ANATOMY OF LEAVES. 287 



hocotyledons, terminate in leaves until the plant attains 

 its ultimate growth. 



There are two natural anatomical subdivisions of 

 the petiolated leaves of monocotyledonous plants : 

 namely, 1- Those in which the ribs run longitudinal- 

 ly, or in a direction from the base to the apex of the 

 leaf; and 2. Those in which they run almost trans- 

 versely, or in the direction from the midrib to the 

 margin. 



1. In this subdivion we perceive, that, in the Gras- 

 ses, the vascular fasciculi resemble, very closely, those 

 of the former division ; the ribs being in longitudinal 

 nearly parallel lines, converging towards the apex of 

 the leaf; and united at irregular distances by oblique- 

 ly transverse threads. If we take a stem leaf of In- 

 dian Corn (Zea Mays), as a specimen, we perceive 

 the petiole, which is broad, expanded, and sheathing, 

 deriving its origin from the whole circumference of the 

 knot of the articulation which produces it ; dilating 

 gradually as it rises upwards, until its edges become a 

 thin fimbriated membrane, and again contracting, but 

 less gradually, at its upper part, or where it is united 

 to the expansion of the leaf. The vascular bundles, 

 which can be readily traced by the naked eye, are 

 composed of two distinct kinds of vessels, which ap- 

 pear as dots in a transverse section of the petiole sit- 

 uated almost close to its external surface. The num- 

 ber of the spiral vessels in each fascicle is generally 

 six, three large and three smaller, symmetrically ar- 

 ranged, as may be seen in a transverse section of one 

 of the fasciculi viewed under the microscope ; and 

 the whole surrounded by a mass of much denser cel- 

 lular matter than the rest of the substance of the peti- 

 ole. The returning, or proper vessels are much 

 smaller and more numerous than the spiral ; and 

 are aggregated into a bundle which occupies a space 



