Lecture 4 Notes – Seed Plants: Gymnosperms & Angiosperms

Administrative & Logistical Announcements

  • Lecture 4 in the Plant Diversity series (continuation from Lecture 3).
  • Labs begin next week – ensure you are enrolled in a lab stream.
    • Lab sign-up and schedules are on Moodle.
  • Weekly Moodle quizzes:
    • 10 random MCQs drawn from ~20 questions that cover the week’s lectures.
    • Unlimited attempts – meant as formative revision.
  • Tuesday of next week: lecturer will teach from Tauranga campus; Hamilton lecture theatre will view live stream (staff will start the video locally).
  • ZorroQ in-class quiz was planned but the backend server was down (blank webpage) – skipped for this session.

Scenic / Motivational Plant Examples

  • Geothermal kānuka (a stunted, heat-tolerant ecotype of the common kānuka tree) – grows flat or ≤ 1 m tall near Taupō geothermal fields.
  • Dwarf mistletoe (tiny, a few cm tall) – epiphytic on kānuka, possum-browsed, resembles the salt-marsh genus Salicornia.

Recap – Big Picture Phylogeny of Land Plants

  • Seven major living lineages (three non-vascular + four vascular):
    1. Liverworts (Bryophyta)
    2. Mosses (Bryophyta)
    3. Hornworts (Bryophyta)
    4. Lycophytes (club, spike & quillworts)
    5. Ferns (incl. whisk & horsetails)
    6. Gymnosperms (naked-seed plants)
    7. Angiosperms (flowering plants)
  • Key evolutionary nodes:
    • Origin of land plants ≈ 450\,\text{Ma}.
    • Vascular tissue appears once; splits into Lycophytes vs. Euphyllophytes (ferns + seed plants).
    • Seed origin node produces Gymnosperms first, then Angiosperms.

Foundational Land-Adaptation Traits (all embryophytes)

  • Alternation of generations (2n sporophyte ↔ n gametophyte).
  • Walled spores (sporopollenin).
  • Multicellular gametangia.
  • Multicellular dependent embryo (hence “Embryophyta”).
  • Apical meristems & 3-D growth.

Seed Plants – The “Upgrade”

  • Metaphor: Toyota Corolla (spore plants) → Ferrari (seed plants).
  • Definition of a seed: \text{Seed}=\text{Embryo}+\text{Food supply}+\text{Protective coat}
    • Embryo is usually dormant.
    • Food supply = endosperm or megagametophyte reserves.
    • Seed coat (testa) = tough, desiccation-resistant integuments.
  • Four derived traits that collectively enable full reproduction without free water:
    1. Reduced gametophytes – micro- to macroscopic, nutritionally dependent on sporophyte.
    2. Heterospory – separate spores: megaspore (♀) vs. microspore (♂).
    3. Ovules (immature seeds) housing the megasporangium, megaspore, & integuments.
    4. Pollen – microgametophyte + sporopollenin wall; delivers non-motile sperm via pollen tube.

Gradual Miniaturisation of the Gametophyte

  • Mosses: dominant n gametophyte, dependent 2n sporophyte (stalk + capsule).
  • Ferns/Lycophytes: free-living, photosynthetic gametophyte but small (heart-shaped prothallus); dominant 2n sporophyte.
  • Seed plants: gametophytes microscopic & retained:
    • Female (megagametophyte) inside ovule.
    • Male (microgametophyte) inside pollen grain.

Generic Alternation-of-Generations Diagram (white-board redraw)

         sporophyte (2n)
              |  meiosis
            spores (n)
              ↓
        gametophyte (n)
         /           \
      egg (n)     sperm (n)
          \         /
          fertilisation
               ↓
            zygote (2n) → embryo → sporophyte

Advantages of Seeds (4-point list)

  • Dormancy – withstand adverse seasons; viable for years–millennia (e.g.
    2,000 yr-old Judean date palm seed successfully germinated).
  • Spatial dispersal – wind, water, animal fur, bird/animal gut, explosive dehiscence.
  • Temporal dispersal – seed bank ensures germination only under favourable conditions.
  • Initial nutrition – endosperm or megagametophyte feeds embryo until photosynthetic.

Fossil Evidence for Seed Evolution

  • First seed-like fossils ~360\,\text{Ma} (Late Devonian).
  • Earliest vascular plant fossils (Cooksonia) ~420\,\text{Ma} → only 60 Myr gap.
  • Lignophyte lineage simultaneously evolves vascular cambium → wood.

Gymnosperms ("Naked Seeds")

General

  • Seeds exposed on cone (sporophyll) scales; not enclosed by ovary wall.
  • Many wind-pollinated; thick-walled pollen.
  • Modern diversity: 4 phyla (~1,000 species total).

Pine (Pinus) Life Cycle Highlights

  • Separate male (pollen) vs. female (ovulate) cones.
  • Timing quirks: pollen released & captured by tiny ovulate cone; fertilisation may occur months later after slow pollen-tube growth.
  • Pollen grain = 4 cells: 2 prothallial, 1 tube, 1 generative (→ sperm).
  • Female cone scale contains: integument, megasporangium, single functional megaspore → megagametophyte → archegonia (eggs) → embryo.

Four Living Gymnosperm Lineages

  1. Cycadophyta – Cycads
    • Palm-like; large compound leaves; massive strobili; “Age of Cycads & Dinosaurs.”
  2. Ginkgophyta – Ginkgo biloba (sole survivor)
    • Fan-shaped leaves; fleshy, malodorous seed coat; rediscovered in Chinese temples; hardy street tree.
  3. Gnetophyta – 3 genera
    • Welwitschia (Namib Desert, 2 ever-growing strap leaves, millennial lifespan).
    • Gnetum (tropical lianas/trees; seeds edible as Indonesian “emping” crackers).
    • Ephedra (desert shrubs – “Mormon tea,” source of ephedrine).
  4. Coniferophyta – Conifers
    • ~600 species; dominate boreal & montane biomes; tallest, largest, & oldest trees (e.g.
      Sequoia).
    • NZ podocarps (kauri, rimu, kahikatea, tōtara) – modified bird-dispersed cones; term Podocarp = “foot-seed” (seed sits atop fleshy peduncle).

Angiosperms ("Enclosed Seeds")

Diagnostic Flower Features

  • Ovary (carpel) encloses ovules → fruit wall (pericarp) after fertilisation.
  • Stamens with anthers produce distinctive tri- or bi-colpate pollen.
  • Highly reduced gametophytes:
    • Female embryo sac = 7 cells / 8 nuclei.
    • Male pollen = 2–3 cells; rapid tube growth (minutes–hours).
  • Double fertilisation → zygote (2n) + nutritive endosperm (3n).
  • Potential for co-evolution with animals (pollinators, frugivores).

Angiosperm Life Cycle Core Steps

  • Microsporogenesis in anther → tetrads of microspores → pollen.
  • Megasporogenesis in ovule → 1 functional megaspore → embryo sac (7/8 configuration).
  • Pollen lands on stigma → tube cell grows through style, generative cell → 2 sperm → double fertilisation.
  • Embryogenesis + endosperm development inside seed; ovary matures into fruit (fleshy or dry).

Origin & Diversification

  • Earliest flowering-plant-like fossils ≈ 140\,\text{Ma} (wetland taxa with mixed sporophylls → proto-flowers).
  • Molecular + fossil data: gymnosperms and angiosperms are sister clades; Gnetophytes are not direct ancestors (convergent features).
  • Explosive radiation in Cretaceous → present-day dominance (~90 % of extant plant species).

Major Modern Clades (4-fold scheme)

  1. Basal angiosperms (e.g.
    Amborella, water lilies).
  2. Magnoliids (magnolias, laurels, black pepper).
  3. Monocots (grasses, orchids, palms) – one cotyledon.
  4. Eudicots (true dicots; beans, roses, myrtles, daisies) – tri-colpate pollen.

Uniqueness of New Zealand Flora (Seed-Plant Perspective)

  • High endemism; mix of ancient conifers & derived angiosperms.
  • Characteristic traits/terms worth noting:
    • Divaricate architecture – tangled, interlacing small-leaf shoots (anti-moa browse hypothesis).
    • Heteroblasty – juvenile vs. adult leaf forms subtly/extremely different.
  • Iconic species: geothermal kānuka, kauri, rimu, kahikatea, dwarf mistletoe.

Ethical, Ecological & Practical Importance of Seed Plants

  • Human dependence on seed plants for:
    • Food staples (grains, legumes, fruits, nuts).
    • Fibre (cotton, linen, paper pulp from conifers).
    • Timber & fuel wood.
    • Medicines (e.g.
      ephedrine, taxol from yew).
  • Conservation imperative: seed-plant diversity underpins future crops, materials & pharmaceuticals; habitat loss + invasive herbivores (e.g.
    possums browsing mistletoes) threaten viability.

Key Terms to Master

  • Sporophyte (2n), Gametophyte (n), Spore, Gamete, Megaspore, Microspore, Megasporangium, Microsporangium, Megagametophyte, Microgametophyte, Ovule, Integument, Seed coat (testa), Pollen, Pollen tube, Archegonium, Embryo sac, Double fertilisation, Endosperm, Strobilus, Cone scale, Stamen, Carpel, Fruit, Dormancy, Lignophyte, Divaricate, Heteroblastic.