Unit 11 Reproduction in plants
Key unit competence: Evaluate various methods of asexual reproduction as means of increasing crop yield.
Key ideas
- Asexual reproduction involves one parent, no gametes, and mitosis: offspring are clones. Sexual reproduction involves meiosis, gametes and fertilisation, usually two parents, giving variation. Asexual methods: binary fission (bacteria, Amoeba), budding (yeast, Hydra), fragmentation (Spirogyra), spore formation (Rhizopus, ferns, mosses), vegetative propagation (flowering plants).
- Natural vegetative organs: stem tubers (Irish potato), root tubers (sweet potato, cassava), bulbs (onion), corms (taro, cocoyam), rhizomes (ginger, couch grass), runners/stolons (strawberry), suckers (banana), plantlets on leaves (Bryophyllum). They store food and have buds that grow into new plants.
- Artificial propagation: stem cuttings (cassava, sugar cane, sweet potato vines), grafting (scion of a good variety on a hardy rootstock: avocado, mango, citrus), budding (a single bud inserted), layering (a branch rooted while still attached), and micropropagation / tissue culture (many disease-free clones from small explants on sterile nutrient media with hormones: bananas, potatoes, cassava).
- Asexual reproduction: advantages: fast; one parent; good qualities preserved; plants mature and yield early; uniform crop; useful for plants that make few seeds. Disadvantages: no variation, so all plants can be destroyed by one disease or change; diseases passed in planting material; overcrowding and competition near the parent; no new better varieties.
- Alternation of generations: plant life cycles alternate between a haploid gametophyte (makes gametes by mitosis) and a diploid sporophyte (makes spores by meiosis). In mosses (bryophytes) the gametophyte is dominant (the green leafy plant) and the sporophyte (capsule on a stalk) depends on it; water is needed for sperm to swim. In ferns (pteridophytes) the sporophyte is dominant (the fern plant with fronds and sori), and the small gametophyte (prothallus) lives independently. In flowering plants the gametophytes are reduced to pollen grains and the embryo sac.
- Flowers: a complete flower has sepals, petals, stamens and carpels; a bisexual (hermaphrodite) flower has both stamens and carpels; unisexual flowers are male or female (maize: tassel and cob/silks). Monoecious plants carry male and female flowers on one plant (maize); dioecious plants carry them on separate plants (papaya). Calyx (sepals), corolla (petals), androecium (stamens), gynoecium (carpels).
- Pollination: transfer of pollen from anther to stigma. Cross-pollination increases variation; it is favoured by dichogamy (stamens and stigma ripen at different times: protandry, protogyny), self-incompatibility, unisexual flowers and dioecious plants, and flower structure. Insect- and wind-pollinated flowers differ in petals, scent, nectar, pollen amount and stigma shape.
- Double fertilisation: the pollen grain germinates on the stigma; the pollen tube grows down the style (guided chemically), enters the ovule through the micropyle; one male nucleus fuses with the egg cell (→ diploid zygote → embryo); the other fuses with the two polar nuclei (→ triploid endosperm nucleus → food store).
- After fertilisation: ovule → seed; ovary → fruit; integuments → testa; ovary wall → pericarp; petals, stamens, style and stigma usually wither. A seed has a testa, hilum (scar), micropyle and embryo (radicle, plumule, cotyledons); monocot seeds (maize) have one cotyledon and endosperm; dicot seeds (bean) have two cotyledons that store food (non-endospermic).
- Fruits: true fruits develop from the ovary only; false fruits include other parts (receptacle: apple, strawberry). Fleshy fruits: berries (tomato, pawpaw), drupes (mango, avocado: stony endocarp). Dry dehiscent (split open: bean pod, legume) and dry indehiscent (do not split: maize grain, sunflower). Dispersal by wind (wings, hairs), animals (fleshy, hooks), water (fibrous, air spaces) and explosive mechanisms reduces competition and colonises new areas.
Common misconceptions
- "All plant life cycles are the same as in animals." No: plants show alternation of a haploid gametophyte and a diploid sporophyte generation.
- "The endosperm is diploid like the embryo." No: the endosperm nucleus is triploid (one male nucleus + two polar nuclei).
- "A false fruit is not a real fruit." No: it is a fruit that includes other flower parts besides the ovary, e.g. the receptacle in strawberry.
- "Maize flowers are bisexual." No: maize is monoecious with separate male (tassel) and female (cob) flowers on the same plant.
Worked examples
Example 1. Explain why farmers in Rwanda use tissue-cultured banana plantlets rather than ordinary suckers.
Solution
Tissue culture produces large numbers of plantlets quickly from a few cells; they are clones of a high-yielding variety; they are produced in sterile conditions, so they are free of diseases (e.g. banana bacterial wilt, nematodes) that suckers can carry; plants are uniform and mature together.
Example 2. Compare the life cycles of a moss and a fern.
Solution
In both, a haploid gametophyte alternates with a diploid sporophyte, and sperm need water to swim to the egg. In the moss the gametophyte is the main green plant and the sporophyte (capsule) grows on it. In the fern the sporophyte is the main plant (fronds with sori), and the gametophyte is a small independent prothallus.
Practice questions
Give four advantages and two disadvantages of asexual reproduction in crop production.
Show answer
Advantages: fast; one parent; desirable qualities kept; early maturity; uniform crop; useful for seedless varieties. Disadvantages: no variation (one disease can destroy all); diseases spread in planting material; overcrowding.
Common mistake: Saying asexual reproduction increases variation.
Describe how grafting is carried out and state two advantages.
Show answer
Cut a scion from a good variety and make a matching cut on a young rootstock; join them so the cambium layers touch; tie firmly and cover to prevent drying; remove growth below the graft. Advantages: keeps good fruit qualities; early fruiting; uses a hardy disease-resistant rootstock.
Common mistake: Not mentioning matching the cambium.
Explain alternation of generations in a fern.
Show answer
The diploid sporophyte (fern plant) produces haploid spores by meiosis in sori; spores grow into a haploid gametophyte (prothallus); it makes gametes by mitosis; sperm swim in water to the egg; fertilisation gives a diploid zygote that grows into a new sporophyte.
Common mistake: Saying the fern plant is haploid.
Distinguish between: (a) monoecious and dioecious plants; (b) complete and incomplete flowers.
Show answer
(a) Monoecious: male and female flowers on the same plant (maize); dioecious: on separate plants (papaya). (b) Complete: has sepals, petals, stamens and carpels; incomplete: lacks one or more of these.
Common mistake: Confusing bisexual with monoecious.
Describe double fertilisation.
Show answer
Pollen grain germinates on the stigma; tube grows through the style and enters the ovule via the micropyle; one male nucleus fuses with the egg to form a diploid zygote; the other fuses with two polar nuclei to form a triploid endosperm nucleus.
Common mistake: Saying both male nuclei fuse with the egg.
State what each part becomes after fertilisation: ovule, ovary, ovary wall, integuments, zygote.
Show answer
Ovule → seed; ovary → fruit; ovary wall → pericarp; integuments → testa; zygote → embryo.
Common mistake: Saying the ovary becomes the seed.
State three mechanisms that favour cross-pollination.
Show answer
Dichogamy (protandry, protogyny); self-incompatibility; unisexual flowers / dioecious plants; flower structure that keeps anthers away from the stigma.
Common mistake: Giving features of insect-pollinated flowers only.
Classify these fruits: mango, bean pod, maize grain, tomato, strawberry.
Show answer
Mango: fleshy drupe; bean pod: dry dehiscent (legume); maize grain: dry indehiscent; tomato: fleshy berry; strawberry: false fruit.
Common mistake: Calling the maize grain a seed only.
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. A plant with male and female flowers on separate plants is: a. dioecious 2. Stamens ripening before the stigma is called: b. testa 3. Rooting a branch while it is still attached to the parent is: c. protandry 4. After fertilisation, the integuments of the ovule become the: d. layering Show answer
1 → a (dioecious) · 2 → c (protandry) · 3 → d (layering) · 4 → b (testa)
- Example: papaya.
- It favours cross-pollination.
- The branch is bent into soil.
- The seed coat.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
favours cross-pollination · grafting · gametophyte · budding of yeast- Joining a scion to a rootstock is .
- In mosses, the dominant generation is the .
- In maize, the tassels shed pollen before the silks on the same plant are ready. This mainly .
Show answer
1: grafting · 2: gametophyte · 3: favours cross-pollination
- Used for mango and avocado.
- The green leafy plant is haploid.
- Dichogamy reduces self-pollination.
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 endosperm nucleus is formed from a male nucleus and: a. genetically identical clones 2. In ferns, the dominant generation is the: b. two polar nuclei 3. Producing many plants from small tissue pieces on sterile media is: c. sporophyte 4. Offspring produced by asexual reproduction are: d. micropropagation (tissue culture) Show answer
1 → b (two polar nuclei) · 2 → c (sporophyte) · 3 → d (micropropagation (tissue culture)) · 4 → a (genetically identical clones)
- It is triploid.
- Fronds carry spore-producing sori.
- Hormones control root and shoot growth.
- Mitosis copies the parent's DNA.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
monoecious · sorus · micropyle · prothallus- The small, heart-shaped gametophyte of a fern is the .
- A plant with male and female flowers on the same plant is .
- The pollen tube enters the ovule through the .
Show answer
1: prothallus · 2: monoecious · 3: micropyle
- It lives independently.
- Example: maize.
- It is a small opening.