200 BARKS. 



(98) Oak Bark. 



The young bark of Quercus robur, Linn. (N.O. Cupuliferse). 



The bark presents the following structure : 



(1) Cork, composed of a number of rows of moderately 

 thickened, brown cells ; in tangential section they are seen to 

 be polygonal. 



(2) Phelloderm, which is generally only slightly developed, 

 and presents the usual characters. 



(3) Cortex, the outer portion of which has developed 

 into a collenchymatous tissue, containing brownish chloro- 

 plastids. The remainder of the cortex is composed of 

 irregular polygonal cells amongst which there are scattered 

 numerous isolated or grouped sclerenchymatous cells; these 

 vary very much in size ; some have very thick walls traversed by 

 branching pits, but others have only moderately thick smooth 

 walls. The parenchymatous cells of the cortex contain chloro- 

 plastids, tannin, and rosette crystals of calcium oxalate. 



(4) Bast Ring, which in young bark is of moderate thickness, 

 and traversed by narrow medullary rays. The bast rays contain 

 numerous tangentially elongated groups of bast fibres with very 

 thick walls upon which cells containing prismatic crystals of 

 calcium oxalate abut. Many of the cells of the bast parenchyma 

 are brown in colour, and nearly all of them as well as of the 

 cells of the medullary rays contain tannin. 



The diagnostic characters of the bark are : 



(a) The presence of abundant tannin. 



(b) The nwnerous sclerenchymatous cells of the cortex. 



(c) The numerous thick-walled bast fibres. 

 (1) The, characteristic cork. 



