Stele Types

As noted earlier, the stele is defined as all tissues inside of, but not including, a distinct physiological barrier or boundary layer such as the endodermis (including the conducting tissue), after a concept called the stelar theory, which was initially developed by Van Tieghem and Douliot (1886a, b). The stelar theory is not used today by most botanists working with living plants as it is sometimes difficult to recognize the outer boundary of the stele as originally defined, and because stelar configuration can vary at different developmental stages of the plant or at different levels within a single axis. Nevertheless, the concept has been useful in comparative and phylogenetic studies of fossil vascular plants. In a general sense, stele types become progressively more complex in the fossil record and certain plant groups are characterized by particular types, so a knowledge of stele types is useful in paleobotany. For a treatment of stelar terminology and classification, see Schmid (1982) or Brebner (1902).

Primitive Vascular Plants (Vascular Cryptogams) The simplest type of stele is a protostele, which consists of a solid core of xylem (no pith) in the center of the axis. Stems of many primitive plants and most roots are protostelic. There are three basic types of protostele: haplostele (FIG. 7.32), actinostele, and plectostele (FIG. 7.33). In a haplostele, the xylem is circular in cross section or cylindrical in three dimensions; phloem is immediately outside the xylem. An actinostele exhibits armlike projections of the xylem in cross section or ridges in three dimensions, with the phloem in

Fern Root Cross Section Stele Endodermis
figure 7.32 Cross section of Gleichenia sp. rhizome showing haplostele (Extant). Bar = 650pm.
Anatomy Lycopodium Serratum
figure 7.33 Cross section of Lycopodium serratum stem showing actinostele with exarch xylem development (Extant). Bar = 200 pm.

the furrows. Many roots are simple actinosteles, as they are usually diarch (two arms), triarch, or tetrarch. A plectostele exhibits many lobes in cross section, and it may appear as if the xylem is in separate plates (FIG. 7.34); phloem occurs between the plates. A three-dimensional view of a plectostele, however, indicates that the plates are interconnected. Both plectosteles and actinosteles occur in the extant Lycopodiales.

A siphonostele occurs in vascular cryptogams that have a pith, with the xylem and phloem forming a continuous cylinder around the pith (FIG. 7.35 ). A solenostele, or amphiphloic siphonostele (FIG. 7.36), has phloem on both the outside and inside of the xylem (FIG. 7.37 ) ; an

Siphonostele Tissue System
figure 7.34 Cross section of Lycopodium sp. stem showing plectostele (Extant). Bar = 300 pm.
Anatomy Lycopodium Serratum
figure 7.35 Cross section of Helmenthostachys siphonostele showing leaf trace (arrow) (Extant). Bar = 780 pm.

ectophloic siphonostele has phloem only on the outside of the xylem. Both ectophloic and amphiphloic siphonosteles are found in the ferns.

When leaves are produced, a leaf trace is given off from the stem stele and it supplies the leaf with xylem and phloem. In a series of cross sections through a fern stem in the region of leaf attachment, parenchyma cells appear in the siphono-stele at the point of leaf trace emission and there is continuity between the pith and cortex at that level. At higher levels, the leaf trace extends upward and outward into the base of the leaf. This interruption of the stelar cylinder is called a leaf gap, even though it is not actually a space as the name implies, but rather parenchymatous tissue. At higher

Marsilea Rhizome
figure 7.36 Cross section of Marsilea quadrifolia rhizome showing amphiphloic siphonostele (Extant). Bar = 300 pm.
Anatomy Marsilea Rhizome Pictures
figure 7.37 Cross section of Psaronius vascular tissue showing preservation of phloem (arrows) (Pennsylvanian). Bar = 1mm.

levels, the xylem and phloem at the edges of the gap appear closer together and eventually the interruption is no longer present. In some taxa, many leaf gaps are present in a single cross section and the stele appears to be dissected into

Dictyostele
figure 7.38 Cross section of Pteridium aquilinum rhizome showing dictyostele (Extant). Bar = 2 mm.
Pteridium Aquilinum Stem Anatomy
figure 7.39 Cross section of Osmunda sp. rhizome showing dissected stele and leaf traces (arrows) (Extant). Bar = 2 mm.

segments. Such a dissected siphonostele is called a dictyostele (FIG. 7.38). Dictyosteles are typical of many extant and fossil ferns (FIG. 7.39).

Since the simplest and oldest stelar type is a protostele, it is hypothesized that plants with siphonosteles evolved from pro-tostelic ancestors, and there are examples in the fossil record that support this hypothesis. Two principal theories have historically been advanced to explain the origin of the siphonos-tele from the protostele (Ogura, 1972). The intra-stelar origin theory (FIG. 7.40 ) suggests that, during the course of ste-lar evolution in some plant groups, cells in the center of the protostele did not mature into tracheids (FIG. 7.41B). The resulting medullated protostele would represent an intermediate

Siphonostele And ProtosteleSiphonostele

figure 7.41 Diagrammatic stages in the evolution of the siphonostele according to the extrastelar theory: A. haplostele; B. departure of leaf trace causing an interruption in the vascular cylinder; C. vascular cylinder closed at arrow; D. siphonostele with C-shaped vascular trace. (From Taylor and Taylor, 1993.)

figure 7.41 Diagrammatic stages in the evolution of the siphonostele according to the extrastelar theory: A. haplostele; B. departure of leaf trace causing an interruption in the vascular cylinder; C. vascular cylinder closed at arrow; D. siphonostele with C-shaped vascular trace. (From Taylor and Taylor, 1993.)

stage between a protostele and a siphonostele, in which the central region contains both tracheids and parenchyma cells. According to this theory, the siphono-stele has evolved by the failure of certain procambial cells to develop into tracheids. Fossil evidence to support this theory can be found within the lycopsids, where protostelic forms occur early in the history of the group, followed by plants with medullated protosteles, and finally by those with siphonosteles (Chapter 9).

The second hypothesis on the origin of the siphonostele, often termed the extrastelar theory (FIG. 7.41), views the siphonostele as evolving by the continued expansion of cortical parenchyma toward the stem center during the production of leaf traces from the surface of a protostele. In this scenario, cortical parenchyma became "trapped" as the xylem became continuous after trace departure (FIG. 7.41C t . The production of a large number of leaf traces from a protostele would eventually result in a stele in which the center contains parenchymatous pith. Some of the Paleozoic ferns best illustrate this pattern of stelar evolution, for example, the early proto-stelic botryopterid ferns and Grammatopteris, considered to be a progenitor of the osmundaceous ferns (Chapter 11).

Seed Plants

The primary vascular tissue in seed plants is arranged in a fundamentally different way from non-seed plants. Xylem and phloem occur in distinct strands called sympodial bundles or sympodial strands (vascular bundles), which are embedded in parenchymatous ground tissue. This stelar type is characteristic of seed plants and is called a eustele. The vascular strands are arranged either in a ring around the central pith, as in gymnosperms and dicotyledonous angiosperms, or scattered throughout the ground tissue (atactostele)

(FIG. 7.42), as in monocots. In a single cross section, a eustele may look like a dictyostele, in that the cylinder of vascular tissue appears dissected. The sympodial strands in seed plants, however, are discrete and continue throughout the stem (FIG. 7.43C, D). The ground tissue is also continuous from the pith to the cortex, that is, around the sympodial bundles. When a leaf trace is produced from a eustele, a stelar

Atactostele
figure 7.42 Cross section of Zea stem atactostele showing scattered vascular bundles (Extant). Bar = 2 mm.

bundle divides in to two, with one part of the bundle separating tangentially to supply the leaf trace and the other part remaining as a sympodial strand (FIG. 7.43C, D) (Namboodiri and Beck, 1968a, b). Vascular bundles in seed plants are most commonly collateral, with primary xylem on the inside and primary phloem on the outside. Bicollateral bundles also occur; these have phloem both internal and external to the xylem.

It was once believed that seed plants had their origin within the ferns (Jeffrey, 1917) and that the eustele evolved by continued dissection of a siphonostele, in part because of the similarity of stelar anatomy in cross section. We now know, based on many lines of evidence, including a better knowledge of early land plant lineages, that the ferns and seed plants had separate evolutionary histories, and the siphonostele and eustele did not have a shared evolutionary history. Progymnosperms and early members of the seed ferns provide evidence of the evolution of the eustele. Namboodiri and Beck (1968c) proposed that the eustele evolved through the continued longitudinal dissection of a protostele (FIG. 7.43B) , that is, the intercalation of parenchyma into the vascular tissue. When leaf traces were produced, no gap was formed in the stele. Several Devonian taxa, including the progymnosperm Aneurophyton (Chapter 12), had ribbed protosteles and could serve as model starting points in this progression (FIG. 7.43A). Continued medulla-tion of a ribbed protostele resulted in a three-stranded vascular system (FIG. 7.43B) , similar to that seen in several species of Stenomyelon, and finally in a central pith with bundles around the periphery, a stele type seen in Archaeopteris. Finally, a

Eustele Evolution

figure 7.43 Suggested stages in the evolution of the gymnosperm eustele. A. Lobed protostele, for example, Stenomyelon primaevum (Chapter 14). B. Longitudinal dissection of protostele to form pith, for example, S. tuedianum. C. Continued dissection giving rise to discrete sympodial bundles. Trace formation via tangential division of sympodia, e.g., Calamopitys. D. Trace formation via radial division of sympodial bundles, resulting in the formation of a primary vascular system like that in most gymnosperms. (Redrawn from Namboodiri and Beck, 1986; in Taylor and Taylor, 1993.)

figure 7.43 Suggested stages in the evolution of the gymnosperm eustele. A. Lobed protostele, for example, Stenomyelon primaevum (Chapter 14). B. Longitudinal dissection of protostele to form pith, for example, S. tuedianum. C. Continued dissection giving rise to discrete sympodial bundles. Trace formation via tangential division of sympodia, e.g., Calamopitys. D. Trace formation via radial division of sympodial bundles, resulting in the formation of a primary vascular system like that in most gymnosperms. (Redrawn from Namboodiri and Beck, 1986; in Taylor and Taylor, 1993.)

change in the production of leaf traces (FIG. 7.43D), so that sympodial bundles divided tangentially to produce traces, is illustrated by several Carboniferous seed ferns, such as Lyginopteris (Chapter 14). Some modern conifers (Chapter 21) have sympodial strands that undulate through the ground tissue.

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Responses

  • Dennis
    What is a stele in plants?
    7 years ago
  • shay
    What type of stele found in lycopodium?
    7 years ago
  • Tyyne Hietanen
    Which species of lycopodium has plectostele?
    3 years ago
  • mandy
    When phloem occurs ouside and inside the xylem the stele is called?
    3 years ago
  • martha
    How is the eustele produced from the prostele?
    2 years ago
  • Idris
    Which of the has amphipholic siphono stelee?
    2 years ago
  • Aziza
    What is the type of stele found in the simplest vascular cryptograms?
    2 years ago
  • Naomi
    Which of the fallowing has amphipholic siphono stele?
    2 years ago
  • Felix
    Which type pf stele found in marsilea schizome?
    2 years ago
  • rian
    What type of stele is found in marsilea rhizome?
    2 years ago
  • Carrie
    What type of stele is found in marsulea rhizome?
    2 years ago
  • miia
    What type of stele is found in marsilea rhizome botany?
    2 years ago
  • eleleta hamid
    Which type of marsilea stem stele is present?
    2 years ago
  • CHARLES ALEXANDER
    What type of stele is found in Marselia rhizome?
    2 years ago
  • piia-noora
    Does marsilea rhizone contain amphiphloic siphonostele?
    2 years ago
  • berengario rossi
    Is stele of marsilea rhizome is amphipholic siphonostele?
    2 years ago
  • Adriana Arcuri
    What kinds of stele are found in osmunda?
    2 years ago
  • Eduardo Calabrese
    Which of the following has amphipholic siphono stele ans rhizome of marsillea?
    7 months ago

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