Sexual & Asexual Reproduction In Plants (chpt. 39 & 40) Flashcards
Female parts of plant
Carpel
• stigma
• style
• ovary
Male parts of plant
Stamen
• anther
• filament
Role of stigma
Captures pollen
Role of style
Connects stigma to ovary
Role of ovary
Site of fertilisation , produces the ovule
Role of anther
Produces male gametes (pollen grains)
Role of filament
Supports anther and supplies it with food and water
Role of sepal
Protects the bud
Role of petal
Attract animal pollinators
↳ brightly coloured and scented
Where are the male gametes found?
In the pollen grains
Male gamete formation → steps
• anther consists of 4 pollen sacs containing diploid microspore mother cells
• divide by meiosis to produce 4 haploid cells which join to form tetrad
• tetrad breaks up and forms 4 microspores
• microspore divides by mitosis to produce 2 haploid nuclei → generative nucleus and tube nucleus
Female gamete formation → steps
• diploid megaspore mother cell in ovule divides by meiosis to produce 4 haploid cells → 3 degenerate and 1 survives as megaspore / embryo sac
• embryo sac undergoes mitosis 3 more times to produce 8 haploid nuclei → 5 die, 2 form polar nuclei, 1 forms egg
Female gametes: 1 egg + 2 polar nuclei
Pollination
Transfer of pollen from an anther to a stigma of a flower of the same species
Cross-pollination
Pollination of another plant of the same species
↳ more variation & greater strength
Self-pollination
Pollination of a plant by itself
↳ less variation
Features of an animal pollinated plant
Large bright scented petals
Sticky stigma
Features of a wind pollinated plant
Large quantity of light pollen produced
Long stamen outside flower
Long feathery stigma
Fertilisation
The fusion of male and female gametes to form a diploid zygote
Steps in fertilisation
• pollen grain lands on stigma
• tube nucleus forms pollen tube which grows down the style to the ovule
• generative nucleus divides by mitosis to form 2 sperm nuclei
• double fertilisation occurs:
Sperm nucleus + egg nucleus → zygote (2n)
Sperm nucleus + 2 polar nuclei → endosperm (3n)
Which part of seed develops into the shoot?
Plumule
Which part of seed develops into the root?
Radicle
Function of testa
Protects embryo
Function of cotyledon
Food storage + seed leaves
Function of embryo
Develops into a new plant
Steps in seed growth
• water softens testa & activates enzymes within the seed
• stored foods are digested by embryo to form new structures
• radicle grows & bursts out of testa
• plumule emerges above ground and leaves start to form
From what structure in the seed does fruit develop?
The ovary
Function of fruit
Protect seeds , allow dispersal
How is seedless fruit produced?
Can be formed by genetically breeding seedless varieties
Dispersal
The transfer of a seed or fruit away from the parent plant
Benefits of dispersal
• minimises chance of overcrowding and competition
• allows plants to grow in new areas
• increases number of species
Example of: wind dispersal
Winged fruits & seeds e.g. sycamore
Example of: water dispersal
Floating fruit e.g. coconut
Example of: animal dispersal
Edible fruits eaten by animals and pass through digestive system, deposited away from parent plant e.g. acorns
Example of: self-dispersal
Dehiscent fruits have an explosive mechanism e.g peas
Dormancy
Resting period when seeds undergo no growth and have reduced cell activity or metabolism
Causes of dormancy
• growth inhibitors
• testa may be impermeable to water or oxygen
• testa maybe too hard for embryo to emerge
• lack of growth regulator
Dormany in agriculture & horticulture
Some seeds need a long cold period to bring on germination
Dormancy can be broken by:
→ soaking seeds in water
→ exposing seeds to light or dark
→ exposing seeds to cold temperature
Advantages of dormancy
• allows seeds to avoid harsh conditions
• allows time for dispersal
• allows time for immature embryo to develop
• maximises the growing season
Germination
The regrowth of the embryo after a period of dormancy if the environmental conditions are suitable
Condition necessary for germination
Water → for enzyme activity
Oxygen → respiration
Suitable temperature → effects germination
Events in germination
• water absorbed through micropyle in testa
• stored food is digested
• digested food moves to the embryo
• embryo grows
• food used in respiration (dry mass falls)
• radicle bursts through testa
• plumule emerges above ground & leaves are produced
• when leaves start to photosynthesise, dry mass increases
Hypogeal germination
Cotyledon remains below the soil e.g broad bean
Epigeal germination
Cotyledons rise above ground e.g sunflower
Vegetative propagation
Asexual reproduction in plants
Asexual reproduction
Production of organisms from only one parent
Example of a modified bud
E.g. garlic
Example of modified leaves
E.g cacti
Example of a modified overground stem
Runners, e.g strawberry
Example of modified roots
Tap roots - swollen main roots, e.g carrot
Methods of artificial vegetative propagation
- Cuttings e.g busy - lizzie
- Grafting e.g apples
- Budding e.g roses
- Micropropogation (tissue culture) e.g commercial crops → bananas, strawberries
Advantages of asexual reproduction
• rapid growth due to maturity
• simple & reliable process
• not dependent on external agents for pollination, fertilisation or seed dispersal
• no waste
Disadvantages of asexual reproduction
• no variation
• overcrowding & competition
• one disease can wipe out all plants
• no seeds formed
Advantages of sexual reproduction
• seeds show variation
• wide dispersal due to seeds and fruits
• some plants may be resistant to disease
• dormancy allows survival in adverse conditions
Disadvantages of sexual reproduction
• plants may take years to develop
• complex process
• depends on external agents
• wastage of fruit / seed by being eaten, disease etc
What is a seed?
A fertilised ovule
Give two examples of advantages of artificial vegetative propagation to horticulturists
Faster production of plants
Larger number of plants produced
Less expensive
No gametes required
