OF THE PRECEDING CHAPTERS. 177 



CHAPTER in. 



Sources of the food of plants. 



Their organic food comes chiefly from the air. 



Carbonic acid, a gas, heavy, extinguishes combus« 

 tion, fatal to life; no color, slight acid taste, and pe- 

 culiar smell. Furnishes carbon to plants. 



This gas is absorbed from the atmosphere by day, 

 through the leaves, and oxygen is at the same time 

 given off; asVoth of carbonic acid exists in the 

 air. 



How the supply of it is kept up; combustion, re- 

 spiration, decomposition. 



The hydrogen of plants is obtained from water. 



The oxygen comes from water, carbonic acid, and 

 almost every form of food. 



Nitrogen is supplied by ammonia and nitric acid. 



Ammonia, a gas, gives the smell to aqua ammonia, 

 and to smelling salts. 



Nitric acid, common aqua fortis. 



CHAPTER IV. 



Of the organic substance of plants ; structure of the 

 stem, the roots, and the branches. 



Principal bodies which make up the organic part 

 of plants. 



Woody fibre the most abundant of all, in stems, 

 stalks, leaves, etc. 



Starch, the leading substance in seeds, and in many 

 tubers. 



Sugar. Gum. Oils. Their nature and import- 

 ance. 



These all composed of carbon, hydrogen, and oxygen 

 only, the two latter being in the proportions to form 



