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. 



119. 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. 



1 20. 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 



