Reproduction in Plants – Condensed Notes

Mitosis vs. Meiosis

• Mitosis: produces genetically identical daughter cells; maintains chromosome number (ploidy unchanged).
• Meiosis: two sequential divisions → four non-identical gametes; chromosome number halved (from diploid $2n$ to haploid $n$).

Alternation of Generations (General Plant Life Cycle)

• Two independent, multicellular phases:
  • Gametophyte (haploid $n$): forms gametes by mitosis.
  • Sporophyte (diploid $2n$): forms spores by meiosis.
• Fertilization $n + n \rightarrow 2n$ links gametophyte → sporophyte.
• Meiosis in sporophyte $2n \rightarrow n$ regenerates haploid phase.

Key Reproductive Structures

• Sporangium: sporophyte organ; sporocytes undergo meiosis → spores.
• Gametangia:
  • Archegonium (female): produces egg; fertilization site.
  • Antheridium (male): produces & releases sperm.
• Ovule (seed plants): integument-wrapped megasporangium + megaspore.
• Pollen grain: microgametophyte; delivers sperm via pollen tube.
• Flower parts (angiosperms):
  • Carpel = stigma + style + ovary (contains ovules).
  • Stamen = anther (microsporangia) + filament.
  • Sepal & petal = sterile whorls.

Life-Cycle Highlights by Major Groups

Bryophytes (mosses)

• Gametophyte dominant, photosynthetic, long-lived.
• Sporophyte small and nutritionally dependent.

Seedless Vascular Plants (ferns)

• Sporophyte dominant, free-living and photosynthetic.
• Gametophyte tiny, independent, on/under soil surface.

Gymnosperms (e.g., pines)

• Sporophyte = familiar tree; cones house gametophytes.
  • Pollen cones → microspores → pollen.
  • Ovulate cones → megaspores → female gametophytes inside ovules.
• Key features: sporophyte dominance, pollen transfer, seeds from fertilized ovules.

Angiosperms (flowering plants)

• Flowers combine both sex organs on sporophyte.
• Male gametophyte = pollen in anthers; female gametophyte (embryo sac) within ovule.
• Double fertilization:
  • Sperm $1$ + egg → zygote (embryo).
  • Sperm $2$ + two polar nuclei → triploid endosperm (food source).
• Mature seed: embryo (root + cotyledons) + endosperm + seed coat; often enclosed within fruit.

Evolutionary Trends in Plant Reproduction

• Shift from water-dependent sperm (bryophytes, ferns) → air-borne pollen (seed plants).
• Reduction of gametophyte size and independence over time.
• Development of protective, nutrient-rich seeds and later, fruits for enhanced dispersal.