Download Plant Reproduction Notes

TME 2010
Plant Reproduction Notes
Structure Of A Flower
The flower forms from a bud on the end of a pedicel or stem. Sepals cover the developing
flower to protect it. The flower has coloured petals to attract insects. Flowers have both 'male'
and 'female' reproductive parts. The male part (called the stamen) consists of a long filament with
the pollen making anthers on the top. The pollen must travel to the female part of another plant
of the same type. Insects or wind can do this. The female part (called the carpel) consists of
three parts. The top part called the stigma, is where pollen sticks. The pollen will grow down
through the style, until it reaches the ovary. Here it will be able to fertilize an ovule which will
develop into a seed.
Pollination
For sexual reproduction in plants to occur, the pollen from one plant must reach the stigma of
another plant of the same species. As plants cannot move like animals they must adopt a
different method of transferring the male sex cells. Wind pollinated plants let their pollen blow
in the wind and hope that some pollen reaches their destination. Insect pollinated plants use
insect to carry the pollen to other plants. They do this by bribing them with food - nectar. As
the insects go to get the nectar from a flower, they get covered in pollen which will rub off on
to a stigma when the insect visits another flower. The plants are adapted to their own particular
type of pollination.
Insect Pollinated Plants
Wind Pollinated Plants
Brightly coloured petals
Dull petals
Nectaries
No nectaries
Short Stamen
Long Stamen which dangle out of the flower
Scented
Not scented
Small stigma
Large feathery stigma
Large pollen grains
Small pollen grains
Sticky pollen grains
Non sticky pollen grains
Small amount of pollen grains produced Large amount of pollen grains produced
Plant Fertilisation
Pollination brings pollen grains to the stigma. There is a male sex cell in each pollen grain. The
sugar coating on the stigma, helps the pollen stick to it. If the conditions are right, the pollen
grain grows a tube. This pollen tube grows down through the style until it reaches the embryo
sac of the ovule in the ovary. The male nucleus from the pollen grain travels down this tube to
the ovule where it will be able to fuse with the egg nucleus located in the embryo sac. The
fertilized egg then divides by mitosis and grows into a seed which will become a new plant.
Once the egg has been fertilized the petals, sepals and stamen wither and fall off. They have
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TME 2010
done their job. The integuments surrounding the ovule (now called a seed) harden and dry to
become the seed covering. In some plants the ovary and receptacle grow into fruit to help with
seed dispersal.
The parent plant needs to disperse the seeds it makes for two reasons.
1. To make sure the species spreads as far as possible.
2. To prevent competition with its own offspring.
Seed Dispersal
There are different ways in which plants disperse their seeds.
1. The wind can carry some seeds like thistle and dandelion parachutes and sycamore
'helicopters' to blow the seeds away from the parent.
2. The receptacle at the base of the flower can develop into fruit which covers the seeds.
Animals which eat the fruit will also eat the seeds. These are protected from digestion
by a thick coating. The animal will eventually deposit the seeds when it defecates. Eg:
strawberry, apple, etc.
3. The seed pods can 'explode' open shooting the seeds away. Eg: Lupin
4. Sticky seeds can stick to the fur of animals. These will eventually fall off. Burrdock and
goose grass use this method.
5. Nuts are gathered by animals like squirrels for the winter. The seeds are hidden but
some will be left and forgotten by the animals and will eventually grow.
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