LECT. XI.] ANATOMY OF LEAVES. 617 



The information we have thus obtained of the 

 structure of these pores, -induces me to believe 

 that they are the respiratory organs of plants. But 

 Phytologists have considered them intended for 

 the functions of absorption and exhalation*; and 

 even Sprengel, who hints something regarding 

 their analogy to the breathing spiracula of insects, 

 rejects the idea of their being respiratory organs, 

 because "they are not in immediate contact with 

 the spiral vessels-f~." If, however, it be admitted, 

 as I have endeavoured to prove, that the spiral 

 vessels are not air-tubes, but sap-vessels, this ob- 

 jection must fall to the ground. The idea that they 

 are absorbing organs, is supposed to gain support 

 from the circumstance, first ascertained by Bon- 

 net J, that leaves absorb more powerfully with 

 their lower than with their upper surface; and 

 Sprengel remarks that " the slits are more nume- 

 " rous in juicy plants, which are nourished more 

 " by the surface of the leaves than by the roots**." 

 But 1 reply, that although leaves absorb chiefly 

 iby their inferior surface, yet, this does not prove 

 that these apertures are the absorbing organs ; for 

 we find none of them on the lower side of the 

 leaves of Nymphsea and other aquatics, which 



* Among those holding this opinion are MM. Bonnet, 

 Decandolle, Sprengel, Mirbel, Mr. Ellis, and Mr. Keith. 



f Elements of the Phil, of Plants, j 31 1 . ** Ibid. I. c. 



t Recherches sur I' Usage des Feuilles, p. 20. 



