THE SOURCES OF PLANT-FOOD. 



39 



prepare material to take its proper place in the build- 

 ing up of animal and vegetable structures. 



81. The atmosphere acts upon both organic and 

 inorganic matter so as to reduce it to simpler forms- 

 As soon as an animal or plant dies, by contact with 

 the air its elements quietly but surely undergo a 

 change, recombining to form new and simpler com- 

 pounds capable of entering again into organic bodies. 



82. Knowing the composition of the soil, of 

 plants, of air, we are prepared to study their relations 

 and the laws of growth and development in plants. 



CHAPTER V. 



THE SOURCES OF PLANT-FOOD AND HOW OBTAINED. 



83. THE earth and atmosphere are, of course, the 

 only possible sources of food for plants, but the in- 

 teresting questions arise, What portion of food does 

 each furnish ? and how is it taken up and used by the 

 plant ? If we include hydrogen, which is one of the 

 elements in watery vapor and in ammonia, there are 

 four elements in air, viz., oxygen, nitrogen, carbon, 

 and hydrogen. A great many experiments have been 

 made to find out whether these four elements enter 

 directly from the air into plants, through the leaves, 

 or are taken in by the roots from the soil. 



84. Carbon, the most abundant element in or- 

 ganic substances, cannot enter the plant in a pure 



