Sexual Reproduction in the Flowering Plant
Sexual reproduction: production of a new individual from two parents.
Structure of the flower
- Receptacle: tissue from which all other parts originate and supports the flower once fully developed.
- Sepal: thick, green, leaf-like structures that protect the developing flower when it is in bud form.
- Petals: large and brightly coloured in animal-pollinated plants; small and usually green in wind-pollinated plants.
- Stamen: male organ consisting of two parts:
- 1. Anther: pollen formation
- 2. Filament: supports the anther in a position where pollen will be easily transferred
- Carpel: female organ consisting of three parts:
- 1. Stigma: pollen is trapped by the stigma
- 2. Style: supports the stigma in a position where pollen will have a good chance of landing
- 3. Ovary: where the eggs and ovules develop
- Gamete: haploid sex cell
- The male gamete is one of the sperm nuclei that develops from the generative nucleus (see below). The generative nucleus is carried by the pollen grain. There are two nuclei within each pollen grain:
1. Tube nucleus
2. Generative nucleus
- The female gamete is the egg cell that is a large cell present in the embryo sac of the ovule
Embryo Sac Development
- Within the ovary are a number of ovules.
- Each ovule is composed of two outer wall called integuments.
- The micropyle is a small opening in the integuments allowing the pollen tube to enter.
- The inner layer of each ovule has a layer called the nucellus – which nourishes the developing embryo sac.
- Within each ovule are a number of diploid cells – one of which develops further to become the megaspore mother cell.
- The megaspore mother cell divides by meiosis to produce 4 haploid cells.
- Three of these haploid cell die and one survives to become the embryo sac.
- The embryo sac (megaspore) grows and the haploid nucleus divides by mitosis to form 2 haploid nuclei.
- The two haploid nuclei undergo mitosis a second timeforming 4 haploid nuclei.
- A third and final round of mitosis occurs to produce 8 haploid nuclei.
- The 8 haploid nuclei take up the positions as shown.
- Cell membranes and a thin cell wall form around 6 of the haploid nuclei.
- The two remaining haploid nuclei remain free and are called polar nuclei.
- The egg cell is present at the bottom of the embryo sac.
Pollen Grain Development
- Anther has 4 chambers called pollen sacs.
- Pollen sacs are where the millions of pollen grains develop and mature.
- Each pollen sac has an outer fibrous layer (dermal tissue) that protects the pollen sacs.
- Inside the protective layer is the tapetum – which nourishes the developing pollen grains.
- On the innermost layer of the pollen sac is a layer of diploid cells (containing two sets of chromosomes) called microspore mother cells.
- Microspore mother cells divide by meiosis (process of halving the number of chromosomes present in a cell) to produce four immature, haploid cells (containing single set of chromosomes), called a tetrad.
- The immature, haploid pollen grains (microspores) then mature over time and develop a tough outer wall called an exine (which is unique to the plant species) and a softer inner wall called the intine.
- Mitosis of the haploid nucleus in each microspore also occurs during maturation – this produces a pollen grain with two haploid nuclei:
- Tube nucleus: burrows into stigma and style and forms the pollen tube.
- Generative nucleus: undergoes mitosis again as it moves down the pollen tube – this forms the sperm nuclei.
- Pollination is the transfer of pollen from anther to stigma of a flower of the same species
There are two types:
- Self-pollination: where a flower allows pollen to fertilise the egg cell within the ovary of the same plant – disadvantageous to species as resulting seeds less likely to form healthy plant
- Cross-pollination: where a flower transfers pollen from anther to stigma of different plant of same species – more advantageous as greater variation is shown
- Wind: pollen is produced in very large amounts by the flower and is usually small, light and smooth to allow easy transfer by wind, e.g., grasses
- Animal: pollen is produced in relatively small amounts grains are larger and stickier and they are usually transferred by insects (examples include dandelions, daisies, tulips, roses)
- Fertilisation: union of the male and female gametes to form a diploid zygote
- Once the pollen grain is trapped by the stigma, the pollen tube forms by action of the tube nucleus
- The generative nucleus enters the pollen tube and divides by mitosis to form two haploid nuclei called sperm nuclei
- The sperm nuclei enter the embryo sac and ‘double fertilisation’ occurs:
1. One fertilises the egg – diploid (2n) zygote results
2. Other fuses with the two polar nuclei to form triploid (3n) endosperm cell which goes on to divide by mitosis and absorbs nutrients and functions as a food store.
- An adaptation of angiosperms to life on dry land is pollen tube formation as no external water is required for fertilisation to occur
- The ovule develops into the seed
- Integuments become the testa (seed coat)
- The diploid zygote becomes the plant embryo
- The embryo develops further into the radicle, plumule, and cotyledon(s)
- Triploid endosperm nucleus divides repeatedly by mitosis to produce many cells that swell with food
Endospermic Seeds versus Non-Endospermic Seeds
- Endospermic seed: the plant embryo increases in size and only absorbs some of the endosperm, e.g. Corn
- Non-Endospermic seed: the plant embryo increases in size absorbing all of the endosperm in the process e.g. Broad bean
Monocot versus Dicot Seeds
- Tend to be endospermic (e.g. corn)
- One cotyledon
- When germinating, the food is obtained mainly from the endosperm
- Tend to have parallel venation in their leaves (e.g. grasses)
- Tend to be non-endospermic (e.g. Broad bean)
- Two cotyledons
- When germinating the food is obtained mainly from the cotyledons
- Tend to have net venation in their leaves (e.g. broad bean)
Fruits are formed from the ovary under the influence of auxins
Fruits can be classified into two groupings:
- True fruits – those that form from the ovary (e.g. oranges)
- False fruits – those that form from the receptacle (e.g. apples)
Advantages of fruit formation for flowering plants
- Fruits protect seeds
- Fruits attract animals to eat them
- Fruits help in the dispersal of seeds away from the parent plant
Dispersal is the transfer of the seeds away from the parent plant
Advantages of dispersal are:
- Avoid competition
- Increases chances of surviving winter
- Colonise new habitats
- Increase the number of the species
Seeds can be dispersed in one of four ways:
Seeds are generally very light and usually have some anatomical adaptation (hairs, wings) that enables them to be transported a long distance from parent plant, e.g. dandelions, sycamore
Seeds are usually enclosed within an air-filled fruit that is capable of floating, e.g. water lillies, coconuts
Seeds may be enclosed within a sticky fruit, e.g. burdock, goosegrass
Seeds may be enclosed by a fleshy fruit, e.g. strawberries, blackberries
Seeds are enclosed within a pod that explodes open when it becomes dry, e.g. pea pods
Dormancy is a resting period in which the seed undergoes no growth and has a very low metabolism
Advantages of dormancy include:
- Allows plant to avoid harsh conditions of winter
- Gives embryo time to fully develop
- Provides extra time for dispersal
- Seedless fruits
- Larger fruits
- Vegetable production
- Ethene as a ripening agent
- Dormancy of seeds in agriculture and horticulture
Seedless Fruits & Larger Fruits
Parthenocarpy is the process of growing fruit that do not have seeds
Parthenocarpy is carried out in two ways:
- Breeding of plants in such a way as to produce seedless fruit (pollination occurs but no fertilisation)
- Use of auxins – auxins are sprayed onto plant and stimulate fruit formation
Parthenocarpy is linked to production of larger fruits as auxins causes fruits to become much bigger than normal during development
Genetic engineering has also been used in producing larger fruit, e.g. tomatoes
Ethene as a Ripening Agent
Ethene is a hydrocarbon (C2H4) gas that causes fruit to ripen (turn from green to characteristic colour)
Germination is the regrowth of the embryo, following a period of dormancy, when the environmental conditions are suitable.
Factors necessary for germination:
- Suitable temperature
Digestion and Respiration in Germination
Digestion of food substrates is required during germination as food stores in the form of oils and starch need to be mobilised and converted to usable forms – like fatty acids and glycerol and glucose
Respiration is required to produce ATP as the embryo is growing and so anabolic reactions are occurring all the time (anabolic reactions require large amounts of ATP).
Dry mass refers to the mass of the germinating seed/seedling. It decreases in the first part of germination (due to nutrients being used up in respiration), but then increases after the seedling begins to photosynthesise.
Stages of Seedling Growth
There are two ways in which a seedling grows after germination:
- Cotyledons remain below the soil (hypogeal germination), e.g. broad bean
- The epicotyl (part of the embryo just above the cotyledons but below the plumule) grows rapidly leaving the cotyledons behind in the soil and pushing the plumule above the soil to photosynthesise.
- Cotyledons move above the soil (epigeal germination), e.g. sunflower
- The hypocotyl (part of the embryo just below the cotyledons but above the radicle) grows rapidly pushing the plumule and the cotyledons above the soil to photosynthesise.
Mandatory Experiment: Investigate Factors Affecting Germination
Equipment and Method:
- Set up 4 test tubes and treat them as described in the diagram below.
- There should be no growth in tubes B, C and D.
- There should be growth in tube A (control)
- Water, oxygen and a suitable temperature are required for germination and growth.
Mandatory Experiment: To Show Digestive Activity of a Germinating Seed
Equipment and Method:
- Set up apparatus as shown in the diagram below.
- Leave both dishes in an incubator at a suitable temperature for a few days.
- After a few days incubation, flood each dish with iodine.
- Allow iodine to sit for a couple of minutes and then pour off the excess.
- The area under the seeds in the control dish should turn blue black
- The area under the seeds in the test dish should remain red-yellow colour
- Germinating seeds digest starch
- Boiled seeds do not digest starch due to the digestive enzymes having been denatured.
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