7.—Similarly represents, without its sheath, an absorbent from the leaf of Clusia flava.
8.—A longitudinal section through the axis of another such organ, showing its annuli of reticulated cells when cut through. The cellular tissue which fills the interior is supposed to be removed.
In concentric bundles one of the elements, either the xylem or the phloem, occupies the centre, and is more or less surrounded by the other, as seen in [Fig. 310]. Meristem tissue is never present, hence concentric bundles are always closed. They, however, occur in the stems of most ferns, and are always surrounded by a pericycle and endodermis, and should be regarded as steles. Concentric bundles with a central phloem occur in the rhizomes of some monocotyles, as Calamus, Iris, Convallaria, &c.
Fig. 317.—Vertical section of Sugar-cane Stem showing parachyma and crystalline cells, × 200 diameters.
The Stele, or Vascular Cylinder, is developed from the phloem of the growing plant, and consists of one or more vasal bundles imbedded in fundamental tissue, the whole being enclosed by a pericycle and an endoderm. The typical stele includes all the tissues evolved by the endodermis, which, however, forms no part of the vascular cylinder itself, but merely surrounds it. The pericycle is always the outermost layer of the tissues of the stele, while the endodermis is the innermost layer of the extra-stelar tissues.
The arboreus type of stem can be best followed by making sections of a twig of the elm (Ulmus campestris), which will be found to be cylindrical hirsute, green or brown according to age, the latter colour being due to the formation of cork. Small brown excrescences are scattered over its surface; these are termed lenticels. The cork will be seen to lie immediately below the epidermis, and to consist of cubical cells, with little or no cell contents; they are arranged in radial rows, without intercellular spaces. The walls of these cork cells will stain yellowish-brown with Schultze’s solution. Treat a thin section with sulphuric acid and the walls will swell out and gradually lose their sharpness of outline, with the exception of the cuticularised outer wall of the epidermis and the cork. This material is occasionally found developed in the twigs of the elm, so that it can be separated as thick radial plates of tissue.
“By comparing sections of twigs cut of various ages, the following information may be gleaned: That cork cambium, or phellogen, appears as a layer of cortical cells below the epidermis, and that these divide parallel to the surface of the stem. The result of successive divisions in this direction is the formation of secondary tissue, which develops externally as cork, internally as phelloderm. The true cork cambium consists of only a single cell in each radial row, from which, by successive division, all these secondary tissues are derived—i.e., cambium of vascular bundles. As stems grow older, layers of cork appear successively further and further from the external surface; not only the cortex, but also the outer portions of the phloem are thus cut off from physiological connection with the inner tissue. The term bark is applied to tissues thus cut off, together with the cork which forms the physiological boundary. The stem of Vitis affords a good example of such successive layers of cork.”
Fig. 318.