Stelar System, Stele, its types and evolution

STELAR SYSTEM
AND STELEB Y H A I D E R A L I M A L I K
D E PA R T M E N T O F B OTA N Y
U N I V E R S I T Y O F S A R G O D H A 1

DEFINITION OF STELAR
SYSTEM:
• According to the older botanists, the vascular bundle is the
fundamental unit in the vascular system of pteridophytes and
higher plants. Van Tieghem and Douliot (1886) interpreted the
plant body of vascular plant in the different way.
• According to them, the fundamental parts of a shoot are the
cortex and a central cylinder, is known as stele. Thus the stele is
defined as a central vascular cylinder, with or without pith
delimited the cortex by endodermis.
2

• The term stele has been derived from a Greek word meaning
pillar.
• Van Tieghem and Douliot (1886) recognized only three types of
steles.
1) Protostele
2) Siphonostele
3) Solenostele
• Some authors recognize only two kinds of stele(protostele and
siphonostele) and consider solenostele as a sub-categorie of
siphonostele.
3

STELAR THEORY
It was proposed by Van Tieghem and Douliot
in1886.
Major highlights of stellar theory are:
Stele is a real entity and present universally in all
higher plants.
Cortex and stele are two fundamental parts of a
shoot system
Stele and cortex are separated by endodermis.
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COMPONENTS OF STELE
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Xylem
Phloem
Pericycle
Medullay rays
Pith

TYPES OF STELES:
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1. Protostele:
 Jeffrey (1898), for the first time pointed out the stelar theory
from the point of view of the phylogeny. According to him,
the primitive type of stele is protostele.
 In protostele, the vascular tissue is a solid mass and the
central core of the xylem is completely surrounded by the
strand of phloem.
 This is the most primitive and simplest type of stellar
organization.
 Pith is absent.

FORMS OF PROTOSTELE
There are several forms of protostele:
(a) Haplostele:
• This is the most primitive type of protostele.
• Named by Brebner in 1902
• Here the central solid smooth core of xylem remains surrounded
by phloem (e.g., in Selaginella spp., Rhynia).
(b) Actinostele:
• This is the modification of the haplostele and somewhat more
advanced in having the central xylem core with radiating ribs.
• A protostele in which xylem appears as stellate or star shaped
with many radiating arms in transverse section and phloem is
present in small patches in between the radiating arms of the
xylem is known as actinostele (e.g., in Psilotum spp. ,
8

(c) Plectostele:
• This is the most advanced type of protostele.
• Here the central core of xylem is divided into number of plates
arranged parallel to each other. The phloem alternates the xylem.
• Zimmermann (1930) called such stele as plectostele e.g.,
Lycopodium
(d) Mixed-pith stele:
• Here the xylem elements (i.e., tracheids) are mixed with the
parenchymatous cells of the pith.
• This type is found in primitive fossils and living ferns.
• They are treated to be the transitional types in between true
protosteles on the one hand and siphonosteles on the other (e.g.,
in Gleichenia spp. and Osmunda spp.).
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2. Siphonostele:
• This is the modification of protostele.
• A stele with central pith surrounded by vascular tissue is called
siphonostele or a medullated protostele is called siphonostele.
• Such stele contains a tubular vascular region and a
parenchymatous central region.
• Jeffrey (1898) interpreted that the vascular portion of
siphonostele possesses a parenchymatous area known as a
immediately above the branch traces only or immediately
leaf and branch traces.
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• On the basis of these branch and leaf gaps Jeffrey (1910),
distinguished two types of siphonosteles.
• In one type, however, the leaf gaps are not found and they
are known as cladosiphonic siphonosteles.
• In the other type both leaf and branch gaps are present
and they are known as phyllosiphonic siphonosteles.
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TYPES OF SIPHONOSTELE
• A siphonostele may be of the following types:
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Ectophloic Amphiphloi

(a) Ectophloic siphonostele:
• In this type of siphonostele, the pith is surrounded by concentric
xylem cylinder and next to xylem the concentric phloem cylinder.
• Phloem is present only external to the xylem (Fig. 3A) e.g.,
Osmunda, Schizaea.
(b) Amphiphloic siphonostele:
• In this type of siphonostele the pith is surrounded by the vascular
tissue. The concentric inner phloem cylinder surrounds the central
pith.
• Next to the inner phloem is the concentric xylem cylinder which is
immediately surrounded by outer phloem cylinder (e.g., in
Marsilea).
17

EVOLUTION OF SIPHONOSTELE FROM
PROTOSTELE:
There are two main theories regarding the evolution of siphonostele from
protostele:
(a) Intraxylary or Intrastelar origin:
 According to this theory the siphonostele is evolved by the conversion of
the central mass of the xylem into parenchymatous pith.
 This theory is also known as expansion theory and it is supported by
Boodle (1901), Bower (1911), Gwynne-Vaughan (1903, 1914). Petry (1914),
Thompson and Gewirtz and Fahn (1960) etc.
(b) Extrastelar Origin:
 This theory is supported by Jaffery (1897, 1899, 1902, 1917). According to
him the pith is originated as a result of invasion of the parenchymatous
cells of the cortex into the stele.
 It takes place through the leaf gaps and branch gaps. This theory is also
known as invasion theory.
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(c) Solenostele:
• The siphonostele which is perforated by scattered
leaf traces is known as solenostele (Gwynne-
Vaughan, 1907; Schoulte, 1938).
• In simple words, siphonostele with leaf gap is called
solenostele
• It is actually a sub categorie of siphonostele.
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TYPES OF SOLENOSTELE
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(i) Ectophlopic solenostele:
• Phloem is present only on outer side
• e.g. Nicotiana & Salix .
(ii) Amphiphloic solenostele:
• Phloem is present on both the sides of the
xylem
• e.g Adiantum, Marsilea

OTHER TYPES
Dictyostele
Eustele
Atactostele
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Dictyostele:
• A solenostele with more overlapping leaf gaps so as to show
more than interruption in one transverse section is known as
dissected siphonostele or dissected solenostele or dictyostele.
• It is broken into network of separate vascular bundles.
• Brebner (1902) called the siphonosteles with overlapping gaps
as dictyosteles.
• The vascular parts of dictyostele between the neighbouring
gaps are known as meristeles which are of protostelic type. The
dictyostele with many meristeles looks like a cylindrical
meshwork.
• These vascular bundles are concentric (central xylem
surrounded by phloem, pericycle and endodermis), called
meristeles (e.g. Dryopteris).
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Eustele:
• It is characteristic of Gymnosperms and dicots stem.
• In this type of stele collateral or bicollateral vascular bundles are
present in a ring (Brebner, 1902)
Atactostele:
• It is characteristic of monocots. In this type of stele vascular
lies scattered in the (Brebner, 1902). It is the highly evolved stelar
organisation. ground tissue
• This is a special case of Eustele in which the stele is more disected
with leaf gaps, so that , in transverse section, the vascular bundles
are scattered though the fundamental tissue (e.g. Monocot stems,
Corn & Grasses).
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