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Reproduction in flowering plants

Reproduction is the process organism (parents) generate new individuals (offspring). There are two distinct types of reproduction:

Asexual Sexual
Requires a single parent

No sex cells

Requires two parents

Two sex cells

Offspring are genetically identical Offspring are genetically different
No fertilisation required Fertilisation fuses the two sex cells (gametes) to form a zygote cell.

Bacteria and most single-celled organisms reproduce asexually. They do not have sexes.

Most animals and plants reproduce sexually. Sexually reproducing species have different sexes (i.e. males and females).

The flower is the reproductive organ of flowering plants.

Flowering plants can reproduce sexually. The flower is a complex structure that contains both male and female sexual structures.

The outside of the flower contains two structures:

  • Sepals : small, modified leaves
  • Petals : large, often brightly coloured modified leaves

Many plants use petals to attract insects.

The inside of the flower contains the sexual structures:

  • Stamen : male sex structure
  • Carpel : female sex structure
The sepals and petals surround the sexual structures of the flower.
The sepals and petals surround the sexual structures of the flower.

The sexual structures of flowers produce the plant's sex cells.

Stamen:

The plant's male sex cells are contained within pollen grains. The pollen grain keeps the male gametes safe while they are delivered to the ovaries. Pollen is produced by a structure at the top of the stamen called the anther.

Wind pollinated plants have very large anthers. This allows them to produce a massive amount of pollen.

The stamen has a long filament that holds the anther up.

Carpel:

The plant's female sex cells are inside ovules. Ovules are housed within the ovary of the carpel.

When some plants are fertilised, the ovary grows to form a fruit. The ovules form the seeds inside the fruit.

The tip of the carpel is called the stigma. The stigma is sticky so that it can catch pollen grains for pollination.

Flowering plants normally have both male and female sexual structures in the flower.
Flowering plants normally have both male and female sexual structures in the flower.

Pollination is the transfer of pollen grains from the anthers of a flower to a stigma.

For plants to sexually reproduce, the nucleus of a pollen grain must fuse with the nucleus of an ovary cell. The first challenge is getting the pollen grain onto the stigma.

The pollen can come from two sources:

  • Self-pollination occurs when a pollen grain pollinates the carpel of the same individual.

    Self-pollination occurs only in flowering plants that have both male and female sex organs.

  • Cross-pollination occurs when a pollen grain pollinates the carpel of a different individual.

    Cross-pollination creates more genetic variation. This makes offspring less vulnerable to pathogens (bacteria, fungi and viruses).

The sexuality of plants can be very confusing! Plants can have a single sex or have both male and female sex organs. In this case, the male and female structures may be on different flowers or on every flower.

Flowering plants can either self-pollinate or cross-pollinate.
Flowering plants can either self-pollinate or cross-pollinate.

Cross-pollination requires a mechanism for transporting pollen from one plant to another. There are two common ways plants do this:

  • Insect pollination uses insects to transfer pollen grains. Most flowering plants are insect pollinated.
  • Wind pollination uses the wind to blow pollen grains between plants.
Insect pollinated Wind pollinated
Brightly coloured, scented flowers to attract insects Small, unscented flowers
Produce a sticky sweet substance called nectar to attract insects Do not produce nectar
Produce moderate amounts of pollen Produce vast amounts of pollen to increase chances of pollination
Stamen and carpel inside the flower Stamen and carpel hang outside the flower to be exposed to wind

Insect pollinated plants are just one type of a larger group of animal pollinated plants. Some plants even rely on bats for cross-pollination!

In order for a pollen grain to fertilise a flower, the gamete cell nucleus needs to be transported from the top of the carpel to the ovaries.

The nucleus travels in a specialised structure called the pollen tube. The pollen tube is produced by the pollen grain when it lands on a carpel.

Important! The pollen grain and pollen tube are NOT the male gamete (sex cell) in flowering plants. The gamete nucleus is inside the pollen tube.

Once the pollen tube reaches an ovule within the ovary, the male nucleus is injected into the ovule. The two haploid gamete nuclei fuse and fertilisation is complete.

Once fertilised, the flower undergoes a dramatic transformation:

  • The fertilised ovule grows to form a seed.
  • The carpel forms a fruit.
  • The petals, sepals and stamen all wither away.
The pollen tube must bury its way through carpel tissue to reach the ovaries.
The pollen tube must bury its way through carpel tissue to reach the ovaries.