Stellar system and Its Evolution -
Q.5. What is stele ? Describe its various types in Pteridophytic plants. (2005, 11)
Related Question -
Q. Describe the plectostele with the help of diagram. (2010, 13)
Q. Describe Amphiphloic Siphonostele with help of diagram. (2012)
Q. Explain illustrated account of siphonostele. (2005, 09)
Q. Describe protostele with diagram. (2007)
Q. Explain illustrated account of amphiphloic stele. (2006)
Q. Describe Dictyostele with diagram. (2006)
Q. Where does amphiphloic siphonostele occur? Describe its structure. (2005)
Q. With the help of diagram differentiate between ectophloic, amphiphloic and siphonostele. (2002)
Ans. The word stele has been derived from Greek language and which means “Pillar”. According to Vantieghem and Douliot (1886) who put forward the “Stelar theory”. The stele is the whole central mass of vascular tissue with or without pith, surrounded on the outer side by endodermis. The endodermis either is the part of stele or cortex is controversal. Thus a stele consists of a layer of pericycle, xylem, phloem and pith.
The following two types of stele occur in pteridophytes.
(1) Protostele: -
It is the simplest, and considered to be the most primitive type of stele. It consists of a solid core of xylem surrounded by a cylinder of phloem, enclosing no pith. Protosteles are considered primitive because of their occurrence in certain earliest of the land plants which appeared about 400 million years ago. All other types of steles have evolve from it in the course of evolutionary specialization. Protosteles are most common in psilophytes and lycophytes but they also occur in the juvenile stems of ferns. Such a stele occurred in such primitive psilophytes as Horneophyton and Rhynia and may be found in Selaginella, Lycopodium among present day forms.
Variations of the protostele include the haplostele, actinostele, plectostele and mixed protostele.
(i) Haplostele: -
A protostele with central solid and smooth core of xylem surrounded by phloem is known as Haplostele. This particular type of protostele has been regarded as the most primitive among the different types. It occurred in primitive psilohytes like Horneophyton and Rhynia and is found in number of living genera eg. Lycopodim cernuum and Selaginella kraussiana.
(ii) Actinostele: -
In a number of pteridophytes the central xylem core of a protostele is not smooth but is thrown into radiating ribs with the protoxylem at the extremities and phloem alternating with its rays when seen in a cross section. Such type of protostele is termed as actinostele. It is found in Lycopodium serratum.
Fig. Diagrammatic views of various types of steles found is pteridophytes. A to D-types of protostele. A-Haplostele, B-Actinostele, C-Plectostele, and D-Mixed-protostle. E to I-types of Siphonostele, E-Ectophloic siphonostele, F-Amphiphloic siphonostele, G-Dictyostele, H-Polycyclic solenostele, and I-Polycyclic dictyostele.
(iii) Plectostele: -
In the stems of some species of Lycopodium eg. L. clavatum when seen in a cross section the xylem occurs in the form of small parallel bands alternating with the phloem plates. This specialized form protostele is usually termed as plectostele.
(iv) Mixed Protostele: -
In Lycopodium cernuum the xylem when seen in cross section appears in the form of irregular groups that are embedded in the ground mass of phloem. This type of protostele is called mixed protostele.
(2) Siphonostele or Medulated Protostele: - Siphonostelic is that stele in which the pith is present in the centre of hollow vascular cylinder so siphonostele is actually modified protostele with pith and therefore botanists belive that siphonastele has arising from protostele either by the intrastelar origin of pith or by the invasion of cortical tissue.
Siphonostele is of the following three types:
1. Ectotophloic Siphonostele: - In this type of stele has central pith which is surrounded by coencentric rings of xylem or phloem e.g., Qsmunda, Equisetum etc surrounded by coencetric rings of xylem or phloem.
2. Amphiphloic Siphonostele: - In this type of stele phloem is present on both the sides of xylem. Pith is in the centre. This type of stele which inner phloem, inner pericycle and inner endodermis lie towards the pith and outer phloem outer pericycle and outer endodermis lie towards the out side of the xylem and the pith is in the centre.
3. Equistelic Siphonostele: - In this type of stele the xylem and phloem cylinders breaks into a large number of collateral vascular bundles which are arranged in a ring e.g., Equisetum.
4. Solenoste1e: - Solenostele is that stele in which the stele is perforated by single leaf. It may be ectophloic or amphiphloic soleno- stele in the same way as in siphonostele, e.g. Ferns.
5. Dictyostele: - Dictyostele is that stele in which these occur more than one leaf gap. The portion of vascular tissue which lies in between two gaps is termed as meristeles. Each meristele is protosteleic in nature e.g., Ferns.
6. Polycyclic Stele: - Polycyclic stele is that in which the meristeles are distributed in more than one rings and are of different sizes.There occur normally two rings in Pteridium aquaillium (fern).
Q.6. Describe the evolution of the stelar sytem in pteridophytes briefly.
Ans. Evolution of Stelar system in Pteridophytes: -
Jeffery (1908) and opinion of other workers clearly indicate that protostele is the primitive type of stele. Simplest form of protostele is haplostelic. During further elaboration the central core of xylem of haptostele becomes star shaped and is known as actinostele. As a result of further elaboration the xylem splits into a number or parallel plates, alternating with phloem. This type of stele is said to be plectostele.
Further by appearances of central pith in protostele siphonostele developed. The siphonostele developed may be ectophloic siphonostele. In
this there is a central pith surrounded by a complete ring of xylem and phloem. This is not interrupted by leaf gaps and also called as cladosiphonac. When this ectophloic siphonostele is interrupted by appearance of leaf gaps and this type of stele is now said to be Phyllosiphonic. In between the two gape the vascular cylinder remains complete and such stele is known as solenostele and siphonostele. Some times number of separate and collateral vascular bundles are formed and it is said to be eustele (as found in seed plants). The vascular bundle when scattered (as in monocot stems) the stele is known as atactostele.
