30 THE VITAL FUNCTIONS. 



pears to 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 oxy- 

 gen, 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 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. 



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 or- 

 der to produce the result intended. 



The water which is absorbed by the roots generally car- 

 ries with it a certain quantity of soluble animal or vegetable 

 materials, which contain carbon. This carbon is transmit- 

 ted to the leaves, where, during the night, it is made to 

 combine with the oxygen they have absorbed. It is thus 



