Pteridophytes

Pteridophytes, commonly known as ferns and their allies, represent a significant group of vascular plants that bridge the gap between bryophytes and seed-bearing plants (gymnosperms and angiosperms). They were the dominant vegetation during the Carboniferous period, forming vast coal deposits. Unlike bryophytes, pteridophytes possess true roots, stems, and leaves, and they exhibit a well-developed vascular system (xylem and phloem) for efficient transport of water and nutrients. Their life cycle involves alternation of generations, with the sporophyte generation being dominant and independent, while the gametophyte is typically small and short-lived.

Classification of Pteridophytes

The classification of pteridophytes has evolved over time, with various systems proposed by botanists. One of the widely recognized systems is by Sporne (1976), which categorizes pteridophytes into four main classes based on their morphological and anatomical characteristics, particularly the structure of their vascular tissues, leaves, and sporangia. These classes are Psilopsida, Lycopsida, Sphenopsida, and Pteropsida.

Class Psilopsida

Psilopsida is considered the most primitive group of pteridophytes. Members of this class are characterized by a dichotomously branched, subterranean rhizome and erect aerial stems. They lack true roots and leaves. The aerial stems are photosynthetic and bear small, scale-like appendages. Sporangia are borne terminally on the aerial stems.

Extinct Forms: This class is predominantly represented by extinct genera like Rhynia and Psilophyton, which are known from fossils of the Devonian period. These ancient plants provide crucial insights into the early evolution of vascular plants.

Extant Forms: The only living genus representing this class is Psilotum. Tmesipteris is sometimes included here, though its exact taxonomic position is debated.

Distribution: Psilotum is cosmopolitan, found in tropical and subtropical regions worldwide, often growing epiphytically on other plants or in soil crevices.

Class Lycopsida

Lycopsida includes clubmosses and their allies. These plants are characterized by small, simple leaves (microphylls) that are not associated with a ligule (a small, tongue-like appendage found at the base of the leaf in many lycophytes). The stems are typically dichotomously branched, and they possess true roots. Sporangia are borne singly in the axils of leaves or at the apex of stems, forming cone-like structures called strobili.

Extinct Forms: Extinct lycophytes, such as Lepidodendron and Sigillaria, were giant tree-like plants that formed a significant part of the Carboniferous flora. They had massive trunks and complex branching patterns.

Extant Forms: Living lycophytes include genera like Lycopodium (clubmosses), Selaginella (spike mosses), and Isoetes (quillworts).

Distribution: Lycophytes are found globally, from the tropics to the arctic, inhabiting diverse environments ranging from forests to arid regions and aquatic habitats.

Class Sphenopsida

Sphenopsida, commonly known as horsetails, are characterized by jointed stems with distinct nodes and internodes. The leaves are small, scale-like, and arranged in whorls at the nodes. The stems are often ribbed and hollow. Sporangia are borne in strobilus-like structures at the apex of fertile shoots.

Extinct Forms: Extinct sphenopsids, such as Calamites, were large, tree-like plants that were abundant in the Carboniferous period.

Extant Forms: The only living genus is Equisetum (horsetails).

Distribution: Equisetum is widespread in temperate and tropical regions of the world, often found in moist environments, along riverbanks, and in open fields.

Class Pteropsida

Pteropsida, the largest and most familiar group of pteridophytes, includes the true ferns. They are characterized by large, compound leaves called fronds, which typically unroll from a fiddlehead or crozier shape in the young stage. The stems are often rhizomatous and subterranean. Sporangia are usually borne in clusters called sori on the underside of the fronds.

Extinct Forms: Many extinct fern families existed, contributing to the Carboniferous flora. Examples include tree ferns and other large fern-like plants.

Extant Forms: This class includes thousands of species found in diverse genera such as Dryopteris, Pteridium, Adiantum, and Osmunda.

Distribution: Ferns are found in almost all terrestrial habitats worldwide, with a particular abundance in moist, shady tropical and temperate regions.

Distribution of Extinct and Extant Pteridophytes

Pteridophytes have a rich evolutionary history, with their peak diversity occurring during the Carboniferous period (approximately 360 to 300 million years ago). This era is often called the "Age of Pteridophytes" due to their dominance in terrestrial ecosystems.

Extinct Forms

The Carboniferous period saw the rise of giant, arborescent (tree-like) forms of Lycopsida and Sphenopsida.

• Lycophytes: Genera like Lepidodendron (scale trees) and Sigillaria were dominant. Lepidodendron had a distinctive diamond-patterned bark formed by leaf scars and a massive trunk that could reach heights of up to 30-40 meters. Sigillaria had a more ribbed trunk with a single row of leaf scars. These plants formed extensive forests, and their buried remains under anaerobic conditions eventually formed coal seams.
• Sphenopsids: Calamites were large, reed-like plants with jointed stems that grew in dense stands, often along riverbanks and in swampy areas. They could reach heights of up to 20-30 meters.
• Psilopsida: Primitive psilophytes like Psilophyton and Rhynia were among the earliest vascular plants, flourishing in the Devonian period. They were relatively small, simple plants that colonized barren land.
• Ferns (Pteropsida): While not as dominant as lycophytes and sphenopsids in terms of forming coal, various extinct fern lineages, including early tree ferns, were also present.

The decline of these dominant extinct forms is attributed to changes in climate, sea levels, and the emergence of more advanced plant groups like gymnosperms.

Extant Forms

Modern pteridophytes are generally smaller and less dominant than their ancient ancestors, but they are incredibly diverse and widespread.

• Psilotum: Found in tropical and subtropical regions, often as epiphytes in humid forests or in soil pockets on rocks.
• Lycophytes:
  • Lycopodium: Widely distributed in temperate and tropical regions, typically found in forests, often growing on the ground or as epiphytes.
  • Selaginella: Primarily found in moist, shady habitats in tropical and subtropical regions, though some species extend into temperate zones. Known for heterospory.
  • Isoetes: Found in aquatic or semi-aquatic environments, often in temporary ponds or slow-moving streams in temperate and tropical regions.
• Sphenopsida: Equisetum is distributed across temperate and subtropical regions of the Northern Hemisphere, commonly found in moist, open areas like meadows, roadsides, and riverbanks.
• Pteropsida (Ferns): This group is the most successful extant pteridophyte lineage. They inhabit nearly every terrestrial environment, from tropical rainforests (where they are most diverse) to deserts, mountains, and even arctic tundra. Many are terrestrial, but some are epiphytic, lithophytic (growing on rocks), or aquatic.

Comparative Morphology and Anatomy of Sporophytes

The sporophyte is the dominant, diploid generation in the life cycle of pteridophytes. It is characterized by true roots, stems, and leaves, and a well-developed vascular system. Comparing these features across the four classes reveals evolutionary trends.

Sporophyte Morphology

Roots:

• Psilopsida: Lack true roots. The underground portion is a rhizome with rhizoids for anchorage and absorption.
• Lycopsida: Possess true roots, typically dichotomously branched. In Isoetes, the root system is unusual, arising from the base of the corm.
• Sphenopsida: Have true roots, which arise adventitiously from the nodes of the underground rhizome.
• Pteropsida: Possess true roots, which may be adventitious or arise from a rhizome.

Stems:

• Psilopsida: Dichotomously branched aerial stems and rhizomes. The aerial stems are photosynthetic.
• Lycopsida: Typically dichotomously branched, but the branching pattern can appear pseudomonopodial. Stems are generally solid.
• Sphenopsida: Jointed stems with distinct nodes and internodes. Stems are often hollow or ridged, with whorls of leaves at the nodes.
• Pteropsida: Often rhizomatous (horizontal underground stems), but some are erect or climbing. Branching patterns vary.

Leaves:

• Psilopsida: Lack true leaves. Possess small, scale-like appendages on the aerial stems, which are non-vascular.
• Lycopsida: Possess microphylls – small, simple leaves, typically entire and vascularized by a single vein. Often have a ligule at the base.
• Sphenopsida: Possess small, scale-like leaves arranged in whorls at the nodes. These are considered reduced megaphylls.
• Pteropsida: Possess megaphylls – large, usually compound leaves (fronds) with a complex venation pattern. These are the most elaborate leaves among pteridophytes.

Sporangia:

• Psilopsida: Terminal sporangia on aerial branches (e.g., synangium in Psilotum).
• Lycopsida: Solitary sporangia, borne in the axils of microphylls or on specialized leaves (sporophylls) aggregated into strobili (cones).
• Sphenopsida: Sporangia borne on specialized structures (sporangiophores) arranged in terminal strobili.
• Pteropsida: Typically borne in clusters called sori on the underside of fronds. Each sorus contains numerous sporangia.

Sporophyte Anatomy

Vascular System (Stele): The arrangement of xylem and phloem in the stem is a key characteristic.

• Psilopsida: Possess a simple actinostele (star-shaped xylem) or plectostele in the stem.
• Lycopsida: Have a protostele, typically a monarch to polyarch xylem core (haplostele, actinostele, or plectostele). In Selaginella, a central vascular strand with a distinct stele is present.
• Sphenopsida: Exhibit siphonosteles, specifically ectophloic or amphiphloic siphonosteles, with large pith. The vascular bundles are separate and form a ring.
• Pteropsida: Generally possess more complex siphonosteles, such as ectophloic or amphiphloic siphonosteles, and in more advanced forms, dictyosteles (a dissected siphonostele forming a network of vascular strands).

Internal Structure of Roots, Stems, and Leaves: Detailed anatomical studies reveal differences in tissue organization, presence of vascular cambium (absent in all pteridophytes), and specialized structures like vallecular canals and carinal canals in Equisetum stems.

Gametophyte Structure and Development

The gametophyte is the haploid, sexual generation in pteridophytes. It is typically small, inconspicuous, and develops from a haploid spore. Its structure and development vary significantly across the classes.

Gametophyte Structure

Psilopsida:

• Structure: The gametophyte is subterranean, dichotomously branched, and thalloid. It is saprophytic or semi-saprophytic, lacking chlorophyll, and obtains nutrition from decaying organic matter, often with the help of mycorrhizal fungi. It bears both antheridia (producing sperm) and archegonia (producing eggs) on its surface.
• Development: Develops from a spore, often remaining underground and developing mycorrhizal associations.

Lycopsida:

• Structure:
  • Lycopodium: The gametophyte is typically small (1-3 mm), subterranean or superficial, and saprophytic or photosynthetic. It is often bisexual, bearing antheridia and archegonia on the same thallus.
  • Selaginella: Exhibits heterospory, producing two types of spores (microspores and megaspores). This leads to unisexual gametophytes. The microspore develops into a male gametophyte retained within the microspore wall, producing antheridia with sperm. The megaspore develops into a female gametophyte retained within the megaspore wall, producing archegonia with eggs.
  • Isoetes: The gametophyte is greatly reduced and endosporic (develops within the spore wall), similar to seed plants.
• Development: Can be exosporic (develops outside the spore wall, as in Lycopodium) or endosporic (develops within the spore wall, as in Selaginella and Isoetes).

Sphenopsida:

• Structure: The gametophyte is typically small (a few millimeters), superficial, lobed, and green (photosynthetic). It is usually bisexual, bearing antheridia and archegonia on different parts of the same thallus.
• Development: Develops exosporically from a spore, typically growing close to the ground.

Pteropsida (Ferns):

• Structure: The gametophyte, called a prothallus, is typically small (usually less than 1 cm), heart-shaped, green, and photosynthetic. It grows independently on the soil surface. It is usually bisexual, with antheridia developing first near the notch and archegonia developing later near the apex.
• Development: Develops exosporically from a spore. The spore germinates to form a filamentous structure, which then develops into the flattened, heart-shaped prothallus.

Gametophyte Development

The development of the gametophyte begins with the germination of a spore.

• Exosporic Development: The spore germinates externally, and the gametophyte grows independently from the spore wall. This is seen in Lycopodium, Equisetum, and most ferns.
• Endosporic Development: The spore germinates internally, and the gametophyte develops within the confines of the spore wall. This is seen in Selaginella and Isoetes, and it is a significant evolutionary step towards the development of seed plants, where the female gametophyte is retained within the ovule.

Sexuality: Most pteridophytes have bisexual gametophytes (homothallic), producing both antheridia and archegonia. However, some species exhibit unisexuality (heterothallic) or protandry (antheridia mature before archegonia) to promote cross-fertilization. Selaginella is distinctly unisexual due to heterospory.

Nutrition: Gametophytes range from photosynthetic (e.g., fern prothallus, Equisetum gametophyte) to saprophytic (e.g., Psilotum gametophyte, Lycopodium gametophyte) or semi-saprophytic, often relying on mycorrhizal associations for nutrient uptake.

Detailed Study of Each Class

1. Class Psilopsida

Considered the most primitive living vascular plants.

Morphology and Anatomy

• Sporophyte: Small, herbaceous, dichotomously branched. Lacks true roots and leaves. Has a subterranean rhizome and erect aerial photosynthetic stems.
• Rhizome: Bears rhizoids for anchorage and absorption, often associated with symbiotic fungi (mycorrhizae).
• Aerial Stems: Dichotomously branched, green, photosynthetic. Small, scale-like appendages (enations) may be present, lacking vascular tissue.
• Sporangia: Terminal, borne on aerial branches. In Psilotum, these are fused into a synangium (a three-lobed structure).
• Vascular Tissue: Simple protostele (e.g., actinostele).

Gametophyte

• Structure: Small, subterranean, dichotomously branched, thalloid. Lacks chlorophyll (saprophytic/semi-saprophytic), relies on mycorrhizal fungi.
• Reproductive Organs: Bisexual, bearing antheridia and archegonia on the same thallus.

Life Cycle

Spore (n) → Germination → Gametophyte (n) → Antheridium (n) & Archegonium (n) → Sperm (n) & Egg (n) → Fertilization → Zygote (2n) → Sporophyte (2n) → Sporangium (2n) → Meiosis → Spore (n)

Example: Psilotum

• Habitat: Tropical and subtropical regions, often epiphytic.
• Sporangium: Synangium, typically tri-lobed.

2. Class Lycopsida

Includes clubmosses and spikemosses. Characterized by microphylls.

Morphology and Anatomy

• Sporophyte: Terrestrial or epiphytic, dichotomously branched. Possesses true roots and stems.
• Stems: Dichotomously branched, often creeping or erect.
• Leaves: Microphylls, simple, entire, borne spirally on the stem. Typically possess a ligule at the base of the leaf.
• Roots: True roots, dichotomously branched.
• Sporangia: Solitary, borne in the axils of sporophylls (modified leaves bearing sporangia) or directly in leaf axils. Sporophylls are often aggregated into strobili (cones) at the apex of branches.
• Vascular Tissue: Protostele (haplostele, actinostele, or plectostele).
• Heterospory: Present in Selaginella and Isoetes, absent in Lycopodium.

Gametophyte

• Lycopodium: Exosporic, small, saprophytic or photosynthetic, bisexual.
• Selaginella: Endosporic, unisexual. Microspores develop into male gametophytes (producing sperm); megaspores develop into female gametophytes (producing eggs).
• Isoetes: Greatly reduced, endosporic, unisexual gametophytes.

Examples:

• Lycopodium (Clubmoss): Homosporous, exosporic gametophyte.
• Selaginella (Spikemoss): Heterosporous, endosporic gametophyte.
• Isoetes (Quillwort): Heterosporous, endosporic gametophyte, aquatic or semi-aquatic.

3. Class Sphenopsida

Includes horsetails. Characterized by jointed stems and whorled leaves.

Morphology and Anatomy

• Sporophyte: Terrestrial, herbaceous, with jointed stems and whorls of leaves at nodes.
• Stems: Differentiated into nodes and internodes. Often hollow or ridged. Two types of shoots: vegetative (green, photosynthetic) and reproductive (bearing strobili).
• Leaves: Small, scale-like, united at the nodes to form a sheath. Arranged in whorls.
• Roots: Adventitious roots arise from the nodes of the underground rhizome.
• Sporangia: Borne on specialized, peltate (shield-shaped) structures called sporangiophores. Sporangiophores are aggregated into terminal strobili (cones).
• Vascular Tissue: Siphonostele (eustele in some interpretations), with vascular bundles arranged in a ring. Distinctive carinal and vallecular canals in the stem.

Gametophyte

• Structure: Small (few mm), lobed, green, photosynthetic, superficial.
• Reproductive Organs: Bisexual (homothallic), bearing antheridia and archegonia on different parts of the same thallus.
• Development: Exosporic.

Example: Equisetum

• Habitat: Moist, open habitats.
• Strobilus: Conical structure formed by aggregated sporangiophores.
• Elaters: Hygroscopic structures associated with spores, aiding in dispersal.

4. Class Pteropsida

Includes the true ferns. Characterized by large, compound leaves (fronds).

Morphology and Anatomy

• Sporophyte: Highly diverse, ranging from small herbaceous plants to large tree ferns. Possesses true roots, stems (often rhizomatous), and large leaves (fronds).
• Stems: Typically rhizomes, creeping underground. Some are erect (tree ferns) or climbing.
• Leaves (Fronds): Large, compound, often pinnately divided. Young fronds are circinately coiled (uncoil from a fiddlehead/crozier). Complex venation patterns.
• Roots: Adventitious roots arising from the rhizome.
• Sporangia: Usually borne in clusters called sori on the underside of fronds. Each sorus contains numerous sporangia. Specialized structures like indusia may cover the sori.
• Vascular Tissue: Complex siphonosteles, often dictyosteles (a dissected siphonostele forming a network of vascular strands).

Gametophyte

• Structure: The prothallus is typically small, heart-shaped, green, photosynthetic, and independent.
• Reproductive Organs: Usually bisexual (homothallic), with antheridia and archegonia developing sequentially.
• Development: Exosporic.

Examples:

• Dryopteris (Wood Fern)
• Pteridium (Bracken Fern)
• Adiantum (Maidenhair Fern)
• Osmunda (Flowering Fern)
• Tree Ferns (e.g., Cyathea, Dicksonia)