The bundle-sheath of the young stem is more easily recognised than in the older stem. It is, in fact, a continuous layer of cells, whose radial walls have a characteristic dark spot on each radial wall. The bundle-sheath lies immediately outside the vascular bundles, curving slightly towards the centre of the stem in the spaces between the bundles. It is more prominent in the stem when very young, as the cells are then filled with starch granules. This layer of cells will be readily seen in sections treated with iodine.
In dicotyls and gymnosperms the medullary rays consist essentially of interfascicular ground tissue. The medullary conjunctive tissue occupies the centre of the stele, constituting the so-called pith, and usually consists of parenchymatous cells, but may contain, in addition, either stone cells, sclerenchyma fibres, laticiferous or glandular tissues.
The Fibro-vascular or Conducting Tissue System.—This system constitutes the fibrous framework of the plant, and is the system by means of which fluids are conducted from one part of the plant to another. Its function is partly to give strength and support, but principally to conduct the crude and elaborated juices to and from the leaves. It is found only in the higher plants, constituting the tough and stringy tissues in stems and roots, and the system of veins in leaves. The fibro-vascular system consists essentially of vascular tissue (ducts, tracheids, and sieve-tubes), and forms long strands—the fibro-vascular bundles—which extend vertically through the fundamental tissues of the plant. The term “fibro-vascular,” as applied to the conducting system, is not strictly correct, since fibres do not always accompany the vascular elements, hence this system is often spoken of as the vascular system, and the bundles as vascular, or more briefly as vasal bundles.
That the arrangement, and course of the vascular bundles in dicotyledous stems are connected with those of the leaves is an obvious fact. It may be seen in sections of Helianthus, but is more markedly shown in plants with regularly decussate leaves, as Cerastium, Clematis, &c. Still, the arrangement of the bundles may differ radically from that of the leaves, and is, to a certain extent, independent of them. This will be noticed in sections of Iberis amara, where the bundles do not run longitudinally, but in tangential spirals. These, as Nägeli pointed out, have no direct relation with the leaves; and he recommends a series of types for investigation, in which it will be seen how closely the arrangement of the bundles is connected with the arrangement of the leaves, and the number of bundles entering the stem from each leaf: Iberis amara, leaves alternate, leaf-trace with one bundle; Lupinus, leaves alternate, leaf-trace with three bundles; Cerastium, leaves opposite, leaf-trace with one bundle; Clematis, leaves opposite, leaf-trace with three bundles; Stachys, leaves opposite, leaf-trace with two bundles.
Fig. 315.
1. Transverse section of the stem of Cedar, showing xylem or wood; 2. Section of stem of Conifer, the phloem and zones of annual growth; 3. Section of an Ivory Nut, cells, and radiating pores; 4. Section of the outer or ligneous portion of same, with radiating cells.
The connection of the leaf and stem will be best seen by cutting longitudinal sections through a young node of Helianthus, so as to include the median plane of the leaf, or of both leaves if opposite to each other, as they often are; steep them in dilute potash for twenty-four hours and mount in glycerine. A medium power will serve for their examination. The course of the vascular bundles will appear dark through the more transparent parenchyma. The continuity of the tissues of the stem and petiole if followed will be seen to have no definite boundary between the two parts; the bundles from the petiole pass into the stem, and no bundle of the upper internode lies in the same vertical plane as that which enters from the petiole between two successive bundles of the vascular ring.
Every complete vasal bundle consists of xylem or wood and phloem or bast.
The former consists essentially of trachery tissue (ducts and tracheids), and may contain in addition both wood fibres and wood parenchyma. The phloem or bast consists essentially of sieve tissue, and usually contains some ordinary parenchyma. In angiosperms companion-cells always accompany the sieve-tubes in the phloem, while in gymnosperms they are absent.