In judging of rhubarb, great stress is laid upon the appearance of the root when broken, and the circumstance of the fractured surface presenting no symptoms of decay, discoloration, or sponginess.[1839] In good rhubarb, the interior is found to be compact, and beautifully veined with reddish-brown and white, sometimes not unmixed with iron-grey. The root when chewed tastes gritty, by reason of the crystals it contains of oxalate of calcium; but it is besides bitter, astringent and nauseous. The odour is peculiar, and except by the druggist, is mostly regarded as very disagreeable.
Microscopic Structure—The tissue of rhubarb is made up of a white parenchyme, brown medullary rays and a few irregularly scattered very large fibro-vascular bundles, which are devoid of ligneous cells.
On a transverse fracture of specimens, which are not too much peeled, a narrow dark cambial zone may be distinguished. In that part of the root, only the medullary rays display the usual radial arrangement, and in the interior of the root no regular structure is met with. There is no well-marked pith, but the central portion of the tissue shows a mixture of white parenchyme and brown medullary rays running in every direction. In full-grown roots, the central part is separated from the cambial zone by the band of stellate patches[1840] already mentioned.
As to the contents of the white cells, they are loaded either with starch or tufted crystals of oxalate of calcium, the amount of the latter being especially liable to variation. Scheele, after having discovered the oxalic acid, pointed out in 1784 that the crystals under notice consist of that acid in combination with lime; he was the first to point out the true composition of those crystals which are of so wide a distribution throughout the vegetable kingdom. The medullary rays contain the substances peculiar to rhubarb, but none of them occur in a crystalline state.
Chemical Composition—The active constituent of the root has long been supposed to reside in the yellowish-red contents of the medullary rays. Schrader as early as 1807 prepared a Rhubarb-Bitter, to which he attributed the medicinal powers of the drug. Since then several substances of the same kind have been separated by various methods, and described under different names: such are the Rhabarberstoff of Trommsdorff, the Rheumin of Hornemann, the Rhabarberin of Buchner and Herberger, the Rhubarb-Yellow or Rheïn, and the Rhabarbaric Acid of Brandes.
Schlossberger and Döpping in 1844 first recognized among the above-named substances a definite chemical body named Chrysophan or Chrysophanic Acid,
| C₁₄H₅ |  | CH₃ | O₂, |
| (OH)₂ |
which had been found in 1843 by Rochleder and Heldt in the yellow lichen, Parmelia parietina. It partly forms the yellow contents of the medullary rays of rhubarb, and when isolated crystallizes in golden yellow needles or in plates. It dissolves in ether, alcohol, or benzol; though scarcely soluble in water, it is nevertheless extracted from the root to some extent by that solvent, probably by reason of some accompanying substance. Alkalis dissolve it, forming fine dark red solutions. Chrysophan, C₁₅H₁₀O₄, is a derivative of anthracene, C₁₄H₁₀, and closely allied to alizarin, C₁₄H₈O₄.
By precipitating alcoholic solutions of extract of rhubarb with ether, Schlossberger and Döpping obtained, together with chrysophan, resinous bodies which they named Aporetin, Phæoretin and Erythroretin.
De la Rue and Müller (1857) extracted from rhubarb, in addition to chrysophan, an allied substance, Emodin, which crystallizes in orange-coloured prisms, sometimes as much as two inches long. Its constitution was subsequently found to agree with the formula