8.4 Pteridophyta: Seedless Vascular Plants

Pteridophytes are the first true land plants possessing vascular tissues (xylem and phloem) but lacking seeds. They represent a significant evolutionary step from bryophytes. This group includes four main sub-divisions: Psilopsida, Lycopsida, Sphenopsida, and Pteropsida (of which the class Fillicinae, or ferns, are a part).

General Characteristics of Pteridophytes

• Vascular Tissue: They are tracheophytes, meaning they possess vascular tissues (xylem and phloem) for the transport of water and nutrients.
• Reproduction: They are seedless and reproduce via spores.
• Life Cycle: Exhibit a distinct heteromorphic alternation of generations, where the diploid sporophyte and haploid gametophyte are morphologically different. The sporophyte generation is dominant.
• Habitat: Typically found in moist, shaded environments as water is required for fertilization.

Sub-Divisions of Pteridophyta

1. Psilopsida (Whisk Ferns)

• Structure: Simplest vascular plants. They lack true roots and leaves.
  • Rhizome: An underground stem for anchorage, possessing rhizoids.
  • Aerial Stems: Green, photosynthetic stems with small scale-like appendages.
• Reproduction: Sporangia (spore-producing structures) are located at the tips or on the sides of aerial branches.
• Gametophyte: A colorless, subterranean structure that lives in a symbiotic relationship with a fungus (mycorrhiza) for nutrition.
• Examples: Psilotum (living), Rhynia (extinct).

2. Lycopsida (Club Mosses, Spike Mosses, Quillworts)

• Structure: Plant body has a branching rhizome with aerial stems.
  • Microphylls: Stems and branches are covered in small, scale-like leaves, each with a single, unbranched vein of vascular tissue.
• Reproduction: Sporangia are borne on terminal clusters of leaves called strobili (cones), which have a club-shaped appearance.
• Gametophyte: Mostly an underground structure.
• Examples: Lycopodium (Club moss), Selaginella (Spike moss).

3. Sphenopsida (Horsetails)

• Structure: Also known as arthrophytes due to their jointed stems.
  • Rhizome: Gives rise to two types of aerial stems: fertile and sterile.
  • Aerial Stems: Slender, green, hollow, and jointed.
  • Leaves: Whorls of scale-like leaves are present only at the nodes (joints). Photosynthesis occurs mainly in the stem.
• Reproduction: Spores are produced in cones (strobili) located at the tips of fertile shoots.
• Gametophyte: A small, thalloid structure that grows on moist soil.
• Example: Equisetum is the only surviving genus.

4. Pteropsida - Class Fillicinae (Ferns)

• General: The most abundant and diverse group of seedless vascular plants, thriving in warm, moist tropics.
• Size: Varies from tiny aquatic forms to large tree ferns.
• Dominance: The sporophyte generation is large, complex, and independent.
• Leaves (Fronds): Large, complex leaves (megaphylls) that serve both photosynthesis and reproduction. Young fronds exhibit circinate vernation (coiled like a fiddlehead).

Life Cycle of a Fern (e.g., Adiantum)

Sporophyte Generation (2n)

• This is the main, recognizable fern plant.
• Structure:
  • Rhizome: An underground, horizontal stem covered in brown scales called ramenta.
  • Roots: Adventitious roots arise from the rhizome.
  • Fronds (Leaves): Large, pinnately compound leaves. The main stalk is the rachis. Leaflets are pinnae (1st order) and pinnules (2nd order).
• Reproduction:
  • Sori: Clusters of sporangia found on the underside of leaflets, often protected by a bent leaflet margin (false-indusium).
  • Sporangium: A capsule on a stalk containing spore mother cells. Its wall has a specialized ring of cells (annulus) and a weak point (stomium).
  • Spore Formation: Spore mother cells undergo meiosis to produce haploid (n) spores.
  • Spore Dispersal: The annulus contracts as it dries, tearing the sporangium at the stomium and flinging the spores out.

Gametophyte Generation (n)

• A germinating spore grows into the gametophyte, also called a prothallus.
• Structure:
  • A small, heart-shaped, green, photosynthetic structure.
  • It is independent and anchors itself with rhizoids.
• Reproductive Organs (on the same prothallus):
  • Antheridia: Globular structures that produce motile, multi-ciliated sperm (antherozoids).
  • Archegonia: Flask-shaped structures containing a single egg (oosphere).

Fertilization

1. Water is essential for fertilization.
2. Antherozoids are released and swim towards the archegonia, attracted by chemical signals (chemotaxis).
3. One antherozoid fertilizes the egg, forming a diploid (2n) zygote.
4. The zygote develops into an embryo, which initially depends on the gametophyte but quickly grows into an independent sporophyte, completing the cycle.

Summary

• Pteridophytes are the first vascular plants, but they are seedless and rely on spores for reproduction.
• The life cycle is an alternation of generations, with the diploid sporophyte being the dominant, long-lived phase.
• The haploid gametophyte (prothallus in ferns) is a small but independent, photosynthetic plant.
• Water is a limiting factor for reproduction, as it is required for the motile sperm to reach the egg.
• The four main groups show a progression in structural complexity.

Biological Significance: Pteridophytes represent a crucial evolutionary link between non-vascular bryophytes and seed-bearing plants. Their development of vascular tissue was a key adaptation for life on land, allowing them to grow taller and colonize more diverse habitats.