10 FIRST PRINCIPLES OF AGRICULTURE. 



different grouping or combining of the constituents in 

 the plant. 



Source of Plant-food Elements. Plants derive the 

 elements of their growth from two sources, the atmos- 

 phere and the soil. The atmosphere, while the original 

 source of carbon, hydrogen, oxygen, and nitrogen, fur- 

 nishes direct chiefly the element carbon. Hydrogen and 

 oxygen are furnished mainly through the soil in the 

 form of water; though the original source is the atmos- 

 phere, where they exist in the form of "water vapor." 

 Nitrogen is also largely taken up by the roots of plants ; 

 though certain classes of plants, as we shall see later, 

 have the power, under proper conditions, of obtaining it 

 from the atmosphere, of which it constitutes about four- 

 fifths of the whole bulk. 



The Proportion of Food derived from the Air. 

 These constituents derived from the atmosphere constitute 

 about ninety-five per cent of the total dry matter of 

 plants, of which carbon constitutes nearly one-half; the 

 remaining five per cent is the " incombustible " or ash 

 constituents. The soil is the exclusive source of these 

 elements; they are, however, indispensable, for without 

 them the carbon of the atmosphere, the hydrogen and 

 oxygen of the water, and the nitrogen of the soil or at- 

 mosphere, would not have been able to enter into plant life. 

 The atmospheric constituents are usually termed the 

 " organic ; " and the soil constituents, the " inorganic," 

 or ash elements. This distinction, though not entirely 

 accurate, serves a good purpose in helping to get a proper 

 understanding of the relations of the compounds formed 

 in the plant. 



