AIR AND SUNLIGHT 15 



exhale carbonic dioxide and at other times oxygen, according to 

 the intensity of the light and the state of development of the plant. 



Oxygen is the life-giving principle in the air for man and animals. 

 It is the most abundant substance of the earth, and comprises by 

 volume about one fifth of our atmosphere. It is present in both 

 plants and animals, both free and also in combination with other 

 elements. Oxygen is just as essential for plants as for animals and 

 it plays an important part in the chemical reactions that occur in 

 the processes by which new cells are built up from the materials 

 elaborated in the older cells. It has been found that seed will not 

 germinate or sprout in the absence of oxygen. Young plants 

 when deprived of it soon wither and perish. The young buds 

 of trees on opening in the spring absorb oxygen from the air and 

 experiments show that they wither and decay when confined in an 

 atmosphere from which the oxygen has been removed. Oxygen 

 is also taken up by flowers and ripening fruit. Mushrooms and 

 lichens absorb oxygen very freely from the air and give off a corre- 

 sponding quantity of carbon dioxide. 



The amount of oxygen contained in vegetable matters is really 

 very large, in fact it is much larger than we might suspect on first 

 thought. About seventy-five pounds of every hundred pounds 

 of vegetable matter consists of water, and since oxygen comprises 

 eight ninths of the weight of water, it is evident that the oxygen 

 from this source alone would amount to 66| pounds or to 66f 

 per cent. When we add to this the amount of oxygen found 

 in combination with other elements in plants and animals we can 

 appreciate the important role it plays in the development of all 

 forms of life. 



Nitrogen is necessary for both plant and animal growth. Ani- 

 mals obtain their supply of nitrogen from plants and from animal 

 food. Nitrogen inhaled with the oxygen is exhaled practically 

 unchanged with the carbon dioxide. Leguminous plants, such as 

 clover, alfalfa, and cowpeas, obtain a part of their supply of nitro- 

 gen from the air, but other families of plants secure nitrogen from 

 certain nitrates in the soil, such as the nitrates of potash, lime, 

 soda, and ammonia. 



Hydrogen does not exist in the atmosphere in a free state, but is 

 found combined with other elements. It unites with oxygen to 



