HOW THE PLANT USES THE FOOD IT MAKES 85 



put green leaves into the jars and place them in sun- 

 light, they change the air back again, so that a candle 

 will burn in it again. The part of the air which is 

 used up in burning or breathing, and which the green 

 leaves in sunlight put back again, is called oxygen. 



FIG. 44. The burning candle, the sprouting seeds, and the frog use up the 

 oxygen of the air so that nothing can burn in it. 



When animals breathe they use up oxygen and give 

 off carbon dioxide. Carbon dioxide is the air food 

 which the green leaves take up and use in making starch. 

 The green leaves give off oxygen in the sunlight, and 

 this is taken up by animals in breathing. 



What the plants give off is needed by animals: 

 what the animals give off is needed by plants. So in 

 making an aquarium we put in both animals and plants, 

 and then both do well. 



