84 CHYMISTRY APPLIED TO AGRICULTURE. 



Plants growing upon marshes and bogs, and conse- 

 quently surrounded the greater part of the time by an 

 atmosphere of vapor, consume less oxygen gas than most 

 other herbaceous plants. In general, the quantity of oxy- 

 gen absorbed by plants, is in proportion to the fertility of 

 the soil in which they grow, and to the quantity of gas 

 contained in the air by which they are surrounded. 

 These inferences have been drawn from the results of nu- 

 merous experiments made by M. de Saussure. 



Healthy roots, separated from their stems, and placed 

 under a bell-glass, diminish the volume of atmospheric air, 

 and form carbonic acid with the surrounding oxygen : in 

 this case they never absorb a volume of oxygen greater 

 than their own. If a root, thus saturated, be placed under 

 another receiver filled with common air, it will form car- 

 bonic acid without changing the volume of the air ; but 

 if it be then exposed to the open air, it will absorb a quan- 

 tity of oxygen gas nearly equal to its volume, as when it 

 was enclosed under the first receiver ; which proves that 

 free atmospheric air can take from roots the carbonic acid 

 which they form. 



It is plain, then, that roots exercise the same action, in 

 regard to oxygen, that leaves do, though they absorb less 

 of it. The only important dflfference is, that the roots do 

 not decompose the carbonic acid ; this office appears to be 

 confined to the leaves, to which the acid is transported, to 

 be decomposed by the solar rays. 



When the root is not separated from the stem, the re- 

 sults differ from the above ; in the last instance, the root 

 absorbs more than once its volume of oxygen ; the reason 

 of this is very simple : the carbonic acid, as soon as it is 

 formed, is dissolved in the juices of the root, passes from 

 that into the stem, thence into the leaves, in which its de- 

 composition is principally performed ; so that the root parts 

 with the carbonic acid as soon as it is formed, and, though 

 it is constantly producing, is never surcharged with it. 



Not only do the roots absorb oxygen from the atmospher- 

 ic air which penetrates to them, but they disengage that 

 which always exists in the water by which they are moist- 

 ened. This leads to the explanation of a fact which I 

 have often observed. When the roots of almost any tree 

 have become surrounded by stagnant water, enclosed be- 

 neath the soil, and secluded from the access of atmospheric 

 air, the tree soon begins to languish, and the leaves to turn 



