AERATION OF THE SAP. 31 



converted into carbonic acid, which, when daylight pre- 

 vails, 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 disinte- 

 gration, to which it is reduced at the moment of its separa- 

 tion 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 available 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 water which 

 conveys it into the interior of the plant. Hence the neces- 

 sity of its having to undergo this double operation of first 

 combining with oxygen, and then being precipitated from 

 its combination in the manner above described. It is not 

 the whole of the carbon introduced into the vegetable sys- 

 tem, in the form of carbonic acid, which has to undergo the 

 first of these changes, a part of that carbon being already in 

 the condition to which that operation would reduce it, and 

 consequently in a state fit to receive the decomposing action 

 of the leaves. The whole of these chemical changes may 

 be included under the general term ^Seration. 



Thus the great object to be answered by this vegetable 

 aeration is exactly the converse of that which we shall af- 

 terwards see is effected by the respiration of animals: in the 

 former it is that of adding carbon, in an assimilated state, to 

 the vegetable organization; in the latter, it is that of dis- 

 charging the superfluous quantity of carbon from the animal 

 system. The absorption of oxygen, and the partial disen- 

 gagement of carbonic acid, which constitute the nocturnal 

 changes effected by plants, must have a tendency to deteri- 

 orate the atmosphere with respect to its capability of sup- 

 porting animal life; but this effect is much more than com- 

 pensated by the greater quantity of oxygen given out by 

 the same plants during the day. On the whole, therefore, 

 the atmosphere is continually receiving from the vegetable 

 kingdom a large accession of oxygen, and is, at the same 

 time, freed from an equal portion of carbonic acid gas, both 



