AERATION OF THE SAP. 29 



of the cells or vessels, to which they are con- 

 ducted from the leaves. This, then, is the first 

 modification in the qualities of the sap which it 

 undergoes in those organs. 



§ 4. Aeration of the Sap. 



A CHEMICAL change much more considerable 

 and important than the preceding is next effected 

 on the sap by the leaves, when they are sub- 

 jected 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 

 obtained directly from the surrounding atmo- 

 sphere. 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 vege- 

 table structures, may be considered as having 

 made a near approach to its complete assimi- 

 Intion, using this term in the physiological sense 

 already pointed out. 



The remarkable discovery that oxygen gas is 

 exhaled from the leaves of plants during the 

 day time, was made by the great founder of 

 pneumatic chemistry. Dr. Priestley : to Senne- 

 bier we are indebted for the first observation 



