CHAPTER XIV 



FOOD ELABORATION AND RESPIRATION 



170. Sources of Raw Material. The ordinary green plant, 

 as we have seen, secures water and certain substances from 

 the soil. It also secures from the air raw material which it 

 utilizes in the elaboration of food material. When a plant is 

 thoroughly dried in an oven, the water passes off; this water 

 came from the soil. The remaining part is called dry sub- 

 stance or dry matter. If the dry matter is burned in an ordi- 

 nary fire, only the ash remains; this ash came from the soil. 

 The part that passed off as a gas in the burning contained 

 the elements that came from the air. It also contained 

 some of those that came from the soil all those (as nitrogen, 

 hydrogen, chlorin) that are transformed into gases by the 

 heat of a common fire. 



171. Carbon. Carbon enters abundantly into the com- 

 position of all plants. Note what happens when a plant 

 is burned without free access of air, or smothered, as in a 

 charcoal pit. A mass of charcoal remains, almost as large 

 as the body of the plant. Charcoal is almost pure carbon, 

 the ash being so small in proportion to the large amount 

 of carbon that we look on it as an impurity. Half or more 

 of the dry substance of a tree is carbon. The carbon goes 

 off as a gas when the plant is burned in air. It does not go 

 off alone, but in combination with oxygen, and in the form 

 called carbon dioxid gas, CO 2 . 



172. The green plant secures its carbon from the air. 

 In other words, much of the solid matter of the plant comes 

 from one of the gases. By volume, carbon dioxid forms 

 only about three-hundredths of 1 per cent of the air. It 



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