Sexual reproduction in flowering plants centres around the flower. Within a flower, there are usually structures that produce both male gametes and female gametes.
Inside the ovary there may develop one or more ovules. Each ovule begins life as a small projection into the cavity of the ovary. As it grows and develops it begins to bend but remains attached to the ovary wall by a placenta.
At the start, the ovule is a group of similar cells called the nucellus. As it develops, the mass of cells differentiates to form an inner and an outer integument, surrounding and protecting the nucellus within, but leaving a small opening called the micropyle.
At the centre of the ovule is an embryo sac containing the haploid egg cell (the female gamete).
Each anther contains 4 pollen sacs. Many pollen grains develop inside each pollen sac. It begins with a mass of large pollen mother cells in each pollen sac. All are diploid.
In each pollen grain the wall thickens and forms an inner layer (the intine) and an often highly sculptured outer layer (the exine). The surface pattern is different on pollen grains from different species. When the pollen grains are mature, the anther dries out and splits open (a process called dehiscence) and the pollen is released.
Many plants favour cross-pollination, so pollen must be transferred to the stigma of another plant if sexual reproduction is to take place. Some flowers rely of the wind to carry pollen grains others rely on insects.
Self-pollination is where the pollen is transferred to the stigmas of the same flower or the stigma of another flower on the same plant. Self-pollination is obviously more reliable, particularly if the nearest plant is not very close.
A potential drawback is that both gametes come from the same parent. If the plant is well adapted to a stable environment, the production of uniform offspring may be advantageous. However, inbreeding will result and if there are disadvantageous recessive characteristics in the parent, they are much more likely to be exposed than if the plant cross-pollinates.
Cross-pollination is less reliable and more wasteful than self-pollination, but it is genetically favourable because genes are transferred and variation increases.
Strategies to favour cross pollination:
- Dioecious plants: Some plants have flowers that are only male - they have only stamen. Other plants of the same species have flowers that are only female - they have only carpels.
- Monoecious plants: Some flowers on a plant are only male; other flowers on the same plant are only female. So, self pollination is avoided by a difference in the timing of their development.
- Protandry: Anthers on some plants mature first. Pollination of immature stigma on the same plant is therefore not possible.
- Protogyny: The stigmas mature first.
- Self-incompatibility: Pollination can occur but the pollen tube doesn't grow well, if at all, so no fertilisation takes place.
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