RHUBARB 189 



The hairs include both non-glandular and glandular, the head in 

 the latter being usually small, but sometimes large and peltate. 

 Calcium oxalate usually occurs in the form of rosette aggregates. 



RHEUM. Rhubarb. The rhizome of Rheum officinale, Rheum 

 palmatum, Rheum palmatum tanguticum, and probably other 

 species of Rheum (Fam. Polygonaceae), perennial herbs (Fig. 80) 

 indigenous to northwestern China and eastern Thibet, and sparingly 

 cultivated in other parts of the world. The rhizomes are collected 

 in autumn from plants that are eight to ten years old, most of the 

 bark is removed, and they are then perforated, strung on ropes and 

 dried either in the sun or by artificial heat. The drug is. exported 

 chiefly from Shanghai. The principal commercial varieties are 

 known as Chinese rhubarb, Canton rhubarb and Shensi rhubarb, 

 the latter being preferred. 



Description. Cut into irregular plano-convex and oblong pieces, 

 frequently with a large perforation, hard and moderately heavy, 

 5 to 15 cm. in length, 5 to 8 cm. in breadth and 3 to 6 cm. in thick- 

 ness, externally mottled from alternating striae of light brown paren- 

 chyma cells and dark-brown medullary rays, occasionally with 

 reddish-brown cork patches and small radiate scars of fibrovascular 

 tissue, smooth and sometimes covered with a light-brown powder; 

 fracture somewhat granular; internally light brown; odor distinct; 

 taste bitter, astringent and gritty. 



Light and spongy rhizomes should be rejected. 



Inner Structure. See Fig. 81. 



Powder. (Fig. 82.) Yellowish-brown; crystals of calcium 

 oxalate in rosette aggregates, 0.050 to 0.150 mm. in diameter; starch 

 grains numerous, spheroidal, from 0.002 to 0.020 mm. in diameter, 

 either single or 2- to 4-compound; tracheae few, reticulate, occasion- 

 ally spiral. The powder is colored reddish with alkalies. A com- 

 mon adulterant is wheat middlings. The exhausted drug is fre- 

 quently added to the powder and may be detected by the somewhat 

 altered starch grains and the decrease in the amount of the aqueous 

 or dilute alcoholic extract, which in genuine rhubarb is about 35 

 per cent. 



Constituents. The principal constituent appears to be a gluco- 

 side (possibly the chrysophan of some authors) or an undetermined 

 substance which yields successive oxidation products, viz.: chry- 

 sophanic acid (di-oxy-methyl-anthraquinone), emodin (tri-oxy- 

 methyl-anthraquinone), and rhein (tetra-oxy-methyl-anthraqui- 

 none) . Chrysophanic acid crystallizes in golden-yellow, clinorhombic 

 prisms and dissolves in alkalies and in concentrated sulphuric acid, 



