Microsporogenesis and the pollen grain (male gametophyte)
Pollen mother cells divide by meiosis into microspore tetrads. Each microspore becomes a pollen grain, the male gametophyte, with a tough sporopollenin exine.
Draw it: microsporogenesis
Meiosis halves the chromosome number, so every microspore and pollen grain is haploid.
Structure of a mature pollen grain
| Part | Description |
|---|---|
| Exine | Hard outer layer of sporopollenin. It resists high temperature and strong acids and alkalis, and no enzyme is known to break it down |
| Germ pores | Spots on the exine where sporopollenin is absent; the pollen tube comes out through one of them |
| Intine | Thin, continuous inner layer of cellulose and pectin |
| Vegetative cell | The bigger cell, with abundant food reserve and a large, irregular nucleus |
| Generative cell | Small, spindle-shaped cell with dense cytoplasm, floating in the vegetative cell; it gives rise to two male gametes |
2-celled vs 3-celled pollen at shedding
Pollen in everyday life
- Allergy: pollen of many plants causes asthma and bronchitis. Parthenium (carrot grass) came into India as a contaminant with imported wheat and is a common cause of pollen allergy.
- Nutrition: pollen is rich in nutrients. Pollen tablets and syrups are sold as food supplements.
- Viability: rice and wheat pollen lose viability within about 30 minutes of release. Some members of Rosaceae, Leguminosae and Solanaceae stay viable for months.
- Storage: pollen can be kept for years in liquid nitrogen (−196 °C) in pollen banks, for use in crop breeding.
Activity: temporary mount to observe pollen germination
This is Experiment 1 (slide preparation) of your practical.
- 1Prepare a sugar (sucrose) solution of about 10%: about 10 g of sugar in 100 mL of water. Your practical manual may add a pinch of boric acid.
- 2Put a drop of the solution in the centre of a clean slide, or in a cavity slide.
- 3Dust pollen from a freshly opened flower such as pea, chickpea, Crotalaria, balsam or Vinca onto the drop.
- 4Cover the slide (for example, in a Petri dish lined with moist filter paper) and wait 15 to 30 minutes.
- 5★Observe under the low power of a microscope. Record: pollen tubes growing out of many pollen grains, through the germ pores.
- 6Conclude: the sugar solution supplies food and moisture, like a receptive stigma, so viable pollen germinates.
★ key step
How to draw a mature pollen grain
- Draw a circle with a thick outer ring; label it exine.
- Leave two or three gaps in the outer ring and label one germ pore.
- Draw a thin inner ring just inside it; label it intine.
- Inside, draw a large irregular nucleus and label it vegetative cell nucleus.
- Draw a small spindle-shaped cell with its own nucleus; label it generative cell. Title: A mature pollen grain.
Many people in Indian cities sneeze and wheeze because of Parthenium (carrot grass) pollen. Plant breeders keep crop pollen in liquid nitrogen pollen banks so they can make crosses months later.
3 marks: “Describe microsporogenesis. Name the two cells of a mature pollen grain and give one feature of each.” Sporogenous cell acts as a pollen mother cell (2n); meiosis gives a microspore tetrad; as the anther dries the microspores separate and develop into pollen. Vegetative cell: large, food-rich, irregular nucleus. Generative cell: small, spindle-shaped, gives two male gametes.
2 marks: “Why are pollen grains well preserved as fossils?” The exine is made of sporopollenin, which withstands high temperatures, strong acids and alkalis; no enzyme is known to degrade it.
1 marks: “Name the materials that make up the intine.” Cellulose and pectin.
Exine is extra tough (sporopollenin); Intine is inside and soft (cellulose and pectin).