Vegetable Physiology. 305 



and the atmosphere. The sprig of mint was exposed to the 

 light of the sun for twelve days consecutively : at the end of 

 each day the carbonic acid was seen to diminish in quantity, 

 t he water rising in the jar to supply the place of what was lost; 

 and at the same time the plant exhaled a quantity of oxygen 

 exactly equal to that of the carbonic acid which had disap- 

 peared. A similar sprig of mint, placed in a jar of the same 

 size filled with distilled water, but without access to carbonic 

 acid, gave out no oxygen gas, and soon perished. When, in 

 another experiment, conducted by means of the same appara- 

 tus as was used in the first, oxygen gas was substituted in the 

 first jar instead of carbonic acid, no gas was disengaged in the 

 other jar, which contained a sprig of mint. It is evident, there- 

 fore, that the oxygen gas obtained from the mint in the first 

 experiment, was derived from the decomposition by the leaves 

 of the mint, of the carbonic acid which the plant had absorbed 

 from the water. 



It is in the green substance of the leaves alone that this pro- 

 cess is conducted ; a process, which, from the strong analogy it 

 bears to a similar function in animals, may be considered as the 

 respiration of vegetables. 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 in some respects the opposite of this ; for they 

 have a tendency to absorb oxygen, and to convert it into car- 

 bonic acid, by uniting it with the carbon they themselves con- 

 tain. This is also the case with the leaves themselves, when- 

 ever they are not under the influence of light. Thus, during 

 the whole of the night, the same leaves which had been ex- 

 haling oxygen all 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 



