LECT. XI.] ANATOMY OF LEAVES. 623 



tuated under it ; and which probably communi- 

 cates with the cuticular cells, as these are in ge- 

 neral found filled with air. The aqueous contents 

 of the cells that form the parenchyma of the leaf, 

 are thus brought into immediate contact with 

 the atmosphere. It is not easy to assign a rea- 

 son why these apertures are found on the under 

 disk only of the leaves of trees, while they ap- 

 pear on both disks of herbaceous leaves; there 

 being lymphatics on both disks of the former 

 as well as of the latter description of leaves. If 

 any connexion could be traced between the re- 

 turning vessels and the apertures, the difficulty 

 would be diminished, the situation of these vessels 

 being on the lower disk of the leaves of trees. 



With regard to the origin of these apertures, 

 M. De Saussure's and M. Kieser's observations 

 would lead us to believe, that they are merely the 

 terminations of numerous vascular processes 

 from the larger fasciculi, which, gradually pene- 

 trating the cuticle, are thus enabled to discharge 

 their fluids. This opinion, however, is altogether 

 hypothetical. They are so far essential that they 

 are found on every leaf in contact with the at- 

 mosphere; their structural characters, position, 

 and situation, are the same on the leaves of every 

 plant of the same species ; and their existence 

 seems to be influenced by no conditional circum- 

 stance except the presence of air; for I have al- 



