BIO4: SEXUAL REPRODUCTION IN PLANTS.

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.

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:

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:

1. 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.
2. 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.
3. 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.
4. 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.
5. The water content of the seed decreases drastically so the seed is prepared for dormancy.
6. 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.

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.

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