It covers the CBCS syllabus for UG Sem II Botany Honors.

1. TYPES OF STELE
AND
STELAR EVOLUTION
Dr. Rimi Roy
J.K. College, Purulia

2. • The stelar theory was proposed by, Van Tiegham
and Douliot in 1886. According to which the root
and stem are fundamentally similar in gross
anatomy, because in both the cortex encloses the
central part of the axis, called the stele. According
to them, stele is the core of the axis, which
includes the vascular system, interfascicular
portion, the pith (if present), and some
surrounding portion of the fundamental tissue in
the vicinity of vascular bundles (pericycle).
• The term stele is applied to the primary tissue
only. On the basis of structural variations, in the
primary vascular system, following types of steles
have been recognized.

3. 1. Protostele
• This is the simplest type of stele.
It consists of a solid central
column of vascular tissue. Pith is
absent. Xylem is located in the
centre surrounded by phloem. On
the basis of shape of xylem, the
following types of protosteles
have been recognized:
• a. Haplostele:
• It has a smooth core of xylem
surrounded by a uniform layer of
phloem, e.g. Rhynia, Cooksonia
and Selaginella kraussiana

4. • b. Actinostele:
• It has a xylem core with
radiating ribs
(starshaped = stellate).
In actinostele, phloem
is present in the form
of separate patches
alternating with
projecting parts of
xylem, e.g. Lycopodium
serratum, Psilotum
nudum

5. • c. Plectostele:
• In this type of
protostele, xylem occurs
in the form of separate
plates which lie parallel
to one another, with
phloem situated
between them. It is
found in Lycopodium
volubile, L. clavatum,
etc.

6. • Mixed protostele with
phloem:
• In this stele xylem
groups are scattered
in the form of
irregular patches that
are embedded in the
ground mass of
phloem, e.g.,
Lycopodium cernuum

7. • Mixed protostele with pith parenchyma:
• In this stele xylem groups are scattered in the
form of irregular patches that are embedded
in the ground mass of parenchyma,
• e.g. Hymenophyllum demissum

8. 2. Siphonostele or medullated
protostele
• A type of stele in which there is present a pith
in the central region, is called a siphonostel. In
siphonostele, vascular tissue is arranged in the
form of a hollow cylinder, with distant pith in
the centre. It is found in the stems of most
members of Filicophyta. It is of the following
two types:

9. • a. Ectophloic siphonostele: It is a type of siphonostele
where xylem cylinder lies next to the pith and is
surrounded by the phloem cylinder on the outer side. It
is found in Osmunda, Equisetum, etc.
• b. Amphiphloic siphonostele: In this type of siphonostele
the pith is surrounded by inner endodermis, inner
pericycle, inner phloem, xylem, outer phloem, outer
pericycle and outer endodermis. So in this case phloem
surrounds the xylem internally as well as externally. It is
found in Adiantum, Marsilea, etc.

10. • In both ectophloic and amphiphloic
siphonosteles, vascular tissue occurs as
continuous cylinder. This is because the leaf
traces do not break the vascular cylinder. Such
plants in which the vascular supply to the leaf
is without any break in vascular cylinder are
called microphyllous.

11. 3. Solenostele
• This is similar to siphonostele in having central pith, but differs in
producing leaf gaps, wherever leaf traces originate. Thus due to
production of leaf gaps, the cylinder becomes dissected at places.
• This type of solenostele may be ectophloic or amphiphloic,
depending upon the type of siphonostele from which it is produced.
Such plants where leaf gaps occur in vascular cylinder are called
magaphyllous

12. • In many siphonostelic Filicophyta, leaves
are inserted on the stem in close
succession. In such cases, leaf gaps overlap
in their longitudinal extent to such a
degree that vascular cylinder of stem
appears dissected into tubular network of
interconnected longitudinal strands
(meristeles), separated from one another
by parenchymatous tissue (leaf gaps).
• These meristeles in a transverse section
appear arranged in a ring. Such a stele is
known as dissected siphonostele or
dictyostele. The vascular parts of a
dictyostele between two neighbouring leaf
gaps, appearing in transverse section as
separate strands, are termed as
meristeles. In a Dictyostele, each meristele
has the general structure of a protostele,
e.g. Dryopteris filix-max, Pteris vittata
4. Dictyostele

13. 5. Eustele
• This is a modification of
siphonostele, in which
vascular system consists of
a ring of collateral or
bicollateral vascular
bundles. These are
separated from one
another by a wide
medullary or interfasicular
regions and leaf gaps are
not clearly distinguishable.
• It is found in the stems of
gymnosperms and
angiosperms

14. 6. Atactostele
• This is the most
complex type of stele.
In this type, the
vascular bundles are
irregularly dispersed
in the ground tissue as
in the stems of
monocotyledons

15. • Polycyclic stele: When more than one
steles are present in the axis of
pteridophytes, e.g. 2 in Selaginella
kraussiana, 16 in S. laevigata, the
condition is called polycyclic stele.

16. Stelar evolution
• According to Jeffrey (1898), protostele is the
most primitive type of stele, from a phylogenetic
stand point, from which other types of steles
have evolved in the course of evolutionary
specialization.
• It is considered to be the fundamental stelar
organisation that was present in the earliest
vascular plants, and is now retained by some
living vascular cryptogams such as Psilotum,
Tmesipteris and Lycopodium, etc.
• the extinct Psilophytales also possessed
protostelic vascular organisation.

17. • In its simplest form, protostele is haplostelic.
During further elaboration, the central core of
xylem became irregular and assumed almost a
star-like shape. Such a modification of stele is
termed as actinostele. As a result of further
evolution, the xylem splits up into a number of
parallel plates alternating with phloem. Such a
modification is called plectostele. It is found in
some species of Lycopodium.
• Haplostele to actinostele, and then to
plectostele, is considered to be one line of
evolution of the protostele.
• It is regarded as Lycopsid line of evolution.

18. • The various types of protosteles may show
relative variation in the position of protoxylem
and metaxylem. It may be exarch with the
protoxylem near the periphery of the xylem
strands, or may be endarch with the
protoxylem at the inner surface. In the
mesarch condition, metaxylem is present both
towards the outer and inner sides of
protoxylem.
• The endarch condition is considered to be the
most evolved and exarch condition as the
primitive in the context of stelar evolution.

19. • Another very important evolutionary change that
occurred in the protostele was the appearance of the
central pith, which in turn led to the development of
more complicated stelar types. Two theories have been
proposed accounting the phylogenetic origin of the
pith. These are: I) Intrastelar theory, II) Extrastelar
theory
• I). Intrastelar theory: According to intrastelar origin of
pith or expansion theory, the inner vascular tissue
metamorphosed into the parenchymatous pith. The
occurrence of mixed pith in some living forms supports
this view. A mixed pith shows tracheids within the
parenchymatous pith. This may be looked upon as a
transition stage between a true protostele and true
siphonostele.

20. • II) Extrastelar theory: According to the
extrastelar origin of pith or invasion theory
the pith is cortical in origin. The
parenchymatous cortex is said to have
intruded through the leaf gaps and branch
gaps into the centre of vascular cylinder to
give rise to the pith. But this cortical invasion
could not have produced polycyclic
siphonostele.
• Eames (1936) considered that in primitive
forms, it may be intrastelar in origin, and in
higher forms extrastelar in origin.

21. • Appearance of pith led to the conversion of the
protostele into a new type of stele, called the
siphonostele. Elaboration of siphonostele also
followed two courses of evolution, as follows:
• i). The appearance of pith resulted in the formation
of a stele consisting of a central pith surrounded by a
complete ring of xylem, which in turn was
surrounded by a complete ring of outer phloem,
pericycle and endodermis. Such a stele was named
as ectophloic siphonostele.
• In its simplest form such a stele is uninterrupted by
leaf gaps and is called cladosiphonic. In the
magaphyllous vascular plants, the ectopholic
siphonostele became interrupted by the appearance
of leaf gaps, which is now called phyllosiphonic.

22. • In case the leaf gaps do not overlap, the stele is,
interrupted only at considerable distances
(nodes) by one leaf gap, so in between the two
leaf gaps, the vascular cylinder remains complete.
Such a stele is also called solenostele or
siphoneustele.
• During the course of evolution, leaf gaps on the
stem overlap and lead to formation of a much
dissected stelar organisation, called eustele. It is
made up of a number of separate and collateral
vascular bundles.
• In certain cases, the vascular bundles are
scattered, as in monocotyledons, this kind of
stele is termed as atactostele.

23. • ii). During another line of evolution, the
medullation of protostele was followed by the
appearance of phloem on either side of the
xylem; likewise internal pericycle and endodermis
also appeared. Such a stele led to the formation
of amphiphloic siphonostele. It may be
cladosiphonic or phyllosiphonic.
• The phyllosiphonic amphiphloic siphonostele
with only one leaf gap at the node is called
amphiphloic solenostele.
• In case the leaf gaps overlap, the resultant stele is
called the dictyostele.

24. • In many eusporangiate and leptosporangiate
ferns, the dictyostelic stems are protostelic at
their bases.
• Recent experimental studies also reveal that
dictyostelic condition can be changed to
solenostelic, or even to protostelic condition by
removing the young leaf primordial from shoot
apices.
• All these observations prove that protostele is the
basic or the fundamental stelar type, from which
the complicated steles or vascular systems arose
by elaboration.

25. • Among the pteridophytes, polycyclic stele is the
most advanced condition exhibited by some ferns
like Marattia, Matonia, etc. It also originated
from the protostelic condition by further
elaboration that in Matonia pectinata, there is a
regular transition from protostelic condition to
solenostele and then to polycyclic condition. This
developmental phenomenon is termed as
recapitulation.
• Occurrence of such a developmental
phenomenon leads further support to Jeffrey's
view that protostele is the primitive condition.

26. 1

27. • Thus, the evolution of stellar organisation has
taken place along several independent lines;
even a single genus, like Gleichenia, shows
some species with protostelic structure, and
some with the siphonostelic organisation.
• So the stellar structure has not been of much
significance in establishing phylogenetic
relationships.