RHUBARB 



191 



carbonates purplish colored solutions. 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 car- 



FIG. 81. Rhubarb. Characteristic group of compound fibrovascular bundles 

 seen in transverse section. (PH), a wedge-shaped group of leptome with 

 starch-bearing and crystal cells, cambial layer (c) separating phloem from 

 the xylem with their large tracheae (g) ; medullary rays cells (m) filled with a 

 brownish amorphous substance. After Moeller. 



bonates. Recently another oxymethylanthraquinone-yielding sub- 

 stance, rheopurgarin, has been isolated from Shensi rhubarb. It 

 forms yellow needles, and appears to be composed of four glucosides: 

 (a) one related to emodin, (6) one related to rhein, (c) chrysophaneir 



