302 VEGETABLE ORGANOGRAPHY. 



for the aqueous evaporation : tliis is the first or principal 

 function of leaves. This aqueous exhalation is the more 

 active, the greater the number of leaves, the larger their 

 surface, and the more stomata they have in a given 

 space. 



2d. A consequence of this first point, which may be 

 considered a second use of leaves, is that of causing the 

 ascent of the sap ; for the quantity of water drawn up 

 by a plant under given circumstances, is in general per- 

 ceptibly proportional to the extent of leaves which the 

 plant bears ; and when difierent species are compared, 

 to the total number of stomata. 



.Sd. There are circumstances in which the stomata, 

 instead of exhaling the super-abundant water, appear to 

 absorb that which is externally in contact with them ; it 

 is thus that withered leaves absorb the water with which 

 they are sprinkled ; and it is thus also that Charles 

 Bonnet made branches to live by placing that part of 

 their leaves which was provided with stomata, upon a 

 watery surface. 



4 th. Leaves exposed to the action of light in an 

 atmosphere which contains a small quantity of carbonic 

 acid gas, or in water which holds in solution air mixed 

 with carbonic acid, decompose this gas, exhale the 

 oxygen, and appear to fix the carbon. 



5th. When they are exposed to the air during the night, 

 they absorb a certain quantity of oxygen, which is as much 

 as seven times their volume for trees with annual leaves, 

 but which is gradually less for herbs, trees with persist- 

 ent leaves, marsh and succulent plants. It is probable 

 that this absorption of oxygen, discovered by Theodore de 

 Saussure, contributes to facilitate the decomposition of 

 the substances contained in the sap. 



6th. It results from the difierent facts mentioned 

 above, that the principal elaboration of the sap is 



