This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
The structure of commercial rhubarb indicates that the drug is derived from two species of Rheum. One of these is certainly R. officinale, Baillon; the other is probably R. palmatum, Linne, var. tanguticum, Maximowicz. The former occurs abundantly in central and western China at an elevation of 3,000 to 4,000 metres, and is collected chiefly in the mountains separating Tibet from the Chinese province of Szechuen and extending eastwards to Hupeh. The plant produces a rhizome which may attain a very great size, and to which large thick roots are attached. The plants are dug up towards the end of September and the roots cut off. The rhizomes are cleaned, and the crown and the bark removed, the larger being cut either longitudinally or transversely. They are dried by stringing them on cords which are then stretched from tree to tree, or on hurdles in huts in which a fire is kindled, or on heated stones. The drug is conveyed chiefly by river to Chung-King on the Yang-tse-Kiang river, and thence to Shanghai, whence it is shipped to Europe, after having undergone further trimming and sorting on its journey.
Rheum palmatum, Linne, var. tanguticum, Maximowiez, occurs in the province of Kansu. Large quantities of the drug are collected near Lake Kokonor in Tibet, and are apparently derived from this plant. It is carried over the mountains to the Han river, and thus reaches Hankow. Further quantities of rhubarb are also produced in the province of Shensi, and may possibly be obtained from a third, at present unknown, species of Rheum.
Practically all the Chinese rhubarb now exported passes through Hankow to Shanghai for shipment to Europe, and consists of rhizome only.
Formerly the drug was conveyed by caravan via Persia to the Syrian ports, whence it reached Europe and was known as Turkey rhubarb; some was sent direct from China (China or Canton rhubarb), and some was shipped via India (East Indian rhubarb), the latter being the most common variety as early as 1640. Subsequently, by a commercial treaty between Russia and China, the trade was diverted to the Russo-Siberian route, and the drug was conveyed via Irkutsk to Moscow, still retaining its designation of Turkey rhubarb. Later, however, when the Chinese opened some of their northern ports to European trade, the drug was exported by the route which it now takes, and the trade names of Turkey and East Indian rhubarb indicate nothing more than the routes by which the drug formerly found its way to the European market.
Fig. 187. - Case of Chinese Rhubarb.
The official drug occurs in pieces of very varying size and shape. Those derived from smaller rhizomes that have not been sliced occur in cylindrical, conical, or barrel-shaped pieces varying in size, but frequently from 7 to 10 cm. in length and 3 to 6 cm. in thickness, although of course they may be larger or smaller. The larger rhizomes, which are usually cut longitudinally, yield plano-convex pieces, frequently tapering more or less towards either end; such pieces often measure 8 or 10 cm. in length or breadth and 4 cm. in thickness. These two principal forms are known as ' rounds ' and 'flats' and are commonly sold in separate cases. They are frequently drilled, a piece of string being occasionally found in the perforation. The outer surface is dusted over with a fine bright yellow powder, and bears evidence of having been not only peeled with a knife, but carefully dressed with a file or scraped after the drug has been dried. Here and there small portions of a dark layer may still be found. These characters are common to the commercial varieties of Chinese rhubarb generally, but the following apply more particularly to the finest variety, viz. Shensi rhubarb.
Fig. 188. - Chinese Rhubarb (Shensi flat), showing network of whitish lines.
If the outer surface, freed from the yellow powder that adheres to it, is examined with a lens, a number of minute, longitudinal, dark reddish brown lines and dots can be observed imbedded in a white ground mass, which latter frequently forms a delicate network, in the meshes of which the alternating white and dark red lines alluded to can be seen. The dark lines are medullary rays containing brown colouring matter; the white lines consist of bast parenchyma containing starch and calcium oxalate. Occasionally minute dark points or projections may be detected which, when carefully shaved off with a knife, exhibit radiating red and white lines; these are the remains of fibro-vascular bundles from the leaves (leaf-traces) and pass through the bark into the central portion of the rhizome.
The drug is firm, heavy, and compact, the outer surface showing little sign of shrinkage during the drying; it breaks with an uneven fracture, the fractured surface, which varies from bright pink to dull grey in colour, exhibiting under the lens dark reddish brown lines alternating with white ones ('nutmeg' fracture). The transverse section shows near the periphery a more or less continuous ring of large, conspicuous, star-like spots, each of which consists of dark red lines (medullary rays) radiating from a common centre through a white groundwork (parenchyma). These star-spots are fibro-vascular bundles which also traverse the inner part of the drug in varying directions. Exterior to this ring the drug exhibits a radiate appearance due to the formation of a narrow ring of secondary wood by the cambium, which is sometimes evident as a dark line about 1 mm. from the margin (compare fig. 189), but is often not seen, as not only the whole of the bark but even part of the secondary wood may have been removed by the peeling. If the peeling has been sufficiently deep to remove the bark and the secondary wood as well, then the star-spots appear on the outer surface, but until then they are indistinct. They can always be seen on the flat (inner) surface of the plano-convex pieces. Beyond the secondary wood is the cambium, usually visible as a dark line, and here and there are the narrow remains of the bark. Small cylindrical pieces are generally more favourable for examination than flat slices of a larger rhizome.