The Flower, the Stamen and the Male Gametophyte: Quick Revision

A brief note on The Flower, the Stamen and the Male Gametophyte. Read it before a test. Try each question at the end before you open its answer.

The flower and the anther

• Whorls from outside: calyx, corolla, androecium (male), gynoecium (female).
• Stamen: a filament and an anther. A typical anther is bilobed, dithecous and four-sided, with four microsporangia, two in each lobe.
• Wall of a microsporangium, outside to inside: epidermis, endothecium, middle layers, tapetum.
• The three outer layers protect the sac and help the anther to split open. The tapetum is the innermost layer, with dense cytoplasm and often more than one nucleus, and it feeds the pollen.
• Sporogenous tissue: the compact centre of a young sac. Its cells can become pollen mother cells.

Making pollen

• Pollen mother cell (PMC): $2n$. It divides by meiosis into a tetrad of four microspores, $n$ each. This is microsporogenesis.
• The tetrad separates as the anther dries. Each microspore becomes a pollen grain.
• The microspore divides by mitosis into a big vegetative cell and a small generative cell.
• The generative cell divides by mitosis into two male gametes. In over 60 per cent of flowering plants, pollen is shed before this, at the two-celled stage. The division happens later in the pollen tube.
• Mitosis keeps the number: every nucleus from the microspore onwards is $n$.

The pollen grain

• Size: about 25 to 50 micrometres. It is the male gametophyte.
• Exine: outer, hard, sporopollenin, with germ pores. It is preserved in fossils.
• Intine: inner, thin, cellulose and pectin.
• Vegetative cell: big, food reserve, large irregular nucleus. Generative cell: small, spindle shaped, dense cytoplasm.

A worked example

A plant has 16 chromosomes in each PMC. How many pollen grains do 25 PMCs make, and how many chromosomes has each male gamete?

1. Each PMC makes 4 grains, so $4 \times 25 = 100$ grains.
2. Each male gamete is haploid, with $16 \div 2 = 8$ chromosomes.

Practice with new numbers

A program to try

The program counts the pollen grains and the male gametes made by a number of pollen mother cells.

Mistakes to avoid

• Saying that an anther has two microsporangia. A typical anther has four.
• Putting the tapetum on the outside. It is the innermost wall layer.
• Calling the microspore diploid. Meiosis made it haploid.
• Mixing up the walls. The exine is outside and is sporopollenin. The intine is inside and is cellulose and pectin.

Check yourself

1. How many microspores does one PMC make?

2. Which wall layer feeds the pollen?

3. Which wall of the pollen grain has germ pores?

4. What does the generative cell form?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 1.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 32, Plant Reproduction.