Seedless-Vascular Plants Lecture Notes

Review of Non-Vascular Plants (NVPs)

• Earliest Land Plants: These were the first to colonize land but were restricted to areas near water.
• Fertilization: Required water for external fertilization.
• Structural Composition: Lacked lignin, which limited their height and structural support.
• Life Cycle Dominance: The gametophyte is the dominant generation.
• Sporophyte Characteristics: The sporophyte is small and remains attached to the gametophyte to obtain nutrients.
• Spore Production: All non-vascular plants are homosporous, meaning they produce spores that are all the same size.
• Phylogenetic Transitions:
  • Liverworts: Recognized as the most basal group.
  • Hornworts: Identified as the first group with a photosynthetic and persistent sporophyte.
  • Evolutionary Sequence: Non-Vascular Plants (NVPs) $\rightarrow$ Protracheophytes $\rightarrow$ Seedless Vascular Plants (SVPs).

Evolutionary History and Protracheophytes

• Definition: Protracheophytes are an extinct intermediary group existing between Non-Vascular Plants and Seedless Vascular Plants.
• Morphology: Characterized by dichotomous branching.
• Cellular Evolution: Parenchyma cells evolved into Hydroid cells. These cells contained some lignin but lacked the specialized tracheids found in later vascular plants.
• Embryology: In the transition to vascular plants, the apical meristem (A.M.) switches to the top in embryology.
• Example Species: Aglaophyton.
  • Hydroids in Aglaophyton contained lignin and specialized further to evolve into tracheids in SVPs.

Seedless Vascular Plants (SVPs / Tracheophytes)

• Ecological Impact: Their emergence caused an "explosion" of plant life that turned barren land green.
• Atmospheric Impact: SVPs helped stabilize $O_2$ levels in the atmosphere.
• Biological Foundation: They formed the basis for the web of terrestrial life.
• Adaptation: Significant evolutionary changes allowed these plants to move further away from water sources.

The Rhynie Chert and Early Colonization

• Age: Over $400\,\text{mya}$ (million years ago).
• Source: Silica from volcanic hot springs petrified this specific ecosystem.
• Biodiversity in the Chert: The fossil record from this site includes:
  • Fungi
  • Lichens
  • Plants
  • Arthropods

First SVPs: Rhyniophytes

• Status: Extinct.
• Representative Genus: Cooksonia ($425\,\text{mya}$).
• Defining Features:
  • Tracheids: Contained specialized vascular cells.
  • Primary Organ: The stem was the first organ of the plant.
  • Sporophyte Dominant: This shift occurred because tracheids evolved specifically in the sporophyte generation.
  • Lack of Organs: Had no roots or leaves.
  • Reproduction: Featured terminal sporangia and were homosporous.

Major SVP Clades: Lycophytes and Monilophytes

• Lycophytes (Club Mosses):
  • Leaves: Evolved microphylls (leaves with a single vascular strand/vein).
  • Roots: Evolved from underground stems.
  • Rhizoids: Functioned as possible root hairs.
  • Key Genera: Lycopodium and Selaginella.
• Monilophytes (Ferns and relatives):
  • Leaves: Evolved megaphylls (leaves with branching veins).
  • Megaphyll Evolution: Arose from the overlapping and fusing of branches.
  • Roots: Evolved from stems.
  • Rhizoids: Functioned as possible root hairs.
  • Diversity: Includes Ferns, Horsetails, and Whisk Ferns.

The Carboniferous Era: Height and Gigantism ($360\,\text{mya}$)

• Selection Pressure: The evolution of megaphylls created pressure for plants to grow larger, leading to the development of secondary growth.
• Giant Flora:
  • SVP trees reached heights of $100\,\text{ft}$.
  • Lepidodendron: Featured unifacial vascular cambium (V.C.) and produced $2^\circ$ (secondary) xylem only.
  • Giant Horsetails: Also dominated the landscape.
• Geological Legacy: SVP remains from this era eventually compressed into coal.
• Atmospheric Changes: Huge amounts of $O_2$ were produced, leading to giant fauna:
  • Insects: $3\,\text{ft}$ Scorpions, $6\,\text{ft}$ Millipedes, and giant winged insects.
  • Amphibians: Eogyrinus ($4.6\,\text{m}$), Diadectes ($2-3\,\text{m}$), Limnoscelis ($1.5\,\text{m}$), and Seymouria ($0.6\,\text{m}$).
• End of the Era ($300\,\text{mya}$):
  • The climate cooled and became arid.
  • Reason for Cooling: Giant megaphylls absorbed massive amounts of $CO_2$ and stored it as wood, reducing the greenhouse effect.
  • Mass Die-off: Giant SVPs died out because external fertilization requires water, which became scarce.

Anatomy and Reproduction in Lycophytes

• Derived Traits: Roots, leaves (microphylls), and the strobilus.
• Strobilus (Cone) Definition: A cluster of sporangia and leaves (sporophylls) located on branch tips.
• Sporophylls: Specifically microphylls that bear sporangia.
• Evolutionary Significance: Cones drastically increased spore production compared to the solitary terminal sporangia of ancestral plants (e.g., Cooksonia).

Heterospory vs. Homospory

• Homospory Process: Sporangium with Sporocytes ($2N$) $\xrightarrow{Meiosis}$ Spores ($1N$) $\xrightarrow{Mitosis}$ Gametophyte ($1N$).
• Heterospory (Found in Selaginella): Produces two different spore types:
  • Megasporangium: Contains Megasporocytes ($2N$) $\rightarrow$ Megaspores ($1N$) $\rightarrow$ Megagametophyte ($1N$).
  • Megaspores: Large; provide increased food for the egg.
  • Microsporangia: Contains Microsporocytes ($2N$) $\rightarrow$ Microspores ($1N$) $\rightarrow$ Microgametophyte ($1N$).
  • Microspores: Small; produced by the billions for better dispersal.
• Evolutionary Importance of Heterospory:
  • It is the first step toward the evolution of seeds and pollen.
  • Pollen protects sperm; seeds protect the egg.
  • Led to internal fertilization, allowing "higher" plants to survive in diverse environments.

Monilophytes: True Ferns

• Fiddleheads: Coiled megaphylls that protect the developing leaf as it arises from tangled rhizomes.
• Sori (singular: Sorus): Clusters of sporangia located on the underside of the leaf.
• Annulus: A heavy ring of cells on the sporangium that folds back and springs forward to eject spores. This is a defining feature of "True" ferns.
• Indusium: A protective cover for the sori.
  • Two Types: A specialized flap of tissue (indusium) or the leaf margins curling over the sori.
  • Naked Sori: Sori that lack a protective indusium.
• Gametophyte: Photosynthetic, heart-shaped, and bisexual (contains both Archegonia/eggs and Antheridia/sperm).

simple Ferns: Psilotum and Equisetum

• Psilotum (Whisk Fern):
  • Lifestyle: Tropical epiphyte.
  • Anatomy: Stem only; lacks roots and leaves.
  • Reproduction: Homosporous with sporangia on enations.
  • Enations: Outgrowths of the epidermis that are NOT leaves because they lack lignin.
  • Evolution: Represents reduction or reversal evolution of megaphylls and roots due to its epiphytic nature.
• Equisetum (Horsetails):
  • Anatomy: Jointed stem with silica in the cell walls for anti-predation.
  • Morphology: Features whorled branches and microphylls (reduced from megaphylls).
  • Reproductive Structure: Strobilus contains sporangiophores.
  • Autapomorphies (Derived Traits):
    • Sporangiophores help prevent dehydration.
    • Spores have elaters, which are arm-like extensions that assist in dispersal.