CHAPTER III. 



ABSOPTION OF GASES. 



26. Gases used by the Plant. — Of the gaseous elements 

 which enter into the food of plants, hydrogen is taken up in 

 the form of water or ammonia by ordinary green plants, while 

 it forms a large proportion of the complex substances which 

 are used by parasitic and saprophytic plants. Oxygen is ob- 

 tained from the air in a free state, and in combination in the 

 form of water, carbon dioxide, and the mineral salts. Nitrogen 

 is derived chiefly from compounds in the soil. Leguminous 

 plants and many groups of the lower forms are able to take 

 up this element directly from the atmosphere. The greater 

 part of it used by the higher plants has been fixed in the soil 

 by the action of Bacteria and related forms. At the present 

 time the power of the various groups of plants to take up free 

 nitrogen is not clearly defined. Carbon is obtained by plants 

 which do not contain chlorophyll from the complex compounds 

 which they use as food. This is true of all plants which use 

 complex foods. Green plants, however, obtain their carbon 

 supply from the carbon dioxide of the air. (§ 29.) 



27. Diflfusion of Gases. — If two gases that will mix are sepa- 

 rated by a membrane, they will pass through the membrane by 

 osmose in the same manner as liquids. The air is a mixture 

 of 77.95 parts of nitrogen, 20.61 parts of oxygen, 1.40 parts 



35 



