V.NATO.UY OF LEAVES. 2S9 



the curve of its shoulders as if originating in the white 

 semitransparent space already descrihed. But be- 

 tween these costs there are several smaller vascular 

 cords, which are scarcely visible on the surface, nei- 

 ther producing elevation nor difference of color ; and 

 which can be demonstrated only on the dissection of the 

 leaf. One of the more obvious distinctions, therefore, in 

 the structure of the petiole and the expansion in the 

 leaves of the gramineous tribe of plants is, that, in the 

 petiole, the vessels run in distinct fasciculi, which are all 

 nearly equal in point of size ; whereas in the expan- 

 sion the fasciculi differ considerably in size, the larger 

 only being very visible on the surface. In both, there 

 are transverse threads which connect the longitudinal 

 bundles, and those are conspicuous even to the naked 

 eye in the more succulent leaves, particularly in those 

 which involve the fructification of the Mays when 

 viewed by transmitted light. 



In examining a transverse section of a portion of 

 the expansion of the leaf of Indian Corn, containing 

 one of the visible costre and the interval between it 

 and the next costa, we immediately perceive the dif- 

 ference of structure in the two kinds of fasciculi. The 

 visible costa consists of two large spiral vessels on the 

 same line, and a compact fasciculus of proper vessels 

 on each side of the line of spirals, towards both sur- 

 faces of the leaf; while, in the interval, each fascicu- 

 lus is composed of one small spiral vessel only, sur- 

 rounded with a circle of proper vessels, and placed in 

 the heart of the substance of the leaf. But, besides 

 these, there is another kind of fasciculi, two of which 

 are generally observed in each space between the vis- 

 ible costae, connected with a peculiar cellular appara- 

 tus. These appear to be modifications of the two 

 vascular fasciculi already noticed ; having the same 

 structure as the obscure or invisible fasciculus, and 

 25 



