ANATOMY OF LEAVES. 313 



aire's and Kieser's observations would lead us to be- 

 lieve that they are merely the terminations of numer- 

 ous vascular processes from the larger fasciculi ; which, 

 gradually penetrating the cuticle, are thus enabled to 

 discharge their fluids. This opinion, however, is al- 

 together hypothetical. They are so far essential that 

 they are found on every leaf in contact with the at- 

 mosphere ; their structure, position, and situation, are 

 the same on the leaves of every plant of the same spe- 

 cies ; and their existence seems to be influenced by 

 no conditional circumstance except the presence of 

 air. With regard to the fact, they are not found on 

 submersed leaves, even of land plants which are made 

 to grow in the water, we may observe, that the leaves 

 produced on such plants differ from those which are 

 natural to them, not in the absence of apertures only, 

 but in form, structure, and functions. 



The knowledge of the structure of leaves enables 

 us to form a correct idea of the importance of these 

 organs in the economy of plants. We find the vessels 

 which convey the sap from the roots terminating in 

 the leaf, and spreading out their contents through its 

 cells, to undergo certain chemical changes which are 

 essentially influenced by the action of the air and 

 light ; we find, also, a new system of vessels com- 

 mencing here, which take up again the sap thus con- 

 verted into proper juice and conduct it downwards, 

 depositing in their course the various secretions form- 

 ed from it, either in the sterns or in the roots, as the 

 nature of the plant requires ; and, in aid of these ope- 

 rations, a cuticular system admirably adapted by its 

 transparency to transmit the rays of light into the fo- 

 liar cells, and by its organic apertures to admit the 

 air, and at the same time favor the exhalation of the 

 superabundant water, which the ascending sap neces- 

 sarily contains. But, besides fitting the sap for yield- 

 27 



