AERATION OF THE SAP. .33 



This reversal at night of what was done in the 

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

 with the unity of plan, which we should ex- 

 pect 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 necessary, 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 con- 

 tain 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 carbonic acid, which, 

 when daylight prevails, is decomposed ; 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 disin- 

 tegration, to which it is reduced at the moment 

 of its separation from carbonic acid by the action 

 of solar light on the green substance of the 

 leaves ; for it is in this state alone that it is avail- 

 able in promoting the nourishment of the plant, 

 and not in the crude condition in which it exists 

 when it is pumped up from the earth, along with 

 the v/ater which conveys it into the interior of 

 the plant. Hence the necessity of its having to 

 undergo this double operation of first combining 



VOL. II. D 



