PHYSIOLOGY. 281 



at those times only when nutriment is required for the rear- 

 ing of their young. 



AERATION OF THE SAP. 



A chemical change of great importance is effected on the 

 sap by the leaves, when they are subjected to the action of 

 light. It consists in the decomposition of the carbonic acid 

 gas, which is either brought to them by the sap itself, or ob- 

 tained directly from the surrounding atmosphere. In either 

 case its oxygen is separated, and disengaged in the form of 

 gas ; while its carbon is retained, and composes an essential 

 ingredient of the altered sap, which, as it now possesses one 

 of the principal elements of vegetable structures, may be 

 considered as having made a near approach to its complete 

 assimilation, using this term in the physiological sense already 

 pointed out. 



Two glass jars were inverted over the same water-bath ; 

 the one filled will carbonic acid gas, the other filled with 

 water, containing a sprig of mint ; the jars communicating 

 below by means of the water-bath, on the surface of which 

 some oil was poured, so as to intercept all communication be- 

 tween the water and the atmosphere. The sprig of mint 

 was exposed to the light of the sun for twelve days consecu- 

 tively : at the end of each day the carbonic acid was seen 

 to diminish in quantity, the 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 car- 

 bonic acid which had disappeared. A similar sprig of mint, 

 placed in a jar of the same size, full of distilled water, but 

 without having access to carbonic acid, gave out no oxygen 

 gas, and soon perished. When, in another experiment, con- 

 ducted by means of the same apparatus as was used in the 

 first, oxygen gas was substituted in the first jar instead of 

 carbonic acid gas, no gas was disengaged in the other jar, 

 which contained a sprig of mint. It is evident, therefore, 

 that the oxygen gas obtained from the mint in the first ex- 

 periment was derived from the decomposition, by the leaves 

 of the mint, of the carbonic acid, which the plant had ab- 

 sorbed 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. The effect appears to be 



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