3flS ELEMENTARY ORGANS. CHAP. Ill* 



tachment, that I obtained it only where it had been 

 left by insects that had eaten up the interior pulp. 

 Finally, in the leaves of Lettuce, and in them 

 only, I found the areas and pores, contrary to the 

 representations of Decandolle, situated even upon 

 the nerves ; a fact that destroys the universality of 

 the remark by which they are excluded from that 

 situation, and may be regarded as affording an ad- 

 ditional example of the truth of the maxim, that 

 says there is no rule without its exception. 



The internal and perceptible pores or apertures, 

 forming the medium of communication between 

 the different cells and tubes, seem to have been 

 the discovery of M. Mirbel ; as I do not recollect 

 to have seen any account of them previous to that 

 which is given in his vegetable anatomy. They are 

 And in the found in the porous tubes, as has been already 

 tubes, 8 and noticed as well as in the dissepiments of the cells, 

 sometimes extremely minute, and at other times 

 of considerable diameter, but always bounded by 

 a sort of circular lip or ring. In some plants they 

 are but few and scattered ; and in others they are 

 numerous and arranged in regular rows, which ex- 

 tend always in a transverse but never in a longitu- 

 dinal direction, being meant perhaps merely for 

 the lateral transmission of the sap. 



ARTICLE 2. Imperceptible Pores. The imper- 

 ceptible pores are pores that are not distinguishable 

 by the eye, assisted even with the best glasses ; but 

 which are known to exist by the evidence of ex- 



