Plant Reproduction and Growth
FoundationThe flower is the plant's reproductive structure: pollination delivers pollen to the stigma, and the pollen tube carries sperm into the ovule for fertilisation; afterwards the ovule becomes the seed and the ovary the fruit, dispersed by wind, water, animals or self-ejection. Auxin governs growth: one-sided light bends stems toward the light, and auxin from the shoot tip suppresses side buds — apical dominance.
- Reproduction splits two ways: vegetative propagation is asexual with offspring identical to the parent, while sexual reproduction runs through pollination and fertilisation for rich variation.
- A flower holds sepals, petals, stamens (filament plus anther) and carpels (stigma, style, ovary); the anther makes pollen and the stigma receives it.
- Insect-pollinated flowers are showy with sticky large pollen; wind-pollinated ones are small and dull with abundant light pollen and feathery exposed stigmas.
文體小知識：科學知識說明文——逐一認識花的各部分結構，追蹤傳粉、受精到種子和果實形成與散播的全流程，並以生長素解釋向光性與頂芽優勢。
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1Plants reproduce in two ways. Asexual reproduction skips gamete fusion, propagating through vegetative parts — potato tubers and ginger rhizomes sprouting into new plants — so the offspring are genetically identical to the parent and keep its good traits stable. Sexual reproduction fuses gametes, so offspring combine genes from both parents, rich in variation and adaptability; flowering plants run this process through their flowers.
2A flower is organised in tiers: the sepals outermost protect the bud; inside them the petals attract pollinators, large and colourful in insect-pollinated flowers. The stamen is a filament topped by the anther, which makes pollen; at the centre stands the carpel — stigma, style and ovary — with the sticky stigma catching pollen and the ovary holding the ovules. A flower with all of these parts is complete.
3Pollination is the transfer of pollen from anther to stigma, by insects or by wind. Insect-pollinated flowers are showy and scented with nectar, their pollen large and sticky to hitch rides; wind-pollinated flowers are small and drab with no nectar, shedding abundant light pollen, and their feathery stigmas often hang outside the flower to catch it. On the stigma the pollen grain grows a pollen tube down the style to the ovule, the sperm travels through it, and fusion with the egg completes fertilisation.
4After fertilisation the ovule becomes the seed and the ovary the fruit. The seed holds a testa and an embryo — plumule, epicotyl region, radicle and cotyledons — with food stored in the cotyledons or endosperm. Seeds and fruits spread four ways: wind dispersal rides wings or parachutes; water dispersal floats; animal dispersal hooks onto fur or tempts with flesh; self-dispersal bursts from drying pods. Dispersal sends offspring away from the parent, cutting competition and opening new habitats.
5Germination needs suitable water, oxygen and warmth: the seed imbibes water, enzymes digest the stored food, the radicle breaks out and grows down, and the plumule rises as a seedling. Growth direction is governed by auxin, made in the shoot tip, which promotes cell elongation at low concentration. One-sided light shifts auxin to the shaded side, whose cells elongate faster, so the stem bends toward the light — phototropism. Auxin from the tip also streams downward and suppresses side buds, the apical dominance that pruning the tip releases.
1Summary 重點摘要
The flower is the plant's reproductive structure: pollination delivers pollen to the stigma, and the pollen tube carries sperm into the ovule for fertilisation; afterwards the ovule becomes the seed and the ovary the fruit, dispersed by wind, water, animals or self-ejection. Auxin governs growth: one-sided light bends stems toward the light, and auxin from the shoot tip suppresses side buds — apical dominance.
2Vocabulary cards Vocabulary
Asexual reproduction through vegetative parts such as roots, stems and leaves, giving offspring genetically identical to the parent.
The male reproductive part of a flower, a filament topped by the pollen-producing anther.
The female reproductive part of a flower — stigma, style and an ovary holding the ovules.
The pollen-producing, pollen-holding tip of the stamen.
The transfer of pollen from anther to stigma of the same or another flower, by insects or wind.
The tube a pollen grain grows down the style to the ovule, carrying the sperm through it.
The fusion of sperm with the egg cell inside the ovule, forming the zygote that starts the seed.
The spreading of seeds or fruits away from the parent, by wind, water, animals or self-ejection.
The resumption of growth in a seed given suitable water, oxygen and warmth, developing into a seedling.
The plant hormone made in shoot tips; at low concentration it promotes cell elongation and controls phototropism and apical dominance.
3Step by step Steps
- Step 1: Two routes of reproductionAsexual reproduction skips gametes and propagates through vegetative parts: potato tubers, ginger rhizomes and strawberry runners all grow into new plants genetically identical to the parent, holding its good traits steady — why growers graft fruit trees from cuttings. Sexual reproduction fuses gametes, so offspring inherit genes from both parents and vary widely. For example, grafting from one fine lychee tree gives trees bearing identical fruit; sow its seeds instead and quality scatters — the difference between the two routes in one orchard.
- Step 2: Naming the parts of a flowerA flower layers from outside in: sepals outermost wrap the bud before it opens; petals recruit pollinators; the stamen is a filament holding up the anther, where pollen forms; and at the centre the carpel rises — a sticky stigma on top, a style down to the ovary, which houses one to many ovules. For example, dissect a lily: count the perianth first, find anthers and stigma, then slit the ovary open and the white ovules are visible to the naked eye — each structure paired with its job.
- Step 3: Insect-pollinated and wind-pollinated flowersThe pollination route dictates the flower's look. Insect-pollinated flowers hire insects: large colourful petals, scent and nectar, with pollen grains large, rough and sticky to cling to visitors. Wind-pollinated flowers hire the wind: small, dull, nectarless, shedding abundant light smooth pollen, with feathery stigmas often hanging outside the flower to catch it. For example, rice flowers are tiny grey-green and barely noticed while their pollen drifts everywhere; apple orchards blaze pink-white and swarm with bees — two strategies, each efficient.
- Step 4: From pollination to fertilisationPollination ends when pollen reaches a stigma; landing on a compatible stigma, the grain absorbs moisture and grows a pollen tube down the style to the ovule. Inside the tube the sperm reaches the egg cell in the ovule, and their fusion is fertilisation. The stigma's sticky fluid even screens pollen by species, blocking mismatches. For example, when drawing the route in an exam, keep the chain unbroken: anther → stigma → pollen tube down the style → ovule → egg cell.
- Step 5: After fertilisationAfter fertilisation the flower wraps up: the ovule becomes the seed, the ovary becomes the fruit, and sepals, petals, stamens and style mostly wither. The seed carries a testa and an embryo of plumule, epicotyl region, radicle and cotyledons; monocots store food mainly in the endosperm, dicots in their two cotyledons. For example, split a soaked bean: two plump cotyledons sandwich a tiny plumule and radicle, while the pod around them is the ovary turned fruit holding several seeds.
- Step 6: Four ways of dispersal, then germinationSeeds and fruits travel four ways: by wind on wings or parachutes, as dandelions do; by water on buoyant husks, as coconuts do; by animals, hitching hooks on fur or tempting fruit-eaters, as burrs do; and by their own force, pods twisting dry and flinging seeds out, as in the legumes. Dispersal cuts competition between parent and offspring and opens new habitats. Germination needs water, oxygen and warmth: imbibed seeds switch their enzymes on, digesting the stored food; the radicle breaks out first and the plumule follows. For example, in a windowsill bean test, the soaked batch cracks its testa in two days while the dry batch never stirs.
- Step 7: Auxin, phototropism and apical dominanceAuxin is made in the shoot tip and promotes cell elongation at low concentration. One-sided light shifts auxin across to the shaded side, where cells elongate faster, bending the stem toward the light — phototropism, gathering more sun for the leaves. Auxin also streams down from the tip and suppresses side buds — apical dominance, concentrating growth upward. For example, seedlings in a box open only on the right bend visibly rightward within days, and tea farmers pinch out shoot tips so side branches bush out — apical dominance released to work.
4Diagrams Diagrams
5Worked example Example
Rice is wind-pollinated: its flowers are small, dull, scentless and nectarless; apple flowers are insect-pollinated. (1) Name the structures that produce and receive pollen. (2) Starting at the stigma, write the complete route by which sperm reach the ovule. (3) After fertilisation, what do the ovary and the ovule become? (4) State four methods of seed dispersal with one example each.
- Pollen is produced in the anther at the top of the filament; it is received by the stigma at the top of the carpel, whose sticky surface holds the grain and lets it germinate.
- The route: pollen lands on the stigma → grows a pollen tube → the tube grows down the style → reaches an ovule in the ovary → the sperm travels through the tube into the ovule → fuses with the egg cell.
- After fertilisation: the ovule becomes the seed and the ovary becomes the fruit; a grain of rice is a fruit enclosing one seed.
- Four methods with examples: wind — the parachuted fruit of dandelions; water — the floating coconut; animals — the hooked burr; self-ejection — the twisting dry pod of legumes.
- Answer: The anther makes pollen and the stigma receives it; sperm reach the egg along the route stigma → pollen tube → style → ovule. After fertilisation the ovule becomes the seed and the ovary the fruit. Dispersal runs four ways — wind (dandelion), water (coconut), animals (burr) and self-ejection (bursting pod). The small rice flower, abundant light pollen and feathery protruding stigma are classic wind adaptations; in structure questions, pair every part with its function for pollination or dispersal.
6Common mistakes Mistakes
7Quick check Quick check
Q1What parts make up the stamen and the carpel?
Q2What adaptations does a wind-pollinated flower show?
Q3What external conditions does germination need?
Q4A seedling is put in a dark box open only on the right. How does its stem grow, and why?
Q5What does removing the shoot tip do to a plant, and why?
8Recap Recap
- Reproduction splits two ways: vegetative propagation is asexual with offspring identical to the parent, while sexual reproduction runs through pollination and fertilisation for rich variation.
- A flower holds sepals, petals, stamens (filament plus anther) and carpels (stigma, style, ovary); the anther makes pollen and the stigma receives it.
- Insect-pollinated flowers are showy with sticky large pollen; wind-pollinated ones are small and dull with abundant light pollen and feathery exposed stigmas.
- Fertilisation route: pollen tube from stigma down the style to the ovule, sperm meets egg; then the ovule becomes the seed, the ovary the fruit, dispersed by wind, water, animals or self-ejection.
- Germination needs water, oxygen and warmth; auxin from the shoot tip bends stems toward one-sided light, drives apical dominance suppressing side buds — pinching the tip frees them.