Plant Diversity I – Mosses & Ferns (Non-Vascular and Seedless Vascular Plants)

Learning Objectives

  • Explain phylogeny of land plants & their terrestrial adaptations (all modules)
  • 4 unique features that separate plants from algae (Module 1)
  • Characteristics & life-cycle phases of non-vascular plants (Module 2)
  • Ecological importance of mosses (Module 2)
  • Characteristics & life-cycle phases of seedless vascular plants—ferns & allies (Module 3)
  • Ecological importance of seedless vascular plants (Module 3)
  • Reference text: Campbell Biology, 11th Ed., Ch 29 “Plant Diversity I – How Plants Colonized Land”

Taxonomy & Biodiversity: Making Sense of Diversity

  • Classification aids in organising life; reflects evolutionary history (phylogeny)
    • Phylogeny inferred from morphology + molecular evidence
  • Transition from 5-kingdom (Monera, Protista, Fungi, Plantae, Animalia) → 3-domain system
    • Domains: Archaea, Bacteria, Eukarya
      • Within Eukarya: Plants, Fungi, Animals, and multiple protist lineages (red algae, green algae, ciliates, diatoms, forams, euglenozoans)
  • Hierarchical ranks: Domain → Kingdom → Phylum (Division) → Class → Order → Family → Genus → Species
    • Binomial rules: italics, genus capitalised, species lowercase (e.g. Homo sapiens)

Defining a Species

  • Highly contentious; several operational concepts
    • Morphological species concept – based on observable form
    • Biological species concept – reproductive isolation (barriers: habitat, timing, mating systems)
    • Ecological species concept – niche-based definition

Biodiversity Crisis

  • Accelerated extinction rates due to human activity
    • ≈500 000 total species threatened (≈3 000 vertebrates, 40 000 plants)
  • Types of biodiversity: genetic, species, habitat
  • Australia labelled “megadiverse”
    • Holds 5\text{–}8\% of world’s species; ≈80\% endemic

Major Photosynthetic Groups

  • Algae (Protista)
  • Bryophytes – seedless non-vascular plants (mosses, liverworts, hornworts)
  • Seedless vascular plants – ferns + allies
  • Gymnosperms (naked-seed plants)
  • Angiosperms (flowering, enclosed seed plants)

From Algae to Land Plants

  • Algae: photosynthetic, eukaryotic; lack protected embryos, vascular tissue, seeds, flowers
  • Green algae (Chlorophytes + Charophytes) share chlorophyll a with land plants; closest relatives
  • “Greening” timeline
    • \approx3\,\text{billion yr} land lifeless
    • Cyanobacteria terrestrial \approx1.2\,\text{billion yr} ago
    • Small plants/animals \approx500\,\text{mya}
    • Today \approx290\,000 plant spp.

Key Innovations Distinguishing Plants from Algae

  1. Gametangia – multicellular sex organs
    • Archegonia (female) / Antheridia (male)
  2. Multicellular, dependent embryos (hence “embryophytes”)
  3. Walled spores with sporopollenin; produced inside sporangia
  4. Apical meristems – persistent regions of cell division that generate above- & below-ground organs

Adaptations to Terrestrial Life (Class Exercise Answers)

  • Problems: desiccation, UV, gravity, reproduction without water, nutrient acquisition, gas exchange
  • Structures/solutions: cuticle, sporopollenin-coated spores, vascular tissue, lignin, roots, stomata, gametangia, seeds (later), supportive tissues

Alternation of Generations: Universal Plant Life Cycle

  • Two multicellular stages alternate:
    • Sporophyte – diploid (2n); produces spores by meiosis
    • Gametophyte – haploid (n); produces gametes by mitosis
  • Generic flow:
    \text{Sporophyte}(2n) \xrightarrow[meiosis]{} \text{Spores}(n) \xrightarrow[mitosis]{} \text{Gametophyte}(n) \xrightarrow[mitosis]{} \text{Gametes}(n) \xrightarrow[fertilisation]{} \text{Zygote}(2n) \rightarrow \text{Embryo}(2n) \rightarrow \text{Sporophyte}(2n)

Non-Vascular Plants (Bryophytes) – Module 2

Overview

  • First terrestrial invaders (≈475–362 mya)
  • Three lineages:
    • Hepatophyta (liverworts) – \approx9000 spp.
    • Bryophyta (mosses) – \approx15\,000 spp.
    • Anthocerophyta (hornworts) – \approx100 spp.
  • Shared traits
    • Chlorophyll a, protected embryos
    • Lack vascular tissue, seeds, flowers
    • Dominant gametophyte (haploid carpet); microscopic, dependent sporophyte

Moss Morphology

  • “Carpet” = leafy gametophytes; sporophytes = elongated stalk + capsule
  • Leafy shoot:
    • Leaf – one-cell thick, photosynthetic, non-vascular
    • Stem – support
    • Rhizoids – anchor + limited uptake (not true roots)

Moss Life Cycle (Detailed)

  1. Spore germinates → protonema
    • Filamentous, branched, photosynthetic; resembles green-algal filaments
  2. Buds on protonema develop into leafy gametophytes
  3. Gametangia at shoot tips:
    • Antheridia ⇒ many biflagellate sperm
    • Archegonia ⇒ single non-motile egg
  4. Water film required; sperm swim to archegonium
    • Fertilisation → zygote retained in archegonium
  5. Sporophyte forms – permanently attached, nutritionally dependent on gametophyte
    • Foot (embedded), seta (stalk), capsule (sporangium)
    • Meiosis in capsule ⇒ millions of resistant spores (sporopollenin-walled)

Why Mosses Need Water for Sex

  • Sperm flagella permit swimming only through free water (dew, rain)

Ecological Significance of Mosses

  • Pioneer species on bare rock/soil; help retain N
  • Peat moss (Sphagnum) forms bogs:
    • Cover \approx3\% land yet store \approx30\% of global soil carbon
    • Global warming may dry bogs → decomposition releases CO_2

Seedless Vascular Plants – Module 3

Evolutionary Milestone

  • Vascular tissue originated \approx425\,\text{mya}; allowed height & wider dispersal
  • Gametophyte reduced; dominant sporophyte (2n)

Vascular Tissues

  • Xylem
    • Dead tracheids; conduct water + minerals upward
    • Lignified walls ⇒ mechanical support, tall growth
  • Phloem
    • Living cells; distribute sugars, amino acids, organics
  • Evolutionary advantages: support, water/nutrient transport, elevated spore release, light competition

Organs

  • Roots – anchor + absorb water/nutrients (advance on bryophyte rhizoids)
  • Leaves – primary photosynthetic surface; greatly increase light capture
  • Some species possess rhizomes (horizontal stems) producing leaves/roots

Two Living Seedless-Vascular Phyla

  1. Lycophyta (lycophytes): club mosses, spike mosses, quillworts
    • Today mostly small (≈1200 spp.), but Carboniferous trees reached 40\,\text{m}
  2. Monilophyta (monilophytes): ferns, horsetails, whisk ferns
    • Most widespread seedless vascular group (≈12 000 spp.)

Fern Morphology

  • Sporophyte sizes: 1\,\text{cm} aquatic species → 24\,\text{m} tree ferns; fronds up to 5\,\text{m}
  • Structures:
    • Rhizome, fronds (leaflets = pinnae), roots
    • Circinate vernation (fiddle-head coiling) unique
  • Sporangia clustered in sori under fronds, often shielded by indusium; meiosis → spores

Fern Life Cycle (Key Stages)

  1. Haploid spores (n) dispersed
  2. Germination → bisexual gametophyte (prothallus)
    • Thin, heart-shaped, photosynthetic, rhizoids
    • Antheridia & archegonia on underside
  3. Water enables sperm to swim to egg; fertilisation → zygote
  4. Zygote develops into new sporophyte; initially nourished by gametophyte but becomes independent

Paleo-Impact: Carboniferous “Coal Forests”

  • Devonian & Carboniferous lycophyte + fern trees formed first forests
    • Sequestered CO_2 → global cooling
    • Plant remains compressed into coal; modern burning releases stored carbon

Sneak Peek: Evolution of Seed Plants

  • Seed plants arose \approx360\,\text{mya}; today divided into two sister clades

Key Evolutionary Innovations

  1. Further reduced gametophyte, enlarged woody sporophyte (some with secondary xylem “wood”)
  2. Heterospory
    • Megaspore → female gametophyte
    • Microspore → male gametophyte
  3. Protection of reproductive stages
    • Megaspore inside ovule → after fertilisation becomes seed (embryo + food + coat)
    • Microspore develops within pollen grain (sporopollenin wall) → drought-proof male phase
    • Internal fertilisation; embryo sheltered in seed

Gymnosperm Groups (4 extant)

  • Cycads (“dinosaur plants”), Ginkgo (maidenhair tree), Conifers (pines & relatives), Gnetophytes
    • Cycads & Ginkgo retain motile sperm; others have non-motile sperm delivered via pollen tube

Angiosperm Diversity

  • One phylum (Anthophyta) but >250\,000 spp.; two large clades
    • Monocots
      • 1 cotyledon; parallel veins; scattered vascular bundles; fibrous roots; pollen with 1 opening; floral parts \times3
    • Eudicots
      • 2 cotyledons; net venation; vascular bundles in ring; taproot; pollen with 3 openings; floral parts \times4 or \times5

Summary of Modules 2 & 3

  • Bryophytes
    • Non-vascular, small; gametophyte dominant; require water for fertilisation; ecologically key in colonisation & carbon storage
  • Seedless vascular plants
    • Xylem & phloem, roots, leaves; sporophyte dominant; two phyla (lycophytes, monilophytes); formed ancient forests influencing climate
  • Trends across plant evolution
    • ↑ Sporophyte dominance, ↓ Gametophyte size
    • Development of vascular tissue, seeds, flowers
    • Enhanced adaptation to terrestrial environments and global ecological roles