Unit 4: Pteridophytes

1. General Characteristics & Origin
2. Early Land Plants
3. Study of Genera
4. Stelar Evolution
5. Heterospory and Seed Habit

Syllabus Reference: General characteristics; Classification; Cooksonia and Rhynia; Psilotum, Lycopodium, Selaginella, Equisetum, Marsilea, Pteris; [cite_start]Apogamy/apospory; telome theory; stelar structure [cite: 482-484].

1. General Characteristics & Origin

"Botanical Snakes": Pteridophytes are the first Vascular Cryptogams (Seedless vascular plants).
Dominant Phase: Sporophyte (2n), which is differentiated into true root, stem, and leaves.
Vascular Tissue: Xylem (tracheids only, usually no vessels) and Phloem (sieve cells only, no companion cells).
Reproduction: via Spores produced in Sporangia.
Leaves: Can be Microphylls (small, single vein, e.g., Lycopodium) or Megaphylls (large, complex veins, e.g., Ferns).

2. Early Land Plants (Fossils)

Cooksonia: The oldest known vascular plant fossil. Simple, leafless, dichotomously branched stem with terminal sporangia.
Rhynia: Found in Rhynie Chert (Scotland). Rootless, leafless. Had underground rhizomes and aerial dichotomous branches. Sporangia were terminal.

3. Study of Genera

Genus	Group	Key Features
Psilotum	Psilopsida	"Living Fossil". Rootless and leafless (has scales). Synangium (fused sporangia). Primitive.
Lycopodium	Lycopsida	"Club Moss". Homosporous. Sporophylls arranged in cones (strobili).
Selaginella	Lycopsida	"Spike Moss". Heterosporous (Micro & Megaspores). Ligulate leaves. Rhizophore present.
Equisetum	Sphenopsida	"Horsetail". Jointed stem with ridges/furrows. Silica in stem. Elaters attached to spores.
Marsilea	Pteropsida	Water fern. Heterosporous. Spores enclosed in a specialized structure called Sporocarp.
Pteris	Pteropsida	Common fern. Sori forms a continuous line along the leaf margin (Coenosorus).

4. Stelar Structure and Evolution

A Stele is the central vascular cylinder (Xylem + Phloem + Pericycle) of the stem/root.

Types of Stele:

Protostele (Most Primitive): Solid core of xylem surrounded by phloem. No Pith. (e.g., Lycopodium, Rhynia).
Siphonostele: Protostele with a central Pith. (e.g., Equisetum).
- Solenostele: Non-overlapping leaf gaps.
- Dictyostele: Overlapping leaf gaps causing the stele to break into distinct strands (Meristeles). Found in Pteris.

5. Heterospory and Seed Habit

Homospory: Production of one type of spore (e.g., Lycopodium, Pteris). Gametophyte is monoecious.
Heterospory: Production of two types of spores (e.g., Selaginella, Marsilea):
- Microspores: Small, give rise to Male Gametophyte.
- Megaspores: Large, give rise to Female Gametophyte.

Significance (Origin of Seed Habit):

Heterospory is the precursor to seed formation. In Selaginella, the megaspore is retained within the megasporangium for some time, and fertilization occurs inside. This retention and protection of the female gametophyte is the first step towards the evolution of the seed habit seen in Gymnosperms and Angiosperms.

6. Special Reproduction Concepts

Apogamy: Development of Sporophyte (2n) directly from Gametophyte (n) without fertilization.
Apospory: Development of Gametophyte (n) directly from Sporophyte vegetative cells (2n) without meiosis/spores.