at: RATI ON OF THE SAP. 25 



or vessels, to which they are conducted 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 the decompo- 

 sition 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 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. 



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 Sennebier we are in- 

 debted for the first observation that the presence 

 of carbonic acid is required for the disengagement 

 of oxygen in this process, and that the oxygen is 

 derived from the decomposition of the carbonic 

 acid ; and these latter facts have since been fully 

 established by the researches of Mr. Woodhouse, 

 of Pensylvania, M. Theodore de Saussure, and 



