Biol120 - Lecture 15: Angiosperms

Evolutionary History & Diversity

  • Plants originated ~470\ \text{mya}; seed plants ~360\ \text{mya}; angiosperms later dominate.
  • Angiosperms = flowering plants; sister group to gymnosperms.
  • Adaptive innovations: flowers, fruits, double fertilisation, enclosed seeds.

Shared Seed-Plant Traits

  • Reduced n gametophytes retained on 2n sporophyte.
  • Heterospory: microspores → male, megaspores → female.
  • Ovules, pollen, and seeds resist desiccation & enable terrestrial life.

Major Angiosperm Clades

  • Basal angiosperms: Amborella, water lilies, star anise relatives, magnoliids.
  • Monocots (~80{,}000 spp.): grasses, palms, orchids, lilies.
  • Eudicots (~260{,}000 spp.): roses, legumes, eucalypts, most trees.

Monocots vs Eudicots

  • Embryo: 1 vs 2 cotyledons.
  • Leaf venation: parallel vs netlike.
  • Vascular tissue: scattered vs ring.
  • Roots: fibrous vs taproot.
  • Pollen: 1 vs 3 openings.
  • Floral parts: multiples of 3 vs 4/5.

Flower Morphology

  • Female carpel: stigma + style + ovary (with ovules).
  • Male stamen: filament + anther (with microsporangia).
  • Flower structure underlies later fruit form (simple, aggregate, multiple, accessory).

Development of Gametophytes

• Female (inside ovule)

  • Megasporocyte (2n) → meiosis → 4 megaspores; 1 survives.
  • Surviving megaspore → 3 mitoses → embryo sac (n) with 7 cells / 8 nuclei: 3 antipodals, 2 polar nuclei, 1 egg, 2 synergids.
    • Male (inside anther)
  • Microsporocyte (2n) → meiosis → 4 microspores.
  • Each microspore → pollen grain with 2 n cells: tube cell + generative cell (later 2 sperm).

Pollination & Double Fertilisation

  • Pollen lands on stigma → tube cell forms pollen tube through style.
  • Generative cell divides → 2 sperm travel down tube.
  • Double fertilisation:
    1. Sperm + egg → zygote 2n.
    2. Sperm + polar nuclei → endosperm 3n (nutritive tissue).
  • Result: seed = embryo 2n + endosperm 3n + seed coat (parent 2n).

Fruit Development

  • Ovary matures into fruit; type depends on floral structure:
    • Simple (single carpel) e.g., pea.
    • Aggregate (multiple separate carpels) e.g., raspberry.
    • Multiple (inflorescence) e.g., pineapple.
    • Accessory (non-ovary tissue) e.g., apple.

Key Points for Review

  • Recognise basal angiosperms vs monocot/eudicot lineages.
  • Distinguish monocot & eudicot anatomical traits.
  • Name flower organs and their roles.
  • Trace female & male gametophyte formation.
  • Sequence: pollination → pollen tube → double fertilisation → seed/fruit.
  • Understand how flower form influences fruit morphology.