The endodermis is contained in the stele

Section from the vascular cylinder of a cactus (Opuntia echiops). Due to the lower rate of putrefaction of lignified parts compared to the other tissues, lignified sprouts exposed in this way are very often found in natural cactus locations.

The statements made so far about the construction of vascular bundles are based on the evaluation of cross-sectional preparations of stems and roots. As we have seen, these are suitable for representing the position and organization of the vascular bundles (position of xylem, phloem, cambium, etc.), but they say nothing about the spatial (three-dimensional) arrangement and the origin of the vascular bundle system. It is not easy to get a clear picture of this. Serial cuts and the resulting reconstruction of the vascular bundle course are a necessary, but very time-consuming process.

Another possibility is the maceration of the stem, that is, practically a controlled putrefaction process, in which it is assumed that the vascular bundles are among the most resistant tissues and survive the maceration process largely undamaged.

Despite methodological adversity, the paths mentioned have been followed with success. Today we know the course of the vascular bundles of numerous plant species from the most varied of classes; additional information was obtained by evaluating fossils.

A first comprehensive, richly illustrated overview of the course of the vascular bundles can be found in the "Comparative Anatomy of the Vegetation Organs of Phanerogams and Ferns", published in 1877 by the plant anatomist A. de BARY, who worked in Strasbourg. He recognized that the organization of the vascular bundles in the individual plant groups can be traced back to certain basic patterns. The terms axial strand (axial strand), leaf trace (strand) and leaf gap, which are still used today, also come from him. Nowadays it is customary to speak of the stelar theory when dealing with the course and formation of vascular bundles.

The concept of the stele goes back to P. van TIEGHEM, professor at the Musée d'Histoire Naturelle de Paris and his student H. DOULIOT (1886). Your suggestions have been revised and modified many times. The terminology proposed by van TIEGHEM is now only of historical interest.

Especially at the turn of the century there was hardly a well-known morphologist who did not deal with this problem. Particularly noteworthy are E. STRASBURGER (Professor of Botany at the University of Bonn) with his 1891 work "About the construction and the devices of the conduction pathways in the plant", EC JEFFREY (University of Toronto, later Harvard University), the 1898 one A number of new terms were defined, including protostele and siphonostele (which we will deal with in detail later), as well as the Englishmen G. BREBNER (1902) and FO BOWER.

The different use of terms and an independent nomenclature contributed more to the confusion than to the clarification, especially at that time. The situation stabilized around 1910, and definitions were agreed which have entered the literature as the "British System".

The vascular bundle system, including the tissue associated with it and the medulla surrounded by the vascular bundle cylinder, are summarized under steles. The expression stele comes from the Greek, like so many expressions in plant anatomy, and means column. The stelar theory states that the primary plant body in the stem and in the root is constructed according to the same principle, because both contain a central column (stele) which is embedded in primary bark tissue. The stelar theory is not concerned with secondary growth in thickness. The stelar pattern can be viewed as a conservative feature, the complexity of which increased in the course of evolution.

The protostele, a simply organized guidance system. The protostele is a simple, unbranched, centrally located axial strand of xylem, which is encased by the phloem or in which phloem parts are interspersed in the xylem. A protostele does not contain a marrow. They are found in primitive vascular plants, e.g. fossil ones Rhynia, as well as in young fern shoots, in the stems of some simple aquatic plants and in most of the roots of angiosperms. One variant, the plectostele, is typical of the lycopodia (bear moss family)

Simple stele from the root of Lycopodium.

Plectostele from one Lycopodium Sprout: The xylem looks plate-shaped (plecto = plate). Each of the plates is surrounded by phloem. The structure is much more complex than that of Psilotum (see below). The leaf marks (vascular bundles that lead from the central stele into the leaf veins) are remarkably small.

© David T. WEBB, University of Hawaii at Manoa: BOT 311 Form & Function in Algae & Plants

The siphonostele in the broadest sense. The siphonostele is the prototype of the vascular bundle systems of ferns and all further developed vascular plants (seed plants). It consists of several axial vascular bundles that are arranged in the stem in the form of a hollow cylinder and enclose a medullary tissue.

Actinostele of Psilotum: The star-shaped xylem (Actino = star) is surrounded by phloem.

Ferns (here: Dicksonia) are characterized by a siphonostele. It consists of a compact cylinder made of conductive fabric. Part of the stele radiates into the leaf bases that are arranged around the trunk. The leaf tracks are V-shaped and the overall structure resembles a sun with rays emanating from it.

© David T. WEBB, University of Hawaii at Manoa: BOT 311 Form & Function in Algae & Plants

Leaf and branch tracks branch off from the axial vascular bundles and extend into the leaves or side branches. A leaf trail always begins at the junction and extends to the leaf base (leaf attachment point). A leaf can be supplied by one or more leaf traces. The axial bundles (axial vascular bundles) extend over the entire length of a stem section. If one visualises how vascular bundles arise in the course of ontogenesis, it becomes immediately clear that they are composed of segments that were originally laid out as leaf traces.

The transition from leaf trace to vascular bundle occurs as soon as a leaf trace placed towards the apical point makes contact with the existing vascular bundle system.

Axial bundles are usually networked with one another through cross connections, which creates a communicating line system (K. J. DORMER, 1954). The areas where leaf traces branch off are the nodes or nodes and the sections between them are the internodes. The structure of the nodes in the stems of the dicotyledons is relatively simple, complex networks occur predominantly in the monocotyledons.

Some examples: Simple relationships occur in primitive fossil precursors of gymnosperms (progymnosperms), e.g. in the species Callixylon browniiwhose vascular bundle system was reconstructed by C. B. BECK in 1979. It shows vascular bundles arranged in parallel, between which there are only a few connections (anastomoses). A considerable increase in the complexity of the branching pattern occurs in the gymnosperms.

A distinction is made between the open vascular bundle systems and the closed ones, in which the leaf tracks are connected to one another through anastomoses. At Ginkgo biloba each leaf is supplied by two leaf traces derived from separate but adjacent axial bundles. In the Cycadales (e.g. Dioon spinulosum) there are five leaf traces per leaf, which arise from distant axial strands. The leaf traces initially lie against the vascular bundle cylinder in a ring before they branch off together towards the leaf (H. A. DORETY, 1919). Here we are already dealing with a rather complex organization of the node structure.

Angiosperms are characterized by a high degree of variability in the construction of the vascular bundle cylinders, although the basic patterns are largely identical. However, there is a major difference between the dicot and the monocot. The situation with the dicotyledons is closest to the basic pattern, is clearest and is therefore discussed first:

The variations are based on the direction and pitch of the screw, the number of tracks per blade and the nature of the blade attachment point. In this context, the term leaf gap must be used, which refers to a recess in vascular bundle cylinders above the leaf attachment point.

Left: Siphonostele without leaf gaps. Middle and right: Siphonosteles with traces of leaves (after Y. OGURA, 1938).

As a rule, angiosperms contain five vascular bundles. This number is viewed as a basic unit from which variants with more or fewer strands are derived. Open systems occur in 91 percent of all species, closed systems (various leaf traces and leaf trace complexes connected to one another) in the others (both in herbs and wood plants). The number of vascular bundles per vascular bundle strand is there even with four as the basic number.

In many cases, leaf gaps are very large (e.g. in many angiosperms, including cacti, but also in cycads and ferns). The vascular bundle cylinder therefore looks like a network.

In summary, the following trends can be identified in the course of the evolution of seed plants (C. B. BECK et al., 1982):

  1. Open vascular bundle systems are primitive, closed ones derived.

  2. The unilacunar type (only one leaf gap) is primitive, multilacunar are considered to be derived. Exceptions, transitions and regressions can be found in many dicotyledons. Often one can find two or more types within one plant. The unilacunar knot system appears in older geological layers than the trilacunar. It is the most common type in primitive seed plants.

  3. Vascular bundle systems with five strands are presumably primitive, those with more or fewer strands are derived.

  4. Steles (vascular bundle columns) with leaf traces that pass through one or only a few internodes in the longitudinal direction can be regarded as primitive, those that pass through many internodes as derived.

  5. In the case of seed plants, a helical (screw-like) arrangement of the leaf systems is considered more primitive than any other arrangement.

In the monocotyledons, the vascular bundles seem to be distributed over the entire stem, approaching each other near the periphery. If you look at the course of a single axial bundle, you can see a clear, helical shape. A vascular bundle never lies only centrally or only peripherally. Depending on where you cut the stem, you will find it sometimes in the center, sometimes in a peripheral position. Leaf traces branch off at regular intervals. They too run long distances in the stem, but are usually peripheral. This is also the main reason why a particularly large number of vascular bundles can be seen there in cross-sections. One part of them represents axial strands, another shows leaf traces. In cross-sections, they are usually indistinguishable from one another, although axial strands usually have a slightly larger diameter. Axial bundles can be linked to one another by bridge bundles.

Taking this construction plan into account, the statement obtained by looking at cross-sections that the vascular bundles are scattered in the stalk appears misleading. The construction plan shows that here, as with the dicotyledons (and the gymnosperms), there is a clear rotational symmetry, in which the shape of the vascular bundle cylinder is, however, largely modified, even appears to be canceled in cross-section. Each vascular bundle is placed in a peripheral position during ontogenesis, and only during subsequent growth do individual sections come in stretches to the center of the stem.