Define the term ‘stele’. Give an illustrated account of various types of steles met within pteridophyte in an evolutionary sequence.
Q. Define the term ‘stele’. Give an illustrated account of various types of steles met within pteridophyte in an evolutionary sequence.
Or, Describe the structure of different types of steles in Lycopodium.
Ans. The word stele has been derived from Greek language and which means “Pillar”. According to Vantieghem and Douliot (1886) who put forward the ‘steler 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 controversial. Thus a stele consists of a layer of pericycle, xylem, phloem and pith (if present).
Types of steles :
The following types of steles are quite common in pteridophytes.
(i) Protostele
(ii) Siphonostele
(iii) Solenostele
(iv) Dictyostele
(v) Polycyclic stele
(i) Protostele:
Protostele, according to Jeffry (1903) and most of other present day workers, is to the most primitive and simplest type of stele. It consists of solid mass of xylem, completely surrounded by phloem, pericycle and endodermis in regular order (Fig.). Protostele is permanently remained in the adult stems of many living pteridophytes e.g. Lygodium, Lycopodium. Hymenophyllum, Selaginella, etc.
The protostele is the fundamental types of stele for the vascular plants and all others steles have been derived from it during the course of evolution. The protostele may exist in any of the following form :
1. Haplostele: Haplostele is that in which the central xylem has a smooth outslide and surrounded by concentric layers of phloem e.g., Lygodium, Selaginella kraussiana and sps. of Lycopodium. This Haplostele is regarded as the most primitive of all the protostele.
2. Actinostele:Actinostele is that in which the central xylem cone has radiating ribs and assumes a star shaped appearance e.g. Lycopodium serratum, Psilotum.
3. Plectostele: Plectostele is that in which the Xylem is divided into a number of separate plates, which lie parallel to or radiate to one another e.g., Lycopodium volubile.
4. Mixed protostele: Mixed protostele is that in which the Xylem and phloem are uniformly distributed. Xylem elements remain embedded in phloem tissue, e.g., Lycopodium cernuum. The stele is without pith.

Fig. A-Haplostele, B-Actinostele, C-Plectostele, D-Mixed protostele.
(ii) Siphonostele or Medullated protostele:
Siphonostelic is that stele in which the pith is present in the centre of hollow vascular cylinder. So siphonostele is actually a modified protostele with pith and therefore botanists believe that siphonostele has arising from protostele either by the intrastelar origin of pith or by the invasion of cortical tissue (extrastelar origin of pith). The first view, which has been proposed by Bordle, has been accepted by most of the workers.
Siphonosteles may of the following three types :
1. Ectophloic siphonostele: This types of stele has central pith, which is surrounded by concentric rings of xylem and phloem.
2. Amphiphloic siphonostele: In this type of stele the centre is occupied by pith. In this case the xylem is surrounded on both external and inner sides by the rings of phloem, pericycle and endodermis, e.g. Marsilea.
3. Eustelic siphonostele: In this type of stele, the xylem and phloem cylinders breakes into a large number of collateral vascular bundles, which are arranged in a ring e.g. Equisetum.

(iii) Solenostele:
Solenostele is that stele in which the stele is perforated by single leaf. It may be ectophloic or amphiphloic solenostele in the same way as in siphonostele e.g. Ferns.
(iv) Dictyostele :
Dictyostele is that stele in which there occur more than one leaf gap. The portion of vascular tissue which lies in between two gaps is termed as Meristeles. Each Meristele is protostelic in nature, e.g., Ferns.
(v) Polycyclic stele:
Polycyclic stele is that stele in which the meristeles are distributed in more than one rings and are of different sizes. There occur normally two rings in Pteridium aquilinum (fern).
Follow on Facebook page – Click Here
Google News join in – Click Here
Read More Asia News – Click Here
Read More Sports News – Click Here
Read More Crypto News – Click Here