the leaves of vascular plants. 271 



Section VL 

 Of the Cavities of Leaves. 



Most leaves are flat or thick, and in each case their 

 internal surface does not present any closed cavity, and 

 even their surface has none which is open externally ; 

 but there are some leaves which, from different causes, 

 form exceptions to those two general laws. 



In order to commence with the more simple cases, 

 which tend to render the others clear, we see that several 

 plants, which have the petiole large and foliaceous, have 

 this organ curved in such a manner as to form a kind of 

 longitudinal tube, with a fissure on its inner surface; 

 sometimes this tube sheathes the stem, as in the 

 Gramineas, or certain Amomeae, where it bears the 

 name of Sheath ; sometimes it diverges from the stem 

 at its origin, and has the appearance of an empty tube, 

 split upon one side: this takes place in Sarracenia, 

 where it may be considered as a tube formed either of a 

 fohaceous petiole, or by the limb of the leaf; it can 

 contain but httle or no liquid on account of the lateral 

 fissure. In some plants, with palminerved leaves, the 

 nerves are numerous, very near each other at the base, 

 and disposed in such a manner that the limb forms 

 a horn-shaped reversed cone, as is seen in Pelargonium 

 cucullatum. Some other plants present accidentally an 

 analogous disposition, but with the two borders of the 

 horn united together, so as to form an entire horn, or 

 reversed cone; this accident is not rare in the Lune 

 (Tilia). 



It happens in some cases that the top of a common 

 petiole, instead of being prolonged into a tendril, spreads 

 out into a kind of hollow foliaceous disc^ which forms a 



