AERATION OF TflE 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 subjected 

 to the action of light. It consists in tJie decom- 

 position of the carbonic acid gas, which is either 

 brought to them by the sap itself, or obtained 

 directly from the surrounding atmosphere. In 

 either case its oxygen is separated, and is dis- 

 engaged 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 ap- 

 proach to its complete assimilation, 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 



