Plant Life Cycle: Week 1 lecture notes

Big-Picture Learning Objectives (Week 1)

• Explain that terrestrial plants evolved from green algae.

• Distinguish vascular vs. non-vascular plants.

• Identify key evolutionary milestones in plant diversification.

• Describe the alternation-of-generations life cycle common to all plants.

Biological Context – The Tree of Life

• Three Domains: Bacteria, Archaea, Eukarya.

  • Plants sit inside Domain Eukarya → Kingdom Plantae.

• Cell-level distinction:

  • Prokaryotes (Monera): lack nucleus/organelles.

  • Eukaryotes: possess membrane-bound nucleus & organelles (includes plants, animals, fungi, protists, most algae).

Ancient Oxygen Engineers – Stromatolites & Cyanobacteria

• Stromatolites: layered, stony microbial mats created by photosynthetic cyanobacteria (once called blue-green algae).

• Credited with raising atmospheric \text{O}_2 from 1\% → 21\%, enabling aerobic life.

• Darwin envisioned such environments as part of Earth’s early “primordial soup.”

Endosymbiotic Theory (Origin of Eukaryotic Organelles)

1. Plasma-membrane infolding in an ancestral prokaryote → internal compartmentalisation.

2. Engulfment of an aerobic bacterium → evolved into mitochondrion.

3. Engulfment of a photosynthetic bacterium → evolved into chloroplast.

• Outcome: a complex, energy-efficient eukaryotic plant cell.

Evolutionary Timeline of Plants

• Life arose in oceans; first known green algae ≈ 600\ \text{MYA}.

• Land colonisation by plant ancestors ≈ 500\ \text{MYA}.

• Origin of embryophytes (land plants) from charophyte-like green algae ≈ 470\ \text{MYBP}.

• Key milestones:

  • Bryophytes (non-vascular) – first land plants.

  • Vascular tissues (xylem & phloem) emerge ≈ 400\ \text{MYA} → seedless vascular plants (ferns, horsetails, club mosses).

  • Seeds evolve ≈ 300\ \text{MYA} → gymnosperms.

  • Cones diversify ≈ 200\ \text{MYA}.

  • Flowers & fruits appear ≈ 100\ \text{MYA} → angiosperms.

Shared Traits – Green Algae & Land Plants

• Plasmodesmata for cell-to-cell communication.

• Chloroplasts → photosynthesis using chlorophyll a & b.

• Oogamy: small, motile sperm + large, non-motile eggs.

• Sporopollenin walls (desiccation resistance) start in algal spores.

• Waxy cuticle & other water-loss adaptations evolve as plants move onto land.

Algal Life Cycle (Haplontic Model)

1. Multicellular haploid (n) thallus produces gametes via mitosis.

2. Fertilisation → zygote (2n).

3. Meiosis within zygote → new haploid cells.

4. Haploid cells grow into next haploid thallus → no multicellular diploid generation (contrast with plants).

Bryophytes – First Land Invaders (Non-Vascular)

• Groups: mosses, liverworts, hornworts.

• Traits:

  • Multicellular, photosynthetic; rely on surface diffusion for water/solutes.

  • No true cuticle or stomata (except sporophyte capsule stomata).

  • Leaf-like blades (not true leaves) & rhizoids (not true roots).

  • Dominant gametophyte (n); small, dependent sporophyte (2n) grows on it.

• Ecological note: often form carpets in moist habitats; contribute to soil formation and water retention.

Moss Life Cycle (Alternation of Generations)

1. Gametophyte (n) produces antheridia (sperm) & archegonia (eggs).

2. Water-film fertilisation → zygote (2n).

3. Zygote develops into sporophyte (seta + capsule) on parent gametophyte.

4. Meiosis in capsule → haploid spores, dispersed by wind.

5. Spores germinate → protonema → mature gametophyte.

Liverwort Highlights

• Flat thalloid or leafy forms; possess gemmae cups for asexual dispersal.

• Separate antheridiophores and archegoniophores elevate sex organs.

Vascular Innovation – Xylem & Phloem

• Xylem: dead, hollow, lignified tubes; transport water & minerals upward from roots.

• Phloem: living, cellulose-reinforced cells; distribute photosynthates (sugars) and signalling molecules bidirectionally.

• Consequences:

  • Structural support → taller growth, canopy competition.

  • Internal plumbing → occupation of drier niches and true organ differentiation (roots, stems, leaves).

Seedless Vascular Plants (Pteridophytes)

• Representatives: ferns, horsetails, whisk ferns (Psilotum), club mosses (Lycopodium), kangaroo fern (Microsorum).

• Dominant sporophyte; free-living but small gametophyte.

Fern Life Cycle in Brief

1. Sporophyte (2n) bears sori (clusters of sporangia) under fronds.

2. Meiosis → haploid spores.

3. Spores germinate → heart-shaped prothallus (gametophyte, n) with rhizoids.

4. Prothallus produces antheridia & archegonia; water required for sperm swim.

5. Fertilised zygote develops into new sporophyte; prothallus eventually degenerates.

The Seed & Flower Revolutions (Forward Look)

• Seeds: multicellular structures with embryo + food supply + protective coat; arose \approx 300\ \text{MYA}.

• Gymnosperms ("naked seeds") dominate \approx 200\ \text{MYA} with cone-bearing morphologies.

• Angiosperms (flowers & fruits) diversify \approx 100\ \text{MYA}, becoming today’s most species-rich plant group.

Concept Integration & Real-World Relevance

• Oxygenic photosynthesis of early cyanobacteria and later plants underpins current atmospheric composition and all aerobic life.

• Alternation of generations influences genetic diversity and adaptation rates.

• Vascular innovations permitted the formation of forests, affecting global climate (carbon sequestration) and soils.

• Seeds & flowers enabled long-distance dispersal (wind, animals), co-evolution with pollinators, and the agricultural species humans rely upon.

Ethical & Philosophical Considerations

• Custodianship of land: acknowledging Indigenous knowledge systems aligns with studying plant evolution on those very landscapes.

• Preserving bryophyte-rich habitats and ancient plant lineages safeguards biodiversity hotspots and unique biochemical pathways (medicinal potential).

End of Week 1 study notes – consolidate before moving into detailed photosynthesis (Week 2) and seed evolution (Week 3).