Description—The fruit when fresh is of the thickness of the little finger, obscurely triquetrous, opening longitudinally by two unequal valves. It is fleshy, firm, smooth, and plump; when cut transversely it exudes an inodorous slimy juice, abounding in spiculæ of oxalate of calcium.[2451] It is one-celled, with a three-sided cavity, from each wall of which projects a two-branched placenta, each branch subdividing into two backward-curling lobes. There are thus in all 12 ridges, which traverse the fruit lengthwise, and bear the seeds. Fine hair-like papillæ line as a thick fringe the three angles of the cavity, and secrete the odorous matter, which after drying is diffused through the whole pod. The papillæ likewise contain drops of oil, which is freely absorbed by the paper in which a pod is wrapped. That the odorous matter is not resident in the fleshy exterior mass we have ascertained by slicing off this portion of a fresh fruit and drying it separately; the interior alone proved to be fragrant.
The vanilla of commerce occurs in the form of fleshy, flexible, stick-like pods, 3 to 8 inches long, and ³/₁₀ to ⁴/₁₀ of an inch wide, of a compressed cylindrical form, attenuated and hooked at the stalk end. The surface is finely furrowed lengthwise, shining, unctuous, and often beset with an efflorescence of minute colourless crystals. The pod splits lengthwise into two unequal valves, revealing a multitude of minute, shining, hard, black seeds of lenticular form, imbedded in a viscid aromatic juice.
The finest vanilla is the Mexican. Bourbon Vanilla, which is the more plentiful, is generally shorter and less intense in colour, and commands a lower price.
Microscopic Structure—The inner half of the pericarp contains about 20 vascular bundles, arranged in a diffuse ring. The epidermis is formed of a row of tabular thick-walled cells, containing a granular brown substance. The middle layer of the pericarp is composed of large thin-walled cells, the outer of which are axially extended, while those towards the centre have a cubic or spherical form. All contain drops of yellowish fat and brown granular masses, which do not decidedly exhibit the reaction of tannin. The tissue further encloses needles of oxalate of calcium and prisms of vanillin.
On the walls of the outer cells of the pericarp[2452] are deposited spiral fibres, which occur still more conspicuously in the aërial roots and in the parenchyme of the leaves of other orchids. The placentæ are coated with delicate, thin-walled cells.
Chemical Composition—Vanilla owes the fragrance for which it is remarkable to Vanillin, which is found in a crystalline state in the interior or on the surface of the fruit, or dissolved in the viscid oily liquid surrounding the seeds. It was formerly regarded as cinnamic or benzoic acid, and then as cumarin, until Gobley (1858) demonstrated its peculiar nature.
The admirable researches of Tiemann and Haarmann performed in Hofmann’s laboratory at Berlin (1874-1876) have shown that vanillin is constituted according to the formula
 | OCH₃ | |
| C₆H₃ | OH. | |
| CHO |
It is the aldehyde. It is the aldehyde of methyl-protocatechuic acid, and like other aldehydes yields a crystallized compound with the bisulphites of alkalis. This is obtained by shaking an ethereal extract (e) of vanilla, with a saturated solution of bisulphite of sodium. The vanillin compound remaining in aqueous solution is mixed with sulphuric acid and ether; the latter on evaporation affords crystals of vanillin. They melt at 81°, and may be sublimed by cautiously heating them. Vanillin is but sparingly soluble in cold water, and requires about 11 parts of it at 100° C. for solution; it strikes a fine dark violet with perchloride of iron.
The said chemists have further demonstrated that vanillin may be formed artificially. In the sapwood of pines there occurs a substance called Coniferin, C₁₆H₂₂O₈ + 2 H₂O, first observed in 1861 by Hartig. By means of emulsin coniferin taking up H₂O, can be resolved into sugar and another crystallizable substance:—C₁₆H₂₂O₈ + H₂O = C₆H₁₂O₆ + C₁₀H₁₂O₃. The second substance thus derived may be collected by means of ether, which dissolves neither coniferin nor sugar. By oxidizing it, or coniferin itself, by bichromate of potassium and sulphuric acid, Vanillin is obtained. The latter has been for sometime manufactured in that way by Tiemann, but now eugenol ([see p. 285]) is used for that purpose. Another source for vanillin is benzoin ([p. 409]).