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Sexual Reproduction in Flowering Plants
Most exam boards only require knowledge about reproduction in Angiosperms - the flowering plants.
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.
For those plants that cross-pollinate, some are wind pollinated, others are insect pollinated. Here are some of the differences:
|Feature:||Wind pollinated flowers:||Insect pollinated flowers:|
|Petals:||Small inconspicous, sometimes absent. If present, not brightly coloured.||Large, brightly coloured, conspicous and attractive to insects.|
|Pollen:||Produced in large quantities, light, smooth pollen grains.||Less produced, pollen grains larger, sculptured walls to ald attachment to insects and to stigma.|
|Anthers:||Move freely, so pollen is easily dispersed.||Fixed to filaments and positioned to come into contact with visiting insects.|
|Stigma:||Large often branched and feathery, hanging outside the flower to trap pollen.||Small inclosed within the flower, positioned to come into contact with visiting insects.|
If the pollen grain lands on a compatible stigma, a pollen tube will grow so that eventually the egg cell, hidden away in the embryo sac, can be fertilised. A tube emerges from the grain, its growth being controlled by the tube nucleus at the tip of the tube. It may grow downwards in response to chemicals made by the ovary (a response known as chemotropism).
During the growth and extension of the tube, the generative nucleus, behind the tube nucleus, divides by mitosis to produce 2 male haploid gametes. The pollen tube enters the ovule through the micropyle and penetrates the embryo sac wall. The tip of the tube bursts open, the tube nucleus dies and what follows is called double fertilisation.
1 male gamete fuses with the egg cell to produce a diploid zygote.
1 male gamete fuses with both the polar nuclei to produce the triploid primary endosperm nucleus.
Immediately after fertilisation, the ovule is known as the seed.
The following happens:
- The zygote divides many times by mitosis to produce an embryo. It differentiates to become a plumule (young shoot), radicle (young root) and either 1 or 2 cotyledons (seed leaves). It is attached to the wall of the embryo sac by a suspensor.
- The primary endosperm nucleus divides many times by mitosis to produce endosperm tissue. In some seeds this endosperm is a food store for later use by the seed. In others it may gradually disappear as the cotyledons develop.
- To accommodate all this growth the embryo sac expands and the nucellus is crushed out of existence, giving its nutrients to the embryo and endosperm.
- The integuments surrounding the embryo sac become the tough and protective testa (seed coat). The micropyle remains though so that oxygen and water can be taken in at germination.
- The water content of the seed decreases drastically so the seed is prepared for dormancy.
- The ovary wall becomes the pericarp - the fruit wall, the whole ovary now being the fruit. The function of the fruit is to protect the seeds and to aid in their dispersal, e.g. by an animal. That is why they can be brightly coloured and sweet; animals will eat them and scatter the seeds either at the time of eating or when they are passed out of the gut in defecation, unharmed.
When conditions are right, the seed will take up water through the micropyle by imbibition. This triggers the beginning of the growth of the seed.
The cell swells and the testa splits. With the addition of water, large molecules of carbohydrate, protein and fat can be hydrolysed (broken down) to produce substances for respiration.
The water activates such enzymes as a-amylase to catalyse this digestion.
The growing embryo releases a hormone called gibberellic acid and some enzymes are produced and released in response to this.
The soluble products of digestion are delivered to the cotyledons, root and shoot. They respire aerobically and grow in size.
By the time the food store has been used up, the shoot has grown enough to push the first leaves into the sunlight. Photosynthesis can then start.