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BiologyPlant Kingdom / Morphology
Match the following reproductive parts of a flower with their correct functions:
A. Stigma → I. Produces pollen grains
B. Anther → II. Connects stigma to ovary
C. Style → III. Receives pollen grains
D. Ovary → IV. Contains ovules, develops into fruit
Options
1
A-III, B-I, C-II, D-IV
2
A-I, B-II, C-III, D-IV
3
A-II, B-III, C-IV, D-I
4
A-IV, B-I, C-II, D-III
Correct Answer
A-III, B-I, C-II, D-IV
Solution
1

A. Stigma: sticky surface that receives pollen = III

B. Anther: produces pollen grains = I

2

C. Style: connects stigma to ovary, pollen tube grows through = II

D. Ovary: contains ovules, develops into fruit = IV

Answer: A-III, B-I, C-II, D-IV

Stigma=pollen receptor | Anther=pollen producer | Style=connector | Ovary=fruit precursor
Theory: Plant Kingdom / Morphology
1. Flower Structure

Flower: reproductive unit of angiosperms. Parts: Calyx (sepals) - outermost whorl, green, protects flower bud. Corolla (petals) - attract pollinators, colourful. Androecium (stamens) - male: filament + anther (bilobed, 4 microsporangia). Gynoecium (pistil/carpel) - female: stigma + style + ovary. Floral formula: represents flower structure symbolically. Complete flower: has all 4 whorls. Incomplete: missing one or more. Bisexual (perfect): both stamen and pistil. Unisexual (imperfect): staminate (male) or pistillate (female). Actinomorphic (regular): radially symmetrical. Zygomorphic (irregular): bilaterally symmetrical (pea flower).

2. Microsporogenesis and Pollen

Anther wall layers: epidermis, endothecium (helps pollen release), middle layers, tapetum (innermost, nutritive). Pollen mother cell (2n) in microsporangia undergoes meiosis to form tetrad of microspores (n). Each microspore develops into pollen grain: 2-celled (vegetative + generative) or 3-celled (vegetative + 2 sperm). Pollen wall: exine (outer, sporopollenin - most resistant biological polymer) + intine (inner, cellulose + pectin). Pollen apertures: colpus (groove) or porus (pore) for pollen tube emergence. Pollen morphology used in palynology (fossil pollen identification) and plant taxonomy.

3. Megasporogenesis and Embryo Sac

Ovule parts: funicle (stalk), hilum, chalaza, micropyle, integuments (1 or 2), nucellus, megaspore mother cell. MMC (2n) undergoes meiosis to form linear tetrad of 4 megaspores (n). Usually 3 degenerate, 1 functional megaspore (chalazal). Functional megaspore undergoes 3 mitoses: 2-nucleate, 4-nucleate, 8-nucleate embryo sac. 8-nucleate becomes 7-celled embryo sac: egg apparatus (3 cells: 1 egg + 2 synergids), central cell (2 polar nuclei), antipodals (3 cells). Polygonum type most common.

4. Pollination

Transfer of pollen from anther to stigma. Self-pollination (autogamy): same flower or same plant. Cross-pollination (xenogamy): different plant. Agents: Anemophily (wind): light pollen, no petals, feathery stigma, many pollen. Entomophily (insect): colourful petals, nectar, scent, sticky pollen. Ornithophily (birds): red/orange flowers, lots of nectar. Chiropterophily (bats): dull, nocturnal, musty scent. Hydrophily (water): rare, Vallisneria (underwater), Zostera (marine). Outbreeding mechanisms: dichogamy (stamens/pistils mature at different times), herkogamy (physical barrier), self-incompatibility (S-gene system prevents self-pollen germination).

5. Fertilisation and Double Fertilisation

Pollen germinates on stigma. Pollen tube grows through style (guided by synergid chemotaxis - LURE peptides). Enters ovule through micropyle. Bursts into synergid. Two sperm released. Sperm 1 + egg cell = diploid (2n) zygote = embryo. Sperm 2 + 2 polar nuclei = triploid (3n) primary endosperm nucleus = endosperm. This is double fertilisation - unique to angiosperms. Discovered by Nawaschin (1898). Triple fusion: 1 sperm + 2 polar nuclei = triple fusion. Significance: efficient - endosperm only forms when embryo forms, no waste.

6. Fruit and Seed Development

After fertilisation: Ovule becomes seed. Integuments become testa (seed coat). Ovary wall becomes pericarp (fruit wall). Ovary becomes fruit. Zygote develops into embryo through proembryo, globular, heart, torpedo stages. Endosperm: provides nutrition to embryo. Endospermic seeds (albuminous): endosperm present in mature seed (coconut, castor, wheat, rice, maize). Non-endospermic (exalbuminous): endosperm absorbed by cotyledons during embryo development (pea, bean, groundnut). Cotyledons: seed leaves. Monocot: 1 cotyledon (scutellum). Dicot: 2 cotyledons. Perisperm: remnant nucellus tissue in some seeds (black pepper, coffee).

7. Fruits Classification

Simple fruits: from single flower, single ovary. Fleshy: berry (tomato, banana, grape - no stone), drupe (mango, coconut, peach - with hard stone = endocarp), pome (apple, pear - thalamus forms edible part). Dry dehiscent: legume/pod (pea, bean - opens along two sutures), capsule (cotton, poppy), follicle (calotropis - splits one side), siliqua (mustard). Dry indehiscent: achene (sunflower seed in shell), caryopsis (grain - pericarp fused to seed coat, wheat, rice, maize), nut (oak, chestnut - hard pericarp). Aggregate fruits: from multiple free carpels of single flower (raspberry = aggregate of drupelets). Multiple/composite fruits: from inflorescence (pineapple - fleshy rachis + flowers, mulberry - fleshy perianth, jackfruit - fleshy bracts). False/pseudofruits: fleshy part not from ovary (apple - thalamus, strawberry - thalamus, cashew - peduncle).

8. Apomixis and Polyembryony

Apomixis: asexual seed formation (seeds without fertilisation). Types: Agamospermy (embryo without fertilisation from egg or sporophytic cells). Vegetative apomixis (bulbils, vegetative propagules). Examples: dandelion, grasses, some Citrus (polyembryony from nucellus). Polyembryony: more than one embryo in seed. True polyembryony: multiple eggs fertilised. False polyembryony: additional embryos from nucellus cells (adventive embryony - common in mango, orange, onion). Cleavage polyembryony: zygote cleaves to give multiple embryos. Significance for agriculture: if hybrid crops could be made apomictic, hybrid seed could be saved and replanted without genetic segregation (major goal for crops like rice, maize). Currently hybrid seed must be bought fresh each year.

Frequently Asked Questions
1. What is the pollen tube path from stigma to egg cell? ⌄
Pollen grain lands on stigma. Pollen recognises stigma signals (compatible = germinates). Pollen tube emerges from germination aperture (colpus or porus). Grows through style using tip growth - secretes enzymes (cutinase, cellulase, polygalacturonase) to digest cell walls. In 3-celled pollen: 2 sperm already present. In 2-celled pollen: generative cell divides during pollen tube growth. Pollen tube guided by chemotaxis: synergid cells secrete LURE peptides (defensin-like proteins) that attract the pollen tube. Tube enters ovule through micropyle (porogamy - most common) or through chalaza (chalazogamy - Casuarina, some nuts) or through integument (mesogamy). Bursts inside synergid (activated by FERONIA receptor), releases 2 sperm cells.
2. Why is double fertilisation considered an advancement in angiosperms? ⌄
Double fertilisation is unique to angiosperms (and Gnetales). Advantages: (1) Efficiency: endosperm (3n) only forms when a sperm fertilises the polar nuclei. This means nutritive endosperm is only produced when an embryo is actually formed. Compare gymnosperms: large food-storing tissue (megagametophyte) forms even if no fertilisation occurs - wasteful! (2) Triploid endosperm: having extra genome copy (3n) compared to embryo (2n) - some suggest this provides extra gene expression for rapid cell proliferation. (3) The endosperm is genetically diverse (contains genes from both parents plus maternal polar nuclei contribution) - this may reduce sibling competition via kin selection arguments. Double fertilisation likely contributed to the evolutionary success of angiosperms as the dominant land plant group.
3. How do stigma and pollen recognise compatibility? ⌄
Pollen-stigma recognition: complex molecular dialogue. On stigma surface: secreted proteins, lipids (pollen coat proteins on anther, exine lipids). On pollen: coat proteins from tapetum (tapetin, oleosins), exine-embedded proteins. Recognition system: in dry stigma (Brassica, Arabidopsis): S-locus encodes SRK (S-Receptor Kinase) on stigma + SCR/SP11 (S-locus Cysteine-Rich protein) on pollen. If SRK and SCR are from same S-haplotype = self = rejection (phosphorylation cascade activates ARC1 ubiquitin ligase, degrades ARC1 substrates needed for germination). If different S-haplotypes = cross-compatible = pollen germinates. This molecular self-incompatibility system promotes outbreeding and genetic diversity.
4. Explain the difference between ovary, ovule, seed and fruit? ⌄
Ovary: part of pistil (carpel), contains ovules. Becomes the fruit after fertilisation. Ovule: structure inside ovary containing the female gametophyte (embryo sac with egg cell). Becomes the seed after fertilisation. Embryo sac: female gametophyte inside ovule. Contains egg cell + synergids + central cell (polar nuclei) + antipodals. Seed: mature ovule after fertilisation. Contains embryo + food reserve (endosperm or cotyledons) + seed coat (from integuments). Fruit: mature ovary (sometimes includes other flower parts). Contains seeds. Protects seeds, aids dispersal. Summary: flower forms on plant. Within flower, pistil contains ovary. Ovary contains ovules. After fertilisation: ovule becomes seed, ovary becomes fruit. Multiple ovules in one ovary = multiple seeds in one fruit (tomato).
5. What is the significance of the tapetum in anther? ⌄
The tapetum (innermost layer of microsporangium wall) is arguably the most important anther layer. It directly surrounds and nourishes the developing microspores. Functions: (1) Nutritive: secretes nutrients into the anther locule for developing pollen. (2) Sporopollenin synthesis: provides precursors (carotenoids, fatty acids) for sporopollenin biosynthesis that makes up the pollen exine wall. (3) Pollenkitt: secretes the oily coating on pollen surface that: sticks pollen together, protects pollen, contains recognition proteins for pollinator attraction and pollen-stigma interaction. (4) Ubisch bodies (orbicules): small sporopollenin granules on tapetum inner surface. (5) Compatibility proteins: secretes proteins that coat pollen during maturation - important for pollen-stigma recognition. Tapetum is amoeboid (moves between microspores) in some species. Tapetum dysfunction = male sterility (commercially valuable for hybrid seed production).
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