26 



NOURISHMENT OF PLANTS. 



ity he contemplates the infinite diversity and variety 

 he encounters in the wondrous beauties of the vege- 

 table world, how must he be filled with amazement 

 and admiration on contemplating the simplicity of 

 the means employed by Divine Omnipotence in pro- 

 ducing this multifarious variety. As the main pil- 

 lars, in a strict sense, upon which plants, nay, still 

 more, all living creatures upon the earth, build up 

 the structure of their bodies, four elements alone de- 

 serve special notice. They are called oxygen^ hydro- 

 gen^ carbon^ and nitrogen. Taken collectively, they 

 receive the more general appellation of organic con- 

 stituents, because they must be viewed as the prin- 

 cipal elements of all organic substances. They are 

 also denominated combustible elementary substances, 

 because upon heating in the air they entirely burn 

 away and disappear; that is, they are converted into 

 gaseous combinations. They may, again, be called 

 putrescible elementary substances, because they are 

 capable of corruption, putrescence, or putrefaction, by 

 which process they are equally converted, although 

 more slowly than by combustion, into gaseous com- 

 binations. Finally, they sometimes take the name 

 of atmospheric elementary substances, because they 

 are contained in atmospheric air. 



1. Oxygen, when uncombined, is an invisible kind 

 of air or gas, without taste or odor. We find it in 

 our atmospheric air, of which it constitutes one fifth. 

 Every one knows that men and animals cannot live 

 without air, and that vegetable and animal sub- 



