BAEKS. 183 



gate 3, and arranged in radial rows. In surface viev/ they are 

 polygonal and isodiametric. Their walls are generally thin, but 

 sometimes they exhibit a thickening on one or more sides ; 

 they are often brown, and sometimes contain a brown amor 

 phous substance. 



(2) Cortex. The cortex consists of parenchymatous tissue in 

 which groups of collenchyma are sometimes developed on the 

 outer margin, that is abutting on the phellogen from which the 

 cork has developed. The remainder of the cortical paren- 

 chyma is composed of rounded or tangentially elongated 

 cells which are readily distinguished from the cork-cells by 

 their larger size and more rounded shape, as well as by the 

 absence of suberisation. This tissue often exhibits distinct in- 

 tercellular spaces. It frequently contains groups of sclerenchy- 

 matous cells, usually in greater number and larger size than 

 the bast ring. Secretory tissue of various kinds is also often 

 found in this region. 



(3) Bast. The bast ring is composed of medullary rays 

 alternating with bast rays; the latter consist of bast 

 parenchyma and sieve tissue, with which there may be associ- 

 ated bast fibres, sclerenchymatous cells, and the varying forma 

 of secretory tissue. 



The cells of the bast parenchyma are usually smaller than 

 those of the cortical parenchyma, and more regularly arranged. 

 The bast fibres may be isolated or grouped. They vary in size, 

 shape and thickness of wall, as well as in other details. The 

 sieve-tubes, which are present in every bark, also exhibit differ- 

 ences in size as well as in the size, number, and position of the 

 sieve-plates. They require special treatment to enable them to 

 be well observed, but may, under certain circumstances, prove 

 useful in the identification of a bark. 



The tissue of the medullary rays is usually found in the 

 powdered drug in the form of plates of parenchymatous cells, 

 which are all elongated in the same direction, and transversely 

 to the long axes of the bast fibres or bast parenchyma. Thejr 

 may contain starch, colouring matter, calcium oxalate, etc. 



