Structure of Iris.
By Edson S. Bastin.
The Blue Flag, Iris versicolor, Linné, is one of the commonest of monocotyls in the eastern United States. The range of its habitat is from Canada to Florida, and from the Atlantic as far West as Minnesota and the Indian Territory. Its rhizomes are horizontally creeping, from 16 to 24 cm. long, more or less branched, and composed of joints, each from 3 to 10 cm. in length, and representing a year's growth. Each joint at or near its base is cylindrical or only slightly flattened, but toward its apex is larger and widened horizontally. At the anterior end on the upper surface of each joint is a more or less cup-shaped scar of a flowering stem. At this end also may occur two, or sometimes four, lateral branches arranged opposite each other in pairs. The surface of the joints is densely covered with scales consisting of the fibrous bases of the decayed leaves, and from the inferior surfaces, chiefly from the broader, flattened portion of the joints, spring numerous, sparingly branching, wrinkled rootlets, averaging 10 or 12 cm. long and about 1 ½ mm. in thickness. These, together with the scales, are usually removed in preparing the drug for market. The dried drug, therefore, shows, except occasionally near the apex of the rhizome, only the crowded ring-like scars of the leaves and the small circular scars of the rootlets.
The rhizomes are also longitudinally wrinkled from shrinkage in drying, are commonly banded transversely with different shades of brown on the outside; the fracture is short, and the fractured surface is usually brownish or grayish brown.
A transverse section of the rhizome shows a distinct cylinder-sheath separating the central cylinder, which contains numerous scattered vasal-bundles, from the cortex, which contains relatively few. The thickness of the cortex, compared with the central cylinder, is about as one to five. The sheath proper consists of a single row of tangentially elongated and thickish-walled cells, but is strengthened interiorly by two or three thicknesses of tangentially elongated, somewhat fibrous cells.
The vasal-bundles of the central cylinder are much more crowded toward the exterior of the cylinder next the sheath, and are mostly smaller than the more scattered ones toward the centre of the stem. The bundles consist of that modification of the concentric type in which the xylem elements are exterior, and the phloem tissues central, and, as seen in transverse section, the bundles are either circular or somewhat elliptical in outline. The ducts are of rather small size. Each bundle has an imperfectly developed sheath of thin-walled cells, differing little from the cells of the adjacent parenchyma except in their smaller size.
Aside from the xylem elements of the bundles and the cylinder-sheath with its strengthening layer of fibrous elements, the tissues of the rhizome are unlignified. The cortex and fundamental tissues of the central cylinder consist of loosely arranged parenchyma. The cells of this parenchyma are notably unequal in size, and the intercellular spaces, though often large, are not regular either in size or in arrangement as they commonly are in the stems of other aquatic and marsh plants.
The parenchyma cells abound in rounded granular particles which look remarkably like starch grains, but which do not polarize light, and which stain brownish instead of blue with potassium-iodide iodine. In chloral-hydrate iodine they swell and gradually disappear, but without acquiring the blue color of ordinary starch. If sections be treated with a 15 per cent. solution of alpha-naphthol, afterwards with sulphuric acid, and then heated, the grains disappear and an intense violet color will be gradually developed in the tissues. This test justifies the suspicion that the grains, though behaving in some respects like proteid, may really be carbohydrate in their character, related to, if not in fact a modification of starch. But this matter requires further investigation.
There occur in the parenchyma, both of the cortex and of the central cylinder, rather numerous isolated crystals of calcium oxalate in the form of large-sized, mostly elongated and pointed prisms, which, between the crossed Nicols, show beautiful polarization effects.
The cross-section of a rootlet shows a structure so characteristic that it might be employed readily in the identification of the drug. The epidermis consists of two or three layers of rather small and thickish-walled cells. The cortical parenchyma consists of very unequal-sized, quite loosely arranged cells, with irregular intercellular spaces. The central bundle is from ten to fifteen rayed. The rays terminate interiorly in about six or eight large ducts, which form a circle about a small pithy central portion. The endodermis is composed of cells very distinct from those of the adjacent tissues. Its cells are of nearly equal size and excessively thickened in their inner and radial walls, which are also lignified, while their exterior walls remain thin and unlignified.