Plant Reproduction Flashcards
Advantages and disadvantages of sexual reproduction
Advantages
- Fusion of gametes combines genetic information from both parents, resulting in increased variety in offspring (better adapted to different conditions than parents)
- Greater chance of survival in changing conditions
Disadvantages
- Second parent required and finding a mate uses energy
- Takes longer to produce offspring
Advantages and disadvantages of asexual reproduction
Advantages
- One parent required, no need to find a mate or wait for pollination
- Large numbers of organism can be produced in a short period of time
- All offspring genetically identical, so should survive well in same conditions as parents
Disadvantages
- Lack of variation in offspring means that any change in environment reduces chance of survival and affects all individuals equally
- No variation so not suited to moving away and living in different conditions
What happens during sexual reproduction in flowering plants?
- Produce male and female gametes
- Male pollen transferred to female part of flower so that pollination can take place
- Male and female gamete fuse during fertilisation to form a zygote
- Zygote develops to form an embryo within a seed, which protects embryo and provides food during germination
- Seeds are dispersed so that they germinate away from parent
The male parts of a flower
Ring of stamens, consisting of anther and filament. Pollen grains contain the male gametes of the flowering plant, which develop in the pollen sacs of the anther. When all of the pollen grains in the anther are mature, the anther splits open to release them.
The female parts of a flower
The female part is called the carpel, with its own style and stigma. The stigma is where the pollen lands during pollination. The ovary at the base of the carpel contains one or more ovules and each ovule contains an egg sac that surrounds the egg cell.
How are wind pollinated plants adapted for this?
- Small petals that don’t get in the way when the wind blows the pollen.
- Many anther that are often large and hang outside the flower so that pollen is easily dispersed
- Stigmas are large and feathery often hanging outside the flower to trap pollen
- Produce large amounts of lightweight pollen
How are insect pollinated plants adapted?
- Large petals for insects to land on
- Brightly coloured petals to attract insects
- Often scented to attract insects
- Nectaries at the base of the flower produce a sugary liquid to attract insects
- Pollen grains have sticky or spiky outer walls
How does fertilisation happen?
- A pollen grain lands on top of the stigma, and if the egg cell is ready and the pollen grain is a suitable type, then the grain starts to grow a pollen tube. This grows down through the style and ovule wall.
- The tube grows towards the micropyle, a tiny hole in the layers that protect the egg sac.
- The male gamete passes into the egg cell and joins with it. This is fertilisation and produces a zygote.
Describe the structure of a seed
The ovule wall hardens as the zygote grows to form the seed coat. The ovary tissue surrounding the ovule often develops into a fruit.
Inside the seed there is the cotyledon, and the embryo including the plumule (shoot) and radicle (root)
Germination
Germination begins when the seed absorbs large amounts of water, this acts as a solvent for other chemicals and the right conditions for enzymes to start working. The presence of light is not needed for germination because seeds may be below ground and cannot get their food by photosynthesis. The energy they need comes from breaking down food reserves in the cotyledons. The roots start to grow first so that the plant can get water.
Conditions for germination experiment
Set up 4 test tubes, with each containing 10 cress seeds on cotton wool
Label the test tubes A, B, C and D
For test tube A, leave the cotton wool dry
For test tube B, add enough water to the cotton wool so that it becomes moist
For test tube C, add enough water to cover the cotton wool and seeds, then carefully add a layer of oil on top of the water
For test tube D, add enough water to the cotton wool so that it becomes moist
Leave tubes A, B and C at room temperature or incubated at a specific temperature (e.g. 20°C)
Place tube D in a fridge (approximately 4°C)
Leave all tubes for a set period of time (e.g. 3 - 5 days)
Ensure the cotton wool in tubes B and D remains moist throughout this time by adding more drops of water if required
Compare the results and see which tube has the greatest number of germinated seeds
The test tubes are set up so that each of the factors required for germination (water, oxygen and warmth) can be tested, by selectively removing each in turn
In test tube A, water is removed
Test tube B is the control tube, where all factors are present
In test tube C, oxygen is removed (oxygen cannot pass through the oil and water layers on top of the seeds)
In test tube D, warmth is removed
As germination cannot occur if the conditions are not right (i.e. if even one of the three factors required is absent), it would be expected that only the seeds in the control tube will germinate
How can plants reproduce asexually naturally?
Some plants produce runners which are stems that grow along the ground. Hen new leaves grow from the stems, roots can grow too. Until the new roots are well developed the plant gets water from the parent plant. Eventually the stem connecting the tow will die off and the two are genetically identical.
How can plants reproduce asexually artificially?
New plants can be grown from cuttings. Pieces of leaf, root or stem are cut from a healthy plant treated with plant hormones and planted in compost. The cuttings are kept moist and after a few weeks, if conditions are right then the cutting will have developed new roots and then new leaves and shoots. The new plant will be genetically identical to the parent plant.
