Unit 5: Pteridophytes and Gymnosperms

1. Pteridophytes: General characteristics

Definition: Pteridophytes (ferns and fern allies) are the first vascular land plants (tracheophytes). They are "vascular cryptogams" (they have vascular tissue but no flowers or seeds).

• Dominant Generation: The main plant body is the diploid (2n) sporophyte. It is independent, photosynthetic, and differentiated into true roots, stems, and leaves.
• Gametophyte: The haploid (n) gametophyte (called a prothallus) is small, reduced, and usually free-living and photosynthetic.
• Vascular Tissue: They are the first plants to possess xylem (for water conduction) and phloem (for sugar conduction).
• Reproduction:
  • Asexual: The sporophyte produces haploid (n) spores (by meiosis) in sporangia. Sporangia are often clustered on leaves called sporophylls (which may form a cone/strobilus).
  • Sexual: The gametophyte bears the archegonia and antheridia. Like bryophytes, they are still dependent on water for the motile sperm to swim to the egg.
• Life Cycle: A distinct heteromorphic alternation of generations (the sporophyte and gametophyte look very different).

2. Pteridophytes: Classification (upto orders)

The Division Pteridophyta is classified into four major classes:

1. Class 1: Psilopsida
   • Most primitive. Simple, rootless sporophyte, dichotomously branched. E.g., *Psilotum*.
   • Orders: Psilotales.
2. Class 2: Lycopsida (Club Mosses)
   • Sporophyte has roots, stem, and small leaves (microphylls). E.g., *Lycopodium*, *Selaginella*.
   • Orders: Lycopodiales, Selaginellales.
3. Class 3: Sphenopsida (Horsetails)
   • Stem is jointed with nodes and internodes. Leaves are in whorls. E.g., *Equisetum*.
   • Orders: Equisetales.
4. Class 4: Pteropsida (True Ferns)
   • Largest group. Sporophyte has large, divided leaves (megaphylls) called fronds. E.g., *Dryopteris*, *Pteris*.
   • Orders: Filicales, Marsileales.

3. Pteridophytes: Economic importance

• Ornamentals: Many ferns are grown in gardens and as houseplants for their beautiful, feathery foliage (e.g., Boston fern).
• Soil Binding: Ferns and fern allies help stabilize soil and prevent erosion.
• Biofertilizer: The aquatic fern Azolla has a symbiotic relationship with the nitrogen-fixing cyanobacterium *Anabaena*. It is grown in rice paddies as a green manure.
• Fossil Fuels: The great "coal forests" of the Carboniferous period were dominated by giant pteridophytes (like *Lepidodendron*). Their remains formed the coal deposits we use today.
• Scouring: *Equisetum* (horsetail) stems are rich in silica and were historically used for cleaning and polishing (scouring) pots and pans.

4. Pteridophytes: Heterospory and seed habit

Heterospory

• Homospory: The "primitive" condition. The plant produces only one type of spore. This spore germinates into a gametophyte that is (usually) monoecious (bisexual), bearing both antheridia and archegonia.
  • Example: *Lycopodium*, *Rhynia*, most ferns.
• Heterospory: The "advanced" condition. The plant produces two different types of spores in two different sporangia.
  • Microspores: Small, male spores. Germinate to form a male gametophyte (which only bears antheridia).
  • Megaspores: Large, female spores (with stored food). Germinate to form a female gametophyte (which only bears archegonia).
  • Example: *Selaginella*, *Marsilea*.

Seed Habit

Heterospory is the most important evolutionary prerequisite for the formation of a seed. A seed is a fertilized, protected, and nourished embryo. The "seed habit" evolved from heterospory by three key steps (all of which are seen in *Selaginella*):

1. 1. Production of two types of spores (Heterospory).
2. 2. Retention of the megaspore: The large megaspore is not released from the megasporangium (which is now called an ovule).
3. 3. Endosporic Development: The female gametophyte develops entirely inside the megaspore wall, *while it is still on the parent sporophyte*.

Fertilization: The sperm swims to the female gametophyte (still on the parent plant). The zygote develops into an embryo, which is now protected by the gametophyte tissue, all inside the old megasporangium. This entire structure is, essentially, a primitive seed.

5. Pteridophytes: Stelar evolution

Definition: The stele is the central core of the stem and root, containing the vascular tissues (xylem and phloem).

A major evolutionary trend in pteridophytes is the increasing complexity of the stele.

1. Protostele: The most primitive type. A solid core of xylem surrounded by phloem. No central pith.
   • Haplostele: Simple, circular core of xylem (e.g., *Rhynia*, *Selaginella*).
   • Actinostele: Xylem core is star-shaped (e.g., *Lycopodium*).
   • Plectostele: Xylem is broken into parallel plates (e.g., *Lycopodium*).
2. Siphonostele: A more advanced type. A ring of vascular tissue surrounding a central core of non-vascular parenchyma (the pith).
   • Solenostele: A simple ring. When a leaf departs, it leaves a "leaf gap."
3. Dictyostele:
   • A complex siphonostele with many, overlapping leaf gaps.
   • This breaks the vascular ring into a network of smaller vascular strands. This is characteristic of most fern rhizomes.

6. Pteridophytes: A general account of fossil pteridophytes - *Rhynia*

• What it is: *Rhynia* is a fossil pteridophyte from the Devonian period (around 400 million years ago). It is one of the oldest and simplest vascular plants known.
• Classification: Psilopsida.
• Morphology (Sporophyte):
  • Extremely simple body.
  • No roots: Had rhizoids for anchorage.
  • No leaves.
  • Stem: The entire plant was just a slender, green, photosynthetic, dichotomously branched stem (rhizome and aerial branches).
  • Stele: A simple haplostele (primitive protostele).
  • Reproduction: Bore simple, oval-shaped sporangia at the tips of the aerial branches. It was homosporous.
• Significance: It shows us what the earliest vascular land plants looked like, representing a crucial step up from non-vascular bryophytes.

7. Pteridophytes: *Lycopodium* (Morphology and reproduction)

(Class: Lycopsida, "Club Moss")

Morphology (Sporophyte):

• Differentiated into true roots, stem, and leaves.
• Stem: Can be erect or creeping, and is dichotomously branched.
• Leaves: Small, simple, scale-like leaves (microphylls) that densely cover the stem.
• Stele: A protostele (often an actinostele or plectostele).

Reproduction:

• Strobilus (Cone): The sporangia are borne on the upper surface of specialized leaves called sporophylls, which are tightly clustered at the stem tips to form a cone or strobilus.
• Spores: *Lycopodium* is homosporous (produces only one type of spore).
• Gametophyte (Prothallus): The spore germinates into a small, subterranean (underground) gametophyte that is non-photosynthetic and depends on a symbiotic fungus (mycorrhiza) for food. It bears both antheridia and archegonia.

8. Pteridophytes: *Selaginella* (Morphology and reproduction)

(Class: Lycopsida, "Spike Moss")

Morphology (Sporophyte):

• Similar to *Lycopodium*, but more delicate.
• Leaves: Typically has two different sizes of leaves (heterophyllous) arranged in four rows.
• Rhizophore: A unique, leafless, pro-like structure that grows down from the stem and gives rise to roots.
• Stele: A protostele (haplostele).

Reproduction:

• Strobilus (Cone): Bears two types of sporangia on its sporophylls.
  • Microsporangia: Contain numerous, small microspores (n).
  • Megasporangia: Contain (usually) four large megaspores (n).
• Significance: *Selaginella* is heterosporous.
• Gametophytes:
  • Male Gametophyte: Develops endosporically (inside the microspore wall). It is just a few cells and produces motile sperm.
  • Female Gametophyte: Develops endosporically (inside the megaspore wall). It breaks open the wall to expose the archegonia.
• Seed Habit: As mentioned before, the retention of the megaspore and endosporic development in *Selaginella* is a clear forerunner of the seed habit.

9. Gymnosperms: General characteristics

Definition: Gymnosperms ("naked seeds") are vascular, seed-producing plants (spermatophytes) where the ovules are not enclosed within an ovary. The seeds are "naked," often borne on the surface of scales (e.g., in a cone).

• Dominant Generation: The diploid (2n) sporophyte is dominant (e.g., the pine tree). It is differentiated into root, stem, and leaves.
• Gametophyte: The haploid (n) gametophyte is extremely reduced, non-photosynthetic, and parasitic (dependent) on the sporophyte.
• Reproduction:
  • They are heterosporous (microspores and megaspores).
  • Microspore: Develops into the pollen grain (the highly reduced male gametophyte).
  • Megaspore: Develops into the female gametophyte (the endosperm), which is retained inside the megasporangium (ovule).
• Pollination: The transfer of pollen to the ovule. It is typically by wind (anemophily).
• Fertilization: Water is not required. The pollen grain grows a pollen tube to deliver the non-motile (in most) male gametes to the egg.
• Seed: The fertilized ovule develops into a seed, which contains the embryo, a food supply (endosperm), and a protective seed coat.

10. Gymnosperms: Affinities

Affinities with Pteridophytes (Similarities)

• Both have a dominant, independent sporophyte.
• Both show a clear alternation of generations.
• Both possess vascular tissue (xylem and phloem).
• Both have archegonia (in most gymnosperms).
• Some pteridophytes (*Selaginella*) show heterospory and the origin of the seed habit.

Affinities with Angiosperms (Similarities)

• Both have a dominant sporophyte, and a highly reduced gametophyte.
• Both possess vascular tissue (though gymnosperm xylem lacks vessels and phloem lacks companion cells, *except* in *Gnetum*).
• Both produce seeds.
• Both use a pollen tube for fertilization (siphonogamy), making them independent of water for reproduction.

11. Gymnosperms: *Cycas*

• Group: Cycadopsida.
• Morphology: Palm-like, unbranched, columnar stem with a crown of large, pinnately compound leaves.
• Anatomy: Stem has a large pith and cortex with many mucilage canals. Leaflets (pinnae) have a single, unbranched midrib.
• Reproduction (Dioecious):
  • Male Plant: Produces a large, compact male cone (strobilus) in the center.
  • Female Plant: Does not produce a true cone. It produces a loose cluster of megasporophylls (leaf-like structures) that bear large, red, naked ovules along their margins.
  • Fertilization: Has primitive features. Pollen tube is present, but it acts as a haustorium (absorbs nutrients). It bursts to release large, motile, multiflagellate sperm (a pteridophyte-like feature).

12. Gymnosperms: *Pinus*

• Group: Coniferopsida.
• Morphology: A large, branched tree. Has two types of shoots:
  • Long shoots: Have unlimited growth.
  • Dwarf shoots: Have limited growth. They bear the needles (leaves) in clusters (e.g., 2, 3, or 5 needles).
• Anatomy: Stem shows secondary growth (wood). Wood is pycnoxylic (dense, compact). Needle leaves are xerophytic (adapted to drought).
• Reproduction (Monoecious):
  • Male Cones: Small, clustered at the base of new shoots. Produce vast amounts of winged pollen.
  • Female Cones: Large, woody cones. Composed of ovuliferous scales (which bear two naked ovules) and bract scales.
  • Fertilization: Wind pollination. A pollen tube grows and delivers two non-motile male gametes to the archegonium.

13. Gymnosperms: *Gingko*

• Group: Ginkgopsida.
• "Living Fossil": *Ginkgo biloba* is the only surviving species in its entire order.
• Morphology: A large, deciduous (loses leaves in winter) tree.
  • Leaves: Unique, fan-shaped leaves with dichotomous venation (a primitive trait).
• Reproduction (Dioecious):
  • Male Plant: Produces small, catkin-like pollen cones.
  • Female Plant: Produces paired ovules on a long stalk.
  • Fertilization: Similar to *Cycas*. A pollen tube grows, but it releases motile, flagellated sperm.
  • Seed: The fertilized ovule develops into a seed with a fleshy, foul-smelling outer coat (it is *not* a fruit).

14. Gymnosperms: *Gnetum*

• Group: Gnetopsida.
• Significance: This group (*Gnetum*, *Ephedra*, *Welwitschia*) is considered the most advanced gymnosperm, showing many angiosperm-like features.
• Morphology: Mostly woody climbers (vines) with large, broad, oval leaves that look like those of a dicot (with reticulate venation).
• Anatomy (Angiosperm-like features):
  • Possession of vessels in its xylem (like angiosperms).
  • Lack of typical phloem companion cells (but has some "albuminous cells").
• Reproduction (Angiosperm-like features):
  • Cones are complex and "flower-like."
  • Archegonia are absent (in some species).
  • A type of double fertilization has been reported, though it's different from the angiosperm version (it doesn't form triploid endosperm).