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In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue (pith) and a pericycle, which, if present, defines the outermost boundary of the stele. Outside the stele lies the endodermis, which is the innermost cell layer of the cortex.

The concept of the stele was developed in the late 19th century by French botanists P. E. L. van Tieghem and H. Doultion as a model for understanding the relationship between the shoot and root, and for discussing the evolution of vascular plant morphology. Now, at the beginning of the 21st century, plant molecular biologists are coming to understand the genetics and developmental pathways that govern tissue patterns in the stele.[citation needed] Moreover, physiologists are examining how the anatomy (sizes and shapes) of different steles affect the function of organs.

Contents

The earliest vascular plants had stems with a central core of vascular tissue. This consisted of a cylindrical strand of xylem, surrounded by a region of phloem. Around the vascular tissue there might have been an endodermis that regulated the flow of water into and out of the vascular system. Such an arrangement is termed a protostele.

There are usually three basic types of protostele:

  • haplostele – consisting of a cylindrical core of xylem surrounded by a ring of phloem. An endodermis generally surrounds the stele. A centrarch (protoxylem in the center of a metaxylem cylinder) haplostele is prevalent in members of the rhyniophyte grade, such as Rhynia.
  • actinostele – a variation of the protostele in which the core is lobed or fluted. This stele is found in many species of club moss (Lycopodium and related genera). Actinosteles are typically exarch (protoxylem external to the metaxylem) and consist of several to many patches of protoxylem at the tips of the lobes of the metaxylem. Exarch protosteles are a defining characteristic of the lycophyte lineage.
  • plectostele – a protostele in which plate-like regions of xylem appear in transverse section surrounded by phloem tissue, thus appearing to form alternating bands. These discrete plates are interconnected in longitudinal section. Some modern club mosses have plectosteles in their stems. The plectostele may be derived from the actinostele.
Three basic types of siphonostele. The siphonostele shown on the left may also be called an amphiphloic siphonostele. The eustele shown on the right is collateral, i.e. with all the phloem on one side of the xylem.

Siphonosteles have a central region of ground tissue called the pith, with the vascular strand comprising a hollow cylinder surrounding the pith. Siphonosteles often have interruptions in the vascular strand where leaves (typically megaphylls) originate (called leaf gaps).

Siphonosteles can be called ectophloic (phloem present only external to the xylem)[citation needed] or they can be amphiphloic (with phloem both external and internal to the xylem). Among living plants, many ferns and some Asterid flowering plants have an amphiphloic stele.

An amphiphloic siphonostele can be called a solenostele, or this term may be used to refer to cases where the cylinder of vascular tissue contains no more than one leaf gap in any transverse section (i.e. has non-overlapping leaf gaps). This type of stele is primarily found in fern stems today. Where there are large overlapping leaf gaps (so that multiple gaps in the vascular cylinder exist in any one transverse section), the term dictyostele may be used. The numerous leaf gaps and leaf traces give a dictyostele the appearance of many isolated islands of xylem surrounded by phloem. Each of the apparently isolated units of a dictyostele that serve a single leaf can be called a meristele. Among living plants, this type of stele is found only in the stems of ferns.

Most seed plant stems possess a vascular arrangement which has been interpreted as a derived siphonostele, and is called a eustele – in this arrangement, the primary vascular tissue consists of vascular bundles, usually in one or two rings around the pith. In addition to being found in stems, the eustele appears in the roots of monocot flowering plants. The vascular bundles in a eustele can be collateral (with the phloem on only one side of the xylem) or bicollateral (with phloem on both sides of the xylem, as in some Solanaceae).

There is also a variant of the eustele found in monocots like maize and rye. The variation has numerous scattered bundles in the stem and is called an atactostele (characteristic of monocot stems). However, it is really just a variant of the eustele.

Stele biology Article Talk Language Watch Edit In a vascular plant the stele is the central part of the root or stem 1 containing the tissues derived from the procambium These include vascular tissue in some cases ground tissue pith and a pericycle which if present defines the outermost boundary of the stele Outside the stele lies the endodermis which is the innermost cell layer of the cortex The concept of the stele was developed in the late 19th century by French botanists P E L van Tieghem and H Doultion as a model for understanding the relationship between the shoot and root and for discussing the evolution of vascular plant morphology 2 Now at the beginning of the 21st century plant molecular biologists are coming to understand the genetics and developmental pathways that govern tissue patterns in the stele citation needed Moreover physiologists are examining how the anatomy sizes and shapes of different steles affect the function of organs Contents 1 Protostele 2 Siphonostele 3 See also 4 Citations 5 ReferencesProtostele Edit The earliest vascular plants had stems with a central core of vascular tissue 3 4 This consisted of a cylindrical strand of xylem surrounded by a region of phloem Around the vascular tissue there might have been an endodermis that regulated the flow of water into and out of the vascular system Such an arrangement is termed a protostele 5 There are usually three basic types of protostele haplostele consisting of a cylindrical core of xylem surrounded by a ring of phloem An endodermis generally surrounds the stele A centrarch protoxylem in the center of a metaxylem cylinder haplostele is prevalent in members of the rhyniophyte grade such as Rhynia 6 actinostele a variation of the protostele in which the core is lobed or fluted 7 This stele is found in many species of club moss Lycopodium and related genera Actinosteles are typically exarch protoxylem external to the metaxylem and consist of several to many patches of protoxylem at the tips of the lobes of the metaxylem Exarch protosteles are a defining characteristic of the lycophyte lineage plectostele a protostele in which plate like regions of xylem appear in transverse section surrounded by phloem tissue thus appearing to form alternating bands 8 These discrete plates are interconnected in longitudinal section Some modern club mosses have plectosteles in their stems The plectostele may be derived from the actinostele Siphonostele Edit Three basic types of siphonostele The siphonostele shown on the left may also be called an amphiphloic siphonostele The eustele shown on the right is collateral i e with all the phloem on one side of the xylem Siphonosteles have a central region of ground tissue called the pith with the vascular strand comprising a hollow cylinder surrounding the pith 9 Siphonosteles often have interruptions in the vascular strand where leaves typically megaphylls originate called leaf gaps Siphonosteles can be called ectophloic phloem present only external to the xylem citation needed or they can be amphiphloic with phloem both external and internal to the xylem 9 Among living plants many ferns and some Asterid flowering plants have an amphiphloic stele An amphiphloic siphonostele can be called a solenostele or this term may be used to refer to cases where the cylinder of vascular tissue contains no more than one leaf gap in any transverse section i e has non overlapping leaf gaps 9 This type of stele is primarily found in fern stems today Where there are large overlapping leaf gaps so that multiple gaps in the vascular cylinder exist in any one transverse section the term dictyostele may be used 10 The numerous leaf gaps and leaf traces give a dictyostele the appearance of many isolated islands of xylem surrounded by phloem Each of the apparently isolated units of a dictyostele that serve a single leaf can be called a meristele 11 Among living plants this type of stele is found only in the stems of ferns Most seed plant stems possess a vascular arrangement which has been interpreted as a derived siphonostele and is called a eustele in this arrangement the primary vascular tissue consists of vascular bundles usually in one or two rings around the pith 12 In addition to being found in stems the eustele appears in the roots of monocot flowering plants The vascular bundles in a eustele can be collateral with the phloem on only one side of the xylem or bicollateral with phloem on both sides of the xylem as in some Solanaceae There is also a variant of the eustele found in monocots like maize and rye The variation has numerous scattered bundles in the stem and is called an atactostele characteristic of monocot stems However it is really just a variant of the eustele 12 13 See also EditVascular tissue Vascular bundleCitations Edit Foster amp Gifford 1974 p 58 Gifford amp Foster 1988 p 42 Bold Alexopoulos amp Delevoryas 1987 p 320 Stewart amp Rothwell 1993 pp 85 89 Gifford amp Foster 1988 p 44 Arnold 1947 pp 66 68 Beentje 2010 p 7 Beentje 2010 p 89 a b c Beentje 2010 p 109 Beentje 2010 p 39 Beentje 2010 p 71 a b Bold Alexopoulos amp Delevoryas 1987 p 322 Gifford amp Foster 1988 p 45 References EditArnold Chester A 1947 An Introduction to Paleobotany 1st ed New York and London McGraw Hill Book Company Beentje Henk 2010 The Kew Plant Glossary Richmond Surrey Royal Botanic Gardens Kew ISBN 978 1 84246 422 9 Bold Harold C Alexopoulos Constantine J amp Delevoryas Theodore 1987 Morphology of Plants and Fungi 5th ed New York Harper amp Row ISBN 0 06 040839 1 Foster A S amp Gifford E M 1974 Comparative Morphology of Vascular Plants 2nd ed San Francisco W H Freeman ISBN 978 0 7167 0712 7 Gifford Ernest M amp Foster Adriance S 1988 Morphology and Evolution of Vascular Plants 3rd ed New York W H Freeman and Company ISBN 0 7167 1946 0 Stewart Wilson N amp Rothwell Gar W 1993 Paleobotany and the Evolution of Plants 2nd ed Cambridge Cambridge University Press ISBN 0 521 38294 7 Retrieved from https en 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