The fibro-vascular bundles of jalap are neither numerous nor large; they are accompanied by thin-walled cells, so that firm woody rays do not occur. Parenchymatous cells are abundant, and, on a longitudinal fracture especially, if subsequently moistened, are seen to constitute concentric layers. The laticiferous cells are always found in the outer part of each layer. The suberous coat with which the drug is covered is made up of the usual tabular cells.
The parenchyme of jalap is loaded with starch grains; in the pieces which have been submitted to heat in order to dry them, the starch appears as an amorphous mass, and the drug then exhibits a horny consistence and greyish fracture, instead of being mealy. Crystals of calcium oxalate are frequently met with. The laticiferous cells contain the resin of jalap in a semi-fluid state, even in the dry drug; drops of the resinous emulsion flow out of the cells, if thin slices are moistened by any watery liquid.
Chemical Composition—Jalap owes its medicinal efficacy to a resin, which is extractable by exhausting the drug with spirit of wine, concentrating the alcoholic solution to a small bulk, and pouring it into water. The resin precipitated in this manner is then washed and dried; it is contained in jalap to the extent of 12 to 18 per cent.[1640]
From this crude resin, which is the Resina jalapæ of the pharmacopœias, ether or chloroform extracts 5 to 7 (12, Umney) per cent. of a resin which, according to Kayser,[1641] partially solidifies when in contact with water in crystalline needles. We can by no means confirm Kayser’s statement. The residue (insoluble in ether) is one of the substances to which the name Jalapin has been applied.[1642] W. Mayer, 1852-1855, who designated it Convolvulin,[1643] found it to have the composition C₃₁H₅₀O₁₆. When purified, it is colourless; it dissolves easily in ammonia as well as in the fixed alkalis, and is not re-precipitated by acids, having been converted by assumption of water into amorphous Convolvulic Acid, which is readily soluble in water. Both convolvulin and convolvulic acid are resolved by moderate heating with dilute acids, or with emulsin, into crystallizable Convolvulinol, C₂₆H₅₀O₇, and sugar. Convolvulinol in contact with aqueous alkalis is converted into Convolvulinolic Acid, C₂₆H₄₈O₆, which is slightly soluble in water and crystallizable.
When convolvulin or its derivatives is treated with nitric acid, it yields several acids, one of which is the Sebacic Acid,
| C₈H₁₆ |  | COOH |
| COOH, |
which is to be obtained by treating castor oil or other fatty substances in the same manner. Sebacic acid forms crystalline scales, soluble in boiling water, melting at 128°. That from jalap was first thought to be a peculiar acid, and therefore termed ipomic or ipomœic acid. Its identification is due to Neison and Bayne (1874).
Convolvulin (dry) melts at 150° C., but a small amount of water renders it fusible below 100° C. It is insoluble in oil of turpentine and in ammonia. It dissolves in dilute nitric acid without becoming coloured or evolving gas. Convolvulin possesses in a high degree the purgative property of jalap, but this is not the case with convolvulinol.
The other constituents of jalap include starch, uncrystallizable sugar, gum, and colouring matter. The sugar, according to Guibourt, exists to the extent of 19 per cent.
Commerce—We have no means of knowing to what extent jalap is produced in Mexico. The imports of the drug into the United Kingdom amounted in 1870 to 169,951 lb. Very considerable quantities have of late (1873) appeared in the London drug-sales.