4.2 Flashcards
What is pollination
The transfer of pollen grains from the anther to mature stigma of a plant of the same species
Why is pollination necessary
So that pollen grains containing male gametes are brought into contact with the female part of the flower so fertilisation can occur.
How does self pollination happen
Pollen from the anthers is transferred to the stigma of the same flower, or another flower on the same plant
Negatives of self pollination
No genetic diversity so likely to go extinct if change in environment
How does cross pollination occur
When pollen is transferred from the anthers of one flower to the stigma of another flower on a different plant of the same species
How are flowers highly adapted for cross pollination
By either insects or wind
How does insect pollination occur?
Bees feed on sugary nectar. As bee enters flower the anthers brush against the back of it, leaving sticky pollen. When bee enters another flower it brushes some of the pollen against the ripe stigma
Why are anthers outside the flower in wind pollinated flowers
Allows the wind to blow away the small, smooth and light pollen
Why do the feathery stigmas hang outside the flowers of wind pollinated flowers
Provide a large S.A for catching pollen grains that are blown into their path
How is variation achieved in self pollinated plants
By mutation and crossing over during meiosis
How is variation achieved in cross pollination
Fusion of gametes from different plants if the same species
Advantages of self pollination
Preserve good genomes suited to a stable environment
Advantages of cross pollination
Greater evolutionary significance as some genomes will be more successful than others
Development of male gamete
- in pollen sac of anther, diploid pollen mother cells undergo meiosis and each firms a tetrad containing 4 haploid cells which become 4 pollen grains.
- tapetum provides nutrients and regulatory molecules to the developing pollen grains and has a role in forming pollen cell wall which allows pollen grains to he transferred from one flower to another.
- in pollen grain the haploid nucleus undergoes mitosis to produce two nuclei - a generative nucleus and tube nucleus. Generative nucleus produces two male nuclei by mitosis.
- when pollen mature, outer layers of anther dry causing tension in lateral grooves and dehiscence occurs which pulls anther walls apart and an opening called stomium exposes pollen grains; they’re carried away by insects or wind
Development of female gamete
- ovary contains one or more ovules and in each ovule, a megaspore mother cell undergoes meiosis, making 4 haploid cells.
- 3 cells disintegrate.
- the remaining cell undergoes 3 rounds of mitosis to produce 8 haploid nuclei and one of these us the oosphere.
- 2 of the haploid nuclei fuse to make a diploid nucleus called polar nucleus.
- these nuclei are in the embryo sac surrounded by the nucellus. Around nucellus are integuments and between them is a micropyle
How do you ensure cross pollination
- dichogamy: stamen and stigma ripening at different times.
- anther is below the stigma so pollen can’t fall on to it.
- separate make and female flowers on the same plant
What is fertilization
Process in which a male gamete fuses with a female gamete to produce a zygote
Process of double fertilization
- when a compatible pollen grain lands on the stigma it germinates in sucrose solution secreted by stigma & produces pollen tube.
- the pollen tube grows out of the pollen grain through a gap in the cell wall and down the style.
- pollen tube nucleus codes for the production of hydrolases and digests its way through the tissues of the style.
- pollen tube grows through the micropyle and passes into embryo sac and the tube’s nucleus disintegrates.
- tip of pollen tube opens, releasing the 2 male gametes in the embryo sac
- one of the male gametes fuses with the female gamete to form a zygote and the other fuses with the diploid polar nucleus to form a triploid nucleus which generates endosperm tissue when divided by mitosis which provides nutrition for embryo.
Why is it called double fertilization
Two fusions have occurred. One to form zygote and one to form endosperm
