THE SOIL AS RELATED TO PLANTS. 79 



more numerous, but they form a very s'mall per 

 cent, of the quantity of plant tissues (not over 

 four or five per cent, altogether), while the ele- 

 ments obtained indirectly or directly from the 

 air form 95 per cent, or more of the quantity, 

 i. Air-derived Elements. 



(1) CARBON. Nearly half of the solid mate- 

 rial of plants is carbon. It is found in the oils, 

 starch, sugar, and albuminoids. \ The leaves take 

 in carbon dioxide from the air and decompose 

 it (in the light) into its elements, carbon and 

 oxygen, building up other compounds with the 

 carbon and giving off the greater part of the 

 oxygen. 



(2) OXYGEN too may be directly taken from 

 the air by leaves, buds, and flowers, or by the 

 roots. It is also taken in in large quantities in 

 the water absorbed. Oxygen forms a part of 

 nearly all the compounds found in plants. 



(3) HYDROGEN, in combination with oxygen 

 forming water, is an important element in 

 plants. There is no other compound so abun- 

 dant in plants as that of water, and none whose 

 function is more important, since it holds in so- 

 lution other elements, or compounds, of plant- 

 foods, and acts as a medium for transporting 

 them to every tissue and cell of the plant. 



(4) NITROGEN is an essential element in all the 

 green and woody parts of plants in fact, of all 

 the protoplasm, or living substance, of the plant. 



