BSCBO-103: Pteridology and Telome Theory

Course-III: Pteridology, Gymnosperms and Palaeobotany (BSCBO-103)

• Course covers Pteridology, Gymnosperms, and Palaeobotany
• Block I focuses on Pteridophytes, specifically the Telome theory and Stelar System

Contents

• Introduction to the topic
• Origin of Telomes and Primitive land plants
• Process of Telome Theory (details below)
• Merits of Telome Theory (details below)
• Demerits of Telome Theory (details below)
• Stelar Evolution (details below)

Introduction

• Earliest land plants (rootless, dichotomously branched, single terminal sporangium, protostele) from Silurian and Devonian periods are important
• Telome theory by Walter Zimmermann (1930, 1952), a comprehensive theory of land plant evolution, is the most significant
• Theory is based on fossil records, outlining major steps in vascular plant evolution
• All vascular plants evolved from a simple, leafless Rhynia-type ancestral form made up of sterile and fertile axes (telomes)
• Modifications produce more complex plants with roots, stems, leaves, protective sporangia, and complex vascular systems

Process of Telome Theory

• Overtopping: One dichotomizing branch strengthens and becomes the main axis, while other branches become shorter lateral branches. This develops an axis with lateral appendages (leaves), e.g., pinnately compound fern leaves. Overtopped mesomes form the rachis, and the leaflets are formed from the overtopped mesomes.
• Planation: Telomes and mesomes rearrange from a three-dimensional to a single plane of organization. Dichotomous branching in multiple planes (cruciate dichotomy) is replaced by single plane dichotomies (fan shape). This process leads from radial to bilateral symmetry in plant organs, especially concerning the evolution of leaves.
• Syngenesis: Fusion of telomes or telome trusses, facilitated by connecting tissue (called syngenesis or webbing), connects telomes and mesomes. Parenchyma tissue forms the webbing between components, optionally fusing their steles. Development of a duck-foot shape structure results.
• Reduction: Suppressed terminal meristem activity of telomes leads to shorter branches. Telome trusses reduce into the needle-like leaves of conifers. Microphyllous leaves of Lycopsida and Sphenopsida evolved through telome truss reduction.
• Curvation: Fertile telomes curve or bend downwards (Wilson, 1953). Recurvation is the inward bending of telomes, which leads to the inversion of sporangia. Incurvation shifts sporangia from a terminal to a ventral position on fern leaves.

Merits of Telome Theory

• Interprets origin and evolution of land plant sporophytes
• Defines the structure of the sporophytes of primitive land plants
• Explains morphological features of lower vascular plants, particularly Ophioglossaceae and coenopterid ferns
• Highlights the plant body axis with descending root and ascending shoot portion, with modified stem appendages

Demerits of Telome Theory

• Does not explain whorled or spiral arrangement of sporangia, which is common in ancient/primitive plants
• Applications to Lycopsida are highly criticised
• Does not fully explain how a telome-like body structure developed from simpler precursors.
• Criticised by Stewart (1964) for failing to explain the dictyostelic condition.

Stelar Evolution

• Stele is the central vascular cylinder (pillar/column)
• Components include xylem, phloem, pericycle, and sometimes pith
• Endodermis separates cortex from the stele
• Van Tieghem and Douliot (1886) developed the stelar theory
• Stem stele connects with leaf stele via leaf trace

Types of Steles in Pteridophytes

• Protostele: The most basic stele type in which vascular tissue forms a solid cylinder

  • Haplostele: Solid, smooth, spherical xylem core with continuous concentric phloem (e.g., Lygodium, Selaginella)
  • Actinostele: Modified Haplostele, xylem core has radiating ribs/arms, phloem between the ribs (e.g., Psilotum)
  • Plectostele: Xylem core is broken into plate-like lobes, parallel to one another, phloem alternates with plates e.g. , Lycopodium volubile
  • Mixed protostele: Xylem and phloem masses are distributed evenly, scattered xylem groups within phloem ground (e.g., Lycopodium cernuum)
  • Protostele with mixed pith: Parenchyma cells interspersed with tracheids (e.g., Lepidodendrom)

• Siphonostele: A protostele with a central pith

  • Ectophloic siphonostele: Pith surrounded by concentric xylem and then phloem on the outer sides (e.g., Osmunda, Schizea)
  • Amphiphloic siphonostele: Pith is surrounded by vascular tissue with concentric inner phloem cylinder and concentric outer xylem cylinder (e.g., Marsilea rhizome)

• Solenostele: Perforated siphonostele where the pith is perforated with leaf trace locations.

  • Ectophloic solenostele: Phloem is present only on one side of the xylem
  • Amphiphloic solenostele: Phloem is present on both sides of the xylem

• Dictyostele: Solenostele with overlapping gaps, forming a network of vascular strands. Individual vascular strands are called meristeles

• Eustele: Consists of a ring of collateral or bicollateral vascular bundles on the periphery of the pith. Interfascicular areas and leaf gaps are indistinct (e.g., Equisetum)

• Atactostele: Scattered vascular strands (e.g., most monocots)

• Polycyclic Stele: Two or more concentric vascular cylinders (e.g., Pteridium, Matonia)

• Polystele: Multiple steles in the stem (e.g., certain Selaginella species).

Evolution of the Stelar system

• Protostele is the simplest stele
• Other stele types evolved via evolutionary specializations from the protostele
• Shoot growth leads to increased size, causing internal differentiation of the stele.
• Parenchyma develops within the xylem core and differentiates into a central pith(medulla). This transformation leads to the formation of siphonostele

Other Modifications of Siphonostele

• The origin and formation of different types of steles from siphonosteles is debated. Various hypotheses, either extra-stelar or intra-stelar, explain pith formation
• Selaginella is used as a cladosiphonic exemplar, highlighting its leaf gaps and how contrasting this structure is to the phyllosiphonic arrangement of other plants.

This quiz covers critical concepts from the BSCBO-103 course, focusing on Pteridology, Gymnosperms, and Palaeobotany. Key topics include the Telome theory, the origins of early land plants, and the stelar system. Test your understanding of these foundational theories in plant evolution.