish, and 

 water insects, such as dragon-fly larvse, may be placed in the jar. 

 When all is ready, allow the aquarium to stand for several days 

 in a, very light place. As the plants begin to grow they will give 



Fia. 41. 



-A sunlight aquarium. The plants furnish oxygen and the animals use it. 

 The animals produce oarbon dioxide and the plants use it. 



off oxygen to the water. This in turn is breathed by the small ani- 

 mals present. If the aquarium is nicely balanced in the two main 

 forms of life, the supply of carbon dioxide given off by the animals 

 will cause the plants to thrive, and the oxygen supplied by the plants 

 to the animals will satisfy their breathing requirements. If any of 

 the fish come to the surface to breathe there are probably too many 

 animals present. The inside of the aquarium may be kept clean by 

 rubbing with a stick wrapped with cloth. Snails and pollywogs 

 are scavengers and will eat much of the waste matter inside the glass 

 and on the plants. Too much food should not be supplied the animals, 

 as this pollutes the water. 



