In this article we will discuss about the two principal types of stelar organisation in vascular plants: 1. Protostele 2. Siphonostele.
Type # 1. Protostele:
It 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.
The name protostele was suggested by Jeffery (1897, 1899). Berbner (1902) suggested that the protosteles are two types:
(a) Haplostele, and
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.
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:
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.
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:
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.
Siphonostele is of two types:
(a) Ectophloic siphonostele, and
(b) Amphiphloic sipnonostele.
(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).
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.
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
(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 (Fig. 4B).
A siphonostele with more overlapping leaf gaps so as to show more than interruption in one transverse section is known as dissected siphonostele or dictyostele. The vascular parts of dictyostele between the neighbouring leaf gaps are known as meristeles.
The meristeles are typically protosteles e.g., Pteris, Ophioglossum lusitanicum, Adiantum capillaris-veneris, Dryopteris chrysocoma etc. (Fig. 5 A,B).
Evolution of Dictyostele from Siphonostele:
Ultimately the siphonostele gives rise to dictyostele. In some of siphonostelic members due to the dwarf axis, the shoot and leaves become over-crowded resulting into the formation of several leaf gaps. The vascular supply given for a leaf from the main stele is called leaf trace.
The parenchymatous region left behind in the main stele after the departure of the leaf trace is called leaf gap. Similarly the vascular supply is also given to branch. Vascular supply given out for a branch from main stele is called branch trace.
The parenchymatous region left out in the main vascular cylinder due to departure of branch traces is called as branch gap forming the dictyostele (Brebner, 1902). This type of stele may further result into polycyclic condition by the formation of several rings.
A number of separate steles are arranged in more or less regular cycles one within another. Such a condition is known as polycyclic and stele is called polycyclic stele. e.g., Pteridium aquilinum and Matonia pectinata. In this type of stele if the outer cylinder is solenostelic it is called polycyclic solenostele Fig. 6 A) and if outer cylinder is dictyostelic it is known as polycyclic dictyostele (Fig. 6 B)
Two or more protosteles lie side-by-side in a stem except at branching place where all steles are connected e.g., Selaginella laevigata. (Fig. 6 C).
It is characteristic of Gymnosperms and dicots. In this type of stele collateral or bicollateral vascular bundles are present in a ring (Brebner, 1902, Fig. 7 A).
It is characteristic of monocots. In this type of stele vascular bundle lies scattered in the ground tissue (Brebner, 1902). It is the highly evolved stelar organisation. (Fig. 7 B).
Thus, it is evident, that protostelic condition has given rise to siphonostelic, dictyostelic and ultimately to polycyclic condition during the course of evolution.