Types of Stelar System and its Evolution in Pteridophytes

The stele is defined as a central vascular cylinder, with or without pith. Endodermis is the boundary between cortex and stele. Xylem and phloem are the components of vasculature. In Pteridophytes, the nature and arrangement of xylem and phloem differ in different groups. Types of stele in Pteridophytes
Basically, there are four types of steles:
1. Protostele, 2. Siphonostele 3. Dictyostele 4. Eustele

I. Protostele: The vasculature consists of central core of xylem completely surrounded by phloem. This is the most primitive and simplest of steles.
Types of Protostele
a. Haplostele: This is the most primitive type of protostele. Here the central solid core of xylem is surrounded by a layer of phloem, e.g., Salaginella sp.
b. Actinostele: Central xylem core with radiating ribs or a star shaped appearance surrounded by phloem. More advanced than haplostele.

e.g., Psilotum stem lower regions
c. Plectostele: Xylem is plate like arranged parallel to each other and each plate is surrounded by phloem. More advanced than actinostele.
e.g., Lycopodium clavatum

d. Mixed Protostele: In this case xylem and phloem are uniformly distributed. Xylem elements break up into small groups and all get surrounded by phloem. Most advanced among protostele and considered as a transitional type between true protosteles and siphonosteles
Mixed protostele
e.g., Lycopodium cerennum, Gleichenia sp., Osmunda sp.

e. Polystele: Generally in a protostele, the stem has a single stele in the centre. But in Selaginella, the stem axis has several steles in parallel arrangement (di-stelic or polystelic). Each stele is a protostele with xylem core surrounded by phloem with pericycle and endodermis.
Selaginella poly stele
Eg: Selaginella stem
II.Siphonostele: It is a modified protostele with a central pith region or medullated protostele.
• Pith region is often parenchymatous rarely sclerenchymatous
• Pith formed may be intra stelar in origin or by invasion of cortical tissue.
  1. 1) Simple Siphonostele: This type of stele has centre pith which may be parenchymatous or sclerenchymatous. That is tube like or siphonous. Otherwise this is a protostele with a pith at the centre or medullated protostele. E.g: Psilotum a. Ectophloic Siphonostele: This type of stele has centre pith which is surrounded by concentric rings of xylem followed by phloem.
    b. Amphiphloic Siphonostele: In this type of stele the centre is occupied by pith. Xylem is surrounded on both external and inner sides by rings of phloem
    Amphiphloic Siphonostele
e.g. Marsilea rhizome
2. Solenostele: A siphonostele possesses a leaf gap on one point. It may be ectopholic or amphipholic Solenostele

• Amphiphloic solenostele: In this type of stele, the centre is occupied by pith. Xylem is surrounded on both sides by rings of phloem. In Adiantum and Marselia rhizome at leaf arising regions.
• Ectophloic solenostele: Xylem is surrounded only on the outer side by phloem. In Osmunda

III. Dictyostele: Siphonosteles with overlapping leaf gaps form dictyostele. The portion of vascular tissue which lies in between two gaps is termed as Meristele. Each meristele is protostelic in nature e.g. Fern

a) Simple dictyostele: Dictyosteles arranged in single ring.

b) Polycyclic dictyostele: Dictyosteles arranged in two or more rings.

A typical polycyclic stele possesses two or more concentric rings of vascular tissue eg: Pteris rhizome.

IV: Eustele: The vascular system consists of a ring of collateral, conjoint vascular bundles situated on the edges of the pith.
Eustele in Equisetum stem
Eg: Equisetum stem
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