Angiosperm Reproduction & Diversity – Detailed Study Notes

Course & Assessment Logistics

  • Lecture position
    • 9ᵗʰ content lecture (10ᵗʰ if counting the introductory session)
    • Final lecture on plant evolution; two remaining lectures will shift to general plant topics
  • Assessment timetable
    • Practice test and other sample resources released on Moodle “today”
    • Three in-course tests across semester; best 2/3 marks kept, lowest dropped
    • Test location → regular lecture rooms; on-paper, circle-Y/N/MCQ + short-answer blanks
    • Time allocation: 50 \text{ min} for 40 marks ⇒ ≈ 1 mark per minute; only 2-3 MCQs per lecture can be asked
    • Essay due: 14 Aug (week before mid-semester break)
  • Lab update
    • Upcoming hormone lab uses bean (eudicot) seedlings; glass-house heater failure was managed by moving seedlings inside to avoid frost damage

Big-Picture Theme: Angiosperm Dominance

  • Vivid metaphor (David Attenborough in Kew herbarium): evolutionary “family tree” grows slowly through bryophytes, lycophytes, ferns, seed plants, then “explodes” at angiosperms, filling the room to the ceiling
  • Hypotheses for success
    • Higher productivity and growth rates
    • Leaf design efficiency
    • MOST emphasised: extraordinarily flexible and specialised reproduction

Learning Outcomes (LO)

  • Describe angiosperm reproductive biology & life cycle in detail
  • Identify key derived traits vs other land-plant lineages
  • Name & recognise flower parts
  • Explain major evolutionary trends in floral morphology & pollination strategies
  • Sketch basic seed and fruit morphology & diversity (monocot vs eudicot)

Three Signature “F”s of Angiosperms

  1. Flowers – aggregated reproductive structures
  2. Double Fertilisation + Endosperm – two sperm nuclei → one 2n zygote + one 3n nutritive tissue
  3. Fruits – post-fertilisation ovary (often additional tissues) enclosing seeds and aiding dispersal

Flower Anatomy Refresher

  • Organised in four concentric whorls (outside → inside):
    • Sepals (calyx) – usually green, protective bud covering
    • Petals (corolla) – often colourful, attractants
    • Stamens (androecium) – filament + anther (microsporangia)
    • Carpels (gynoecium) – stigma (pollen landing), style (tube path), ovary (contains ovules)
  • Each ovule = integuments + megasporangium + female gametophyte ➔ develops into a seed

Evolutionary Trends Within Angiosperms (4 predictable axes)

  1. Symmetry shift
    • Basal → radial (actinomorphic); e.g.
      Water-lily (Nymphaea)
    • Derived → bilateral (zygomorphic); e.g.
      Orchids — forces pollinators to approach in one orientation, enhancing precision 
  2. Numerical reduction
    • Many similar organs → few fixed-number organs (often multiples of 3 in monocots, 4/5 in eudicots) 
  3. Fusion of parts
    • Free petals/sepals → fused tubes, spurs, pockets (e.g. Convolvulus corolla tube) 
  4. Repositioning / Ovary insertion
    • Superior ovary (above other parts) → inferior ovary (embedded below), distancing delicate ovules from foraging visitors 

Seed-Plant & Angiosperm Life-Cycle Review

  • Shared land-plant alternation: sporophyte 2n ↔ gametophyte n
  • Seed-plant specialisations (gymnosperms + angiosperms)
    1. Microscopic dependent gametophytes embedded in sporophyte
    2. Heterospory (micro vs mega)
    3. Ovules (immature seeds) + pollen (air/animal mobile male gametophytes)

Angiosperm Detailed Cycle (key terminology in bold)

  1. Microsporogenesis in anther
    • Microsporangia undergo meiosis → 4 microspores n each
    • Microspore mitosis → 2-celled pollen grain (tube cell + generative cell) + sporophytic exine wall
  2. Megasporogenesis in ovule (inside ovary)
    • One megasporocyte meioses → 4 megaspores, 3 degenerate, 1 survives
    • Surviving megaspore → mitoses → 8-nucleate embryo sac (female gametophyte)
      • 1 egg cell
      • 2 synergids (guide tube)
      • 2 polar nuclei (central cell)
      • 3 antipodals (often nutritive)
  3. Pollination
    • Pollen lands on stigma, hydrates, tube grows rapidly (fastest known plant cells) through style
  4. Double fertilisation
    • Sperm #1 + egg → 2n zygote
    • Sperm #2 + 2 polar nuclei → 3n endosperm (nutritive)
  5. Seed formation
    • Zygote → embryo (globular → heart → torpedo stages)
    • Endosperm proliferates/stores starch, oil, protein
    • Integuments harden → seed coat
  6. Fruit development
    • Ovary wall (= pericarp) + adjunct tissues enlarge, becoming dispersal unit

Why double fertilisation?

  • Resource-use efficiency hypothesis: endosperm only forms if fertilisation succeeds, preventing wastage
  • Human relevance: cereal endosperm = bulk carbohydrate we eat (rice, wheat, maize)

Specialised Pollination Examples & Syndromes

  • Pseudocopulation Orchids (Flying-duck, others)
    • Flowers mimic female wasps visually & chemically; males attempt copulation ➔ pollen transfer
    • Co-evolution & genetics nuance: Hymenopteran sex determination (haploid ♂ vs diploid ♀) can skew sex ratios, potentially feeding back to pollination success
  • Wind pollination (~20 % spp.)
    • Reduced petals, exposed feathery stigmas, massive pollen output (e.g. grasses)
  • Bee syndrome
    • Bright UV patterns, landing platforms, radial or weak bilateral symmetry, sweet odour
  • Nocturnal moth/bat
    • White/cream, heavy scent or copious dilute nectar (e.g. Dactylanthus – NZ, pollinated by short-tailed bat)
  • Fly/carrion
    • Dark maroon/brown, foul smell (rotting flesh/faeces), thermogenic heat sometimes
  • Bird (e.g. honeyeaters, hummingbirds)
    • Tubular, red/orange, dilute nectar, little scent (birds poor olfactory)

Case Study – Avocado (Persea americana)

  • Protogynous dichogamy: each flower opens twice → Day 1 female, overnight closure, Day 2 male
  • Cultivar groups
    • Type A: ♀ morning / ♂ afternoon
    • Type B: ♀ afternoon / ♂ morning
  • Promotes outcrossing but climate can desynchronise phases → pollination failure, low fruit set in NZ

Seed Morphology & Germination Patterns

  • Eudicot, cotyledon-storage (bean)
    • Thick cotyledons store starch/lipids; endosperm consumed during embryo development
  • Eudicot, endosperm-storage (castor-bean)
    • Persistent 3n endosperm; thinner absorptive cotyledons
  • Monocot grain (maize, wheat)
    • Bulk of seed = endosperm; single cotyledon = scutellum (absorbs nutrients)
    • Protective sheaths: coleoptile (shoot) & coleorhiza (root)
  • Germination styles
    • Epigeal – hypocotyl hook lifts cotyledons above soil (bean)
    • Hypogeal – cotyledons stay below soil; plumule pierces via coleoptile (monocot)

Fruits & Dispersal Diversity

  • Pericarp (ovary wall) differentiates into exocarp, mesocarp, endocarp → numerous forms
    • Berry (kiwifruit): entire pericarp fleshy
    • Drupe (peach): stony endocarp encloses seed
    • Legume (kōwhai): dry dehiscent; floats for hydrochory
    • Samara (maple): winged for anemochory
    • Aggregate (raspberry) & multiple (pineapple) fruits from multiple ovaries/flowers
  • Dispersal agents & examples
    • Water: coconut, kōwhai pods down streams
    • Wind: dandelion pappus, maple samaras
    • External animal (epizoochory): hook-grass burrs cling to fur/clothes
    • Internal animal (endozoochory): tōtara & most NZ forest trees via birds; wētā documented as seed vectors
    • Ants (myrmecochory): elaiosome-bearing seeds (e.g. some Australian Acacia)

Quiz & Concept Checks (embedded during lecture)

  1. Pollen-grain staining experiment
    • Sporophytic exine outside, gametophytic cytoplasm inside → blue exterior/red interior (option C)
  2. Term equivalence: embryo sac ≡ female gametophyte
  3. Basic plant tissue systems: dermal, ground, vascular (students answered correctly)

Terminology To Master (non-exhaustive)

  • Floral parts: sepal, petal, stamen, anther, filament, carpel, stigma, style, ovary, ovule
  • Symmetry: actinomorphic, zygomorphic
  • Reproduction: microspore, megaspore, pollen grain, embryo sac, synergid, polar nuclei, double fertilisation, endosperm, plumule, radicle, cotyledon, testa, pericarp
  • Fruit types: berry, drupe, legume, samara, achene, aggregate vs multiple fruit

Ethical & Practical Implications

  • Co-evolutionary arms races (e.g. orchid–wasp deception) raise questions about exploitation vs mutualism in evolutionary ethics
  • Human-food reliance on endosperm underscores importance of genetic diversity & crop pollination services (bee decline, climate effects on avocado, etc.)
  • Conservation: specialised pollination systems (e.g. Dactylanthus & bats) make certain plants vulnerable when animal partners decline