74 BIOLOGY OF PLANTS. 



of air, and carbonic acid will be given off without altering 

 the volume of the air ; but if they are placed in the open air, 

 they will absorb a volume of oxygen equal to themselves, as 

 in the first instance ; hence, the atmosphere abstracts the 

 carbonic acid which the roots form.* 



If the roots are connected with the stems and leaves, they 

 will constantly absorb oxygen, and the quantity will amount, 

 in time, to much more than their volume, because the carbon- 

 ic acid which they form passes into the juices, ascends to the 

 leaves, where it is decomposed by the action of light, or tran- 

 spired with the water, if the plant is in the shade ; hence, they 

 never become saturated. But these facts are equally well ac- 

 counted for on other theories. 



Second theory. The oxygen of the air, which is absorbed 

 by the roots, combines with the hydrogen of the water, which 

 the vital power decomposes, while the oxygen of the decompo- 

 sed water combines with carbon and forms carbonic acid ; 

 hence, the agency of the oxygen of the air is to keep up the 

 supply of water. 



Third theory. The oxygen thus absorbed by the roots, is 

 directly assimilated to the vegetable products, or, if any chan- 

 ges take place, the oxygen is neither converted into carbon- 

 ic acid by combining with the carbon, nor into water by unit- 

 ing with the hydrogen of the decomposed water. These 

 changes may take place, but the theory supposes that the oxy- 

 gen in some forin is assimilated to the vegetable organs. 



Fourth theory. It is possible, however, that neither of 

 the above theories explain the reason of the necessity of oxy- 

 gen to the roots of plants, in order to promote their growth. 

 The oxygen of the air effects changes upon the humus of the 

 soil, in preparing the food, and this may be the reason for its 

 influence. 



But whatever view is taken, the agency of oxygen upon 

 the roots of plants, explains the reason why the earth must be 



^ Chaptal. 



