AERATION or THE SAP. 20 



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 water which conveys it into the interior of the 

 plant. Hence the necessity of its having to undergo 

 this double operation of first combining with oxy- 

 gen, and then being precipitated from its combina- 

 tion in the manner above described. It 'is not the 

 whole of the carbon introduced into the vegetable 

 system, 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 Aeration. 



Thus the great object to be answered by this 

 vegetable aeration is exactly the converse of that 

 which we shall afterwards see is effected by the 

 respiration of animals: in the former it is that of 

 adding carbon, in an assimilated state, to the vege- 

 table 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 disengagement of carbonic acid, which 

 constitute the nocturnal changes effected by plants, 

 must have a tendency to deteriorate the atmo- 

 sphere with respect to its capability of supporting 

 animal life ; but this effect is much more than 

 compensated by the greater quantity of oxygen 

 given out by the same plants during the day. On 

 the whole, therefore, the atmosphere is continually 



