Sexual Reproduction in Flowering Plants

• Asexual reproduction involves only one parent
• Sexual reproduction – union of 2 sex cells capable/gametes

• 2 parents
• Meiosis essential as it halves chromosome number
• Off-spring show variation

• Gametes – haploid cells capable of fusion

• Gamete + gamete = zygote

Carpal = pistil

1. Receptacle – floral parts arise from this & it supports these parts
2. Sepals – protect flower when it’s a bud
3. Petals – attract animals in animal-pollinated flowers
4. Stamen – male parts of flower made up of anther & filament
5. Carpel – female parts of flower made up of stigma, style, ovary      

• meiosis produces pollen grains
• pollen grains divide by mitosis (form male sex cells/gametes)

• Anther has 4 chambers (pollen sacs)
• Epidermis & fibrous layer protect pollen sac
• Tapetum stores food and supplies energy for cell division
• Pollen mother cells divide by meiosis to produce tetrad of pollen (4 in cluster)
• Tetrad separates – 4 separate pollen grains

Female gamete formation

• Ovule surrounded by 2 walls – integuments
• Micropyle – opening in integuments where pollen tube can enter
• Nucellus – supplies nutrient for later growth in ovule

1. Megaspore mother cell (diploid) undergoes meiosis to produce 4 haploid cells
2. 3 cells degenerate leaving embryo sac
3. Embryo sac undergoes mitosis 3 times to form 8 haploid nuclei
4. 2 polar nuclei & egg cell = female gametes

• Pollination – transfer of pollen from anther to stigma of a flower from same species
• Self-pollination – between same plant
• Cross-pollination – between different plants of the same species

Table: adaptations of flowers to wind or animal pollination

Wind	Animal
Petals: small (absent), not brightly coloured (green), no scent, no nectaries	Petals: Large, brightly coloured, scented, have nectaries
Pollen: large amounts, light, small, dry, smooth	Pollen: small amounts, heavy, large, sticky, spiny
Anthers: large, outside petals, loosely attached to filament	Anthers: usually small, inside petals, firmly attached to filament
Stigmas: large and feathery, outside petals	Stigmas: usually small and sticky, inside petals

 

• Fertilisation – union of male & female gametes to form a diploid zygote

1. Pollen lands on stigma
2. Tube nucleus moves down style towards chemicals released from ovule (chemotropism) forming pollen tube
3. Generative nucleus divides by mitosis as moves down pollen tube to form 2 sperm nuclei
4. Tube nucleus degenerates at micropyle

Double fertilization

• One sperm nucleus (n) + egg nucleus (n) = diploid zygote (2n)
• Other sperm nucleus (n) + 2 polar nuclei (n) = triploid endosperm nucleus (3n)

Seed formation

• Fertilised ovule forms seed
• Integuments dry up and form testa
• Zygote (2n) forms radicle (future root) & plumule (future shoot) by repeated mitosis
• Embryo cells form cotyledons (seed leaves)
• Endosperm nucleus (3n) forms endosperm (acts as food store) by repeated mitosis

 

• Non-endospermic seed – no endosperm when fully formed e.g. broad bean
• Endospermic seed – contains some endosperm when fully formed e.g. maize, corn

• Monocots – food stored in endosperm
• Dicots – store food in cotyledons (sometimes endosperm aswell as cotyledons)

Development of fruit

• Petals wilt – embryo develops – seed develops from ovule – fruit develops from ovary – fruit ripens + flower parts fall away

• true fruit = grape
• False fruits develop from other parts of flower besides ovary e.g. apple, strawberry

Changes in flower after fertilisation

Before	After
Ovule	Seed
Integuments	Testa (seed coat)
Nucellus	Endosperm (cotyledons)
Egg	Zygote (embryo – plumule, radicle, cotyledons)
Polar nuclei	Endosperm
Ovary	Fruit
Ovary wall	Pericarp (fruit wall/coat)

Parthenocarpy - development of a fruit without a seed i.e. egg not fertilised

Can be formed: - genetically, spray plants with growth regulators (ethene)

Dispersal – transfer of seed/fruit away from parent plant to :

• Avoid competition
• Increase chance of survival
• Find new areas for growth

• 4 methods: wind (dandelion, sycamore), water (water lilies), animal (sticky fuits e.g buttercup) or (edible fleshy fruits e.g. strawberry, blackberry), self-dispersal e.g. peas

Dormancy – resting periods where seeds undergo no growth & have reduced cell activity/ metabolism

Causes:            Growth inhibitors                      

Testa impermeable to water/oxygen

Testa too tough – embryo can’t emerge

Lack of growth regulator

Advantages:    Plants avoid harsh conditions

                        Time for embryo to develop fully

                        Maximises growing season

Germination – regrowth of embryo after period of dormancy if environmental conditions are suitable.

Conditions required W.O.W

Water                                                 Oxygen                                               Warmth (temp.)

• Also dormancy must be complete

 

 

Main events in germination

1. Seed absorbs water
2. Stored foods digested by enzymes in seed
3. Digested foods transferred to embryo from cotyledons/endosperm
4. -some digested foods make new structures

-some used in respiration

 

5. Radicle grows down & bursts through testa
6. Plumule emerges above ground
7. New leaves form

2 forms of germination:        -cotyledons stay below ground e.g. broad bean

-cotyledons move above ground & form first leaves e.g. sunflower

Experiments related to this chapter:

• Effect of water, oxygen & temperature on germination (no growth if all 3 are not present)

• Show germinating seeds produce digestive enzymes (dead (boiled) seeds don’t digest starch – all agar is blue black. In the live seed (unboiled) starch is digested (clear areas occur on blue black agar where digestion has occurred)

Useful videos:            

https://www.youtube.com/watch?v=Gq8NWh98wQs

https://www.youtube.com/watch?v=V5yya4elRLw&feature=related