12 FIRST PRINCIPLES OF AGRICULTURE. 



their influence, are shown by omitting in each case, in 

 another series of experiments, one of the mineral ele- 

 ments; the crop secured in each experiment is found to 

 be imperfect. Perfect growth is only reached when all 

 the elements named are present. 



Food obtained from the Atmosphere. The leaves 

 and roots are called organs of nutrition; the leaves take 

 material from the air, the roots from the soil. 



The dry substance of a plant is made up chiefly of 

 carbon, and the proper absorption of this element depends 

 upon the activity of the leaves. The leaf is made up of 

 rows of cells, placed side by side, which in the under 

 portion are loosely held together, forming "air spaces" 

 between them, and over the whole leaf there is a thin, 

 transparent skin. This skin, called the "epidermis," is 

 not entire, but is dotted with little holes called " stomata ; " 

 through these the atmospheric air passes freely into the 

 cellular spaces in the leaf, and through the porous walls 

 of the leaf cells, which contain a green substance called 

 "chlorophyll." 



In these cells, containing chlorophyll, the carbonic acid 

 of the air is broken up through the influence of light and 

 warmth, and the oxygen set free and the carbon retained. 

 This process is called "assimilation," and is active only 

 during the daytime ; during the night the food, formed by 

 day, undergoes a change, whereby it can be transferred in 

 solution to the places where it is needed. Daylight, as 

 also the presence of iron in the plant, is necessary for 

 the formation of chlorophyll. 



In the daytime, growing plants purify the air by con- 

 suming the carbon ; in the night, the process is reversed, 



