Stelarevolution 120401081016-phpapp02
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The document discusses the evolution of stelar organization in pteridophytes. It describes how Van Tieghem and Douliot proposed in 1886 that the stele, rather than the vascular bundle, is the basic structural unit of the plant body. The stele is defined as the central vascular cylinder surrounded by cortex. There are several types of protosteles and siphonosteles that evolved, with the protostele being the most primitive form and siphonostele being more advanced with the introduction of a central pith. The document outlines the key characteristics and examples of different protostele and siphonostele types.
Stelarevolution 120401081016-phpapp02
- 1. STELAR EVOLUTION IN PTERIDOPHYTES DR. Manoj Joshi DEPT. OF BOTANY KCMT
- 2. • According to the older botanists, the vascular bundle is the fundamental unit in the vascular system of pteridophytes and higher plants. Van Tieghem and Douliot (1886) interpreted the plant body of a vascular plant in the different way. • The stelar theory was proposed by Van Tiegham and Douliot 1886 while earlear worker Thought that vascular bundel is the basic unit of vascular orgnization. • But according to Van Tiegham and Douliot 1886 the stele is the basic unit structure. The fundamental part of the shoot is cortex and stele . Stele is comprise not only xyleme and phloem element but also pericycle and pith whatever is present . • Jeffery 1898 viewed the stelar orgnization from the point of view of elolution • According to them the fundamental parts of a shoot are the cortex and a central cylinder is known as stele. • The name stele has been derived from a Greek word meaning pillar. This way, the stele is defined as a central vascular cylinder, with or without pith and delimited the cortex by the endodermis. • Van Tieghem and Douliot (1886) recognized only three types of steles. They also thought that the monostelic shoots were rare in comparison of polystelic shoots. It is an established fact that all shoots are monostelic and polystelic condition rarely occurs.
- 3. BASIC TYPES OF STELES PROTOSTELE – Central Xylem, surrounding Phloem. No Pith. PrimitiveJeffrey (1898), for the first time pointed out the stelar theory from the point of view of the phylogeny. According to him the primitive type of stele is protostele. In protostele, the vascular tissue is a solid mass and the central core of the xylem is completely surrounded by a layer of phloem. This is the most primitive and simplest of steles. • SIPHONOSTELE – Protostele with Central Pith. Advanced
- 5. • There are several forms of the protostele which are as follows: • 1. Haplostele: • This is the most primitive type of protostele. Here the central solid smooth core of xylem is surrounded by a layer of phloem, e.g., Salaginella sp. • 2. Actinostele: • This is the modification of the haplostele and somewhat more advanced in having the central xylem core with radiating ribs, e.g., Psilotum sp. • 3. Plectostele: • This is the most advanced type of protostele. Here the central core of xylem is divided into a number of separate plates arranged parallel to each other. The phloem alternates the xylem, e.g., Lycopodium sp. • 4. Mixed-pith stele: • Here the xylem elements (i.e., tracheids) are mixed with the parenchymatous cells of the pith. This type is found in primitive fossils and living ferns. They are treated to be the transitional types between true protosteles and siphonosteles, e.g., Gleichenia sp., Osmunda sp.
- 6. TYPES OF PROTOSTELES The simplest type of stele central core of xylem surrounded by phloem and pericycle . Jeffery 1903 called it protostele two type of protostele has been identify by Brebnerin pteridophyte. • Haplostele – Central circular Xylem. Rhynia selegenella sp. • Actinostele – Star shaped Xylem. Ex. Psilotum, Lycopodium serratum Brebner 1902 • Plectostele- Xylem broken into many parallel plates. By Zimmerman 1930 Ex. L. clavatum • L. volubile etc • Mixed Protostele – Small irregular patches of Xylem. Ex. L. cernuum in hymenophyllum demissum mixed protostele with pith is formed
- 8. TYPES OF SIPHONOSTELES • With the introduction of a central pith a protostele seems to have given rise to a siphonostele. • Thus a stele with a central pith surrounded by vascular tissue is discribed as siphonostele (jeffery 1998)transition of protostele into a siphonosteleis seem in plant like Botrychium, Gleichenia, Osmunda, Schizaea etc. • The siphonostele are of two type • Cladosiphonic – No leaf traces • Phyllosiphonic – with leaf traces • Ectophloic – Phloem outside xylem . Ex. Equisetum • Amphiphloic – Outer and inner rings of Phloem, Xylem central. Ex. Marselia
- 9. Solenostele – Non- overlapping leaf traces. • Solenostele – Non-overlapping leaf traces. Ex. Adiantum • The vascular plants have been divided into two groups on the basis of the presence or absence of the leaf gaps. • These groups are • 1. Pteropsida • 2. Lycopsida. • The ferns, gymnosperms and angiosperms are included in Pteropsida, whereas the lycopods, horse tails, etc., are included in Lycopsida. • The simplest form of siphonostele has no leaf gaps, such as some species of Selaginella. However, among the simplest siphonostelic Pterospsida and siphonostelic Lycopsida, the successive leaf gaps in the stele do not overlap each other and are considerably apart from each other. • According to Brebner (1902), Gwynne-Vaughan (1901) such siphnosteles which lack overlapping of gaps are known as solenosteies. They may be ectophloic or amphiphloic in nature. Some authors (Bower, 1947; Wardlaw, 1952; Esau, 1953), however, interpreted the solenostele as an amphiphloic siphonostele.
- 11. TYPES OF SIPHONOSTELE • Solenostele – Non-overlapping leaf traces. Ex. Adiantum • Dictyostele- Overlapping leaf traces Ex. OphioglossumIn the more advanced siphonosteles of Pteropsida, the successive gaps may overlap each other. Brebner (1902) called the siphonosteles with overlapping gaps as dictyosteles. In such cases the intervening portion of the vascular tissue meristele is of protostelic type. The dictyostele with many meristeles look like a cylindrical meshwork. • Polycyclic – Two or more concentric rings of vascular tissue. Complex type. Ex. Pteris This type of stelar organization is the most complex one amongst all pteridophytes. Such type of steles are siphonostelic in structure. Each such stele possesses an internal vascular system connected with an outer siphonostele. Such connections are always found at the node. A typical polycyclic stele possesses two or more concentric rings of vascular tissue. This may be a solenostele or a dictyostele. Two concentric rings of vascular tissue are found in Pteridium aquilinum and three in Matonia pectinata.
- 13. Eustele: • According to Brebner (1902) there is one more modification of the siphonostele, known as eustele. Here the vascular system consists of a ring of collateral or bicollateral vascular bundles situated on the priphery of the pith. In such steles the interfascicular areas and the leaf gaps are not distinguished from each other very clearly. The example of this type is Equisetum.
- 14. Origin of Siphonostele • Intra stelar Origin – Pith developed from inner xylem cells. • Boodle 1901 Gwynne & Vaughani 1908, Bower 1911, Fahn(1960) • Extra stelar Origin – Pith formed by outer cortical cells which entered inside through leaf gaps. • Jeffery 1902,1910,1917