RELATION OF PLANT TO ANIMAL LIFE 3 



hydrochloric acid, marsh gas, sulphuretted hydrogen, all acids 

 and most organic (animal and vegetable) compounds contain 

 this element. It is necessary for plant and animal life and it is 

 used by animals in the form of water and as a constituent of 

 feeds. 



Oxygen. About one-fifth of the atmosphere is made up of 

 this element, in the free state, mechanically mixed with nitrogen. 

 It is found in enormous quantities in combination with other 

 elements. It constitutes about eight-ninths by weight of water 

 and nearly one-half of the earth's crust. All combustion and 

 decay require oxygen. The plant stores up oxygen in combina- 

 tion with other elements and it enters into many of the com- 

 pounds of the animal body. Without oxygen plants and animals 

 would die. The plant takes in oxygen in combination with car- 

 bon as carbonic acid gas, through the openings on the under 

 sides of the leaves ; the carbon is absorbed and the excess of 

 oxygen given oft'. The animal inhales air which contains oxygen, 

 which serves to purify the blood, and exhales carbonic acid which 

 is thrown off by the blood through the lungs. So the supply of 

 oxygen and carbon is continually being used and formed. 



Nitrogen. About four-fifths of the atmosphere is made up 

 of nitrogen in the free state. In combination this element is 

 found in many substances such as ammonia, sodium nitrate, 

 potassium nitrate, and many organic compounds. Certain plants 

 namely the legumes, of which the pea, bean, alfalfa, clovers, 

 cowpea, soy bean, etc. are members, have the power of gather- 

 ing nitrogen from the air, by means of certain growths (tuber- 

 cles) on their roots. Our other plants are not capable of obtain- 

 ing nitrogen in the free state. This element is one of the most 

 important for us to consider. When in combination with other 

 elements in plants, it is one of our most valuable compounds for 

 animals' food. In fertilizers it is the most expensive and fugitive 

 of essential elements. It tends to produce vigorous growth of 

 plants. 



Carbon is found in the free state in charcoal, graphite and 

 diamonds. In combination with oxygen we find carbon as car- 

 bon dioxide (carbonic acid gas) in the air. It is given off by 



