32 THE VITAL FUNCTIONS. 



be proportionate to the number of stomata which 

 the plant contains. It is a process which takes 

 place only in a living plant ; for if a leaf be 

 bruised so as to destroy its organization, 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 produce, indeed, an effect which is in some 

 respects the opposite 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, ab- 

 sorb a portion of that element. The oxygen 

 thus absorbed enters immediately into combina- 

 tion 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 

 presented to the leaf, when daylight returns, 

 and when a fresh decomposition is again effected. 



