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 corrtents 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 bjroad 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. 



