=710. Central cylinder or stele.=—As the central cylinder is organized from the plerome it becomes differentiated into the vascular bundles, the pith, the pith-rays (medullary rays) which radiate from the pith in the center between the bundles out to the cortex, and the pericycle, a layer of cells lying between the central cylinder and the cortex. The bundles then are farther organized into the xylem and phloem portions with their different elements, and the fascicular cambium (meristem) separating the xylem and phloem, as described in Chapter V. Such a bundle, where the xylem and phloem portions are separated by the cambium is called an open bundle (as in fig. 58). Where the phloem and xylem lie side by side in the same radius the bundle is a _collateral_ one. Dicotyledons and conifers are characterized by open collateral bundles. This is why trees and many other perennial plants continue to grow in diameter each year. The cambium in the open bundle forms new tissue each spring and summer, thus adding to the phloem on the outside and the xylem on the inside. In the spring and early summer the large vessels in the xylem predominate, while in late summer wood fibers and small vessels predominate and this part of the wood is firmer. Since the vascular bundles in the stem form a circle in the cylinder, this difference in the size of the spring and late summer wood produces the “annual” rings, so evident in the cross-section of a tree trunk. Branches originate at the surface involving epidermis, cortex, and the bundles. In monocotyledonous plants (corn, palm, etc.) the bundles are not regularly arranged to form a hollow cylinder, but are irregularly situated through the stele. There is no meristem, or cambium, left between the xylem and phloem portions of the bundle and the bundle is thus _closed_ (as in fig. 60), since it all passes over into permanent tissue. In most monocotyledons there is, therefore, practically no annual increase in diameter of the stem. [Illustration: Fig. 419. Section of stem (rhizome) of Pteris aquilina. _sc_, thick-walled sclerenchyma; _a_, thin-walled sclerenchyma; _par_, parenchyma.] =711. Ferns.=—In the ferns and most of the Pteridophytes an apical meristem tissue is wanting, its place being taken by a single apical cell from the several sides of which cells are successively cut off, though Isoetes and many species of Lycopodium have an apical meristem group. In most of the Pteridophytes also the bundles are _concentric_ instead of collateral. Fig. 418 represents one of the bundles from the stem of the polypody fern. The xylem is in the center, this surrounded by the phloem, the phloem by the phloem sheath, and this in turn by the endodermis, giving a concentric arrangement of the component tissues. A cross-section of the stem (fig. 419) shows two large areas of sclerenchyma, which gives the chief mechanical support, the bundles being comparatively weak.