Sexual Reproduction in Flowering Plants Sexual Reproduction
Sexual reproduction Anther Gametes
Stamen Carpel
Stigma Keywords Pollen grain
Pollination Fertilisation
Seed formation Seed dispersal
Germination Aims
• State the structure & function of the floral parts including: Sepal, petal, stamen, carpel)
• Outline pollen grain development from microspore mother cells
• Outline embryonic sac development
What is meant by sexual reproduction? • It involves the fusion of a male and female gamete to form a zygote.
• Gametes are haploid cells.
• Sexual reproduction allows for variation and is more advantageous to a species than asexual reporduction. Structure of a flower
Structure of a flower Stamen and Carpel Functions of the flower:
Receptacle: Supports the floral parts of the flower.
Sepal: To protect the flower
Petals: To attract the insects and animals for pollination (to attract insects the flower is large and bright) Functions of the flower:
• Stamen : To produce the pollen grains in the anthers. (Male part of flower)
Functions of the flower:
Filament: Holds the anther in place
Anther: produces pollen (male gamete) Functions of the flower:
Carpel : The female part of the flower Functions of the flower:
Stigma: Where pollen lands after pollination
Style: Pollen travels down this.
Ovary Contains ovules
Male Gamete formation
Meiosis takes place in some cells to produce pollen grains which have a haploid nucleus.
Each pollen grain divides by mitosis to form the male gametes.
Pollen Grain development
Anther consists of 4 chambers called pollen sacs
Each pollen sac is surrounded by a protective layer called the fibrous layer.
Pollen grain development
Tapetum is a layer of cells used to provide food for cell division in the pollen sac. Pollen grain development
There are diploid microspore mother cells in each pollen sac which are diploid.
These cells divide by meiosis to produce haploid tetrad cells.
Each tetrad forms four separate haploid pollen grains (microspores)
Pollen grain development
Each pollen grain has a thick outer wall called the exine (allows it to survive for long periods)
Pollen grain then divides by mitosis to produce two haploid nuclei (the tube and generative nuclei).
Once pollen grains have matured the anther dries and shrivels and the pollen are released.
Development of the embryo sac
The ovary contains an ovule. The ovule has a small opening called a micropyle for the pollen tube to enter.
The ovule has a megaspore mother cell which divides by meiosis to form four haploid cells.
Three of these cells degenerate and the remaining one is called the embryo sac. Development of embryo sac Structure of the ovule Megaspore mother cell (2n)
Meiosis
Integuments Embryo sac (n)
micropyle Development of the embryo sac
The haploid nucleus in the embryo sac divides by mitosis three times to form 8 haploid nucleus.
Mitosis happens 3 times
Polar nuclei (n)
Egg cell (n) Development of the embryo sac
Five of these nuclei degenerate.
The three remaining nuclei form the female gametes.
Two polar nuclei Egg cell Development of the embryo sac Pollination
Pollination is the transfer of pollen from an anther to a stigma of a flower from the same species.
Self pollination: involves the transfer of pollen from an anther to a stigma on the same plant.
Cross fertilisation: involves the transfer of pollen from an anther to a stigma on a different plant of the same species. Pollination
Anther Stigma Methods of Pollination
Wind: wasteful of pollen (e.g grasses and oak)
Animals: Less pollen wasted, insects are most common pollinators (e.g dandelions, primroses and buttercups)
Wind Pollinated Plants
Petals: small and green, no scent or nectar
Pollen: large amounts, light, small and dry
Anther: large and outside petal
Stigma: large and feathery and outside petal. Wind Pollinated plants Adaptions of wind pollinated plants Animal Pollintion
Petals: large, brightly coloured, scented and have nectar.
Pollen: small amounts, heavy, large and sticky
Anthers: small and inside petal
Stigmas: small and sticky and inside petal Animal Pollination Animal Pollination Pollination Video
Wild orchid wasp mimic - David Attenborough - BBC – YouTube
Plant Reproduction: Methods of Pollination – YouTube nilu's classes:- class xii , chem.., Plant Reproduction Methods of Pollination - YouTube Did you Know:
Hay Fever: An allergic reaction to allergens such as pollen grains.
Affects up to 10% of population.
Causing sneezing, blocked or runny nose. Fertilisation
Fertilisation is the fusion of the male and female gametes to form a diploid zygote.
Male gamete is the pollen grain
Female gamete = 2 polar nuclei and the egg cell
Fertilisation
• Pollen grains lands on a stigma and a pollen tube begins to grow down the style towards the ovule.
• Growth of the pollen tube is controlled by the tube nucleus which degenerates when it reaches the micropyle.
• Ovule releases chemicals to cause pollen tube to gro towards the micropyle.
Fertilisation
Pollen grain
Stigma
Style Polar nuclei
Ovary
Egg cell
Fertilisation Fertilisation
Fertilisation
• The haploid generative nucleus divides by mitosis to form two haploid sperm nuclei (male gametes)
• The presence of the pollen tube means that water is not needed for the male gametes to move towards the egg.
Double Fertilisation
1) One sperm nucleus (n) fuses with the egg nucleus (n) to form a diploid zygote (2n).
2) The second sperm nucleus (n) fuses with the two polar nuclei (both n) to form a triploid (3n) endosperm.
Double Fertilization in Flowering Plants - YouTube
Double fertilisation
Double Fertilization in Angiosperms - YouTube Fertilisation
Seed Formation
• After fertilisation, the ovule becomes the seed.
• Zygote divides by mitosis to form an embryo which will form the plumule (future shoot) and the radical (future root).
• Some of embryo also develops into the cotyledon. Seed formation
• The endosperm (3n) divides by mitosis to produce many endosperm cells.
• These absorb the nucellus and act as a food store (starch and oils). Seed Development Seed Formation
Before Fertilisation After Fertilisation Ovule Seed Integuments Testa (seed coat) Nucellus Endopserm (cotyledon) Egg Embryo (plumule, radicle) Polar nuclei Endosperm Ovary Fruit Ovary wall Pericarp (fruit coat or wall) Non endospermic seed
It has no endosperm when fully formed.
All the endosperm is absorbed by the cotyledon.
Example: Broad bean, peanut and sunflower. Endospermic Seed
It contains some endosperm when fully formed.
Cotyledon only absorbed some of the endosperm.
Example: Maize and corn
NOTE: when your eating popcorn you are eating the endosperm of a seed. Non endospermic and endospermic seeds Summary of seed development
Plumule (future shoot)
Embryo Radical (future root)
Cotyledon (food store or seed leaf) Seed
Endosperm Food store (Monocots) Endospermic seed
Monocot and dicot seeds
Monocot seeds have one cotyledon
Example: cereals, grasses and daffodils.
Dicots have two cotyledons
Example: broad bean and sunflower.
NOTE: A cotyledon is a seed leaf used for food storage Monocots and Dicots
Monocots: food is not stored in cotyledon but is instead stored in the endosperm.
Dicots: food is stored in the cotyledons. Fruit Formation
The ovary becomes the fruit.
Wall of the ovary becomes the wall of the fruit (pericarp).
Fruits are used to protect the seed and help in seed dispersal.
Fleshy Fruits: Peaches, Tomatoes and grapes
Dry Fruits: Green beans, monkey nuts and popcorn Fruit Formation
Once the fruit has formed the other flower parts disappear and fall away.
False Fruits
In some exceptions, some fruits develop form other parts of flower apart from the ovary.
These fruits are known as false fruits.
Example: apples develop when base of flower join together and swell.
Strawberries develop from a swollen receptacle. Seedless Fruit
• Development of a fruit without a seed is called parthenocrapy.
• The egg is not fertilised.
How seedless fruits are formed?
1) Can be formed genetically (bananas, oranges and grapes)
2) Spray plants with growth regulators (auxins) which allows fruits to form without seeds and fertilisation occurring (grapes, peppers, peaches) Fruit and seed dispersal
Dispersal is the transfer of a seed or fruit away from the parent plant.
Why is dispersal important: 1) Avoids competition between each seed and also with the parent plant. 2) Increases the chance of survival. 3) Finds new area to grow. 4) Increases the numbers of the species. Types of dispersal:
1) Wind
2) Water
3) Animal
4) Self dispersal Wind dispersal
Normally these seeds are small, light and have wings.
Some of these seeds may also have parachutes
His allows them to be easily dispersed.
However, the young plant has very little food to keep it nourished in the early days of growth. Wind dispersal Water dispersal
These have light air filled fruits which float.
Allows them to be dispersed over large distances by rivers and the sea.
Animal dispersal
Animals carry seeds over large distances.
These seeds have two adaptions: 1) Sticky fruits: may stick to animals hair or fur and be carried away. Animal dispersal
2) Edible and fleshy fruits: Brightly coloured, plenty of food with strong scents to attract animals.
These seeds are released with faeces and acts as a fertiliser for the seed.
Examples: strawberries and acorns. Self dispersal
Some fruits have an explosive mechanism that sprays seeds a short distance away from parent plant.
Examples: peas and beans Video on Seed dispersal
• Seed Dispersal - YouTube Dormancy
• Dormancy is a resting period when seeds under go no growth and have reduced cell activity or metabolism Causes of Dormancy
1) Growth inhibitors (abscisic acid) slow down growth until they are broken down by water.
2) Testa maybe impermeable to water which prevents growth.
3) Testa maybe to tough to allow embryo to grow.
Advantages of Dormancy:
1) Allows the plant to avoid harsh conditions in the winter.
2) Give the embryo time to develop fully.
3) Provides time for seed to be dispersed.
4) Maximises the growing season by starting to grow in spring. Dormancy in agriculture and horticulture
Some seeds need a cold period to break dormancy which allows for the production of growth promoters (auxins).
This can be done by placing them in a fridge.
Germination
Germination is the regrowth of the embryo, after a period of dormancy, if conditions are suitable.
Conditions for germination
1) Water: allows enzyme reactions to occur, seeds absorb water from soil.
2) Oxygen: needed for aerobic respiration, absorbed from the soil.
3) Temperature: needed to allow enzyme reactions to occur (5 to 30oC)
4) Dormancy: must be complete.
Conditions needed for germination
Events in Germination
NOTE: seeds store their own food in the form of starch, oils and proteins.
1) Seeds absorb water through the micropyle (activates enzymes).
2) Food reserves are digested (starch to glucose, oils to fatty acids and glycerols, proteins to amino acids)
3) Products of digestion are moved to the embryo
Events of germination
4) Glucose and amino acids are used to make cell walls and enzymes.
5) Glucose also used to make ATP (energy)
6) Dry weight of seed falls due to food used in respiration.
7) The radicle bursts through the testa.
8) The plumule appears above the ground and forms leaves. A germinating seed
Plumule
Testa Radicle The dry weight of the seed falls due to the foods used in respiration but then begins to increase due to photosynthesis. Changes in dry weights of germinating seeds
Embryo
Weight of food store (endosperm/cotyledon) decreases, the weight of embryo increases. Events in seed germination
Testa 1) Water is absorbed Cotyledon
2) Food is digested
3) Food moved to the embryo
4) Embryo (radicle + plumule) start to grow Cotyledons remaining below ground Example: germination of the broad bean
Plumule
Epicotyl (The region between the cotyledon and the plumule)
Radicle
Germination of a broad bean
• Plumule remains hooked to protect the delicate leaves.
• Once above the ground the plumule straightens.
Cotyledons that move above the soil
Example: germination of the sunflower
Hypocotyl hook (This is the region between the radicle and the cotyledon)
The hypocotyl causes the cotyledon to be carried above the soil. • Once above the ground, the pericarp (fruit wall) falls to the ground.