The Female Gametophyte, Pollination and Pollen-Pistil Interaction: Quick Revision

A brief note on The Female Gametophyte, Pollination and Pollen-Pistil Interaction. Read it before a test. Try each question at the end before you open its answer.

The pistil, the ovule and the embryo sac

• Pistil: stigma, style and ovary. The ovules are on the placenta, inside the locule.
• Ovule (megasporangium): funicle, hilum, integuments, micropyle, chalaza, nucellus and embryo sac.
• Megasporogenesis: the MMC ($2n$) divides by meiosis into four megaspores ($n$). One is functional, and three degenerate.
• Embryo sac: the functional megaspore divides by mitosis three times, free nuclear, to give 8 nuclei. Seven divisions of single nuclei make 8 nuclei.
• Mature embryo sac: 7 cells and 8 nuclei. Egg apparatus (egg and 2 synergids) at the micropylar end. 3 antipodals at the chalazal end. Central cell with 2 polar nuclei.
• The filiform apparatus on the synergids guides the pollen tube.

Pollination

• Autogamy: same flower. Geitonogamy: another flower of the same plant. Xenogamy: another plant.
• Only xenogamy brings genetically different pollen. Geitonogamy is genetically like autogamy.
• Cleistogamous flowers (Viola, Oxalis, Commelina) never open and set seed without pollinators.
• Wind: light pollen, feathery stigma, one ovule, a dense inflorescence. Water: about 30 genera, for example Vallisneria and Zostera. Animals: large, scented, coloured flowers with nectar and sticky pollen.

Outbreeding and the pollen tube

• Devices: different timing, different positions, self-incompatibility and unisexual flowers.
• Castor and maize are monoecious (no autogamy). Papaya is dioecious (no autogamy and no geitonogamy).
• Path of the tube: stigma, style, ovary, micropyle, synergid (through the filiform apparatus).

A worked example

A plant has 12 chromosomes in the cells of its root tip. How many chromosomes has the central cell of its embryo sac?

1. Each polar nucleus is haploid, with $12 \div 2 = 6$ chromosomes.
2. The central cell has two polar nuclei, so it has $2 \times 6 = 12$ chromosomes.

Practice with new numbers

A program to try

The program counts the cells and the nuclei of a typical embryo sac.

Mistakes to avoid

• Saying that the embryo sac has eight cells. It has seven cells and eight nuclei.
• Saying that monoecious plants prevent geitonogamy. They prevent only autogamy.
• Saying that cleistogamous flowers are cross-pollinated. They never open.
• Saying that the tube enters the egg directly. It enters a synergid.

Check yourself

1. Which end of the embryo sac has the egg apparatus?

2. How many megaspores of a tetrad are functional in most plants?

3. Which kind of pollination brings genetically different pollen?

4. Through which structure does the pollen tube enter the ovule?

Sources

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