Stamen → Microsporangium → Pollen Grain class 12 biology cbse

Here are the important NEET/CBSE points from this passage, arranged in a simple flow:

Stamen → Microsporangium → Pollen Grain

1. Stamen

Stamen = Filament + Anther

  • Filament → long, slender stalk
  • Anther → terminal, generally bilobed structure
  • Filament is attached to thalamus or petal.
  • Number and length of stamens vary among species.

2. Anther

A typical angiosperm anther is:

  • Bilobed
  • Each lobe has 2 theca
  • Therefore, anther is dithecous
  • Total 4 microsporangia → 2 in each lobe
  • In transverse section → tetragonal/four-sided
  • Microsporangia later develop into pollen sacs
  • Pollen sacs contain pollen grains

3. Wall layers of Microsporangium

From outside → inside:

Epidermis → Endothecium → Middle layers → Tapetum

LayerMain function
EpidermisProtection
EndotheciumProtection + helps in dehiscence
Middle layersProtection
TapetumNourishes developing pollen grains

⭐ Most important: Tapetum is the innermost wall layer.

The centre of a young microsporangium contains sporogenous tissue.

4. Microsporogenesis

Sporogenous tissue → Pollen Mother Cell (PMC) → Meiosis → Microspore tetrad → Pollen grains

Key points:

  • PMC is diploid (2n)
  • PMC undergoes meiosis
  • Produces 4 haploid (n) microspores
  • Four microspores remain together initially → microspore tetrad
  • Later they separate and develop into pollen grains

Definition ⭐

Microsporogenesis: Formation of microspores from a pollen mother cell through meiosis.

5. Pollen Grain

Pollen grain = Male gametophyte 🌱♂️

Generally:

  • Spherical
  • About 25–50 μm diameter
  • Has a two-layered wall

Pollen wall

Exine → outer wall

  • Made of sporopollenin
  • Extremely resistant
  • Resistant to high temperature, strong acids and alkalis
  • No known enzyme degrades sporopollenin
  • Has germ pores

Intine → inner wall

  • Thin and continuous
  • Made of cellulose + pectin

Germ pore

Aperture in exine where sporopollenin is absent.

Function: Allows emergence of the pollen tube during germination.

6. Mature Pollen Grain

A mature pollen grain may be:

2-celled stage

Vegetative cell + Generative cell

Vegetative cell

  • Larger
  • Abundant food reserve
  • Large, irregular nucleus

Generative cell

  • Smaller
  • Spindle-shaped
  • Dense cytoplasm
  • Has a nucleus

3-celled stage

Generative cell undergoes mitosis → 2 male gametes

So:

2-celled pollen = Vegetative cell + Generative cell

3-celled pollen = Vegetative cell + 2 male gametes

⭐ >60% angiosperms: pollen grains are shed at 2-celled stage.

🔥 Most important NEET facts

  1. Stamen = Filament + Anther
  2. Typical anther is bilobed and dithecous.
  3. Anther has 4 microsporangia.
  4. Microsporangia develop into pollen sacs.
  5. Tapetum → nutrition to developing pollen grains.
  6. PMC is 2n.
  7. Microspores are n.
  8. PMC undergoes meiosis.
  9. 4 microspores → microspore tetrad.
  10. Microsporogenesis = PMC → microspores by meiosis.
  11. Pollen grain = male gametophyte.
  12. Exine → sporopollenin.
  13. Intine → cellulose + pectin.
  14. Germ pore → sporopollenin absent.
  15. Generative cell → 2 male gametes by mitosis.
  16. Most angiosperms shed pollen at 2-celled stage.
  17. Rice and wheat pollen may lose viability within ~30 minutes.
  18. Some Rosaceae, Leguminosae and Solanaceae pollen can remain viable for months.
  19. Pollen can be stored in liquid nitrogen at −196°C → pollen banks.

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