256 RfllZOMES AND BOOTS. 



(123) Rhubarb Rhizome (Chinese). 



The rhizome of one or more species of Rheum (N.O. 

 Polygoneee) deprived of its cork and of the greater part of its 

 cortex. 



Tne rhizome presents the following structure: 



(1) Cortex, of which at most only a very narrow layer is left, the 

 majority or sometimes the whole of it having been removed by 

 peeling. It consists of polygonal cells containing starch and 

 calcium oxalate. The starch is in simple and compound 

 grains; the former are rounded, hive a very distinct hilum 

 and measure from 10 to 18/x in diameter ; the compound 

 gra ; ns contain from two to five component granules. The 

 calcium oxalate usually assumes the form of large rosettes, 

 sometimes attaining as much as 60^, in diameter. The cortex 

 also contains large mucilage cells. 



(2) Bast Ring. Towards the cambium tne cortex passes 

 imperceptibly into the bast ring, which is traversed by medullary 

 rays from three to five cells wide. These cells are filled with 

 reddish-brown substances, and hence the rays are visible under 

 the lens as reddish-brown lines. The bast parenciyma contains 

 starch ; the sieve tubes are mostly obliterated and there are no 

 bast fibres. 



(3) Cambium, which is visible to the naked eye as a dark line 

 very near the periphery of the rhizome, is formed of five or six 

 rows of cells which are free from starch and calcium oxalate. 



(4) Wood, forming a very narrow ring within the cambium ; it 

 is traversed by dark yellow medullary rays, between which 

 numerous vessels of varying size are scittered, either isolated 

 or in groups. Abutting on the inner margin of the wood is a 

 circle of abnormal bundles which constitute the remarkable 

 stellate spots visible on a transverse sectio i ; these bundles 

 have their bast in the centre and wood at the periphery. 



(5) Pith, which is largely developed and is irregularly traversed 

 by stellate bundles which pursue various course ^ ; the medullary 

 rays of these bundles are filled with reddish-brown colouring 

 matt r which gives rise to the characterist c marbled ap ear- 

 ance of the rhizome ; the white portions between these rays are 

 parenchymatous tissue filled with starch and oalcium oxalate. 



The diagnostic characters of powdered rhubarb are : 



(a) The very lirgc rosettes o f calcium oxalate. 



(b) The starch grains. 



(c) The vessels. 



(d) The medullary ray cells with reddish-broiun contents. 



