Description—The ovary of the flower is one-celled with numerous ovules, which as they advance towards maturity become separated by the growth of intervening septa. The ripe legume is cylindrical, dark chocolate-brown, 1½ to 2 feet long by ¾ to 1 inch in diameter, with a strong short woody stalk, and a blunt end suddenly contracted into a point. The fibro-vascular column of the stalk is divided into two broad parallel seams, the dorsal and ventral sutures, running down the whole length of the pod, The sutures are smooth, or slightly striated longitudinally; one of them is formed of two ligneous bundles coalescing by a narrow line. If the legume is curved, the ventral suture commonly occupies its inner or concave side. The valves of the pods are marked by slight transverse depressions (more evident in small specimens) corresponding to the internal divisions, and also by inconspicuous transverse veins.
Each of the 25 to 100 seeds which a legume contains, is lodged in a cell formed by very thin woody dissepiments. The oval, flattish seed from ³/₁₀ to ⁴/₁₀ of an inch long, of a reddish-brown colour, contains a large embryo whose yellowish veined cotyledons cross diagonally, as seen on transverse section, the horny white albumen. One side is marked by a dark line (the raphe). A very slender funicle attaches the seed to the ventral suture.
In addition to the seeds, the cells contain a soft saccharine pulp which in the recent state fills them up, but in the imported pods appears only as a thin layer, spread over the septum, of a dark viscid substance of mawkish sweet taste. It is this pulp which is made use of in pharmacy.
Microscopic Structure—The bands above described running along the whole pod, are made up of strong fibro-vascular bundles mixed with sclerenchymatous tissue. The valves consist of parenchymatous cells, and the whole pod is coated with an epidermis exhibiting small tabular cells, which are filled with dark granules of tannic matter. A few stomata are also met with. The thin brittle septa of the pod are composed of long ligneous cells, enclosing here and there crystals of oxalate of calcium.
The pulp itself, examined under water, is seen to consist of loose cells, not forming a coherent tissue. They enclose chiefly granules of albuminoid matters and stellate crystals of oxalate of calcium. The cell-wall assumes, on addition of iodine, a blue hue if they have been previously washed by potash lye. The seeds are devoid of starch, but yield a copious amount of thick mucilage, which surrounds them like a halo if they are macerated in water.
Chemical Composition—No peculiar principle is known to exist either in the woody or the pulpy portion of cassia fistula. The pulp contains sugar in addition to the commonly occurring bodies noticed in the previous section.
Uses—The pulp separated from the woody part of the pods by crushing the latter, digesting them in hot water, and evaporating the strained liquor, is a mild laxative in common domestic use in the South of Europe,[863] but in England scarcely ever now administered except in the form of the well-known Lenitive Electuary (Confectio sennæ) of which it is an ingredient.
Commerce—Cassia fistula is shipped to England from the East and West Indies, but chiefly from the latter. The pulp per se has been occasionally imported, but it should never be employed when the legumes for preparing it can be obtained.
Substitutes—The pods of some other species of Cassia share the structure above described and have been sometimes imported.
Those of C. grandis L. f. (C. brasiliana Lamarck), a tree of Central America and Brazil, are of much larger size, showing when broken transversely an elliptic outline, whose longer diameter exceeds an inch. The valves have very prominent sutures and transverse branching veins. The pulp is bitter and astringent.