624 CONSERVATIVE ORGANS, [LECT. XI. 



ready demonstrated the incorrectness of the as- 

 sertion, that etiolated leaves are devoid of aper- 

 tures, or at least the remark is not universally 

 applicable. With regard to the fact, that they 

 are not found on submersed leaves, even of land 

 plants which are made to grow in the water, I may 

 merely 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 en- 

 ables us to form a correct idea of the importance 

 of these organs in the economy of the 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 commencing here, which 

 take up again the sap thus converted into proper 

 juice and conduct it downwards, depositing in their 

 course the various secretions formed from it, either 

 in the stem or in the roots, as the nature of the 

 plant requires; and, in aid of these operations, a 

 cuticular system admirably adapted by its trans- 

 parency to transmit the rays of light into the foliar 

 cells, and by its organic apertures to admit the 

 air, and at the same time favour the exhalation of 

 the superabundant water, which the ascending 



2 



