Paleobotany, Bryophyta and Pteridophyta BBO1T02

Syllabus topics (official RTMNU)

1. Palaeobotany: Concept and Importance. Geological Time Scale.
2. Contributions of Birbal Sahni
3. Types of fossils: Impression, Compression, Petrifaction, Cast, Mold and Amber.
4. Fossil plants: Glossopteris (Leaf, Scutum).

Palaeobotany: Concept and Importance. Geological Time Scale.

Palaeobotany (Gk. palaios = ancient, botane = plant) is the branch of botany that deals with the study of fossil plants of past geological ages. A fossil is any remains, trace or impression of an organism preserved in rocks of the earth's crust. The term was popularised by the study of plant remains in sedimentary rocks.

Concept: When plants die, most of them decay. But if a plant part is quickly buried in fine sediments (mud, silt, volcanic ash) under water, decay is slowed because oxygen and microbes are less. Over millions of years the sediment becomes rock and the plant part is preserved as a fossil. Fossils are found mostly in sedimentary rocks.

Importance of Palaeobotany:

• Evolution: gives direct evidence of plant evolution, e.g. origin of vascular plants (Rhynia), origin of seed habit (seed ferns).
• Connecting links: reveals extinct groups such as Pteridospermales (seed ferns), Cycadeoidales and Glossopteridales.
• Stratigraphy: index fossils help in dating and correlating rock layers.
• Fuel exploration: coal is formed from Carboniferous and Permian plants; fossil spores and pollen (palynology) help locate coal and petroleum.
• Palaeoclimate and palaeogeography: fossils tell about past climates; Glossopteris flora supports the theory of continental drift (Gondwanaland).
• Phylogeny and classification: helps in arranging plant groups in a natural order.

Geological Time Scale: The history of the earth (about 4.6 billion years) is divided into Eras → Periods → Epochs. Each era has a characteristic flora.

Contributions of Birbal Sahni

Prof. Birbal Sahni (1891–1949) is known as the Father of Indian Palaeobotany. He was born at Bhera (now in Pakistan), studied at Lahore and at Cambridge under Prof. A.C. Seward.

• Worked as Professor of Botany at Lucknow University; founded the Birbal Sahni Institute of Palaeobotany (BSIP), Lucknow in 1946 (foundation stone laid by Pt. Jawaharlal Nehru in 1949).
• Studied the Glossopteris flora of the Gondwana rocks of India and supported continental drift.
• Described the Rajmahal Hills (Jharkhand) Jurassic flora; discovered and named Pentoxylon (Pentoxylales), a new group of gymnosperms, and Williamsonia sewardiana.
• Worked on Homoxylon (a vesselless angiosperm-like wood) and on fossils of the Deccan Intertrappean beds (e.g. Enigmocarpon).
• Studied the age of the Saline Series of the Salt Range (Punjab) using microfossils.
• Also contributed to archaeology – studied coin moulds of Rohtak.
• Elected Fellow of the Royal Society (FRS) in 1936 – first Indian botanist to receive it; President of the Indian Science Congress (1940).

Types of fossils: Impression, Compression, Petrifaction, Cast, Mold and Amber.

Fossil plants: Glossopteris (Leaf, Scutum).

Systematic position: Gymnosperms → Pteridospermales (Glossopteridales). Glossopteris (Gk. glossa = tongue) is a form-genus for tongue-shaped leaves of Permian age found in the Gondwana rocks of India, Africa, South America, Australia and Antarctica. Its wide distribution supports the idea of a single southern continent, Gondwanaland.

Leaf:

• Simple, tongue-shaped (lingulate) to spathulate, 2–40 cm long, margin entire, apex obtuse or acute.
• Prominent midrib made of several parallel veins.
• Lateral veins arise from the midrib at acute angles, arch towards the margin and form a reticulate (net) venation with elongated meshes.
• Leaves borne in spirals or pseudo-whorls on woody stems; the fossil roots of this plant are called Vertebraria (they look jointed in cross section); the wood is Araucarioxylon-type.
• Stomata on lower surface (hypostomatic), sunken.

Scutum (fertile structure):

• The scutum is a female (ovulate) fructification of Glossopteridales.
• It is attached to the midrib of a fertile leaf by a stalk (adnate pedicel).
• It is made of two halves: a sterile cap/scale half and a fertile half bearing many ovules (seeds) on the inner surface; the margin is often fringed.
• Examples of related fructifications: Scutum, Ottokaria, Lidgettonia (female); Eretmonia and Glossotheca (male, bearing pollen sacs).

5 likely exam questions

Long answer

1. What is palaeobotany? Explain its importance and give an outline of the Geological Time Scale with dominant plant groups.
2. Describe the different types of fossils and the process of their formation. Add a note on Glossopteris.

Short answer / short notes

3. Write the contributions of Prof. Birbal Sahni to palaeobotany.
4. Differentiate between impression and compression fossils.
5. Describe the leaf and scutum of Glossopteris.

Syllabus topics (official RTMNU)

1. Bryophytes – Bryophytes as amphibians of kingdom Plantae, General characteristics of Hepaticopsida, Anthocerotopsida and Bryopsida, alternation of generation and classification (Proskauer 1957) of Bryophytes.
2. Distribution, morphology, anatomy, reproductive structures and life-cycles of Riccia and Funaria
3. Economic Importance of Bryophytes.

Bryophytes – Bryophytes as amphibians of kingdom Plantae, General characteristics of Hepaticopsida, Anthocerotopsida and Bryopsida, alternation of generation and classification (Proskauer 1957) of Bryophytes.

Bryophytes (Gk. bryon = moss, phyton = plant) are the simplest embryophytes: non-vascular, green land plants with a dominant gametophyte and a dependent sporophyte. Examples: Riccia, Marchantia, Anthoceros, Funaria.

Why "amphibians of the plant kingdom"?

• They live on land (soil, rocks, bark) but in moist, shady places.
• They need water for fertilization – the biflagellate antherozoids swim to the archegonium.
• No true roots and no vascular tissue (xylem/phloem), so they absorb water over the whole surface.
• Thus like animal amphibians, they live on land but depend on water to complete their life cycle.

General characters: thalloid or leafy gametophyte; rhizoids instead of roots; multicellular jacketed sex organs (antheridia and flask-shaped archegonia); embryo retained in archegonium; sporophyte (sporogonium) differentiated into foot, seta and capsule and is partially or wholly dependent on gametophyte; spores homosporous.

Alternation of generations: Bryophytes show a heteromorphic alternation of generations. The haploid (n) gametophyte is dominant, independent and produces gametes by mitosis. Fusion of gametes gives a diploid (2n) zygote which forms the sporophyte (sporogonium). Spore mother cells undergo meiosis to form haploid spores, which germinate into new gametophytes.

Gametophyte (n) → sex organs → gametes (n) → fertilization → zygote (2n) → sporophyte (2n) → meiosis → spores (n) → gametophyte (n).

Classification (Proskauer, 1957): Proskauer divided Bryophyta into three classes:

1. Hepaticopsida (liverworts) – e.g. Riccia, Marchantia, Pellia, Porella
2. Anthocerotopsida (hornworts) – e.g. Anthoceros, Notothylas
3. Bryopsida (mosses) – e.g. Funaria, Polytrichum, Sphagnum

Distribution, morphology, anatomy, reproductive structures and life-cycles of Riccia and Funaria

Riccia (Hepaticopsida, Marchantiales, Ricciaceae)

• Distribution: about 200 species, cosmopolitan; on moist soil and walls. Indian species: R. discolor, R. himalayensis; R. fluitans is aquatic.
• Morphology: small, green, dorsiventral, dichotomously branched thallus forming rosettes; a dorsal median groove on upper side ending in an apical notch with growing point; ventral surface bears rhizoids (smooth-walled and tuberculate) and one row of multicellular scales.
• Anatomy (V.S.): two zones – (a) upper photosynthetic zone of vertical unbranched chlorophyllous filaments separated by narrow air canals, the top cell of each filament larger and colourless (forms a loose epidermis); (b) lower storage zone of compact colourless parenchyma storing starch, with the lower epidermis giving rise to rhizoids and scales.
• Vegetative reproduction: by death and decay of older parts (fragmentation), adventitious branches, persistent apices, tubers.
• Sexual reproduction: monoecious or dioecious. Sex organs are embedded in the dorsal groove in acropetal order. Antheridium – stalked, oval, with one-layered jacket enclosing androcytes that form biflagellate antherozoids. Archegonium – flask-shaped with venter (egg and venter canal cell) and neck (4 neck canal cells, 6 vertical rows of neck cells).
• Sporophyte: the simplest among bryophytes – only a spherical capsule (no foot, no seta). The capsule wall is single-layered and disintegrates; spore mother cells form spore tetrads. No elaters (some sterile nurse cells). Spores released after decay of thallus.
• Germination: spore → short germ tube → young thallus.

Funaria (Bryopsida, Funariales, Funariaceae) – the "cord moss"

• Distribution: cosmopolitan, about 117 species; F. hygrometrica common on moist soil, walls and burnt ground.
• Morphology: gametophyte has two stages – (1) protonema, a green branched filament with oblique cross walls, bearing buds; (2) gametophore, an erect leafy shoot 1–3 cm with a central axis, spirally arranged sessile leaves with a midrib, and multicellular branched rhizoids at the base.
• Anatomy of axis: epidermis, cortex (outer thick-walled, inner thin-walled parenchyma) and a central strand of narrow conducting cells (hydroids, not true xylem). Leaf is one cell thick except at the midrib.
• Reproductive structures: monoecious and autoicous – antheridia on the main shoot (male branch) and archegonia on a lateral branch of the same plant. The antheridial head has club-shaped antheridia mixed with paraphyses and surrounded by perigonial leaves. Archegonia are at the tip of the female branch among paraphyses and perichaetial leaves.
• Sporophyte: foot, long seta and pear-shaped capsule. Capsule parts: (a) apophysis – basal sterile part with stomata; (b) theca – middle fertile part with central columella, spore sac and air spaces with trabeculae; (c) operculum – lid, separated by the annulus; under it the peristome of two rings of 16 teeth each (diplolepideous). A calyptra (from archegonium) covers the capsule. Peristome teeth are hygroscopic and help spore dispersal.
• Life cycle: spore → protonema → buds → leafy gametophore → sex organs → fertilization → zygote → sporophyte → spores (meiosis).

Economic Importance of Bryophytes.

• Soil formation: pioneers on bare rocks in xerarch succession (with lichens); help form soil.
• Soil conservation: dense mats prevent soil erosion.
• Peat: Sphagnum (peat moss) forms peat used as fuel and in horticulture; it can hold water up to 20 times its weight – used as packing material for live plants.
• Medicinal: Sphagnum was used as surgical dressing (antiseptic, absorbent); Marchantia was used for liver ailments; some mosses have antibiotic substances.
• Indicators: sensitive to SO₂, used as indicators of air pollution; some indicate soil type (calcicole/acidic).
• Research: Physcomitrella patens and Marchantia polymorpha are model plants in genetics.
• Used as food for some animals and as decorative material.

5 likely exam questions

Long answer

1. Why are bryophytes called amphibians of the plant kingdom? Give the general characters of Hepaticopsida, Anthocerotopsida and Bryopsida with Proskauer's (1957) classification.
2. Describe the external morphology, internal structure and life cycle of Funaria with labelled diagrams.

Short answer / short notes

3. Describe the V.S. of thallus of Riccia.
4. Describe the structure of the sporophyte of Riccia.
5. Write the economic importance of bryophytes.

Syllabus topics (official RTMNU)

1. Pteridophytes – General characteristics of Psilopsida, Lycopsida, Sphenopsida and Pteropsida
2. Classification (Smith 1955) and Economic importance
3. Alternation of generation in Pteridophytes (Homosporic and Heterosporic)
4. Stelar system in Pteridophytes

Pteridophytes – General characteristics of Psilopsida, Lycopsida, Sphenopsida and Pteropsida

Pteridophytes (Gk. pteron = feather, phyton = plant) are the first true vascular land plants without seeds – "vascular cryptogams". The main plant body is the diploid sporophyte, differentiated into root, stem and leaves, with xylem (tracheids) and phloem (sieve cells). Spores are borne in sporangia; the gametophyte (prothallus) is small and independent. Water is still needed for fertilization.

Classification (Smith 1955) and Economic importance

G.M. Smith (1955) divided the vascular cryptogams into four divisions:

1. Division Psilophyta – Class Psilophytinae: orders Psilophytales (Rhynia, Horneophyton) and Psilotales (Psilotum, Tmesipteris).
2. Division Lepidophyta – Class Lycopodinae: orders Lycopodiales (Lycopodium), Selaginellales (Selaginella), Lepidodendrales (Lepidodendron), Lepidocarpales, Isoetales (Isoetes) and Pleuromeiales.
3. Division Calamophyta – Class Equisetinae: orders Hyeniales, Sphenophyllales (Sphenophyllum) and Equisetales (Equisetum, Calamites).
4. Division Pterophyta – Class Filicinae: orders Coenopteridales, Ophioglossales, Marattiales and Filicales (e.g. Osmunda, Pteris, Marsilea, Salvinia, Azolla).

Economic importance of Pteridophytes:

• Biofertilizer: Azolla harbours the nitrogen-fixing cyanobacterium Anabaena azollae; used in paddy fields.
• Food: sporocarps of Marsilea eaten by tribals; young fronds of Diplazium eaten as vegetable.
• Medicinal: rhizome of Dryopteris filix-mas gives anthelmintic drug (against tapeworm); Lycopodium spores used as dusting powder; Equisetum used as diuretic.
• Ornamental: ferns like Adiantum (maidenhair), Nephrolepis, Selaginella grown in gardens.
• Industrial: stems of Equisetum (rich in silica) used for scrubbing utensils ("scouring rush"); Lycopodium spores formerly used in fireworks and flash powder.
• Coal: Carboniferous pteridophytes (Lepidodendron, Calamites) formed coal deposits.
• Phytoremediation and indicators: Pteris vittata accumulates arsenic; Equisetum indicates gold-rich soil.
• Selaginella bryopteris ("Sanjeevani") is a resurrection plant used in folk medicine.

Alternation of generation in Pteridophytes (Homosporic and Heterosporic)

Pteridophytes show heteromorphic alternation of generations in which the sporophyte (2n) is dominant, independent and long-lived, while the gametophyte (n) is small, short-lived and also independent (unlike bryophytes).

(a) Homosporous type (e.g. Pteris, Lycopodium, Equisetum):

• Sporophyte produces only one kind of spores in sporangia after meiosis.
• Each spore germinates into a free-living, green, usually bisexual (monoecious) prothallus bearing both antheridia and archegonia.
• Fertilization in presence of water → zygote (2n) → embryo → new sporophyte.
• Cycle: Sporophyte (2n) → sporangium → meiosis → spores (n) → bisexual prothallus (n) → antherozoids + egg → zygote (2n) → sporophyte.

(b) Heterosporous type (e.g. Selaginella, Marsilea, Isoetes):

• Two kinds of spores: small microspores in microsporangia and large megaspores in megasporangia.
• Microspore → reduced male gametophyte (antheridium with antherozoids); megaspore → female gametophyte with archegonia.
• Gametophytes are endosporic (develop inside the spore wall) and unisexual, depending on food stored in the spore.
• Cycle: Sporophyte (2n) → micro- and megasporangia → meiosis → microspores and megaspores (n) → male and female gametophytes → gametes → zygote → sporophyte.

Stelar system in Pteridophytes

The stele is the central vascular cylinder of stem and root, including xylem, phloem, pericycle, medullary rays and pith, bounded by endodermis (Van Tieghem and Douliot, 1886, Stelar theory). Pteridophytes show the greatest variety of steles.

1. Protostele – solid central xylem core surrounded by phloem, no pith. The most primitive type.

• Haplostele: smooth round xylem core (e.g. Rhynia, Selaginella spp.).
• Actinostele: star-shaped (ridged) xylem core (e.g. Psilotum, Lycopodium serratum).
• Plectostele: xylem broken into parallel plates alternating with phloem (e.g. Lycopodium clavatum).
• Mixed protostele: xylem and phloem cells mixed (e.g. Lycopodium cernuum).

2. Siphonostele – stele with central pith (medullated).

• Ectophloic: phloem only outside xylem (e.g. Osmunda, Equisetum).
• Amphiphloic: phloem on both sides of xylem, with outer and inner endodermis (e.g. Marsilea rhizome).
• Solenostele: amphiphloic siphonostele with non-overlapping leaf gaps (e.g. Adiantum spp.).
• Dictyostele: siphonostele broken by many overlapping leaf gaps into a network of separate strands called meristeles (e.g. Pteris, Dryopteris).

3. Polystele – more than one stele in the axis, e.g. Selaginella kraussiana (distele) and S. laevigata (polystele), each stele suspended in an air cavity by trabeculae.

Evolution: Protostele (haplostele) is the most primitive; siphonostele evolved from it by formation of pith (intrastelar or extrastelar origin); dictyostele is the most advanced type.

5 likely exam questions

Long answer

1. Give the general characters of Psilopsida, Lycopsida, Sphenopsida and Pteropsida. Outline the classification of Pteridophytes proposed by Smith (1955).
2. What is a stele? Describe the different types of steles found in pteridophytes with suitable diagrams and comment on their evolution.

Short answer / short notes

3. Explain alternation of generation in a homosporous pteridophyte.
4. Differentiate between homosporous and heterosporous pteridophytes.
5. Write the economic importance of pteridophytes.

Syllabus topics (official RTMNU)

1. Fossil Pteridophyte: Rhynia
2. Morphology, anatomy, reproductive structures and life-cycle in Selaginella and Pteris
3. Heterospory and Seed habit.

Fossil Pteridophyte: Rhynia

Systematic position: Psilophyta (Psilopsida) → Psilophytales → Rhyniaceae. Found in the Rhynie chert (Aberdeenshire, Scotland) of Lower Devonian age; described by Kidston and Lang (1917–1921). Preserved as petrifactions in silica. Species: Rhynia gwynne-vaughanii (R. major is now placed in Aglaophyton).

• Morphology: small herbaceous plant (about 20 cm tall); a horizontal rhizome bearing tufts of rhizoids and erect, dichotomously branched aerial axes. No roots and no leaves. Small hemispherical outgrowths on axes in R. gwynne-vaughanii.
• Anatomy of aerial axis (T.S.): single-layered epidermis covered by cuticle and with stomata; cortex differentiated into outer compact and inner loose (photosynthetic) zone with intercellular spaces; central haplostele (protostele) with a small core of annular tracheids surrounded by phloem. Xylem is centrarch. No endodermis is clearly seen.
• Reproductive structures: sporangia are borne singly at the tips of aerial branches; elongated-cylindrical, with a thick multi-layered wall, no dehiscence mechanism (annulus absent). Spores are of one type (homosporous), in tetrads, with triradiate mark – showing meiosis.
• Gametophyte: not known for certain in older accounts; small free-living axes from the same chert (Remyophyton) are now thought to be its gametophytes.
• Importance: one of the simplest and oldest vascular plants; supports the Telome theory (Zimmermann) of origin of leaves and steles.

Morphology, anatomy, reproductive structures and life-cycle in Selaginella and Pteris

Selaginella (Lycopsida → Selaginellales → Selaginellaceae) – "spike moss"; about 700 species, mostly in moist tropical areas; S. bryopteris, S. kraussiana; xerophytic S. lepidophylla (resurrection plant).

• Morphology: sporophyte with prostrate or erect branched stem bearing small ligulate microphylls. In dorsiventral species leaves are in four rows: two rows of small dorsal leaves and two rows of large ventral leaves (anisophyllous). A colourless, leafless rhizophore arises at branching points and bears adventitious roots at its tip. Each leaf has a small tongue-like ligule at its base on the upper side.
• Anatomy of stem: epidermis with cuticle (no stomata), cortex, then a large air cavity; stele (protostele, mono- to polystelic) is suspended by elongated endodermal cells called trabeculae with Casparian strips. Xylem is exarch; vessels occur in some species (e.g. S. oregana).
• Reproductive structures: sporophylls form a terminal four-angled strobilus. Plant is heterosporous: microsporangia (many microspores) and megasporangia (usually only 4 megaspores, as only one megaspore mother cell is functional). Usually megasporangia are at the base and microsporangia above.
• Gametophytes: endosporic. Microspore → male gametophyte with one prothallial cell and an antheridium producing biflagellate antherozoids. Megaspore → female gametophyte (cellular), often begins development while still in the sporangium; archegonia develop at the exposed tri-radiate region.
• Life cycle: fertilization → zygote → embryo with suspensor, foot, root, stem, cotyledons → new sporophyte. Retention of megaspore in the sporangium and presence of suspensor show approach towards seed habit.

Pteris (Pteropsida → Filicales → Pteridaceae) – "brake fern"; about 280 species; P. vittata, P. cretica, P. quadriaurita in India on moist walls and hills.

• Morphology: sporophyte with an underground rhizome covered by brown hairs (ramenta), adventitious roots and large pinnately compound fronds showing circinate vernation (coiled when young). Frond has petiole and rachis with pinnae.
• Anatomy of rhizome: epidermis, hypodermis of sclerenchyma, ground tissue with sclerenchyma patches, and a dictyostele (or solenostele) of meristeles; each meristele is concentric amphicribral (phloem surrounds xylem) with its own endodermis and pericycle. Xylem is mesarch.
• Anatomy of petiole: a single horse-shoe (U-shaped) shaped vascular bundle.
• Reproductive structures: sporangia in a continuous marginal sorus (coenosorus) along the leaf margin on lower side, protected by the false indusium formed by the reflexed margin. Sporangium (leptosporangiate) has a stalk, a biconvex capsule with an incomplete vertical annulus and a thin-walled stomium; 48–64 spores, homosporous.
• Gametophyte: spore → green, heart-shaped, bisexual prothallus with rhizoids; antheridia near the posterior end among rhizoids, archegonia near the apical notch. Antherozoids are multiflagellate.
• Life cycle: fertilization in water → zygote → embryo (foot, root, stem, leaf) → young sporophyte; prothallus dies.

Heterospory and Seed habit.

Heterospory is the production of two kinds of spores – small microspores (male) and large megaspores (female) – by the same plant. Seen in Selaginella, Isoetes, Marsilea, Salvinia, Azolla and fossils like Lepidocarpon.

Origin of heterospory: explained by degeneration of spore mother cells in the megasporangium due to nutritional differences – fewer spores receive more food and become larger. Experiments on Selaginella (Goebel) showed that poor light and nutrition favour the formation of microsporangia. Fossil evidence (Calamostachys casheana) shows intermediate spore sizes.

Seed habit: A seed is a fertilized ovule (integumented megasporangium) containing an embryo. Steps leading to seed habit:

1. Production of two kinds of spores (heterospory).
2. Reduction in number of megaspores to one per megasporangium.
3. Retention of the megaspore inside the megasporangium.
4. Development of female gametophyte inside the megaspore (endospory) while still attached to the parent.
5. Protection of the megasporangium by an integument.
6. Fertilization and development of embryo inside the megasporangium.

Selaginella shows approach to seed habit: heterospory; endosporic gametophytes; reduction of megaspores to four (and to one in S. rupestris and S. monospora); retention of megaspore in the strobilus in S. rupestris and fertilization there; embryo with a suspensor. It lacks an integument, and there is no resting period of the embryo – so it is not a true seed.

5 likely exam questions

Long answer

1. Describe the morphology, anatomy and reproductive structures of Selaginella. How does it show an approach towards seed habit?
2. Describe the external features, anatomy of rhizome, sporangium and life cycle of Pteris with labelled diagrams.

Short answer / short notes

3. Describe the morphology and anatomy of the fossil pteridophyte Rhynia.
4. What is heterospory? Explain its significance.
5. Describe the structure of the sporangium of Pteris.