MSCBOT-503: Pteridophytes, Gymnosperms, and Palaeobotany Study Guide

Introduction to Pteridophyta

• Definition: The word Pteridophyta originates from the Greek words Pteron (meaning feather) and Phyton (meaning plant). These plants are characterized by their feather-like fronds (leaves).
• Classification Status: Pteridophytes are categorized within Cryptogams alongside Thallophyta (algae, fungi) and Bryophyta. They are specifically designated as higher cryptogams or vascular cryptogams because they possess a well-developed conducting system, making them the first true land plants.
• Spore Bearing: Unlike spermatophytes, all cryptogams reproduce via spores and do not produce seeds.
• Geological History: Pteridophytes have a fossil history spanning approximately $380\,million\,years$. Fossils are found in rock strata from the Silurian and Devonian periods of the Palaeozoic era, which is often termed the "Age of Pteridophyta."
• Evolutionary Stature: Modern day lycopods are considered relicts of giant fossil arborescent lycopods like Lepidodendron. Genera like Psilotum and Tmesipteris are surviving remains of psilopsids that conserve primitive features of early land plants.
• Position in Plant Kingdom: Pteridophytes occupy a position between bryophytes and gymnosperms.
  • Similarities with Bryophytes: Presence of a sterile jacket around antheridia and archegonia; requirement of water for fertilization; alternation of generations; spore formation.
  • Similarities with Gymnosperms: Sporophytic plant body is independent and dominant; differentiation into roots, shoots, and leaves; presence of vascular tissues (Tracheophyta).

General Characteristics and Habitat of Pteridophytes

• Sporophyte: The main independent plant body is the sporophyte ($2n$), which possesses a vascular system.
• Morphological Differentiation: Differentiated into true roots, shoots, and leaves, though primitive members (Psilophytales, Psilotales) lack true roots and well-developed leaves.
• Habit and Growth: Mostly herbaceous, except for a few woody tree ferns. Symmetry can be dorsiventral or radial.
• Leaves:
  • Microphyllous: Small leaves with an unbranched midrib (e.g., Lycopodium, Selaginella, Equisetum).
  • Megaphyllous: Large leaves with branched midribs, typical of true ferns.
  • Vernation: Young fern leaves show circinate vernation (coiled inwards).
• Roots: Primary embryonic roots are short-lived and usually replaced by adventitious roots.
• Spores: Plants are either homosporous (all spores same size) or heterosporous (microspores and megaspores).
• Sporangia: Produced on specialized leaves called sporophylls. In aquatic ferns, sporangia are housed in bean-shaped structures called sporocarps. In true ferns, they cluster on the lower leaf surface in sori.
• Gametophyte: Spores germinate into a prothallus.
  • Homosporous plants are usually monoecious.
  • Heterosporous plants are usually dioecious (microspore forms male prothallus; megaspore forms female prothallus).
• Fertilization: Requires water ($H_2O$). Fertilization results in a diploid zygote ($2n$) or oospore, leading to an embryo.
• Habitats:
  • Terrestrial: Growing in cool, moist, shady places (Lycopodium clavatum).
  • Lithophytic: Growing on rocky patches (Selaginella kraussiana).
  • Epiphytic: Growing on trees (Psilotum nudum, Lycopodium phlegmaria).
  • Aquatic (Water Ferns): Marsilea, Salvinia, Azolla, Regnellidium.
  • Xerophytic: Selaginella rupestris, Marsilea rajasthanensis.

Detailed Classification Systems for Pteridophytes

• Early Taxonomists: Classified vascular plants into Pteridophyta (seedless) and Spermatophyta (seeded).
• Sinnott ($1935$): Introduced "Tracheophyta" to include all vascular plants.
• Eames ($1936$): Divided Tracheophyta into four groups: Psilopsida, Lycopsida, Sphenopsida, and Pteropsida.
• Wardlaw ($1955$): Followed ICBN recommendations, ensuring division names end in -phyta, subdivisions in -phytina, and classes in -opsida.
• Smith ($1955$): Recognized four divisions:
  1. Division Psilophyta: Classes Psilophytopsida and Psilotopsida.
  2. Division Lycophyta: Classes Eligulopsida and Ligulopsida (based on the presence of a ligule).
  3. Division Sphenophyta (Calamophyta): Classes Sphenophyllopsida and Calamopsida.
  4. Division Pterophyta (Filicophyta): Classes Eusporangiopsida, Protoleptosporangiopsida, Leptosporangiopsida, and Primopteropsida.
• A.R. Smith ($2006$): A revised phylogenetic classification recognizing four classes: Psilotopsida ($2$ orders), Equisetopsida ($1$ order), Marattiopsida ($1$ order), and Polypodiopsida ($7$ orders, including Osmundales, Salviniales, and Polypodiales).

Stelar Evolution and Sori in Pteridophytes

• Definition of Stele: Derived from the Greek word for "pillar," it is the central vascular cylinder of xylem, phloem, pericycle, and pith.
• Van Tieghem and Douliot ($1886$): Proposed the Stelar Theory.
• Protostele (Most Primitive): No pith.
  • Haplostele: Solid smooth core of xylem (Rhynia).
  • Actinostele: Star-shaped xylem core (Psilotum).
  • Plectostele: Xylem divided into parallel plates (Lycopodium clavatum).
  • Mixed Protostele: Xylem groups scattered in phloem (Lycopodium cernuum).
• Siphonostele (Medullated Protostele): Contains a central pith.
  • Ectophloic: Phloem outside xylem (Osmunda).
  • Amphiphloic: Phloem on both sides of xylem (Marsilea).
• Solenostele: Perforated siphonostele due to leaf traces.
• Dictyostele: Siphonostele broken into separate vascular strands called meristeles by overlapping leaf gaps (Dryopteris).
• Eustele: Vascular system split into distinct collateral bundles (Equisetum).
• Atactostele: Scattered vascular strands, typical of monocotyledons.
• Polystele: Multiple steles in one axis (Selaginella).
• Sorus Structures:
  • Protects clusters of sporangia.
  • True Indusium: A unique protective cover.
  • False Indusium: A rolled-over leaf margin covering sporangia (Adiantum).
  • Simple Sorus: All sporangia mature simultaneously.
  • Gradate Sorus: Basipetal development with oldest at the apex.
  • Mixed Sorus: Sporangia of different ages present without definite arrangement.

Heterospory and the Seed Habit

• Definition: The production of two different types of spores (microspores and megaspores) by a single species.
• Living Examples: Selaginella, Isoetes, Marsilea, Salvinia, Azolla, Regnellidium, Pilularia, and Stylites.
• Significance: It is the essential prerequisite for the origin of seeds.
• Evolution of Seed Habit Requirements:
  1. Heterospory.
  2. Reduction to a single functional megaspore per megasporangium.
  3. Permanent retention and germination of the megaspore within the megasporangium.
  4. Nourishment of the female gametophyte from the sporophyte.
• The Selaginella Case: S. rupestris approaches the seed habit by retaining the megaspore and developing the young sporophyte on the parent plant (comparable to vivipary). However, it lacks a true integument and a resting period, preventing it from producing a "true" seed.

Detailed Taxonomy of Fossil and Living Pteridophytes

• Psilophytopsida (Extinct): Includes Cooksonia (oldest land plant, $433-393\,MYA$, lacks leaves and roots) and Rhynia (discovered in Scotland, $1917$; R. major is $50\,cm$ tall, R. gwynne-vaughani is $20\,cm$ tall).
• Aglaophyton major: Formerly Rhynia major; reclassified as non-vascular (more like moss hydromes) by D.S. Edwards ($1986$).
• Psilotopsida: Includes Psilotum. Characterized by synangium (trilobed sporangia), rootless habit with rhizoids, and dichotomous branching.
• Lycopsida:
  • Lycopodium (Club Moss): Homosporous, various protostele types (plectostele in L. clavatum).
  • Lepidodendron (Fossil): Arborescent (tree-like) reaching $30\,m$; diamond-shaped leaf scars; Stigmaria roots.
  • Selaginella (Spike Moss): Heterosporous; presence of ligules; rhizophore as a transition organ between stem and root.
  • Isoetes (Quill Wort): Grows in swamps; fleshy lobed corm; shows peculiar secondary growth.
• Sphenopsida:
  • Equisetum (Horsetail/Scouring Rush): Jointed stems with nodes/internodes; carinal and vallecular canals in anatomy; spores possess elaters for dispersal.
  • Hyenia (Fossil): Horizontal rhizome; sterile forked leaves.
  • Sphenophyllum (Fossil): Slender scrambling stems; triarch exarch protostele; heterophyllous wedge-shaped leaves.
• Pteropsida (Ferns):
  • Dryopteris (Male Shield Fern): Dictyostele; sori with kidney-shaped indusium; leptosporangiate development.
  • Hymenophyllum (Filmy Fern): Leaves one-cell thick; sori marginal and gradate.
  • Adiantum (Maidenhair Fern): Sori on false indusia; "walking habit" in A. caudatum.
  • Azolla: Symbiotic with Anabaena azollae for nitrogen fixation.

General Account of Gymnosperms

• Definition: Greek Gymnos (naked) and Sperma (seed). Unlike angiosperms, ovules are exposed on megasporophylls before and after fertilization.
• Morphology: Slow-growing woody perennials (trees and shrubs); herbs are absent. Differentiated into tap roots, branches (long and dwarf shoots), and leaves.
• Leaf Diversity: Microphyllous or megaphyllous; venation can be parallel (Welwitschia), reticulate (Gnetum), or dichotomous (Ginkgo).
• Wood Anatomy:
  • Manoxylic: Soft, porous wood with abundant parenchyma and wide medullary rays (e.g., Cycas).
  • Pycnoxylic: Compact wood with small amounts of parenchyma and narrow rays (e.g., Pinus).
• Vascular Elements: Xylem lack vessels (except Gnetales); phloem lacks companion cells.
• Reproduction: Heterosporous; microspores ($pollen\,grains$) and megaspores. Seeds represent three generations:
  1. Outer integument (old sporophyte, $2n$).
  2. Endosperm (female gametophyte, $n$).
  3. Embryo (new sporophyte, $2n$).
• Classification Sets:
  • Sahni ($1920$): Stachyospermae (axillary ovules) and Phyllospermae (leaf-borne ovules).
  • Sporne ($1965$): Three divisions: Cycadopsida, Coniferopsida, Gnetopsida.

The Pteridospermales (Seed Ferns) and Pentoxylales

• Pteridospermales: Gymnosperms with fern-like foliage. Prevalent in the Carboniferous.
  • Lyginopteris oldhamia: Stem with "dictyoxylon" cortex; seeds named Lagenostoma lomaxi; microsporangia named Crossotheca.
  • Medullosa: Polystelic trees; large seeds (Pachytesta).
  • Glossopteris Flora: Distinctive Permian vegetation of Gondwanaland; tongue-shaped leaves with reticulate venation; roots known as Vertebraria.
  • Caytoniaceae: Discovered in Yorkshire; palmately compound leaves (Sagenopteris); fruits contain closed seeds.
• Pentoxylales: Discovered by Prof. Birbal Sahni ($1948$) from Rajmahal hills, India.
  • Stem (Pentoxylon sahnii): Polystelic ($5$ or $6$ primary steles) with exocentric secondary wood.
  • Leaves (Nipaniophyllum raoi): Simple lanceolate with prominent midribs.
  • Female Cones (Carnoconites): Mulberry-like; over $20$ sessile ovules attached to a central axis.
  • Male Flower (Sahnia nipaniensis): Branched sporangiophores fused at the base.

Major Groups of Higher Gymnosperms

• Bennettitales (Cycadeoideales): Extinct Mesozoic group.
  • Features: Bisporangiate "flowers"; syndetocheilic stomata; stalked ovules interspersed with interseminal scales.
  • Genera: Cycadeoidea (short barrel trunk), Williamsonia.
• Cycadales: Living relics like Cycas, Zamia.
  • Features: Palm-like appearance; girdling leaf traces; diploxylic vascular bundles in leaves; circinate vernation.
  • Motility: Multi-flagellated spermatozoids (largest in the plant kingdom).
• Cordaitales: Paleozoic arborescent gymnosperms ($30\,m$ tall).
  • Features: Large strap-shaped leaves ($1\,m$ long); pycnoxylic wood; compound unisexual cones.
• Ginkgoales: Represented by the living fossil Ginkgo biloba.
  • Features: Dioecious; deciduous fan-shaped leaves; dichotomous venation; endospermic "tent pole" in the ovule; motile sperm.
• Coniferales: Dominant forest-makers (Pinus, Cedrus).
  • Features: Monoecious/dioecious; winged pollens; compound female cones with bract and ovuliferous scales.
• Taxales: Taxus baccata (Yew).
  • Features: No resin canals; single terminal ovule surrounded by a red fleshy aril.
• Gnetopsida: Considered the link to angiosperms.
  • Ephedra: Jointed stems; reduced scaly leaves; double fertilization (though no resulting endosperm); source of ephedrine.
  • Welwitschia: Native to Namib desert; gigantic turnip-like stem with only two lifelong leaves.
  • Gnetum: Tropical climbers; angiospermous features like vessels in xylem and broad net-veined leaves; absence of archegonia.

Introduction to Palaeobotany

• Definition: The study of plant fossils, derived from the Latin Fossilis ("to dig up").
• Preservation Conditions (Fossilization):
  • Anoxic Environment: Depletion of oxygen prevents decay.
  • Rapid Deposition Rate: Protects organisms from scavengers/bacteria.
• Types of Fossils:
  • Petrifaction: Organic material replaced by minerals (calcium carbonate or silica).
  • Compression: Carbonaceous film flattened by pressure.
  • Cast/Incrustation: Mineral filling of a cavity left by decayed material.
  • Impressions: Imprints of external features without organic material.
  • Amber: Hardened resin from Pinus succinifera preserving insects/plant parts.
  • Coal Balls: Petrified spherical plant organs found in coal seams.
  • Pseudofossils: Mineral depositions mimicking plant remains.
• Fossil Research Methods:
  • Radio Carbon Dating: Based on the decay of $C^{14}$ (half-life of $5,730\,years$).
  • Ground Thin Section: For petrified samples.
  • Peel Section: Using nitrocellulose film applied to etched surfaces.
  • Maceration: Acid digestion of sediments to isolate spores and pollens.

Geological Time Scale

• Earth's Age: Approximately $4.6\,billion\,years$.
• Eras:
  • Azoic: Era of no life.
  • Archaeozoic/Proterozoic: Combined as Precambrian; first aquatic plants (algae, fungi).
  • Palaeozoic: "Age of Pteridophyta."
    • Carboniferous: Formation of coal swamps by lycopods and horsetails.
    • Permian: Rise of early conifers.
  • Mesozoic: "Age of Gymnosperms."
    • Triassic/Jurassic: Dominance of Cycads and Conifers.
    • Cretaceous: Rise of Angiosperms.
  • Cenozoic: Era of mammals and herbaceous plants.
• Revolutions: Major geological disturbances separating eras (e.g., Appalachian Revolution, Rocky Mountain Revolution).
• Father of Paleobotany: Sternberg; Father of Indian Paleobotany: Prof. Birbal Sahni.