INFLUENCE OF THE ATMOSPHERE. 75 



Stirred about them. The oxygen of the air is thus either 

 brought into direct contact with all parts of them, so as to 

 answer the conditions of some one of the above theories, or 

 else it is thus brought into contact with the humus, and pro- 

 motes its decay and conversion into vegetable food. 



The leaves of plants also absorb oxygen from the atmos- 

 phere, especially during the night season, and sometimes in 

 the shade, at the same time they transpire carbonic acid, and 

 the volume of carbonic acid thrown off, is just equal to that 

 of the oxygen consumed. The changes which take place in 

 this process are also subjects of theory, although it is pretty 

 well established that the last of the three following is the true 

 one. 



First theory. The oxygen, absorbed by the leaves, enters 

 into combination with the carbon, formerly introduced into 

 the sap of the plant, and there being no light to decompose 

 the carbonic acid thus formed, it is exhaled, or given back to 

 the atmosphere. This is precisely the change, which is sup- 

 posed by some to take place in the lungs of animals, and is 

 a true process of respiration. Some of the oxygen however 

 must remain uncombined in the juices, as the amount of 

 Jg part of the quantity absorbed can be disengaged from the 

 plant by means of heat. 



Second theory. The oxygen which is absorbed by the 

 leaves, enters into combination with the hydrogen of the wa- 

 ter which the vital power decomposes, in the same manner 

 as when introduced into the roots. So that water is decom- 

 posed by the plant, its oxygen assimilated, while at the same 

 time, the hydrogen combines with the absorbed oxygen, and 

 forms water. This, however, is a very doubtful theory of 

 these changes, though a possible one. 



Third theory. The oxygen thus absorbed, combines with 

 the vegetable substances in the leaves. This appears to be a 

 purely chemical process, as it takes place equally well in the 

 dead, as in the living plant. If the substance of the leaves 



