THK ATMOSrUEKE AS KKLATBD TO VEGETATION. 99 



Not absorbed ( Nitrogen. 

 by Plants. ( Water in state of vapor. 



(Oxygen, [ by folia;;e and green parts, but only in the 

 Ozone? ) light. .. , _ 



M.\RSH Gas in traces by aquatic plants ? 

 j Water, «.s- vapor, from surface of plant at all times. 

 (.Carbonic Acid, from the growing parts at all times. 



CHAPTER II. 



THE ATMOSPHERE AS PHYSICALLY RELATED TO 

 VEGETATION. 



§ 1- 

 MANNER OF ABSORPTION OF GASEOUS FOOD BY THE PLANT. 



Closing here our study of the atmosphere considered as 

 a source of the food of plants, we still need to remark 

 somewhat upon the physical properties of gases in rela- 

 tion to vegetable life ; so far, at least, as may give some 

 idea of the means by which tliey gain access into the 

 phmt. 



Physical Constitution of the Atmosphere.— That the 

 atmosi)here is a mixture and not a chemical combination 

 of its elements is a fact so evident as scarcely to require 

 discussion. As we have seen, the proportions whicli sub- 

 sist among its ingredients are not uniform, although they 

 are ordinarily maintained within very narrow limits of va-= 

 riation. This is a sufficient proof that it is a mixture^ 

 Th« remarkable fact that very nearly the same relative 

 quantities of Oxygen, Nitrogen, and Carbonic Acid, 

 steadily exist in the atmosphere is due to tlie even balance 

 which obtains between growth and decay, between life 

 and death. The equally remarkable fact that .the gases 



