Seed Plants

Evolution of Seeds

• Key transformative innovation

  • Allowed plants to colonise diverse terrestrial habitats and become dominant primary producers.

  • First seed plants appear ≈ $360\,\text{MYA}$ (Late Devonian).

  • Seeds = embryo + stored nutrients + protective coat → ↑ survival & dispersal potential.

  • Domestication of seed plants ≥ $8{,}000\,\text{years}$ BP → permanent human settlements.

• Progressive reduction of the gametophyte

  • Bryophytes: sporophyte dependent on gametophyte.

  • Ferns: large independent sporophyte & small independent gametophyte.

  • Seed plants: highly reduced (often microscopic) gametophyte dependent on sporophyte → protects gametophyte from environmental stress.

• Pollen advantage

  • Microspores → pollen grains (male gametophytes).

  • Pollination = transfer of pollen to ovules; eliminates need for water film; wind/animal vectors can transport long distances.

  • Germinating pollen forms tube → delivers non-motile sperm directly to female gametophyte inside ovule.

• Phylogenetic milestones (Fig. 29.6)

  • Origin of land plants ≈ $470\,\text{MYA}$.

  • Vascular plants ≈ $425\,\text{MYA}$.

  • Seed plants ≈ $360\,\text{MYA}$ (Gymnosperms + Angiosperms).

Global & Australian Plant Diversity (approximate)

• Worldwide species

  • Flowering plants: $\sim268{,}600$–$352{,}000$.

  • Gymnosperms: $\sim1{,}050$.

  • Ferns & allies: $\sim12{,}000$–$15{,}000$.

  • Liverworts: $\sim5{,}000$–$7{,}500$.

  • Hornworts: $\sim250$; Mosses $\sim11{,}000$–$15{,}000$.

• Australian context

  • Flowering plants: $18{,}706$ spp (≈ $7\%$ of world total); $93.3\%$ endemic; $15.2\%$ threatened.

  • Gymnosperms: $120$ spp; $96\%$ endemic.

  • Ferns/allies: $498$ spp; $33.8\%$ endemic; $29.5\%$ threatened.

Gymnosperms ("naked seeds")

• Shared features

  • Vascular, seed-producing, no flowers/fruit; ovule/seed exposed on cone scales.

  • Superior adaptation to drier Mesozoic climates vs seedless plants.

• Fossil timeline

  • First seed plants: $360\,\text{MYA}$.

  • Distinct gymnosperm lineages: $\approx305\,\text{MYA}$; dominated $251$–$65\,\text{MYA}$.

Four extant phyla

1. Ginkgophyta

   • Single living species Ginkgo biloba (fan-shaped deciduous leaves).

   • Dioecious: separate male & female trees; female seed coats emit strong odour → urban plantings favour males.

   • Long-lived, "living fossil"; native China but likely no wild populations.

2. Cycadophyta (cycads)

   • Palm-like appearance yet gymnosperms; very slow growth; dioecious.

   • Australian diversity: $76$ spp (families Zamiaceae & Cycadaceae), all endemic; many tissues poisonous.

   • SE-QLD genera/species: Macrozamia miquelli, M. lucida, M. pauli-guilielmi, Lepidozamia peroffskyana, Cycas media.

3. Gnetophyta (poorly known; potential evolutionary link to angiosperms)

   • United by similar reproductive structures.

   • Genera

     • Welwitschia mirabilis: Namib Desert, only two strap-like leaves; extremely long-lived.

     • Gnetum: tropical vines/trees Asia; possible record in Torres Strait.

     • Ephedra: arid-zone shrubs (N. Hemisphere); $65$ spp; source of alkaloid ephedrine (decongestant). E. fragilis naturalised in Victoria.

4. Coniferophyta (conifers)

   • Largest gymnosperm group (~$600$ spp) yet only $\sim45$ spp dominate extensive boreal forests.

   • Adaptations: needle or scale leaves, waxy cuticle, sunken stomata, some photosynthetic stems.

   • Reproduction: male (pollen) & female (ovulate) cones often on same tree; wind pollination; seeds on cone scales.

Australian Native Conifer Families (4 major)

• Podocarpaceae (15 spp)

  • Broad-leaved conifers; fruit-like cones.

  • Eg. Podocarpus elatus (Plum pine), P. lawrencei (Mountain plum pine), Lagarostrobos franklinii (Huon pine).

• Araucariaceae (6 spp) – most ancient

  • Eastern Australia/Oceania + 2 S. American spp.

  • Araucaria: Hoop (A. cunninghamii), Bunya (A. bidwillii), Norfolk Island (A. heterophylla).

  • Agathis robusta (Kauri), Wollemia nobilis (critically endangered; <$100$ wild individuals; protected during 2019–20 fires).

• Cupressaceae (21 spp)

  • Mainly Callitris; dry-adapted; solid cones, minute scale leaves; stem photosynthesis.

• Taxodiaceae (often merged into Cupressaceae)

  • 3 Tasmanian relic spp (Athrotaxis – King Billy pines).

• Non-native notable family

  • Pinaceae (Northern cccx): true pines (Pinus), firs, spruces; Pinus aristata (Bristlecone pine) oldest living trees (up to $3{,}500$ yr at $3{,}000\,\text{m}$ elevation).

Conifer (Pine) Life Cycle – key points

• Sporophyte (2n) dominant; each tree bears pollen & ovulate cones.

• Microsporocytes (2n) in pollen cones → meiosis → microspores (n) → pollen grains.

• Ovulate cone scale has two ovules; megasporocyte (2n) → meiosis → one surviving megaspore (n) → female gametophyte.

• Pollination → pollen tube growth (months); fertilisation occurs >$1$ yr after pollination.

• Mature seed contains

  • Embryo (new sporophyte, 2n)

  • Food supply (female gametophyte tissue, n)

  • Seed coat (integument, 2n)

• Fertilised by pollen grain, dispersal mainly by wind; no fruit formation.

Angiosperms (Phylum Anthophyta)

• Defining traits: vascular, flowers (sexual organs) & fruits (ripened ovaries enclosing seeds).

• Most diverse & widespread plant group: $\approx250{,}000$ described spp.

Current Taxonomy (APG IV – 2016)

• ≈$400$ families organised via molecular phylogenetics; traditional dicot category largely paraphyletic.

• Two major clades highlighted here:

  • Monocots

  • Eudicots (true dicots)

Monocots vs Eudicots (diagnostic table)

• Embryo: $1$ cotyledon vs $2$.

• Leaf venation: parallel vs netlike.

• Stem vascular bundles: scattered vs ring.

• Roots: fibrous vs taproot.

• Pollen apertures: single vs triple.

• Flower parts: multiples of $3$ vs $4$ or $5$.

Local Examples (SE Queensland)

• Monocots

  • Brisbane Lily (Proiphys cunninghamii): shade-adapted bulb; fragrant white flowers.

  • Yellow/Griffith Swamp Orchid (Phaius australis/bernaysii): endangered terrestrial orchid; tall spikes.

• Eudicots

  • Love Flower (Pseuderanthemum variabile): colourful groundcover; butterfly host.

  • Native Violet (Viola banksii/hederacea): creeping shade groundcover.

Anatomical Innovations in Angiosperm Xylem

• Tracheids (support + water) shared with gymnosperms.

• Fibres for additional support.

• Vessel elements → continuous vessels → highly efficient water transport → ecological dominance.

Flower Structure (specialised reproductive shoot)

• Perianth (non-reproductive)

  • Sepals (calyx): protect bud.

  • Petals (corolla): attract pollinators; often inconspicuous in wind-pollinated spp.

• Androecium (male)

  • Stamens = filament + anther; anther produces microspores/pollen.

• Gynoecium (female)

  • Carpels = stigma (pollen receptive) + style + ovary.

  • Ovules inside ovary → seeds; ovary wall → pericarp (fruit).

• Variations covered (Clarke & Lee diagrams)

  • Free vs fused carpels (apocarpous vs syncarpous).

  • Placentation types; anther attachment (basifixed/dorsifixed); dehiscence modes; stamen/staminode fusion.

• Floral symmetry: radial (actinomorphic) vs bilateral (zygomorphic, e.g., orchids).

Inflorescence Architecture (arrangement of flowers)

• Raceme, spike, panicle, corymb, umbel, head (e.g., composites), etc. Developmental sequence (oldest vs youngest flowers) influences pollination strategies.

Angiosperm Life Cycle

• Dominant sporophyte (2n) produces flowers.

• In anthers: microsporogenesis → microspores → pollen (male gametophyte, n).

• In ovules: megasporogenesis → megaspore → female gametophyte (embryo sac).

• Double fertilisation (not detailed in transcript but implied): one sperm + egg → zygote; second sperm + central cell → endosperm (triploid) – key angiosperm feature.

• Post-fertilisation

  • Ovule → seed (embryo + endosperm + seed coat).

  • Ovary → fruit; surrounding tissues may contribute (accessory fruit).

Fruit & Seed Development

• Pericarp (ovary wall) differentiates into exocarp, mesocarp, endocarp.

• Hormonal changes after pollination trigger fruit set; lack of pollination → flower abscission.

• Ripening: enzymatic softening, starch/acid → sugar conversion, colour/aroma changes → attract dispersers.

• Ecological roles: protection, nutrition, dispersal (wind, water, ballistic, animals).

Seed Size Spectrum

• Smallest: orchid seeds – millions per gram; require mycorrhizal fungus for germination.

• Largest: Coco de Mer (Lodoicea maldivica) fruits up to $42\,\text{kg}$.

Principal Fruit Classifications

• True Fruits (from ovary only)

  1. Dry indehiscent – Achene, Nut, Samara.

  2. Dry dehiscent – Follicle, Legume, Capsule.

  3. Fleshy – Berry, Drupe.

• Accessory Fruits – incorporate other floral parts (e.g., Pome in apples, receptacle in strawberries; pineapple = multiple fruit from inflorescence).

• Key to 7 common types

  • Achene (e.g., sunflower), Nut (acorn), Follicle (Grevillea), Legume (pea), Capsule (eucalypt), Berry (tomato), Drupe (peach).

Ethical / Cultural / Practical Notes

• Many cycads & conifers are culturally significant (e.g., Bunya gatherings by First Nations).

• Conservation concern: Wollemia nobilis firefighting protection; endangered orchids cultivated at university gardens.

• Human selection has produced larger, sweeter, seedless or thin-skinned fruits (e.g., modern apples, citrus).

• Cereals (dry fruits) underpin global food security; recognition that “grain” = fruit + seed fused.

Connections to Prior Content & Broader Principles

• Builds on vascular tissue evolution (lectures 1–3) → seed and pollen innovations.

• Illustrates monophyly vs paraphyly concepts (seed plants monophyletic; “dicots” paraphyletic).

• Demonstrates convergent/homoplastic evolution (gnetales vs angiosperms) per Ran et al. $2018$ phylogenomic study.

Summary

Importance of Seeds & Major Evolutionary Steps:

• Key transformative innovation: Seeds allowed plants to colonise diverse terrestrial habitats and become dominant primary producers.

• First seed plants appeared approximately $360\,\text{MYA}$ (Late Devonian).

• Seeds consist of an embryo + stored nutrients + protective coat, significantly increasing survival and dispersal potential.

• Domestication of seed plants over $8\,000\,\text{years}$ BP led to permanent human settlements.

• Progressive reduction of the gametophyte: In seed plants, the gametophyte is highly reduced (often microscopic) and dependent on the sporophyte, which protects it from environmental stress.

• Pollen advantage: Microspores develop into pollen grains (male gametophytes). Pollination (transfer of pollen to ovules) eliminates the need for a water film, allowing wind or animal vectors to transport pollen long distances. Germinating pollen forms a tube that delivers non-motile sperm directly to the female gametophyte inside the ovule.

Gymnosperms Definition, Four Phyla, and Attributes:

• Gymnosperms ("naked seeds")

  • vascular

  • seed-producing plants that lack flowers and fruits

  • ovules/seeds exposed on cone scales.

  • They were superiorly adapted to drier Mesozoic climates compared to seedless plants.

• Dioecious: trees that produce male flowers + trees that produce female flowers

  • Landscaper → plant male trees because the seed coats on female plants decay + produce odour

• Four extant phyla:

  1. Ginkgophyta: Characterized by a single living species, has fan-shaped deciduous leaves and is dioecious. It is long-lived and considered a "living fossil," culturally significant.

  2. Cycadophyta (cycads): Palm-like gymnosperms that grow very slowly and are dioecious. Australian diversity includes $76$ endemic species, many of which have poisonous tissues. They hold cultural significance, for example, through Bunya gatherings by First Nations.

  3. Gnetophyta: A poorly known group that potentially links to angiosperms due to similar reproductive structures. Genera include:

     • Welwitschia mirabilis (Namib Desert),

     • Gnetum (tropical vines/trees),

     • Ephedra (arid-zone shrubs, source of ephedrine).

  4. Coniferophyta (conifers): The largest gymnosperm group ($\sim600$ spp). They exhibit adaptations like needle or scale leaves, waxy cuticles, and sunken stomata. Reproduction involves male (pollen) and female (ovulate) cones, usually on the same tree, with seeds dispersed by wind.

Conifer (Pine) Life Cycle (Key Points):

• The sporophyte ($2n$) is dominant; each tree typically bears both pollen (male) and ovulate (female) cones.

• In pollen cones, microsporocytes ($2n$) undergo meiosis to produce microspores ($n$), which develop into pollen grains (male gametophytes).

• In ovulate cones, each scale contains two ovules. A megasporocyte ($2n$) undergoes meiosis, with one surviving megaspore ($n$) developing into the female gametophyte.

• Pollination is the transfer of pollen to the ovules. A pollen tube then grows, a process that can take several months.

• Fertilisation typically occurs more than $1$ year after pollination.

• A mature seed contains the new embryo (a $2n$ sporophyte), a food supply (derived from the $n$ female gametophyte tissue), and a seed coat (developed from the $2n$ integument).

• Seed dispersal is primarily by wind; there is no fruit formation.

Angiosperms Definition, APG Taxonomy, and Monocots/Eudicots Contrast:

• Angiosperms (Phylum Anthophyta) are vascular plants defined by the presence of flowers (specialized sexual organs) and fruits (ripened ovaries enclosing seeds). They are the most diverse and widespread plant group, with approximately $250{,}000$ described species.

• Current Taxonomy (APG IV – 2016) organizes approximately $400$ families based on molecular phylogenetics. This system shows that the traditional "dicot" category is largely paraphyletic. The two major clades highlighted are Monocots and Eudicots (true dicots).

• Monocots vs. Eudicots (Diagnostic Table):

  • Embryo: Monocots have $1$ cotyledon; Eudicots have $2$.

  • Leaf Venation: Monocots typically parallel; Eudicots typically netlike.

  • Stem Vascular Bundles: Monocots scattered; Eudicots arranged in a ring.

  • Roots: Monocots typically fibrous; Eudicots typically have a taproot.

  • Pollen Apertures: Monocots single; Eudicots triple.

  • Flower Parts: Monocots in multiples of $3$; Eudicots in multiples of $4$ or $5$.

Complete Flower Structure & Inflorescence Types:

• A flower is a specialized reproductive shoot.

• Flower Structure:

  • Perianth (non-reproductive parts):

    • Sepals (calyx): Protect the flower bud.

    • Petals (corolla): Often conspicuous and attract pollinators; inconspicuous in wind-pollinated species.

  • Androecium (male reproductive parts):

    • Stamens: Composed of a filament and an anther; the anther produces microspores/pollen.

  • Gynoecium (female reproductive parts):

    • Carpels: Comprise the stigma (pollen receptive), style, and ovary. Ovules inside the ovary develop into seeds, and the ovary wall develops into the pericarp (fruit).

• Floral Symmetry: Can be radial (actinomorphic) or bilateral (zygomorphic, e.g., orchids).

• Inflorescence Architecture: Refers to the arrangement of flowers on a plant. Examples include raceme, spike, panicle, corymb, umbel, and head (e.g., composites). The developmental sequence of flowers (oldest vs. youngest) influences pollination strategies.

Flowering-Plant Life Cycle (Highlighting Double Fertilisation):

• The dominant sporophyte ($2n$) produces flowers.

• In the anthers, microsporogenesis leads to microspores, which develop into pollen (the male gametophyte, $n$).

• In the ovules, megasporogenesis produces a megaspore, which develops into the female gametophyte (embryo sac).

• Double fertilisation is a key angiosperm feature: One sperm cell from the pollen tube fuses with the egg cell to form the zygote ($2n$), which develops into the embryo. The second sperm cell fuses with the central cell (containing two polar nuclei) to form the endosperm (typically triploid, $3n$), which provides nutrients to the developing embryo.

• Post-fertilisation: The ovule develops into a seed (containing the embryo, endosperm, and seed coat), and the ovary develops into the fruit. Surrounding tissues may also contribute to accessory fruit formation.

Basic Fruit Categories with Examples:

• The pericarp (ovary wall) differentiates into the exocarp, mesocarp, and endocarp.

• Hormonal changes after pollination trigger fruit set; lack of pollination leads to flower abscission.

• Ripening involves enzymatic softening, conversion of starch/acid to sugar, and changes in color/aroma to attract dispersers.

• Fruits serve ecological roles in protection, nutrition, and dispersal (wind, water, ballistic, animals).

• Principal Fruit Classifications:

  • True Fruits (derived from the ovary only):

    1. Dry indehiscent: Do not split open at maturity. Examples: Achene (e.g., sunflower), Nut (e.g., acorn), Samara.

    2. Dry dehiscent: Split open at maturity to release seeds. Examples: Follicle (Grevillea), Legume (e.g., pea), Capsule (e.g., eucalypt).

    3. Fleshy: Have a fleshy pericarp at maturity. Examples: Berry (e.g., tomato), Drupe (e.g., peach).

  • Accessory Fruits: Incorporate other floral parts in addition to the ovary (e.g., Pome in apples, which includes receptacle tissue; strawberries, where the fleshy part is the receptacle; pineapple, which is a multiple fruit from an entire inflorescence).

Application of Terminology During Laboratory/Practical Identification Exercises:

The ability to apply this detailed terminology (e.g., distinguishing fruit types, identifying flower parts, contrasting monocot/eudicot features) is crucial for accurate identification and analysis of plant specimens in laboratory or field practical exercises.

• Define gymnosperms; distinguish 4 phyla; cite ecological/cultural attributes.

• Reproduce conifer life cycle diagram with labels.

• Define angiosperms; outline APG taxonomy; contrast monocots/eudicots.

• Accurately label complete flower & major inflorescence types.

• Draw flowering-plant life cycle highlighting double fertilisation.

• Identify & describe basic fruit categories with examples.

• Apply terminology during laboratory/practical identification exercises.