Sexual Reproduction in Flowering Plants

Chapter overview

Flowering plants reproduce sexually through flowers. The male reproductive part is the stamen and the female reproductive part is the pistil/carpel. Reproduction includes gamete formation, pollination, fertilisation, embryo development, seed formation, and fruit formation.

1. Structure of a flower

Main parts:

2. Stamen and microsporangium

Stamen consists of:

• filament,
• anther.

Anther contains microsporangia.

Each microsporangium has:

• epidermis,
• endothecium,
• middle layers,
• tapetum,
• sporogenous tissue.

3. Microsporogenesis

Microsporogenesis is formation of microspores from microspore mother cells.

$$\text{Microspore mother cell} \xrightarrow{\text{meiosis}} \text{microspore tetrad}$$

Flow:

4. Pollen grain

Pollen grain is the male gametophyte.

It has:

• exine,
• intine,
• vegetative cell,
• generative cell.

Generative cell divides to form two male gametes.

$$\text{Generative cell} \rightarrow 2\ \text{male gametes}$$

5. Pistil and ovule

Pistil has:

• stigma,
• style,
• ovary.

Ovule has:

• funicle,
• hilum,
• integuments,
• micropyle,
• nucellus,
• embryo sac.

6. Megasporogenesis

Megasporogenesis is formation of megaspores from megaspore mother cell.

$$\text{Megaspore mother cell} \xrightarrow{\text{meiosis}} 4\ \text{megaspores}$$

Usually, one megaspore is functional and three degenerate.

7. Embryo sac development

Functional megaspore undergoes three mitotic divisions.

$$1\rightarrow2\rightarrow4\rightarrow8\ \text{nuclei}$$

Mature embryo sac is:

$$7\text{-celled},\ 8\text{-nucleate}$$

Arrangement:

8. Pollination

Pollination is transfer of pollen grains from anther to stigma.

Types:

Agents:

• wind,
• water,
• insects,
• birds,
• bats.

9. Outbreeding devices

Plants avoid self-pollination by:

• dichogamy,
• herkogamy,
• self-incompatibility,
• dioecy,
• production of unisexual flowers.

10. Pollen-pistil interaction

Pollen-pistil interaction determines whether pollen is compatible.

Flow:

11. Double fertilisation

Double fertilisation is a unique feature of angiosperms.

First fusion:

$$\text{male gamete} + \text{egg} \rightarrow \text{zygote}$$

This is syngamy.

Second fusion:

$$\text{male gamete} + \text{secondary nucleus} \rightarrow \text{primary endosperm nucleus}$$

This is triple fusion.

Together:

$$\text{Syngamy} + \text{Triple fusion} = \text{Double fertilisation}$$

12. Post-fertilisation events

After fertilisation:

13. Embryo development

Dicot embryo has:

• embryonal axis,
• two cotyledons,
• plumule,
• radicle.

Monocot embryo has:

• one cotyledon,
• scutellum,
• coleoptile,
• coleorhiza.

14. Apomixis and polyembryony

Apomixis is seed formation without fertilisation.

Polyembryony means occurrence of more than one embryo in a seed.

15. Solved/example questions

Example 1

Why is embryo sac called 7-celled and 8-nucleate?

Because it contains:

$$3\ \text{antipodals} + 2\ \text{synergids} + 1\ \text{egg} + 1\ \text{central cell} = 7\ \text{cells}$$

But central cell has two polar nuclei, so total nuclei:

$$3+2+1+2=8$$

Example 2

What is double fertilisation?

It is the occurrence of syngamy and triple fusion in the same embryo sac.

16. Common mistakes

• Confusing microsporogenesis and megasporogenesis.
• Forgetting pollen grain is male gametophyte.
• Forgetting embryo sac is female gametophyte.
• Writing embryo sac as 8-celled instead of 7-celled, 8-nucleate.
• Confusing pollination with fertilisation.
• Forgetting endosperm forms from primary endosperm nucleus.

17. Quick revision