🌿 Introduction
Pteridophytes are the first vascular land plants that possess xylem and phloem for the transport of water and food. They are also the first plants to have a distinct alternation of generations with dominant sporophytic phase.
They occupy a position between bryophytes (non-vascular plants) and gymnosperms (seed plants) in evolution.
Pteridophytes include horsetails and ferns.
They are found in cool, damp, shady places, though some flourish well in sandy-soil conditions.
- Dominant Phase: In contrast to Bryophytes, the main plant body in Pteridophytes is the diploid sporophyte (2n). It is differentiated into true root, stem, and leaves.
- Vascular Tissues: The presence of xylem and phloem is a defining characteristic.
- Leaves: The leaves can be small (microphylls) as in Selaginella or large (macrophylls) as in ferns.
🌱 General Characteristics
- Habitat: Moist, shady terrestrial places; some aquatic (e.g., Marsilea).
- Body Organization: True roots, stems, and leaves are present.
- Vascular Tissue: Xylem and phloem present (no secondary growth).
- Reproduction: Asexual by spores, sexual by gametes.
- Dominant Generation: Sporophyte (diploid) is independent and photosynthetic.
🧬 Life Cycle of Pteridophytes
Pteridophytes show heteromorphic alternation of generations:
- Sporophyte (2n): Dominant phase, bears sporangia containing spores.
- Gametophyte (n): Small, short-lived, independent, heart-shaped (prothallus) producing antheridia and archegonia.
- Fertilization requires water, producing a zygote that grows into new sporophyte.
Reproduction
Spore Production: The sporophyte bears sporangia that are subtended by leaf-like appendages called sporophylls. In some cases, sporophylls may form distinct compact structures called strobili or cones (e.g., Selaginella, Equisetum).
Spore Germination: Sporangia produce haploid spores by meiosis in spore mother cells. These spores germinate to give rise to a small, multicellular, free-living, mostly photosynthetic thalloid gametophyte called a prothallus.
Gametophyte (Prothallus): The prothallus requires cool, damp, shady places to grow. Due to this specific requirement and the need for water for fertilization, the spread of pteridophytes is restricted to narrow geographical regions.
Sex Organs: The prothallus bears the male sex organs (antheridia) and female sex organs (archegonia).
Fertilization: Water is required for the transfer of antherozoids (male gametes, usually flagellated) released from the antheridia to the archegonium. Fusion of the male gamete with the egg present in the archegonium results in the formation of a zygote (2n).
Sporophyte Development: The zygote subsequently develops into a multicellular, well-differentiated sporophyte, which is the dominant phase.
Homospory vs. Heterospory:
Homosporous: Most pteridophytes produce spores of similar kinds.
Heterosporous: Some genera like Selaginella and Salvinia produce two kinds of spores: large megaspores (female) and small microspores (male).
Seed Habit Precursor: In heterosporous species, the female gametophyte develops within the megaspore, which is retained on the parent sporophyte for variable periods. The development of the zygote into a young embryo takes place within the female gametophyte. This event is considered a precursor to the seed habit seen in gymnosperms and angiosperms, and is thus an important step in evolution.
🌾 Classification of Pteridophytes
Pteridophytes are classified into four major classes:
| Class | Examples | Key Features |
|---|---|---|
| Psilopsida | Psilotum | No true roots or leaves, primitive vascular plants |
| Lycopsida | Selaginella, Lycopodium | Microphyllous leaves; heterosporous species like Selaginella |
| Sphenopsida | Equisetum | Jointed stems, silica deposition, called “horse tails” |
| Pteropsida | Pteris, Adiantum, Dryopteris | Large fronds with sori on underside |
🌿 Heterospory and Seed Habit
- Most pteridophytes are homosporous (produce one type of spore).
- Some (e.g., Selaginella, Salvinia) are heterosporous, producing microspores and megaspores.
- Heterospory is considered a precursor to seed habit as it leads to development of endosporic gametophytes.
🌍 Economic and Ecological Importance
| Importance | Example |
|---|---|
| Soil-binding and erosion control | Pteris, Equisetum |
| Bioindicators of moisture | Selaginella |
| Ornamental plants | Adiantum, Nephrolepis |
| Fossil fuel source | Ancient pteridophytes formed coal deposits |
🧠 Summary
- Pteridophytes are vascular cryptogams with dominant sporophyte.
- Reproduce by spores and show alternation of generations.
- Represent an important evolutionary step toward seed plants.
💡 Solved Examples
Q1. Why are pteridophytes called vascular cryptogams?
A1. They possess vascular tissues (xylem and phloem) but reproduce through spores (cryptogamic reproduction).
Q2. What is heterospory? Give examples.
A2. Production of two different types of spores — microspores and megaspores — as in Selaginella and Salvinia.
Q3. What is the dominant generation in pteridophytes?
A3. Sporophyte is the dominant, independent, photosynthetic generation.
Q4. What is the function of a prothallus?
A4. The prothallus is the gametophyte bearing antheridia and archegonia where fertilization occurs.
Q5. Mention one economic importance of pteridophytes.
A5. Some pteridophytes like Adiantum are ornamental plants; fossil forms contributed to coal formation.