Unit 11 Asexual and sexual reproduction in plants and microorganisms
Key unit competence: Compare asexual and sexual reproduction in plants and microorganisms and explain pollination, fertilisation, fruit and seed formation and germination.
Key ideas
- Asexual reproduction involves one parent, no gametes and no fertilisation; offspring are produced by mitosis and are genetically identical (clones). Sexual reproduction involves the fusion of male and female gametes (fertilisation), usually from two parents, giving variation.
- Asexual methods in microorganisms: binary fission (bacteria, Amoeba, Paramecium: one cell splits into two), budding (yeast, Hydra), spore formation (Rhizopus bread mould, mushrooms), fragmentation (Spirogyra).
- Natural vegetative propagation in plants: stem tubers (Irish potato), root tubers (sweet potato, cassava), bulbs (onion), corms, rhizomes (ginger, couch grass), runners/stolons (strawberry), suckers (banana), leaves (Bryophyllum).
- Artificial propagation: cuttings (cassava stems, sugar cane), grafting (a scion of a good variety joined to a rootstock: avocado, mango, citrus), layering, budding, and tissue culture (many plants from a few cells, e.g. disease-free bananas and potatoes).
- Asexual reproduction: advantages: fast, one parent is enough, good qualities kept, offspring mature quickly; disadvantages: no variation, so a disease can wipe out all; overcrowding near the parent; diseases passed on in planting material.
- A flower is the organ of sexual reproduction. Sepals (protect the bud), petals (attract insects), stamens (male: anther makes pollen; filament), carpel/pistil (female: stigma receives pollen, style, ovary containing ovules). Receptacle holds the parts; nectaries produce nectar.
- Pollination is the transfer of pollen from anther to stigma: self-pollination (same flower or plant) or cross-pollination (another plant of the same species). Insect-pollinated flowers: large bright petals, scent, nectar, sticky spiky pollen, stamens and stigma inside. Wind-pollinated flowers (maize, grasses): small dull petals, no scent or nectar, large amounts of light smooth pollen, long filaments and feathery stigmas outside.
- Fertilisation: a pollen grain germinates on the stigma and grows a pollen tube down the style to the ovule (through the micropyle). In flowering plants, double fertilisation occurs: one male nucleus fuses with the egg cell → zygote (becomes the embryo); the other fuses with the polar nuclei → endosperm (food store).
- After fertilisation: ovule → seed, ovary → fruit, integuments → testa (seed coat), ovary wall → pericarp. Fruits and seeds are dispersed by wind (light, wings or hairs: Spathodea, Tridax), animals (fleshy or hooked fruits: mango, blackjack), water (fibrous, floating: coconut) and self (explosive) (bean pods).
- Germination is the growth of the embryo into a seedling. Conditions: water (activates enzymes, softens testa), oxygen (respiration), suitable temperature (enzymes). In epigeal germination (bean) the cotyledons are carried above the soil; in hypogeal germination (maize) they stay below.
Common misconceptions
- "Pollination and fertilisation are the same." No: pollination is the transfer of pollen; fertilisation is the fusion of nuclei inside the ovule.
- "Fruits are always sweet and fleshy." No: a fruit is any mature ovary with seeds, including bean pods and maize grains.
- "Seeds need light to germinate." No: most seeds need water, oxygen and warmth; light is needed later for photosynthesis.
- "Clones from tissue culture can be different from the parent." No: they are produced by mitosis and are genetically identical.
Worked examples
Example 1. Explain why farmers prefer to grow bananas from suckers or tissue culture rather than from seeds.
Solution
Suckers and tissue culture are asexual: plants are identical to the good parent (high yield, good taste), grow and fruit faster, and cultivated bananas produce few or no viable seeds. Tissue culture also gives disease-free plantlets in large numbers.
Example 2. Compare a maize flower with a bean flower for pollination.
Solution
Maize is wind-pollinated: small dull flowers, no nectar or scent, large amounts of light pollen, hanging anthers and feathery stigmas (silks) outside. Bean flowers are insect-pollinated: coloured petals, nectar and scent, sticky pollen and stigma inside the flower.
Practice questions
Give three differences between sexual and asexual reproduction.
Show answer
Sexual: two parents (usually), gametes and fertilisation, meiosis involved, offspring show variation, slower. Asexual: one parent, no gametes, mitosis only, identical offspring, faster.
Common mistake: Saying asexual reproduction produces variation.
Name four methods of natural vegetative propagation with an example of each.
Show answer
Stem tuber (Irish potato); root tuber (sweet potato); bulb (onion); rhizome (ginger); runner (strawberry); sucker (banana).
Common mistake: Calling cassava a stem tuber.
State the functions of: (a) petals; (b) anther; (c) stigma; (d) ovary.
Show answer
(a) Attract insects. (b) Produce pollen. (c) Receive pollen. (d) Contains ovules; becomes the fruit.
Common mistake: Saying the stigma makes pollen.
Give four differences between insect-pollinated and wind-pollinated flowers.
Show answer
Insect: large bright petals, scent and nectar, sticky spiky pollen in small amounts, stigma and anthers inside. Wind: small dull petals, no scent or nectar, large amounts of light smooth pollen, feathery stigmas and anthers hanging outside.
Common mistake: Saying wind-pollinated flowers have nectar.
Describe double fertilisation in flowering plants.
Show answer
The pollen tube carries two male nuclei into the ovule; one fuses with the egg cell to form the zygote (embryo); the other fuses with the polar nuclei to form the endosperm.
Common mistake: Saying both nuclei fuse with the egg.
State the conditions necessary for germination and why each is needed.
Show answer
Water: activates enzymes, softens testa; oxygen: for aerobic respiration to release energy; suitable temperature: for enzyme activity.
Common mistake: Including light and soil.
State three advantages of grafting.
Show answer
Keeps the good qualities of the scion; uses a strong, disease-resistant rootstock; fruits sooner; allows different varieties on one tree.
Common mistake: Saying grafting produces variation.
Give two adaptations of seeds dispersed by wind.
Show answer
Light and small; wings (Spathodea) or hairs/parachutes (Tridax, dandelion) to be carried far.
Common mistake: Describing hooks.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The male part of a flower is the: a. grafting 2. The Irish potato is a: b. stamen 3. Joining a shoot of one plant to the rootstock of another is: c. stem tuber 4. After fertilisation, the ovule becomes the: d. seed Show answer
1 → b (stamen) · 2 → c (stem tuber) · 3 → a (grafting) · 4 → d (seed)
- It consists of anther and filament.
- It has buds (eyes).
- Used for avocado and mango.
- The ovary becomes the fruit.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
endosperm · seed · fruit · pollination- Transfer of pollen from anther to stigma is .
- After fertilisation, the ovary becomes the .
- The food store formed in double fertilisation is the .
Show answer
1: pollination · 2: fruit · 3: endosperm
- It happens before fertilisation.
- The ovary wall forms the pericarp.
- A male nucleus fuses with polar nuclei.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Yeast reproduces asexually by: a. anther 2. Pollen is produced in the: b. stigma 3. The part of the carpel that receives pollen is the: c. budding 4. Reproduction with one parent and no gametes is: d. asexual reproduction Show answer
1 → c (budding) · 2 → a (anther) · 3 → b (stigma) · 4 → d (asexual reproduction)
- A small bud grows from the parent cell.
- Anthers are on stamens.
- It is often sticky.
- Offspring are clones.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
animals · tissue culture · layering · spores- Bread mould (Rhizopus) reproduces asexually by .
- Seeds with hooks are dispersed by .
- Growing many plants from small pieces of tissue in a sterile medium is .
Show answer
1: spores · 2: animals · 3: tissue culture
- Sporangia release many spores.
- Hooks stick to fur and clothes.
- It gives disease-free clones.