Write an essay on stelar organisation
Answers
Answer:t is the simplest and most primitive type of stele. The vascular cylinder consists of solid core of xylem surrounded by phloem, pericycle and endodermis. There is no pith e.g., Selaginella, Lycopodium, Lygodium, Gleichenia etc.
Haplostele and Actinostele
The name protostele was suggested by Jeffery (1897, 1899). Berbner (1902) suggested that the protosteles are two types:
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(a) Haplostele, and
(b) Actinostele.
(a) Haplostele:
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A protostele having a central smooth core of xylem (almost circular in transverse section) surrounded by the phloem and the pericycle is known as haplostele (Fig. 1A) e.g., Rhynia, Selaginell, chrysocaulos, S.kraussiana.
(b) Actinostele:
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 (Fig. 1 B) e.g., Lycopodium serratum., Psilotum. The actinostele may show some variations.
It is as follows:
(i) Plectostele:
In this type of actinostele xylem is in the form of plates lying parallel to one another and alternate with the phloem plates (Fig. 2A) Zimmermann (1930) called such stele as plectostele e.g., Lycopodium clavatum, Lycopodium volubile.
(ii) Mixed protostele:
Xylem groups are uniformly scattered in the ground mass of phloem (Fig. 2B) e.g., Lycopodium cernuum.
Plectostele, Mixed Protostele and Mixed Protostele with Pith
Type # 2. Siphonostele:
A stele with central pith surrounded by vascular tissue is called siphonostele or a medullated protostele is called siphonostele.
Evolution of Siphonostele from Protostele:
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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:
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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.
Explanation:Siphonostele is of two types:
(a) Ectophloic siphonostele, and
(b) Amphiphloic sipnonostele.
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(a) Ectophloic siphonostele:
Phloem is present only external to the xylem (Fig. 3A) e.g., Osmunda, Schizaea.
(b) Amphiphloic siphonostele:
Phloem is present on both external and internal to the xylem e.g., Marsilea rhizome. In it the pith is surrounded by inner endodermis, inner pericycle, inner phloem, xylem, outer phloem, outer pericycle and outer endodermis (Fig. 3B).
Ectophloic and Amphiphiloic Siphonostele
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Other Modifications of Siphonostele:
(a) Cladosiphonic siphonostele:
The siphonostele without leaf gap is known as cladosiphonic siphonstele (Jeffery, 1910) e.g., Selaginella.
(b) Phyllosiphonic siphonostele:
A siphonostele with smaller or larger leaf gaps is called phyllosiphonic siphonostele e.g., Filicophyta.
(c) Solenostele:
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The siphonostele which is perforated by scattered leaf traces is known as slenostele (Gwynne-Vaughan, 1907; Schoulte, 1938).
It may of two types:
(a) Ectophloic, and
(b) Amphiphloic.
(i) Ectophlopic siphonostele:
Phloem is present only on outer side (Fig. 4A).
(ii) Amphiphloic siphonostele:
Phloem is present on both the sides of the xylem