176 THE ELEMENTS OF VEGETABLE HISTOLOGY 



Cortical Parenchyma. — The parenchymatic tissues 

 of the middle bark vary greatly both in quantity and 

 in details of cell structure. Stone cells are rather 

 frequently found among the cortical cells, and secre- 

 tion cavities, when present, occur in this portion of 

 the bark. The cortical cells contain starch, crystals 

 or other cell contents. This tissue is present in 

 powdered materials in the form of fragments con- 

 sisting of several thin-walled angled or circular cells 

 (Plate 59). 



Medullary Rays. — The ray cells present in barks 

 are usually smaller than those within the cambium. 

 Seen in transverse section, the individual cells are thin- 

 walled, square or oblong in form, and each ray is made 

 up of from one to four rows of cells. In tangential 

 sections the ends of the rays are apparent as oval or 

 elliptical groups of polygonal cells. In radial sections 

 one obtains a side view of the rays and they appear 

 as broad bands of rectangular cells extending at right 

 angles to the surrounding tissues. In powdered 

 materials medullary rays are generally seen on radial 

 view (Plate 59), and are usuallly in combination with 

 parenchyma or other surrounding tissues. Occasion- 

 ally, starch, crystals or other cell contents occur 

 within the ray cells. 



Phloem Elements. — The phloem elements in barks 

 are usually located in the vicinity of the medullary 

 ray cells. The bast fibers occur as groups of cells 

 between the medullary rays or toward the outer ends 

 of these. In transverse sections the individual fibers 

 appear as small, thick-walled cells, readily distinguished 

 from the surrounding elements, with the possible 

 exception of stone cells. The lumen of these fiber 



