Stelar Evolution in Pteridophytes

What is stele? What are the components of stele?

Ø  Stele is the central cylinder or core of vascular tissue in higher plants.
Ø  The stele consists of xylem, phloem, pericycle and medullary rays and pith if present.
Ø  The term ‘stele’ was for the first time used by Van Tieghem and Douliot in 1886 in their ‘Stelar Theory’.

What is ‘stellar’ theory’?

Ø  Proposed by Van Tieghem and Douliot in 1886.

Ø  Major highlights in stellar theory are:

$.  The stele is a real entity and present universally in all axis of higher plants.

$.  The primary components of stele are xylem and phloem.

$.  Tissues like pericycle, medullary rays and pith are also the components of stele.

$.  ‘Stelar theory’ also says that the cortex and the stele are the two fundamental parts of a shoot system.

$.  Both these components (stele and cortex) are separated by the endodermis.

$.  In higher vascular plants (Pteridophytes, Gymnosperms and Angiosperms), the leaf traces are large, and it appears that they play an important role in the vascular system of the axis.

$.  The whole set-up of leaf traces appears as a composite structure in these plants.

$.  Such composite structures do not remain within the limits of stellar theory of Van Tieghem and Douliot.

Different types of steles in plants (Pteridophytes and higher plants)

Ø  On the basis of ontogeny and phylogeney, there are THREE broad categories of steles in vascular plants.

Ø  They are:

(1).  Protostele

(2).  Siphonostele

(3).  Solenostele

Ø  Some authors recognize only two categories (Protostele and Siphonostele). They consider Solenostele as a sub-category of Siphonostele.

(1). Protostele

Ø  A stele in which the vascular core consists of a solid core of xylem and it is surrounded by phloem, pericycle and endodermis is called protostele.

Ø  Pith is absent in protostele.

Ø  Protostele represents the simplest stellar organization in vascular plants.

Ø  Majority of the Pteridophytes show protostelic condition in their rhizome, stem or roots.

Ø  Protostele is considered as the most primitive stellar organization in plants.

Ø  There are FIVE types of protosteles in Pteridophytes, they are: (a) Haplostele, (b) Actinostele, (c) Plectostele, (d) Mixed protostele and (e) Mixed protostele with pith.

(a). Haplostele

Ø  A protostele with a smooth core of xylem surrounded by uniform layers of phloem.

Ø  Named by Brebner in 1902.

Ø  Considered as the most primitive type of protostele.

Ø  Usually present in fossil genera like Rhynia and Horneophyton

Ø  Example: Selaginella, Gleichenia and Lygodium.

(b). Actinostele

Ø  Protostele with xylem core having radial ribs or arms.

Ø  Xylem is star shaped or stellate, hence the name.

Ø  The phloem is NOT present in a continuous manner.

Ø  Phloem occurs as separate patches between the arms of xylem.

Ø  Named by Brebner in 1902.

Ø  Example: Asteroxylon, Psilotum, Lycopodium serratum.

(c). Plectostele

Ø  Xylem occurs as several plates which are more or less parallel to each other.

Ø  Such xylem plates are alternated with phloem patches.

Ø  Named by Zimmermann in 1930.

Ø  Example: Lycopodium clavatum.

(d). Mixed protostele

Ø  Xylem is divided into several units or groups.

Ø  Each xylem units are scatteredly arranged inside the ground mass of phloem.

Ø  Example: Lycopodium cernuum.

(d). Mixed protostele with pith

Ø  Advanced type of stele among protosteles.

Ø  The formation of pith in the stele started here for the first time in the evolution.

Ø  Stele is similar to mixed protostele.

Ø  Here small patches of parenchymatous region occur in association xylem strands.

Ø  Mixed protostele with pith is considered as a connecting link between protostele and siphonostele.

Ø  Example: Hymenophyllum demissum, Lepidodendron selaginoides.

(2). Siphonostele

Ø  A stele with pith (medulla) at the centre is called siphonostele.

Ø  The central core of pith is surrounded by the xylem.

Ø  Siphonostele is an advanced type of stele than protostele.

Origin of siphonostele

Ø  The siphonostele is derived from protostele by the formation of pith in the centre.

Ø  The centrally placed xylem core in a protostele is replaced by parenchymatous pith.

Ø  Different stages of changing of protostele to siphonostele can be observed in the transverse sections at different levels in Gleichenia, Osmunda and Anemia.

Ø  There are TWO views regarding the origin of pith in the siphonostele:

(i). Inter-stelar origin of pith:

$.  According to this theory, the innermost (centrally placed) vascular tissue in a protostele changes into parenchymatous cells.

$.  This theory is proposed by Bower in 1923 and supported by Fahn in 1960.

$.  This theory is the most widely accepted theory of pith development.

(ii). Extra-stelar origin of pith:

$.  According to this theory, the pith is formed as a result of the invasion of cortical parenchymatous cells into the stele.

$.  The invasion of pith occurs through the leaf gap or branch gap.

$.  Thus pith and cortex are homogenous structures according to this theory.

$.  This theory is proposed by Jeffery.

$.  It is not accepted by most of the authors since in many Pteridophytes there is stele without leaf gaps but having pith in the centre.

Different types of siphonosteles

Ø  Based on the position and distribution of phloem, TWO types of siphonostles reported among Pteridophytes are (a) Ectophloic siphonostele and (b) Amphiphloic siphonostele.

(a). Ectophloic siphonostele

Ø  Phloem is present only on the external side of the xylem.

Ø  The pith is at the central position.

Ø  Phloem is externally surrounded by pericycle and endodermis.

Ø  Leaf traces present, but leaf gap absent

Ø  Example: Osmunda, Schizaea

(b). Amphiphloic siphonostele

Ø  Phloem is present on both sides of the xylem (external and internal phloem).

Ø  The central portion of the stele is occupied by pith.

Ø  Xylem on its inner side is surrounded by inner phloem, inner pericycle and inner endodermis.

Ø  Xylem on its outer side is surrounded by outer phloem, outer pericycle and outer endodermis.

Ø  Example: Marsilea, Adiantum

(3). Solenostele

Ø  Solenostele is actually a sub category of siphonostele.

Ø  A siphonostele which is perforated at the place of origin of leaf trace is called solenostele.

Ø  In simple term, siphonostele with leaf gap is called solenostele.

Different types of Solenostele

Ø  Six different types of solenostele can be observed among plant kingdom.

Ø  They are (a) Ectophloic solenostele, (b). Amphiphloic solenostele, (c) Dictyostele, (d) Eustele and (e) Atactostele.

(a). Ectophloic solenostele

Ø  Derived from ectophloic siphonostele.

Ø  Thus phloem is present only on the outer side of the xylem.

(b). Amphiphloic solenostele

Ø  Derived from amphiphloic siphonostele.

Ø  Thus phloem is present on both sides of the xylem.

Ø  Phloem in both sides is intern surrounded by pericycle and endodermis.

Ø  Example: Adiantum pedatum

(c). Dictyostele

Ø  Solenostele that is broken into a network of separate vascular strands are called dictyostele.

Ø  This breaking-up of stelar core is due to the presence of large number of leaf gaps.

Ø  Each such separate vascular strand is called meristele.

Ø  Example: Pteris, Adiantum capillus-veneris.

(d). Poly-cyclic stele

Ø  Here the stele is present as two or more concentric cylinders.

Ø  Poly-cyclic stele will be always solenostelic in nature.

Ø  Poly-cyclic stele may be polycyclic solenostele or polycyclic dictyostele

(d). Eustele

Ø  If the stele is split into distinct collateral vascular bundles, then it is called eustele.

Ø  It is modified ectophloic siphonostele.

Ø  Spitting of the original stelar core takes place due to the overlapping of large number of leaf gaps.

Ø  Individual vascular bundles in the eustele are arranged as broken ring in the ground tissue.

Ø  Example: dicot stem primary structure

(e). Atactostele

Ø  Similar to eustele.

Ø  Both the individual vascular bundles are scatteredly distributed in the ground tissue

Ø  Example: Monocot stem

Test your understanding…

1. What is stelar theory?
2. Define ‘stele’.
3. Who proposed the ‘stelar theory’?
4. What are the main points in ‘stelar theory’?
5. Name the three major categories of steles in vascular plants.
6. What is meant by protostele?
7. Describe different types of Protostels with examples.
8. Differentiate actinostele and haplostele.
9. What is meant by siphonostele?
10. Describe different types of siphonosteles with examples.
11. What is meant by solenostele?
12. Describe different types of solenosteles with examples?
13. Define meristele.
14. What is atactostele?
15. What is meant by Eustele?
16. Describe dictyostele with example.
17. Differentiate ectophloic and amphiphloic solenostele.
18. Write an essay on stelar evolution in Pteridophytes with suitable examples and neat diagrams.

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