Angiosperms – Chapter 30 (Part 2) Comprehensive Notes

Evolutionary Context & Timing

• Angiosperms = “true seed plants in a fruit”
  • Last major lineage to evolve in terrestrial flora.
• First reliable fossil appearance: $160\text{–}140\text{ Ma}$ (Early Cretaceous).
• Current diversity: $\approx 250{,}000$ described species distributed among numerous families.
• Represent the culmination of land-plant adaptation: spores → vascular tissue → seeds → pollen → flowers & fruits (final innovations).

Defining Characteristics of Angiosperms

• Flower (specialized reproductive shoot; 4 modified leaf-whorls):
  • Sepals, petals, stamens (microsporophylls), carpels (megasporophylls).
• Fruit (mature ovary containing seeds) enhances dispersal.
• Dramatically reduced gametophytes housed on the sporophyte.
• Double fertilization → diploid zygote $2n$ + triploid endosperm $3n$.
• Sporophyte is the visually dominant life stage.

Flower Structure & Fruit Types

• 4 modified whorls = sepals, petals, stamens, carpels.
• Fruit categories (examples emphasised by lecturer):
  • Simple: 1 flower, 1 pistil (e.g.
  • Apples, pears – fleshy to attract animals).
  • Aggregate: 1 flower, many carpels (e.g. raspberries, blackberries).
  • Multiple: many flowers fuse (pineapple – mentioned tangentially).
  • Accessory: tissues other than ovary contribute (apple “core vs flesh”).
• Dispersal adaptations:
  • Fleshy & sweet → endozoochory.
  • Wings (maple samara) → anemochory.

Angiosperm Life History Overview

• Sporic life cycle (alternation of generations).
• Dominant phase: diploid sporophyte (the whole flowering plant you pick).
• Gametophytes tiny & nutritionally dependent.
  • Male = pollen grain.
  • Female = embryo sac inside ovule.
• Key ploidy events:
  1. Sporophyte $2n$ ➜ meiosis ➜ micro/mega-spores $n$.
  2. Spores ➜ mitosis ➜ gametophytes $n$.
  3. Fertilization (double): sperm $n$ + egg $n$ ⇒ zygote $2n$; sperm $n$ + polar nuclei $(n+n)$ ⇒ endosperm $3n$.

Detailed Male Gametogenesis (Micro-gametogenesis)

• Occurs in anther’s four microsporangia (pollen sacs).
• Sequence:
  1. Microsporocyte (microspore mother cell) $2n$.
  2. Meiosis ➜ tetrad of microspores $n$.
  3. Each microspore mitotically divides ➜ mature pollen grain (male gametophyte):
  • Tube cell nucleus (controls pollen-tube growth).
  • Generative cell → divides → two sperm $n$ each.
• Pollen wall rich in sporopollenin → desiccation resistance.

Detailed Female Gametogenesis (Mega-gametogenesis)

• Inside ovule (integuments + micropyle):
  1. Megasporocyte $2n$ inside nucellus (megasporangium).
  2. Meiosis ➜ 4 megaspores $n$; 3 degenerate, 1 functional.
  3. Functional megaspore undergoes 3 mitoses (no cytokinesis) ➜ 8 nuclei / 7-cell embryo sac:
  • 1 egg.
  • 2 synergids (guide pollen tube).
  • 3 antipodals.
  • 2 polar nuclei (central cell) – will form endosperm.

Pollination & Double Fertilization

• Pollination = transfer of pollen to stigma.
  • Passive (wind) or active (animal vectors).
  • Can self-fertilize (selfers) or cross (out-crossers); each strategy has pros/cons.
• Pollen hydration & germination on stigma → pollen tube through style & micropyle.
• Double fertilization steps:
  1. Sperm 1 + egg → zygote $2n$ (embryo).
  2. Sperm 2 + 2 polar nuclei → primary endosperm nucleus $3n$ (seed food).
• Coconut water cited as liquid endosperm example (nutritive tissue humans consume).

Embryogenesis & Seed Structure

• First zygotic mitosis ➜ basal cell + terminal cell.
  • Terminal ➜ embryo proper (cotyledons, shoot apex, root apex).
  • Basal ➜ suspensor (anchors embryo, pushes into endosperm).
• Seed components (mature):
  • Seed coat (from parent sporophyte integuments, $2n$ – “grandparental generation”).
  • Endosperm $3n$ (food reserve; sometimes consumed before maturity in some taxa).
  • Embryo $2n$ (new sporophyte).
• Seeds undergo dehydration & dormancy until environmental cue breaks dormancy.

Seed Types & Germination Patterns

• Lecture exemplars:
  1. Garden bean (Phaseolus) – eudicot, epigeous germination.
  • Two fleshy cotyledons store reserve; raised above soil.
  1. Castor bean (Ricinus) – eudicot, hypogeous.
  • Thin cotyledons act as placenta absorbing endosperm; remain belowground.
  1. Maize (Zea) – monocot, hypogeous.
  • Single cotyledon (scutellum) absorbs endosperm.
  • Protective sheaths: coleoptile (shoot), coleorhiza (root).
• Germination terminology:
  • Epigeous: cotyledons emerge above soil ("epi-" = upon).
  • Hypogeous: cotyledons stay below soil ("hypo-" = under).

Evolutionary History & Phylogeny

• Fossil + molecular clock: angiosperms diverged $\approx 180\text{ Ma}$.
• Probable gymnosperm–angiosperm transitional group: Bennettitales (extinct, cone-like bisexual structures).
• Competing hypotheses:
  • Bennettitalean sister relationship (cone-flower analogy).
  • Evo-Devo “Mostly Male” hypothesis (Melbourne NHM): ancestral plants had separate male & female cones; mutation in male added ovules → bisexual flower; supported by MADS-box gene homology.

Basal Angiosperm Groups

• Phylogeny (simplified):
  • Gymnosperms (outgroup).
  • Bennettitales († transitional).
  • Angiosperms sensu stricto.
  • Amborella (1 extant shrub, New Caledonia) – sister to all other angiosperms.
  • Nymphaeales (water lilies).
  • “Archaic grade” incl. Illicium, Austrobaileyales, etc.
• Collectively called basal angiosperms / archaeofructics.

Monocots vs Eudicots (True Dicots)

• Monocots
  • 1 cotyledon.
  • Parallel leaf venation.
  • Vascular bundles scattered.
  • Floral organs in 3’s.
• Eudicots
  • 2 cotyledons.
  • Net-like venation.
  • Vascular bundles in a ring.
  • Floral organs in 4’s or 5’s.
• These traits let you quickly ID field specimens.

Human Domestication & Economic Importance

• Plant domestication ≈ $13{,}000$ years old.
  • Modern crops (corn, rice, sweet potato) look radically different from wild progenitors due to artificial selection.
• Wood (secondary xylem) also an angiosperm product of high economic value.
• Understanding angiosperm life history aids crop breeding & biotechnology.

Derived Traits Recap (Seed-Plant Focus)

1. Reduced gametophytes.
2. Heterospory (micro vs mega spores).
3. Ovules.
4. Pollen.
5. Seeds (with endosperm in angiosperms) + flowers/fruits (unique additions).

Study & Practice Recommendations

• Re-draw life cycle diagrams; annotate ploidy at each stage.
• Memorize differences between micro- and mega-gametogenesis.
• Use real fruits/seeds to classify as simple vs aggregate etc.
• Self-test with concept-check & practice questions from text (Ch. 30; suppl. Ch. 38 for extra terminology).
• Connect genetic control (MADS genes) to morphological evolution for deeper synthesis.