Sexual Reproduction in Flowering Plants

The male gametophyte develops inside the microsporangium of the anther (part of the stamen), where pollen grains are formed. The female gametophyte develops inside the ovule (megasporangium) present in the ovary.

Microsporogenesis is the formation of microspores from the pollen mother cell (PMC) inside the anther. Megasporogenesis is the formation of megaspores from the megaspore mother cell (MMC) inside the ovule.

In both cases, the cell division is meiosis.

At the end of microsporogenesis, a microspore tetrad is formed, and each microspore later develops into a pollen grain.

At the end of megasporogenesis, four megaspores are formed, out of which usually one functional megaspore develops into the embryo sac (female gametophyte).

The developmental sequence is:

1. Sporogenous tissue
2. Pollen mother cell (PMC)
3. Microspore tetrad
4. Pollen grain
5. Male gametes

So the correct order is sporogenous tissue → pollen mother cell → microspore tetrad → pollen grain → male gametes.

Monosporic development means that the embryo sac develops from only one functional megaspore out of the four formed after meiosis. The other three megaspores degenerate.

Chasmogamous flowers are flowers that open at maturity and expose the anthers and stigma.

Cross-pollination cannot occur in cleistogamous flowers because these flowers do not open at all. Their anthers and stigma lie very close to each other, and pollen released inside the closed flower lands on the stigma of the same flower. So cleistogamous flowers are invariably autogamous and there is no chance for pollen from another flower to reach the stigma.

Two strategies mentioned in the chapter to prevent self-pollination are:

- Self-incompatibility
- Production of unisexual flowers

These help discourage self-pollination and promote cross-pollination.

Self-incompatibility is a genetic mechanism that prevents self-pollen from fertilising the ovules. It may stop pollen germination on the stigma or pollen tube growth in the style.

Self-pollination does not lead to seed formation in self-incompatible species because the pistil rejects its own pollen. Since the pollen tube cannot grow and reach the ovule, fertilisation does not occur, and therefore seeds are not formed.