SECT. III. APERTURES. 367 



tions of M. Mirbel, who regards the pores in ques- 

 tion, as being organs both of transpiration and 

 absorption. * 



The doctrine of external and perceptible pores 

 therefore, as existing in the epidermis, may be re- 

 garded as perfectly established. And with the help 

 of a good microscope they are not very difficult 

 of detection, at least in the leaves of herbaceous 

 plants. I have observed them distinctly in the 

 leaves of the following species : Saponaria offici- 

 nalis, Sagittaria sagittifolia, Nymph&a lutea, 

 Glaucium luteum, Cynoglossum officinale, Rumex 

 acetosa, and Tamus communis ; as also in the leaves 

 of the several species or varieties of Brassica and 

 Lactuca, cultivated in the gardens, and of the 

 Oak, Poplar, and Lilac. 



In most of the above examples they are disco- 

 verable on both surfaces of the leaf, exhibiting the 

 oval aperture more or less dilated, as described by 

 Hedwig, together with the communicating ducts. 

 But on the upper surface they are fewer and 

 smaller than on the under surface ; and in the 

 leaves of trees they are fewer and smaller on both 

 surfaces than in the leaves of herbs ; and in no 

 case have I found them so closely crowded together 

 as represented by Hedwig. The areas are gene- 

 rally oval ; but in Nymphaa lutea they are round, 

 and not easily detected. For in the leaves of this 

 plant the epidermis is so extremely difficult of de- 



* Tratte de Phys. Veg. vol. i. p. 81. 



