568 CONSERVATIVE ORGANS. [LECT. X. 



cave surface of the midrib, which are curved out- 

 wards in opposite pairs, at different distances be- 

 tween the basis and the apex of the leaf; but the 

 central fasciculi pass on its apex. These vascular 

 fasciculi are embedded in a cellular tissue; besides 

 which the petiole and midrib of this description of 

 leaves contain peculiar pneumatic or air cells (23. 

 d.d. d.) closely resembling those which I shall after- 

 wards demonstrate to you, constitute a great part 

 of the substance of aquatic plants. In a transverse 

 section of a small part of the expansion of the leaf 

 (24. see page 567), we perceive that the vascular 

 cords (a. a. a) run nearly in the centre between 

 the two plates of cuticle, embedded in an opaque 

 green parenchyma; and that, instead of the 

 pneumatic apparatus of the petiole and midrib, 

 there is a transparent layer of large cells (24. b.) 

 immediately under the cuticle of the upper disk. 

 I should, however, inform you that these pneumatic 

 cells are not present in the petiole and midrib of 

 all leaves with transverse costse, belonging to mo- 

 nocotyledonous plants; but the same general ar- 

 rangement of the vascular cords, and, conse- 

 quently, the same structure of the framework, 

 are seen in all of them. 



With regard to the composition of the fasci- 

 culi; these, as in the other leaves we have ex- 

 amined, consist of spiral and proper vessels; dif- 

 fering, however, in the relative position of the 

 spiral vessels, which in each fasciculus, in these 



