LRCT. X.J ANATOMY OF LEAVES. 559 



surrounds the spiral vessels : the object of which 

 is, probably, to give such a degree of firmness to 

 the petiole, as will enable it to sustain, in the erect 

 position, the expansion of the leaf. If we now 

 make a vertical section of the petiole, so as to 

 divide one of the fasciculi longitudinally, in the 

 thickness of the petiole we perceive that the 

 larger vessels are regular spirals (1 1. e.), furnished 

 with diaphragms (h.) at certain distances, the struc- 

 ture of which, however, we shall perhaps never 

 be able to ascertain, owing to the minuteness of 

 the parts ; the diameter of these vessels, although 

 comparatively large, not exceeding ^-^ of an 

 inch. In this section, also, we perceive that the 

 proper vessels (l 1 .f.) are membranous and porous: 

 and that the cells in immediate contact with both 

 sets of vessels are oblong ( u . d.) ; whereas those 

 (g.) which are between the proper vessels and the 

 cuticle of the outer surface of the leaf, and which 

 form the elevated portion of the costa, although 

 they are not oblong, yet, differ both in size and 

 in regularity of structure from those (n. c.) that 

 form the inner substance of the petiole. Close to 

 the cuticle of the upper disk ( 1 1 . a.) is a mass of 

 oblong cells (b.) resembling those in contact with 

 the vessels. 



Ascending to the expansion of the leaf, which 

 is separated from the petiole by a semitranspa- 

 rent white, condensed, membranous space (see cut 



