APPENDIX A 

 THE GROWTH OF PLANTS AND ANIMALS 



Food of plants. As plants grow, they are constantly 

 taking material from the world about them and build- 

 ing it into themselves. The most common source of 

 material for their use is that sea of air called the atmos- 

 phere, at the bottom of which they live. In it are found 

 mainly oxygen, nitrogen and carbon dioxide, with slight 

 amounts of such other gases as ammonia and coal gas. 



Oxygen (O). Oxygen is an invisible gas which 

 forms about 20 per cent of atmospheric air. It is capa- 

 ble of union with other substances, thereby producing 

 light and heat. This union is called combustion or 

 oxidation. 1 Oxygen is given off to the air by plants 

 when, in order to get the carbon and hydrogen which 

 they need, they separate the carbon dioxide (CO,) of 

 the air into carbon and oxygen, and the water (H a O) 

 into hydrogen and oxygen. Oxygen is thus a waste 

 product from plant growth. 



Nitrogen (N). Nitrogen is an invisible gas which 

 forms about 80 per cent of atmospheric air. It serves 

 to dilute the oxygen of the air and thereby prevents 

 over-rapid combustion, which otherwise would mean the 

 destruction of all things. Plants unite nitrogen with 

 carbon, hydrogen and sulphur, and thereby build up the 

 flesh-forming food of animals. Nitrogen as found in 



1 The chemical compounds formed by oxidation are called oxides. 

 Water (H a O) is an oxide of hydrogen; carbon dioxide (C0 3 ), of 

 carbon; and iron rust (FeaOa), of iron. 



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