LECT. XI.] ANATOMY OF LEAVES. 609 



the exterior green leaves of the common Cabbage 

 and the Lettuce. 



These foliar apertures vary very considerably 

 in form, size, number, and position, in different 

 leaves. They are commonly oblong (46.), but in 

 some instances circular (47-)> an d in the Agave 

 tribe (59- p. 6li), and a few other families of 

 plants, they are quadrilateral. In almost all 

 leaves they are surrounded by a border, in which 

 the vessels forming the cuticular meshes appear 

 to terminate. Placing minute portions of the cu- 

 ticle of different leaves under the microscope, 

 we can readily ascertain the superficial form of 

 these pores : the following are the principal diver- 

 sities of form which I have observed *. 1. A simple 

 slit, more open in the middle than at either 

 end, bisecting an oval shield ; which may, there- 

 fore, be termed the oval scutiform aperture, oscu- 

 lum scutiforme ovatum, as exemplified on the lower 

 disk of the leaves of Sage (48. p. 610), of Lactuca 

 quercina (4Q.), Dandelion, Leontodon Taraxacum, 

 Sweet-scented Coltsfoot, Tussilagofragrans, many 

 of the Grasses, the common Bean, Vicia Faba, &c. 

 2. A simple slit, bisecting an oval shield enclosed 



* In naming the different kinds of pores, I have assumed the 

 fact, that they are respiratory organs, or apertures resembling 

 in their functions the spiracula of insects ; but, on account of 

 the simplicity of their structure, I have preferred the term 

 osculum to that of spiraculum. 



VOL. 1. R R 



