THE STEM 109 
ings of birch bark, which make it so ornamental, are due to the 
lenticels. In most trees they disappear on the older parts, 
where the bark is constantly breaking away and sloughing off. 
t19. Internal structures.— Cut a transverse section 
through your specimen, and notice under the epidermis a 
greenish layer of young bark; beneath this a layer of rather 
tough, stringy bast fibers, and beyond these a harder woody 
substance that constitutes the bulk of the interior; within this, 
_ at the very center of the axis, we find a cylinder of lighter 
texture, the pith, or medulla, occupying the place of the soft 
parenchyma which fills this space in very young stems. 
Between the woody axis and the bark notice a more or 
less soft and juicy ring. 
120. The cambium layer.— This is not always easily 
distinguishable with a hand lens, but is conspicuous in the 
stems of sassafras, slippery elm, and aristolochia. If some 
of these cannot be obtained, the presence of the cambium 
can be recognized by observing the tendency of most stems 
to ‘‘ bleed,” when cut, between the wood and bark. The 
reason for this is because the cambium is the active part of 
the stem, in which growth is taking place, and consequently 
it is most abundantly supplied with sap. In spring, es- 
pecially, it becomes so full of sap that if a rod of hickory 
or elder is pounded, the pulpy cambium is broken up and the 
bark may be slipped off whole from the wood. 
121. Medullary rays. — Observe the whitish, silvery lines 
that radiate in every direction from the center, like the 
spokes of a wheel from the hub. These are the medullary 
rays, and consist of threads of pith that serve as lines of com- 
munication between the “ central cylinder” and the grow- 
ing cambium layer. In old stems the central pith frequently 
disappears and its office is filled by the medullary rays, which 
become quite conspicuous. 
122. Structural regions of a woody stem. — Sketch cross 
and vertical sections of your specimen, as seen under the lens, 
labeling the different parts, Refer to Figs. 125, 126, if you 
