184 BARKS. 



(90) Alder Buckthorn Bark. 



The bark of Rhamnus Frangula, Linn. (N.O. Rhamnese). 

 The bark presents the following structure : 



(1) Cor k, varying in thickness according to the age of the 

 bark. It is composed of a number of rows of flattened cells 

 with thin walls and bright, purplish-red contents ; in surface 

 view the cells are polygonal. 



(2) Phelloderm, usually very narrow ; the cells resemble those 

 of the cork in shape but they do not contain any red substance 

 and the walls are not suberised. 



(3) Cortex, the cells of which are mostly tangentially elon- 

 gated, those abutting on the phelloderm being collenchyma- 

 tously thickened. This tissue is characterised by the presence 

 of numerous cavities containing mucilage, and by the absence 

 of sclerenchymatous cells (compare cascara sagrada bark). Many 

 of the cells contain rosette crystals of calcium oxalate. 



(4) Bast Ring, containing numerous tangentially elongated 

 groups of bast fibres, and traversed by medullary rays one or 

 two cells wide. The bast fibres have very thick walls and are 

 surrounded by cells containing calcium oxalate in very uniform 

 prismatic crystals. The sieve-tubes are large, and hava large 

 sieve-plates on oblique walls. 



The diagnostic characters of powdered alder buckthorn bark 



are : 



(a) The cork cells with characteristic red contents. 



(b) The bast fibres with crystal cells. 



(c) The absence of sclerenchymatous cells. 



(d) The presence of mucilage. 



(e) The bright purplish colour yielded by the contents of 



the parenchymatous cells in contact loith caustic 

 alkali. 



