In vascular plants the xylem and phloem are nearly always found in close association in strands or bundles, but as is to be expected in so complex and varied a group as the Pteridophytes the arrangement of such bundles is by no means stereotyped either in the group or in the different parts or members of the plant-body. A connected vascular system runs, however, through the entire body. In the roots and leaf-blades the vascular system is comparatively simple. In the former a solid, slender xylem core is virtually invested by phloem, with a single layer of pericycle as a mantle in direct contact with a continuous cylindrical endodermis, beyond which is the bulky and often sclerotic cortex. The arrangement is thus virtually radial. The bundles in the leaf-blade are on a some what similar plane of simplicity, but their phloem is directed downwards, their xylem upwards, the tracheids are dominantly spirally thickened, and the phloem is scanty or entirely absent towards the slender bundle-endings. Similarly throughout the entire length of stem there may be a solid xylem core, sur rounded successively by phloem, pericycle and endodermis. Such a vascular system is a solid protostele. The slender stems of most juvenile Pteridophytes are protostelic and despite widening of the stele in the more bulky portions of the adult plant, this primitive protostelic state may be maintained throughout the entire stem. In many cases, however, as in advanced ferns, the vascular system of the juvenile stem alone is a solid protostele. When followed upwards the core of the xylem strand may remain entirely parenchymatous. This parenchymatous core is the me dulla or pith, and the stele is here called a medullated protostele. At higher levels the stem may show a different vascular arrangement, sieve tubes being differentiated in the centre of the medulla, at first as a solid strand, but later or higher as a cylinder with parenchymatous pith at its core and parenchyma between it and the xylem. At a still higher level there may be an inner endodermis between the pith and the inner phloem so that the succession of tissues from within as viewed in a transverse section of the stem may be medulla, inner endodermis, parenchyma (or inner pericycle), inner phloem, parenchyma, xylem, parenchyma, outer phloem, outer pericycle and outer endodermis. Such a local arrangement of the stelar elements is said to be siphonostelic, is considered an evolutionary advance on the protostelic state, and may characterize the entire upper portion of the adult stem.
When the vascular system is entirely protostelic the decurrent bundles from the leaves join the vascular system of the stem without disturbing the continuity of the latter. Thus throughout the entire plant the endodermis is an unbroken mantle. But in the siphonostelic portions of the vascular system of the stem, the pith and cortex are commonly in open parenchymatous con nection through gaps in the stele, and round the edges of these gaps the inner and outer endodermal cylinders are united by endodermal cells. Thus here again the endodermis is a continuous tissue, and isolates completely the pith and the cortex from the vascular tissues. The gaps which occur immediately above the level of insertion of the leaf-trace bundles on the stele of the stem are called foliar gaps, while those which occur elsewhere are called perforations. When there is no overlapping of leaf
gaps the siphonostele is said to be solenostelic; when two or more leaf-gaps overlap, the stele is dictyostelic. Thus locally the vas cular system of a Pteridophytic stem may be solenostelic or dictyostelic : likewise it may be locally a perforated solenostele or a perforated dictyostele. Rarely among ferns is there more than a single vascular cylinder. The condition is described as polycyclic, and an outer and inner cylinder may be even united by oblique lateral strands. In such cases the leaf-trace strands are joined to the outer cylinder, while the inner cylinder may best be considered concerned with internal conduction.
The typical struc ture of the vascular cylinder of the adult primary stem of the Gymnosperms (firs and their allies) and Dicotyledons (the domi nant group of higher flowering plants)—all of which are seed bearing plants—is, like that of the higher ferns, a hollow cylinder of vascular tissue enclosing a central parenchymatous pith. But unlike the ferns the seed-plants have no internal phloem (except as a special development in certain families) and there is no internal endodermis. The xylem and phloem rarely form perfectly continuous layers as in the solenostelic f erns, for the vascular tissue is typically separated into distinct collateral bundles, the xylem of which is usually wedge-shaped in cross section with the first formed xylem (protoxylem) at the inner extremity, while the phloem forms a band on the outer side of the xylem, and is separated from the latter by a band of conjunctive tissue (mesoderm). These collateral bundles are separated from one another by bands of conjunctive tissue called primary medullary rays, which may be quite narrow or of considerable breadth.
When the pith is large-celled, the xylem of the bundles is often separated from it by a distinct layer of conjunctive tissue called the endocycle, and a similar layer, the pericycle, separates the phloem from the cortex. The inner layer of the cortex (phloeo therma) may form a well-marked endodermis, or may differ in other ways from the rest of the cortex. The pericycle, medullary rays, endocycle and mesoderm all form part of one tissue-system, the external conjunctive. The external conjunctive is usually a living and comparatively small-celled tissue, the cells of which are greatly elongated in the direction of the stem-axis and may be collenchymatous, sclerenchymatous, fibrous or food-storing. The conjunctive tissue of the stem is open to secondary changes, bound up with the thickening of the stem as it grows old. Most of the collateral bundles are leaf-trace bundles, i.e., they can be traced upwards from any given point till they pass out of the cylinder, travel through the cortex of the stem, and enter the leaves. The remaining bundles (compensation bundles) which go to make up the cylinder, are united at some level with the leaf trace bundles, and in turn form the traces of leaves at some higher level. Purely cauline vascular strands (i.e., confined to the stem) are rare in the flowering plants, though common in Pteridophytes. The leaf-trace of any given leaf rarely consists of a single bundle only, the number of bundles in a trace is gen erally odd, and the median bundles of the trace are typically the largest.