Kingdom Plantae – Bryophytes, Vascular and Seedless Vascular Plants

Phylogeny and Ancestry of Life

  • All extant organisms trace back to a single “Common Ancestor of All Life.”

  • Three Domains recognised

    • Domain Bacteria

    • Domain Archaea

    • Domain Eukarya

      • Plants

      • Animal

      • Fungi

  • A branch point leading to multiple lineages is a polytomy—an unresolved divergence pattern.

Kingdom Plantae – Core Features

  • Eukaryotic,

  • multicellular

  • photosynthetic

  • cellulose cell walls.

    *These organisms are characterized by their ability to adapt to terrestrial environments, which includes the development of structures like roots, stems, and leaves that facilitate nutrient absorption and gas exchange. *

  • Chloroplasts house photosynthetic machinery (from cyanobacterial endosymbiosis).

  • Adapted to terrestrial life: cuticle, stomata, protected embryos, alternation of generations, spores/seeds.

  • Two broad structural grades

    • Non-vascular (bryophytes)

    • Vascular (pteridophytes, gymnosperms, angiosperms)

  • Fossil evidence: land plants present by 475\ \text{mya} (Ordovician).

Four Major Extant Land-plant Groups & Key Innovations

  • Bryophytes (mosses, liverworts, hornworts) – non-vascular.

  • Pteridophytes (lycophytes, ferns, horsetails, whisk ferns) – seedless vascular.

  • Gymnosperms (conifers & allies) – naked seeds.

  • Angiosperms – flowers & enclosed seeds.

Seed Evolution & Significance

  • Seed = embryo + nutritive tissue + protective coat.

  • Appeared \approx360\ \text{mya} (late Devonian) giving rise to gymnosperms then angiosperms.

  • Allowed reproduction away from water & dormancy during unfavourable periods.

Flower & Angiosperm Radiation

  • Flowers first appear early Cretaceous \approx130\ \text{mya}.

  • Ovary encloses ovules → fruit; drives co-evolution with pollinators & dispersers

    .

  • Angiosperms now dominant plant group

Fundamental Life-cycle Pattern: Alternation of Generations

  • Two multicellular stages alternate:

    • Gametophyte (n) → produces gametes by mitosis.

    • Sporophyte (2n) → produces spores by meiosis.

  • In bryophytes, the gametophyte stage is dominant, appearing as the primary photosynthetic plant.

    • The sporophyte in bryophytes is smaller and remains dependent on the gametophyte for nutrition.

  • For seedless vascular plants like ferns, both the sporophyte (the recognizable leafy plant) and the gametophyte (often a small, heart-shaped structure that produces gametes) are distinct phases

  • In vascular plants, the sporophyte stage is dominant, featuring larger, complex structures capable of independent photosynthesis.

Bryophytes (Mosses, Liverworts, Hornworts): Taxonomy, Distribution, and Lifecycle

  • Three phyla diverged early; not monophyletic collectively.

    • Hepatophyta (liverworts)

    • Anthocerophyta (hornworts)

    • Bryophyta (mosses) – closest bryophyte group to vascular plants.

  • Prevalent vegetation for first 100\ \text{million} years after land colonisation.

  • Australian richness and distribution:

    • Liverworts 841 spp (≈23{-}28\% endemic)

    • Hornworts 30 spp (≈23{-}28\% endemic)

    • Mosses 976 spp (≈22.7\% endemic)

  • Bryophyte Life-Cycle Specifics (e.g., Moss):

    1. Dominant gametophyte (primary photosynthetic plant); sporophyte small & transient, dependent on gametophyte.

    2. Spore germinates into a protonema (filamentous stage).

    3. Protonema develops into meristems, which then grow into gametophores (the leafy structures).

    4. Sex organs: antheridia (produce sperm) and archegonia (produce eggs) form on the gametophyte.

    5. Fertilised egg is retained within the archegonium.

    6. Zygote develops into a sporophyte (consisting of a seta and capsule), which remains dependent on the gametophyte for nutrition.

    7. Spores are produced by meiosis within the capsule and are typically wind-dispersed.

  • Structural & Physiological Notes:

    • One-cell-thick tissues keep cells near water/minerals.

    • Absence of lignin limits height (few cm).

    • Rhizoids for anchorage not absorption.

    • Hornwort & liverwort gametophytes often thalloid (flattened).

  • Ecological Role of Mosses (Sphagnum):

    • Form peatlands (≈3\% land area) storing ≈30\% global soil carbon.

Evolution of Vascular Plants

  • Earth’s surface lifeless for first 3\ \text{billion} years; land cyanobacteria by 1.2\ \text{bya}. First small terrestrial plants, fungi, animals ≈ 500\ \text{mya}.

  • Timeline of Major Evolutionary Innovations:

    1. Origin of land plants \approx475\ \text{mya} (Ordovician).

    2. Development of vascular tissue \approx425\ \text{mya}.

    3. Evolution of seeds \approx360\ \text{mya} (late Devonian).

    4. Appearance of flowers \approx130\ \text{mya} (early Cretaceous).

  • Vascular Adaptations (leading to vertical growth and independence):

    • Lignified xylem (water/mineral transport upward) & phloem (bidirectional sugar/organic transport).

    • Roots (anchorage, absorption) derived from subterranean stems.

    • Leaves (microphylls vs megaphylls) increasing photosynthetic area.

    • Sporophyte dominance and independence.

    • Increased height for spore/seed dispersal and light competition.

  • Shift in Generational Dominance (Evolutionary Trend):

    a) Bryophytes – sporophyte fully dependent on gametophyte.

    b) Ferns – independent sporophyte; small, freeliving gametophyte.

    c) Seed plants – microscopic gametophyte dependent on sporophyte.

Ferns and Allies (Pteridophytes): Characteristics, Lifecycle, and Significance

  • Two extant phyla:

    1. Lycophyta – club mosses, spike mosses, quillworts.

    2. Pterophyta – ferns, whisk ferns (Psilotum), horsetails (Equisetum).

  • Modern forms descended from multiple early vascular lineages; seedless.

  • Sporophyte-Dominant Fern Life-Cycle (e.g., Ferns):

    1. Sporangia (found in sori on the underside of fronds) undergo meiosis to produce haploid spores (n).

    2. Spore germinates, developing into a small, bisexual gametophyte (prothallus).

    3. The prothallus forms antheridia (producing sperm) and archegonia (producing eggs).

    4. Sperm swim via water to fertilize the egg.

    5. The resulting zygote develops into a new sporophyte, which initially grows from the prothallus.

    6. The mature sporophyte becomes independent (the recognizable leafy plant), and the gametophyte disintegrates.

  • Fern Morphology & Identification:

    • Frond parts: stipe (petiole), rachis, pinnae, pinnules; crozier/fiddlehead = circinate vernation.

    • Identification relies on venation patterns (free vs anastomosing), indusia shapes (cup, linear, false), leaf margins (entire, serrate, crenate), and presence of scales/hairs.

  • Cultural References to Ferns:

    • “Silver fern” (Cyathea dealbata) is an emblem of New Zealand, appearing on its national coat of arms & Air NZ logo.

    • Fiddleheads are used as edible greens (caution: some species are carcinogenic).

  • Fern Diversity & Ecology:

    • >12,000 species globally; centres of diversity in tropics, also temperate & arid zones.

    • Growth typically from rhizomes; megaphyllous fronds often highly dissected.

    • Sporangia possess an annulus – a spring mechanism for ejecting spores metres away.

  • Australian Ferns & Allies Snapshot (Examples and Locations):

    • \sim525 native species (≈33.8\% endemic), plus \sim36 naturalised.

    • Hotspots: Wet Tropics (211 spp), Border Ranges (130), Sydney (90), Tasmania (65).

    • Examples:

      • Tree ferns: Cyathea cooperi.

      • Epiphytes: Elk horn (Platycerium bifurcatum), Staghorn (P. superbum).

      • Weedy natives: Bracken (Pteridium esculentum), Fishbone fern (Nephrolepis cordifolia).

      • Indoor ornamentals: Maidenhair (Adiantum spp.), Bird’s Nest (Asplenium australasicum).

    • Whisk fern Psilotum nudum found in dry Eucalyptus forests.

    • Horsetails (Equisetum) are non-native, invasive & toxic to livestock; high silica content once used for scouring pots.

  • Global and Ecological Impact of Early Vascular Forests:

    • Devonian/Carboniferous lycophytes, horsetails, ferns formed first tall forests.

    • Massive CO_2 drawdown via photosynthesis likely triggered global cooling.

    • Burial of lignin-rich biomass produced coal; burning coal today returns that carbon, driving climate change.

Modern Plant Diversity & Habitats

  • >280,000 described species (most terrestrial; some secondarily aquatic like sea-grasses).

  • Occupy deserts, grasslands, forests, alpine zones, and coastal/marine shallows.

  • All land plants derived from a green-algal ancestor.

Global vs Australian Species Numbers

  • Flowering plants: World \sim268{,}600-352{,}000; Australia 18{,}706 (≈7\% of world, 93\% endemic, 15.2\% threatened).

  • Gymnosperms: World 1,021-1,050; Australia 120 (≈96\% endemic).

  • Ferns & allies: World \sim12,000-15,000; Australia 498 (≈33.8\% endemic, 29.5\% threatened).

  • Liverworts, hornworts, mosses likewise show high endemism.