288 ANATOMY OF LEAVES. 



close to the former, between it and the cuticle, and is 

 bounded by a mass of the same dense cellular matter 

 as that which surrounds the spiral vessels ; the object 

 of which is, probably, to give such a degree of firm- 

 ness 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 reg- 

 ular spirals, furnished with diaphragms at certain dis- 

 tances, the structure of which however, we shall per- 

 haps never be able to ascertain, owing to the minute- 

 ness of the parts ; the diameter of these vessels, al- 

 though comparatively large, not exceeding ji ¥ of an 

 inch. In this section, also, the proper vessels are 

 membranous and porous ; and the cells in immediate 

 contact with both sets of vessels are oblong ; whereas 

 those which are between the proper vessels and the 

 cuticle of the outer surface of the leaf, and which form 

 the elevated portion of the costae, although they are 

 not oblong, yet differ both in size and in regularity of 

 structure from those that form the inner substance of 

 the petiole. 



Ascending to the expansion of the leaf, which is sepa- 

 rated from the petiole by a semitransparent white, con- 

 densed, membranous space from which the expansion 

 of the leaf spreads out like a shoulder on each side : 

 we perceive that the midrib, which is not distinguish- 

 able in the lower part of the petiole, becomes very 

 conspicuous on the under disk at this point ; forming 

 almost a knob, which passes into a striated ridge, and 

 extends, gradually diminishing in size, to the apex of 

 the leaf. From 10 to 12 parallel costae are visible 

 on each side of the midrib, which, when magnified, ap- 

 pear like white parallel lines, running through the 

 green smooth substance of the expansion, and taking 



