286 ANATOMY OF LEAVES. 



namely, the exposure of the sap to the light and air. 

 would lead us, a priori, to conclude that the vessels 

 carrying forward the sap must, necessarily, be on that 

 side of the leaf most exposed to these agents ; the 

 sap-vessels receiving their origin in the stem from the 

 vessels of the alburnum, and the returning vessels ter- 

 minating in those of the bark, the disposition could 

 not well be otherwise, seeing that the relative position 

 of the upper and under disk of every leaf, to the cen- 

 tre of the stem, is exactly that of the alburnum and 

 the bark. In leaves, however, which stand vertical- 

 ly, or have no distinction of surfaces, the situation of 

 the spiral vessels is either the reverse, or in the centre 

 of the entire vessels : anatomy thus confirming the 

 idea of the close affinity of such leaves to stems. 



It has already been stated that the bundles and 

 threads of vessels, in leaves belonging to this division 

 of the class under consideration, run in longitudinal 

 lines. These are nui exactly parallel, but approach 

 both at the base and the apex of the leaf; and, also, 

 communicate laterally in their course by small threads, 

 given off at irregular intervals ; as may be seen in a 

 slice of the Lily leaf cut immediately within the cuti- 

 cle of the upper disc, and placed beneath the micro- 

 scope. 



The vascular system, then, of the sessile leaves of 

 monocotyledons, consists of fasciculi composed of spi- 

 ral vessels, accompanied with proper vessels which 

 are not spiral, arranged in longitudinal lines, and con- 

 nected by smaller transverse threads ; the whole form- 

 ing a reticulated texture with irregular rhomboidal 

 meshes. The longitudinal vessels are a continuation of 

 those which are nearest to the surface, in the root, cau- 

 dex, or the stem, from which the leaves immediately 

 spring ; and thus the greater number of the circles of 

 the distinct fasciculi, which compose the stems of men 



