298 KANSAS UNIVERSITY SCIENCE BULLETIN. 



Embedded in the spongy mesophyll is the venation of the leaf 

 (fig. 41). Each leaflet is provided with one prominent midrib 

 (fig. 41, z), which gives rise to lateral branches extending to 

 the margin, these lateral branches giving rise to smaller 

 branches, and so on, the whole system forming a close network 

 of veins. The veinlets lie exceptionally close to the lower epi- 

 dermis (fig. 22, a), the surrounding parenchyma or scleren- 

 chyma sheath often being in contact with the lower epidermal 

 cells. The suiTounding sheath (fig. 38) is similar in many re- 

 spects to that described as surrounding the bundles of the 

 stem, and corresponds to it. Lignification is not so extensive 

 in the sheath cells of the leaf, but the crystals are found in 

 numbers in the thick-walled cells of the parenchyma sheath 

 cells. The sheath about the midrib shows marked sclerosis 

 (fig. 23), as do also the sheaths about the main lateral veins 

 extending from the midrib. Toward the tips of the latter the 

 specially modified cells give way to ordinary border paren- 

 chyma cells (figs. 27 and 28) , such as border the finer veinlets, 

 and are common to the veining systems of many leaves. The 

 tips of the veinlets which end freely in the mesophyll (figs. 29 

 and 30) , unlike those of the midrib and lateral veins ending in 

 the margin of the leaf, are made up of various compact groups 

 of spirally thickened tracheids (fig. 31). 



