476 BOTANY AND PHARMACOGNOSY. 



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. long, 5 to 8 cm. broad and 3 to 6 cm. thick; 

 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 fibro- 

 vascular 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 Figs. 281, A; 289. 



Constituents. — The principal constituent appears to be a 

 glucoside (possibly the chrysophan of some authors) or an unde- 

 termined substance which yields successive oxidation products, 

 viz.: chrysophanic acid (di-oxy-methyl-anthraquinone), emodin 

 (tri-oxy-methyl-anthraquinone), and rhein (tetra-oxy-methyl-an- 

 thraquinone). Chrysophanic acid crystallizes in golden-yellow, 

 clinorhombic prisms and dissolves in alkalies and in concentrated 

 sulphuric acid, the solutions having a deep-red color. It is 

 re-formed in rhubarb after extracting it by exposing the moistened 

 root to air. Emodin occurs to the extent of 1.5 per cent, and 

 forms orange-red needles which are soluble in hot toluene and 

 give with alkalies and alkali carbonates purplish colored solu- 

 tions. Rhein forms yellowish-brown scales which are insoluble 

 in hot toluene, soluble in hot acetic acid and produce purplish-red 

 solutions with the alkalies or alkali carbonates. Rhubarb also 

 contains the tanno-glucoside (tannin) which yields rheumic acid ; 

 the glucoside glucogallin, which yields gallic acid; the glucoside 

 tetrarin, which yields cinnamic acid in addition to gallic acid; 

 an aldehyde rhoeosmin, which has the odor of rhubarb; consid- 

 erable starch ; calcium oxalate in the form of rosette aggregates ; 

 and yields about 15 per cent, of ash. 



Allied Plants. — ^The rhizomes of other species of Rheum 

 are also used to a limited extent, as English or Austrian rhubarb 



