Stamen, microsporangium and pollen grain
Touch an opened hibiscus flower and your finger turns yellow. That yellow dust is millions of tiny male packets waiting for a ride to the female part of another flower.
The stamen (filament + anther) is the male part of a flower. A typical anther is bilobed and has four microsporangia (pollen sacs). Inside them, pollen mother cells undergo meiosis (microsporogenesis) to form microspore tetrads, and each microspore matures into a pollen grain (the male gametophyte).
In the figure : Meiosis happens at the step from the pollen mother cell to the tetrad; the cells of the tetrad are haploid.
In the figure : A pollen grain has a hard exine with a germ pore, a thin intine, and two cells inside.
Rule 1: Four wall layers protect the pollen sac — From outside to inside the layers are epidermis, endothecium, middle layers and tapetum. The outer three protect and help the anther burst open (dehiscence); the innermost tapetum feeds the developing pollen.
- Easy : Which layer of the pollen sac gives food to growing pollen? The tapetum.
- Medium : Tapetal cells are rich in cytoplasm and often have more than one nucleus, which suits their job of nourishing pollen.
- Exam-type : Q: Name the four wall layers of a microsporangium from outside. A: Epidermis, endothecium, middle layers, tapetum.
Rule 2: Pollen wall and cells — The hard outer exine is made of sporopollenin, which resists heat, acids and alkalis, and has gaps called germ pores. The inner intine is a thin layer of cellulose and pectin. A mature pollen grain has a large vegetative cell and a small generative cell.
- Easy : Pollen fossils are found in rocks because sporopollenin does not decay easily.
- Medium : In more than 60 per cent of angiosperms pollen is shed at the 2-celled stage; in the rest the generative cell divides before shedding (3-celled stage).
- Exam-type : Q: Why is the exine hard? A: Sporopollenin protects the male gametes from heat, acids and drying; germ pores let the pollen tube come out.
Pollen grain = male gametophyte (n). Rice and wheat pollen lose viability in about 30 minutes, but pollen of many species can be stored for years in liquid nitrogen (–196 °C) in pollen banks.
Thinking the tapetum is a protective layer. The three outer layers protect; the innermost tapetum nourishes.
Memory clue: E-E-M-T — Epidermis, Endothecium, Middle layers, Tapetum — from the outside in.
Spot the difference
| Point | Exine | Intine |
|---|---|---|
| Position | Outer wall | Inner wall |
| Made of | Sporopollenin | Cellulose and pectin |
| Germ pores | Present | Absent |
Practice questions
- 1What is a microsporangium also called?
- 2Name the male gametophyte of a flowering plant.
- 3Which wall layer nourishes the developing pollen?
- 4What is microsporogenesis?
- 5State the two parts of the pollen grain wall.
- 6How many cells does a pollen grain have when it is shed at the 3-celled stage?
- 7Why can pollen grains be preserved as fossils?
- 8A student says every cell in a tetrad is diploid. Correct the student.
Show answers (try them yourself first)
- 1A pollen sac.
- 2The pollen grain.
- 3The tapetum.
- 4The formation of microspores from a pollen mother cell by meiosis.
- 5The outer exine and the inner intine.
- 6One vegetative cell and two male gametes (three cells).
- 7Their exine is made of sporopollenin, one of the most resistant organic materials, which no known enzyme breaks down.
- 8Meiosis produces four haploid (n) microspores, so every cell of the tetrad is haploid.
Pistil, ovule and the embryo sac
A mango has just one seed, a papaya has hundreds. The difference starts inside the flower: the ovary holds one ovule or many ovules, and each ovule hides a tiny female gametophyte.
The pistil has stigma, style and ovary. The ovary holds ovules (megasporangia) fixed to the placenta by a funicle. An ovule has integuments, an opening called micropyle, a food-rich nucellus and a base called the chalaza. The embryo sac (female gametophyte) forms from one megaspore and is 7-celled and 8-nucleate.
In the figure : One megaspore mother cell gives one functional megaspore, which divides three times to give eight nuclei.
In the figure : Three cells at the micropylar end, one big central cell, and three antipodals at the chalazal end.
Rule 1: Megasporogenesis — The megaspore mother cell (MMC) in the nucellus divides by meiosis into four megaspores. In most plants only one stays functional and the other three degenerate (monosporic development).
- Easy : The MMC is diploid; each megaspore is haploid.
- Medium : Nucellus and MMC are 2n, but the functional megaspore and the embryo sac are n.
- Exam-type : Q: Why must the MMC divide by meiosis? A: So that the female gametes are haploid and fertilisation restores the diploid number.
Rule 2: Structure of the mature embryo sac — The first three divisions are free-nuclear; walls form only after the 8-nucleate stage. Six nuclei become cells and the two polar nuclei lie in the large central cell. The egg apparatus (one egg + two synergids) sits at the micropylar end; the synergids have a filiform apparatus that guides the pollen tube.
- Easy : Embryo sac: 3 + 3 + 1 = 7 cells.
- Medium : Eight nuclei but seven cells, because the central cell holds two nuclei.
- Exam-type : Q: Name the cells at the chalazal end. A: Three antipodal cells.
Embryo sac = 8 nuclei, 7 cells. Egg apparatus (egg + 2 synergids) at the micropylar end; 3 antipodals at the chalazal end; 2 polar nuclei in the central cell.
Saying the embryo sac has eight cells. It has eight nuclei but only seven cells.
Memory clue: 3-1-3 — Three cells up (egg apparatus), one central cell, three antipodals down.
Spot the difference
| Point | Micropylar end | Chalazal end |
|---|---|---|
| Cells present | Egg + 2 synergids | 3 antipodals |
| Role | Receives pollen tube | Base of ovule |
| Special feature | Filiform apparatus | Opposite the micropyle |
Practice questions
- 1What is the stalk of an ovule called?
- 2Which tissue inside the ovule stores food?
- 3How many cells make up a mature embryo sac?
- 4What is the opening in the integuments called?
- 5Name the three parts of the egg apparatus.
- 6What does monosporic development mean?
- 7Why are the first three nuclear divisions called free-nuclear?
- 8What is the ploidy of the nucellus, the MMC, a megaspore and the embryo sac?
Show answers (try them yourself first)
- 1The funicle.
- 2The nucellus.
- 3Seven cells (eight nuclei).
- 4The micropyle.
- 5One egg cell and two synergids.
- 6The embryo sac forms from a single functional megaspore.
- 7The nuclei divide without cell walls forming; walls are laid down only after the 8-nucleate stage.
- 8Nucellus 2n, MMC 2n, megaspore n, embryo sac n.
Pollination and pollen–pistil interaction
Bees buzz from flower to flower; a corn cob wears silky threads that wave in the wind. Both are tricks to bring pollen to the stigma, because pollen cannot walk by itself.
Pollination is the transfer of pollen grains from the anther to the stigma. By source of pollen it is autogamy (same flower), geitonogamy (another flower of the same plant) or xenogamy (different plant). The agents are abiotic (wind, water) or biotic (animals).
In the figure : Only xenogamy brings genetically different pollen to the stigma.
In the figure : The flower's shape and colour match its pollinating agent.
Rule 1: Kinds and agents of pollination — Cleistogamous flowers never open, so they are always autogamous. Geitonogamy is cross-pollination in action but genetically like self-pollination. Wind pollination is common in grasses; water pollination is rare (Vallisneria, Hydrilla, Zostera); bees are the main animal pollinators.
- Easy : Maize and grasses use wind.
- Medium : A flower that never opens (cleistogamous, as in Viola) still sets seeds without any pollinator.
- Exam-type : Q: Why is geitonogamy genetically similar to autogamy? A: The pollen comes from the same plant, so the genes are the same.
Rule 2: Outbreeding devices and pollen–pistil interaction — Plants discourage self-pollination by pollen and stigma ripening at different times, different positions of anther and stigma, self-incompatibility, and unisexual flowers (monoecious like castor and maize stop autogamy; dioecious like papaya stop both autogamy and geitonogamy). The pistil accepts compatible pollen and rejects the wrong type.
- Easy : Papaya has male and female flowers on different plants.
- Medium : Monoecious plants prevent autogamy but not geitonogamy.
- Exam-type : Q: What is self-incompatibility? A: A genetic mechanism that stops self-pollen from germinating or growing a tube in the pistil.
Pollen grains are carried by chance in wind and water, so these plants make a huge amount of pollen. Continued self-pollination causes inbreeding depression.
Calling geitonogamy "self-pollination because it is the same plant" and ignoring that it needs an agent like cross-pollination.
Memory clue: A-G-X — Autogamy = same flower, Geitonogamy = same plant, Xenogamy = different plant (going further away).
Spot the difference
| Point | Autogamy | Xenogamy |
|---|---|---|
| Pollen source | Same flower | Different plant |
| Genetic variation | None new | New combinations |
| Pollinating agent | Not needed | Needed |
Practice questions
- 1Define pollination.
- 2Name one water-pollinated plant.
- 3What are cleistogamous flowers?
- 4Why do wind-pollinated flowers make a lot of pollen?
- 5Which type of pollination brings genetically different pollen to the stigma?
- 6What do pollen and nectar act as for animals?
- 7How does dioecy help a plant avoid inbreeding?
- 8A pistil rejects pollen of another species. At which two points can the rejection occur?
Show answers (try them yourself first)
- 1Transfer of pollen from the anther to the stigma.
- 2Vallisneria (or Hydrilla or Zostera).
- 3Flowers that do not open, so self-pollination happens inside the bud.
- 4Pollen landing on a stigma is a matter of chance, so a large quantity makes up for the loss.
- 5Xenogamy.
- 6Floral rewards.
- 7Male and female flowers are on separate plants, so both autogamy and geitonogamy are impossible.
- 8It can stop pollen germination on the stigma or stop pollen tube growth in the style.
Pollen tube and double fertilisation
A pollen grain landing on a stigma is like a seed starting to sprout, but it must grow a long thin tube down the style to reach a target hidden in the ovary.
A compatible pollen grain germinates through a germ pore and its pollen tube grows through the stigma and style to the ovule, entering through the micropyle into a synergid and releasing two male gametes. One fuses with the egg (syngamy) and the other with the two polar nuclei (triple fusion). This pair of fusions is double fertilisation, found only in flowering plants.
In the figure : Two fusions occur in one embryo sac; hence the name double fertilisation.
Rule 1: Path of the pollen tube — Pollen grain → germinates on stigma → tube through style → enters ovule at the micropyle → discharges two male gametes into a synergid. The filiform apparatus of the synergids guides the tube.
- Easy : The tube enters the ovule by the micropyle.
- Medium : If the pollen is shed at the 2-celled stage, the generative cell divides inside the pollen tube to form the two male gametes.
- Exam-type : Q: What guides the pollen tube into the synergid? A: The filiform apparatus.
Rule 2: Two fusions, two products — Syngamy gives the diploid zygote. Triple fusion gives the triploid primary endosperm nucleus (PEN) in the central cell, which becomes the primary endosperm cell (PEC) and then the endosperm.
- Easy : Zygote is 2n; PEN is 3n.
- Medium : Three nuclei (two polar nuclei + one male gamete) join, so PEN has three sets of chromosomes.
- Exam-type : Q: Why is the process called double fertilisation? A: Two fusions, syngamy and triple fusion, take place in one embryo sac.
Zygote → embryo. PEC → endosperm. Both male gametes are used up in the embryo sac.
Thinking triple fusion involves three male gametes. It involves one male gamete and two polar nuclei.
Memory clue: SZ-TP — Syngamy → Zygote; Triple fusion → PEN (3n).
Spot the difference
| Point | Syngamy | Triple fusion |
|---|---|---|
| Fusing nuclei | Male gamete + egg | Male gamete + 2 polar nuclei |
| Product | Zygote (2n) | PEN (3n) |
| Develops into | Embryo | Endosperm |
Practice questions
- 1What is syngamy?
- 2Name the product of triple fusion.
- 3Through which opening does the pollen tube enter the ovule?
- 4Which cell does the central cell become after triple fusion?
- 5What is the chromosome number of the zygote compared with the egg?
- 6Name the cells that receive the contents of the pollen tube.
- 7Why is double fertilisation called unique to flowering plants?
- 8Both polar nuclei and the male gamete are haploid. Show why PEN is triploid.
Show answers (try them yourself first)
- 1Fusion of a male gamete with the egg cell.
- 2The primary endosperm nucleus (PEN), which is triploid.
- 3The micropyle.
- 4The primary endosperm cell.
- 5The zygote is diploid (2n); the egg is haploid (n).
- 6A synergid.
- 7Syngamy and triple fusion both occur in the embryo sac only in angiosperms.
- 8n + n + n = 3n.
Post-fertilisation: endosperm, embryo, seed and fruit
A tender coconut gives you sweet water and white kernel. Both are endosperm, at two different stages of development.
After fertilisation the PEN forms endosperm (food tissue), the zygote forms the embryo, the ovule becomes the seed and the ovary becomes the fruit. Endosperm development comes before embryo development, so that the embryo is assured of food.
In the figure : Everything in the ovary is transformed together.
In the figure : Early stages of embryo development are alike in monocots and dicots.
Rule 1: Endosperm and embryo — The PEN divides again and again to form free nuclei (free-nuclear endosperm) and then cell walls form (cellular endosperm). Tender coconut water is free-nuclear endosperm and the white kernel is cellular endosperm. A dicot embryo has two cotyledons and an embryonal axis (plumule, epicotyl, hypocotyl, radicle); a grass embryo has one cotyledon called the scutellum with coleoptile and coleorrhiza.
- Easy : The radicle forms the root; the plumule forms the shoot.
- Medium : In maize the single cotyledon is the scutellum, and the shoot tip is covered by the coleoptile.
- Exam-type : Q: Which part of the dicot embryo lies above the cotyledons? A: Epicotyl, ending in the plumule.
Rule 2: Seeds and fruits — A seed has seed coat, cotyledon(s) and embryonal axis. In non-albuminous seeds (pea, groundnut) the endosperm is used up; in albuminous seeds (wheat, maize, castor) some remains. Perisperm is persistent nucellus (black pepper, beet). The ovary wall becomes the pericarp. Fruits formed without fertilisation are parthenocarpic (banana); in false fruits (apple, strawberry, cashew) the thalamus also joins.
- Easy : Pea seed has no endosperm left.
- Medium : Perisperm in black pepper is leftover nucellus, not endosperm.
- Exam-type : Q: What is parthenocarpy? A: Fruit development without fertilisation; such fruits are seedless.
A seed is a fertilised ovule. Seed dehydration and dormancy let us store grain for the whole year. The oldest viable seed recorded is lupine, germinated after about 10,000 years.
Writing that the ovary becomes the seed. The ovary becomes the fruit; the ovule becomes the seed.
Memory clue: O-S, O-F — Ovule → Seed, Ovary → Fruit.
Spot the difference
| Point | Albuminous seed | Non-albuminous seed |
|---|---|---|
| Endosperm in mature seed | Present | Used up |
| Examples | Wheat, maize, castor | Pea, groundnut |
| Food stored in | Endosperm | Cotyledons |
Practice questions
- 1Name the product of the ovule after fertilisation.
- 2What is the wall of the fruit called?
- 3Which part of tender coconut is free-nuclear endosperm?
- 4What is the single cotyledon of grasses called?
- 5Give one example of a parthenocarpic fruit.
- 6What is perisperm?
- 7Why does endosperm develop before the embryo?
- 8Why are seed dehydration and dormancy useful for farmers?
Show answers (try them yourself first)
- 1The seed.
- 2The pericarp.
- 3The coconut water.
- 4The scutellum.
- 5Banana.
- 6The remains of the nucellus that persist in a mature seed (black pepper, beet).
- 7It ensures that food is ready for the developing embryo.
- 8Dry, dormant seeds can be stored all year for food and used to raise a crop in the next season.
Apomixis and polyembryony
A farmer has to buy costly hybrid seeds every year, because the next generation of a hybrid loses the good traits. What if a hybrid could produce seeds that are exact copies of itself?
Apomixis is the formation of seeds without fertilisation; it is asexual reproduction that looks like sexual reproduction. Polyembryony is the presence of more than one embryo in a seed, as in many Citrus and mango varieties, where nucellar cells grow into extra embryos.
In the figure : Apomixis keeps the hybrid's good traits from one generation to the next.
Rule 1: How apomictic seeds form — Either a diploid egg cell forms without meiosis and develops without fertilisation, or nucellar cells next to the embryo sac divide and grow into embryos.
- Easy : Many orange seeds contain several embryos.
- Medium : Nucellar embryos are genetically identical to the mother plant, so they are clones.
- Exam-type : Q: Why are nucellar embryos clones? A: They arise from diploid nucellus cells by mitosis, with no gametes involved.
Rule 2: Why apomixis matters — If hybrid crops become apomicts, farmers can keep and sow their own hybrid seed every year without the characters segregating, which would reduce the seed cost.
- Easy : No need to buy new hybrid seed each year.
- Medium : Hybrid seed production is costly because it must be done afresh every year.
- Exam-type : Q: Mention one use of apomixis in agriculture. A: Fixing hybrid vigour in a crop so that seeds can be reused.
Apomixis is common in some grasses and Asteraceae. Seeds are produced without fertilisation, so the offspring are clones of the parent.
Saying apomixis is a type of fertilisation. It is a form of asexual reproduction.
Memory clue: A-pomixis → A-sexual — Apomixis, like A-sexual, starts with A: no fusion, no variation.
Spot the difference
| Point | Apomixis | Polyembryony |
|---|---|---|
| Meaning | Seed without fertilisation | More than one embryo per seed |
| Example | Some grasses, Asteraceae | Citrus, mango |
| Genetic nature | Clone | Often clones (nucellar) |
Practice questions
- 1Define apomixis.
- 2Define polyembryony.
- 3Name one plant group in which apomixis is found.
- 4From which cells do the extra embryos of an orange seed arise?
- 5Is apomixis sexual or asexual?
- 6Why do hybrid seeds need to be produced every year?
- 7How can apomixis reduce the cost of hybrid seed?
- 8Is an embryo formed from a nucellar cell haploid or diploid? Explain.
Show answers (try them yourself first)
- 1Production of seeds without fertilisation.
- 2The presence of more than one embryo in a seed.
- 3Some grasses or species of Asteraceae.
- 4From the nucellus cells around the embryo sac.
- 5Asexual reproduction.
- 6Seeds from a hybrid crop segregate, so the hybrid characters are lost in the next generation.
- 7Apomictic hybrids keep their characters, so farmers can reuse the seeds and need not buy new ones each year.
- 8Diploid, because the nucellus is diploid and mitosis does not change the chromosome number.