198 Bureau ok Farmers' Institutes. 



plants, such as starch, fibre, sugar, fat or oil, proteids, acids, etc. 

 Hence, its function is to supply its necessary part of the material 

 found in such compounds. Plants take in their supply of caruon 

 dioxide chiefly through their leaves directly from the surround- 

 ing air. In the cells of the leaves, the carbon dioxide, in the 

 presence of sunshine, is separated into its elements, its carbon unit- 

 ing with other elements to form various compounds, such as 

 starch, sugar, woody fiber, etc., the oxygen being largely returned 

 to the air as free or uncombined oxygen. 



Hydrogen. 



(a) Source. — The element hydrogen is nearly always found 

 combined with other elements. It combines with oxygen to form 

 water. Hydrogen also occurs in most animal and vegetable sub- 

 stances, such as various kinds of wood, fruits, etc., in which it 

 is combined with the elements, carbon and oxygen. Hydrogen is 

 always present in all kinds of acids. 



(&) Description. — Hydrogen, in the uncombined form, is a 

 gas that resembles air in that it has neither color, smell nor 

 taste. 



(c) Relation to Fertilizers. — Hydrogen, as a constituent of 

 water has most important work in plant nutrition, and this will 

 be considered a little later. 



(d) Specific Functions in Flant Growth. — This is considered 

 under the subject of water. 



Oxygen. 



(a) Source. — Oxygen is the most abundant of all the elements. 

 The compounds which contain no oxygen are few in number. 

 Oxygen forms nearly one-half of the crust of the earth; eight- 

 ninths of water; about one-fifth of air, and one-third of all animal 

 and vegetable matter. 



Oxygen occurs in the air uncombined with other elements. 

 Oxygen, combined with the elements carbon and hydrogen, or 

 with carbon, hydrogen and nitrogen, is found in substances which 

 go to make up animals and vegetables. 



(b) Description. — As might be inferred from knowing that 

 oxygen in the uncombined state forms part of the air, oxygen 

 is a gas having no color, taste or smell. 



Oxygen is a very active substance from a chemical point of 

 view. It tends to unite with nearly all of the elements. In all 



