THE ATMOSPHERE AS RELATED TO VEGETATION. 99 
Not absorbed 
by Plants. 
NITROGEN. 
WATER in state of vapor. 
OXYGEN, | ee foliage and green parts, but only in the 
ieee 
Exhaled by | 02082! light. 
Plants , MARSH Le in traces by aquatic plants ? 
‘ fovea as vapor, from surface of plant at all times. 
CaRBONIC ACID, from the growing parts at all times. 
CHAPTER IL. 
THE ATMOSPHERE AS PHYSICALLY RELATED TO 
VEGETATION. 
o£ 
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 nced 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 they gain access into the 
plant. 
Physical Constitution of the Atmosphere.—That the 
atmosphere is a mixture and not a chemical combination 
of its elements isa fact so evident as scarcely to require 
discussion. As we have seen, the proportions which 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. 
The remarkable fact that very nearly the same relative 
quantities of Oxygen, Nitrogen, and Carbonic Acid, 
steadily exist in the atmosphere is due to the even balance 
which obtains between growth and decay, between life 
and death. The equally remarkable fact that the gases 
