PLANT ORGANS AND ORGANISMS 
149 
Histology of a Typical Bark, Cascara Sagrada— In transverse 
section passing from outer to inner surface, the following structural 
characteristics are evident: 
1. Cork, or outer bark, composed of several layers of rectangular 
cork cells. The most external layers are dead and appear black 
because they are filled with air. The inner layers of this region 
are living and contain brownish contents. 
2. Cork cambium (phellogen), a layer of delicate cells with pro¬ 
toplasmic contents in the process of division. 
3. Cortex, or middle bark, consisting of two regions, viz.: an outer 
zone of two or three rows of brownish collenchyma cells, and an 
inner broader zone of tangentially elongated cortical parenchyma 
cells. Imbedded within this zone will be noted numerous groups 
oi stone cells. 
4. Phloem, or inner bark, a very broad zone composed of irregular¬ 
shaped, elongated phloem masses separated from each other by 
medullary rays which converge in the outer phloem region. Each 
phloem mass consists of numerous sieve tubes and phloem cells, some 
of which latter contain spheriodal starch grains while others contain 
monoclinic prisms or rosette aggregates of calcium oxalate. Em¬ 
bedded within the phloem masses in tier-like fashion will be noted 
groups of bast fibers, each group of which is surrounded by a row 
of crystal fibers, individual cells of which can only be made out in 
this kind of a section. Each of these contains a monoclinic prism of 
calcium oxalate. The medullary rays possess brownish contents 
which take a red color with an alkaline solution. 
In radial longitudinal section a lengthwise view of the tissues will 
be seen. The medullary rays appear 15 to 25 cells in height and 
crossing at right angles to the other elements. 'The crystal fibers 
here will be seen to be composed of vertical rows of superimposed 
thin-walled cells each of which contains a monoclinic prism of 
calcium oxalate. The bast fibers appear elongated and taper ended 
and are associated with crystal fibers. 
In a tangential longitudinal section which has been cut through 
the phloem, the exact range in width of the medullary rays may be 
ascertained. In this bark the medullary rays are spindle-shaped 
in tangential view and one to four cells in width. 
