LA

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).