2b'2 VEGETABLE 



proportionate to the number of stomata which the plant con- 

 tains It is a process which lakes place only in a living 

 plant , for if a leaf be bruised so as to destroy its organiza- 

 tion, and consequently its vitality, its substance is no longer 

 capable either of decomposing carbonic acid gas under the 

 influence of solar light, or of absorbing oxygen in the dark. 

 Neither the roots, nor the flowers, nor any other parts of the 

 plant which have not this green substance at their surface, 

 are capable of decomposing carbonic acid gas : they pro- 

 duce, indeed, an effect which is in some respects the oppo- 

 site of this ; for they have a tendency to absorb oxygen, and 

 to convert it into carbonic acid, by uniting it with the carbon 

 they themselves contain. This is also the case with the 

 leaves themselves, 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 mattei 

 in the plant, forming with it carbonic acid : this carbonic acid 

 is in part exhaled ; but the greater portion either remains at- 

 tached 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 presented 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 with the unity of plan, 

 which 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 necessary, in order to 

 produce the result intended. 



Thus the great object to be answered by this vegetable 

 aeration is exactly the converse of that which 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 discharging the superfluous quantity 

 of carbon from the animal system. The absorption of oxy- 

 gen, and the partial disengagement of carbonic acid, which 

 constitute the nocturnal changes efYected by plants, must have a 

 tendency to deteriorate the atmosphere with respect to its capa- 

 bility of supporting animal life ; but this effect is much more 

 than compensated by the greater quantity of oxygen given out 



