28 THK VI'IAL FUNCTIONS. 



contain. This is ulso the case with the leaves them- 

 selves, whenever they are not under the influence 

 of light : thus, during the whole of the night, the 

 same leaves, which had been exhaling oxygen 

 during the day, absorb a portion of that element. 

 The oxygen thus absorbed enters immediately into 

 combination with the carbonaceous matter in the 

 plant, forming with it carbonic acid ; this carbonic 

 acid is in part exhaled ; but the greater portion 

 either remains attached to the substance of the leaf, 

 or combines with the fluids which constitute the 

 sap : in the latter case, it is ready to be again pre- 

 sented to the leaf, when daylight returns, and when 

 a fresh decomposition is again effected. 



This reversal at night of what was done in the 

 day may, at first sight, appear to be at variance 

 w ith the unity of plan, w^hich we should expect to 

 find preserved in the vegetable economy ; but a 

 more attentive examination of the process will show 

 that the whole is in perfect harmony, and that 

 these contrary processes are both of them neces- 

 sary, in order to produce the result intended. The 

 water which is absorbed by the roots generally 

 carries with it a certain quantity of soluble animal 

 or vegetable materials, which contain carbon. This 

 carbon is transmitted to the leaves, where, during 

 the night, it is made to combine with the oxygen 

 they have absorbed. It is thus converted into car- 

 bonic acid, which, when daylight prevails, is de- 

 composed ; the oxygen being dissipated, and the 

 carbon retained. It is evident that the object of 

 the whole process is to obtain carbon in that precise 

 state of disintegration, to which it is reduced at the 

 moment of its separation from carbonic acid by the 



