Overview — Continuing the species

Reproduction is production of one’s own kind for continuity of the species and replacement of dead members. Modes vary: cell division, fragmentation, buds, spores, or elaborate sexual organs (stamens and pistils). All organisms pass hereditary material to offspring. This NIOS Module 3 lesson covers vegetative, asexual and sexual modes; Chlamydomonas and Spirogyra; flowering-plant sexual cycle (flower, pollen, ovule, pollination, double fertilisation, seed, fruit); germination; natural and artificial vegetative propagation; and micropropagation.

After this lesson you should define reproduction; compare the three modes; describe lower-plant and angiosperm sexual paths; draw pollen and embryo sac ideas; state pollination types; explain double fertilisation and seed/fruit; and list vegetative and tissue-culture methods. Notes follow textbook order only. Link to L7: flower parts and placentation. Link to L11: photoperiod and flowering factors.

Section 1: Modes of reproduction (19.1)

Three types: vegetative, asexual, sexual.

Asexual and vegetative (no gamete fusion): single parent; offspring genetically identical (clones); only mitosis needed for growth of the new unit.

• Vegetative: new plantlets from somatic cell, bud or organ (root, stem, leaf, bud detaches and grows). Discussed fully for angiosperms later.
• Fragmentation: filament breaks; each piece with ≥1 cell grows (Spirogyra).
• Fission: parent cell content → 2, 4 or 8 daughters (bacteria, yeasts).
• Budding: outgrowth separates (yeast).
• Asexual by spores: flagellate or not; bryophytes/pteridophytes; higher plants heterosporous — microspores → male gametophyte, megaspores → female after meiosis.

Sexual: fusion of haploid male and female gametes → diploid zygote → new individual. Meiosis at some stage; offspring not exact clones — mixed parental characters.

Apomixis: asexual seed formation without pollination/fertilisation (e.g. dandelions). Diploid somatic cell of ovule becomes embryo; ovule → seed still dispersed like sexual seeds. “Away from act of mixing.” MCQ option: development without fusion of gametes — not darkness or cold alone.

Sexual reproduction is not “better” in every niche: clones preserve a proven genotype in stable habitats; sex pays off when environments change because offspring vary. Exam answers should still list single parent and identical offspring for asexual/vegetative, and fusion plus genetic difference for sexual.

Section 2: Chlamydomonas (19.2.1)

Haploid freshwater unicellular alga; pear-shaped; two flagella; eye spot; cup chloroplast with pyrenoid; central nucleus.

Asexual: with water → zoospores (flagellate, thin wall) by mitosis of protoplast (2–16), parent wall ruptures. Thin film → aplanospores (non-flagellate); colonial-looking Palmella stage; flooding restores flagella. Drought → thick dark hypnospores that later release zoospores.

Sexual: isogamy (morphologically identical gametes, e.g. C. eugametos); anisogamy (larger female, smaller male — C. braunii); oogamy (non-motile egg, motile sperm — C. coccifera, C. ooganum). Zygote thick-walled resting stage; only diploid stage; meiosis → 4 haploid zoospores → adult plants.

Isogamy detail: non-motile parents form 32–64 flagellate gametes by mitosis; fusion of gametes from different mothers → quadriflagellate zygote → resting zygospore. Anisogamy: male side more gametes (32–64), female fewer larger (8–16); larger gamete becomes non-motile and is fertilised by smaller. Oogamy: entire female content is one egg; many male gametes enter but one fertilises. In all cases meiosis is in the zygote — key one-liner for “where does meiosis occur in Chlamydomonas?”

Section 3: Spirogyra (19.2.2)

Filamentous freshwater alga; spiral chloroplasts with pyrenoids; large vacuole; central nucleus on cytoplasmic strands.

Vegetative: fragmentation at septa. Sexual conjugation:

• Scalariform: two filaments ladder-like; conjugation tubes; male content moves into female; zygospore in female cells; male filament empty. Meiosis in zygospore → 4 haploid nuclei, 3 degenerate → one filament.
• Lateral: one filament; male and female cells alternate in pairs; lateral tube across septum.

Body cells form gametes without meiosis → plant is gametophyte (haploid), like Chlamydomonas.

Scalariform sequence for long answers: filaments align septum-to-septum → conjugation tubes → cytoplasm rounds into gametes → male amoeboid transfer → zygospore thickens and darkens for unfavourable period → meiosis on return of favour → three haploid nuclei degenerate → one nucleus forms young filament by mitosis. Lateral conjugation uses one filament only, with alternating male/female pairs and tubes beside septa. After scalariform sex, male filament cells are empty; female cells hold zygospores — exam diagram clue.

Section 4: Angiosperm flowering framework (19.3)

Sexual reproduction by fusion of gametes in the flower — reproductive unit. Annuals complete cycle in one season (pea); biennials vegetative then flower second season (radish); perennials live years (mango). Monocarpic reproduce once then die (bamboo, agave, annuals/biennials); polycarpic flower many times (mango, guava).

Juvenile vs adult shoot: adult responds to flowering stimuli; cereals need minimum leaf number. Factors: vernalisation (cold promotes flowering); photoperiodism (day/night length). Sex of flowers: bisexual or unisexual; hormones can reverse sex (gibberellin → male in Cucumis; auxin/ethylene → female; also Cannabis).

Flower whorls on thalamus: calyx, corolla (accessory); androecium, gynoecium (essential). Absence of stamens or carpels fails sexual reproduction.

Juvenile shoots are soft with few young leaves and do not respond to flower stimuli; adult shoots are branched, leaves change form/size, and can flower. Transition from vegetative apex to floral apex may take years in trees but weeks in annuals. Cereals need a minimum leaf number (textbook: about seven) before flowering. Photoperiodism is response to length of light and dark in sequence; vernalisation is cold pretreatment that speeds flowering in some species. External gibberellin can induce male flowers on genetically female Cucumis; auxin or ethylene can produce functional female flowers on male plants — also seen in Cannabis.

Section 5: Male side — microsporangium and pollen (19.4)

Stamen = anther (4 microsporangia/pollen sacs) + filament. Microspore mother cells (MMC) meiosis → haploid microspore tetrads. Wall layers include epidermis, middle layers, tapetum (nourishes pollen). Mature microspore = pollen grain = male gametophyte precursor.

Pollen wall: durable exine (sporopollenin, germ pores) + thin intine. Nucleus divides → large vegetative cell + small generative cell; released at dehiscence. Pollen itself is not the male gamete — it produces male gametes (sperm). Generative cell divides to two sperms (often by three-celled stage on stigma).

Microsporogenesis sequence: four pollen sacs in anther → sporogenous tissue as MMCs with dense cytoplasm → meiosis yields tetrads of haploid microspores → free pollen grains. Wall layers outer epidermis, thin middle layers, tapetum of large nutritive cells. Exine patterns help identify species under microscope. Germ pores are thin areas where the pollen tube emerges. At dehiscence the stomium splits along a line of weakness — mechanical release of dry pollen into air or onto insect bodies.

Section 6: Female side — ovule and embryo sac

Pistil: stigma, style, ovary with ovules. Ovule: nucellus, integuments, micropyle, funiculus, chalaza. MMC meiosis → 4 megaspores; usually 3 degenerate; functional megaspore → 3 mitoses → 8 haploid nuclei → embryo sac.

Mature embryo sac: 7 cells, 8 nuclei — egg apparatus (egg + 2 synergids) at micropyle; 3 antipodals at chalaza; central cell with 2 polar nuclei (may fuse as secondary nucleus). Synergids guide pollen tube; antipodals degenerate for nutrition; egg + sperm → zygote; secondary nucleus + sperm → primary endosperm nucleus (3n).

Ovule development story: projection on placenta → nucellus covered by integuments leaving micropyle → raised on funiculus. Opposite micropyle is chalaza. Only one hypodermal cell usually becomes MMC. Three successive free nuclear mitoses in the functional megaspore create eight nuclei before walls form around all but the central pair. A fully developed embryo sac with nucellus, integuments and funiculus awaits pollination then fertilisation. Label exam diagrams carefully: micropyle, egg, synergids, polar nuclei, antipodals, integuments, funicle.

Section 7: Pollination (19.4.1)

Transfer of pollen from anther to stigma. Self: same flower or same plant (pea, gram). Cross: another plant of same species (palm, maize). Importance: fertilisation → seed; gene recombination (cross); pollen-tube hormones stimulate fruit.

Agencies: anemophily (wind — small dull flowers, lots of light pollen, large hairy stigma — grasses); entomophily (insects — large showy, nectar — Salvia lever mechanism with bees); hydrophily (water — Hydrilla, Vallisneria); zoophily (birds, squirrels). Artificial hybridisation by humans.

Promote cross: unisexuality; dichogamy (sexes mature at different times); self-sterility. Ensure self: cleistogamy (closed flowers); homogamy (same-time maturity — groundnut).

Dichogamy examples: anther matures before stigma in sweet pea and Salvia; carpel matures before anther in custard apple. Self-sterility: pollen fails on own stigma even if placed carefully (Petunia, apple). Unisexual flowers may be on same plant (maize) or different plants (papaya, palm). Insect flowers may secrete nectar; Salvia shows a special lever mechanism when bees visit. Wind flowers produce huge pollen numbers because most grains never reach a stigma. Water-pollinated pollen floats until it hits female flowers of Hydrilla or Vallisneria. Humans perform artificial pollination deliberately for hybrids — same biological transfer, controlled by breeders.

Section 8: Fertilisation and post-fertilisation (19.4.2–19.4.5)

On right stigma pollen germinates → pollen tube through style → micropyle → releases 2 sperms. If pollen is not already three-celled, generative division yields two male gametes plus vegetative/tube cell. Tube nucleus leads the tip then degenerates; two sperms occupy the tip. Tube enters via a synergid and bursts. Syngamy: sperm + egg → 2n zygote. Triple fusion: sperm + 2 polar nuclei → 3n primary endosperm nucleus. Together = double fertilisation (unique to angiosperms).

Significance of fertilisation: stimulates ovary→fruit growth; recombines parental genes in the zygote. Endosperm development starts before embryo so food is ready (nuclear type common in maize, wheat, rice — free nuclei then walls; cellular from first division; helobial intermediate). Endosperm may be used up in dicot seeds (pea, bean) or persist massively (cereals, coconut). Embryo: zygote → upper embryonal + lower suspensor cell; suspensor pushes embryo into endosperm; embryo differentiates radicle, plumule, cotyledon(s). Integuments harden as seed coat. Polyembryony: more than one embryo — adventive (synergids/antipodals) or cleavage (zygote splits).

Seed = ripened ovule. Gram (dicot): testa/tegmen, hilum, micropyle, two fleshy cotyledons. Maize (monocot grain): fused coats/pericarp, endosperm + aleurone, scutellum, coleoptile/coleorhiza. Fruit = ripened ovary; protects and disperses seeds. Ripening: starch→sugar, esters for flavour, chlorophyll breakdown. Parthenocarpy: fruit without fertilisation (banana, grapes) — seedless commercial value.

Section 9: Germination (19.4.6)

Dormancy: dry inactive embryo until moisture, temperature, oxygen suitable. Steps: imbibition → swelling → enzymes mobilise food → radicle then plumule emerge. Epigeal: cotyledons above ground (bean, castor). Hypogeal: cotyledons stay below (maize, rice).

Imbibition occurs through micropyle and seed coat; seed swells as hydrated; reserve starch, protein and fat convert to soluble glucose, amino acids and fatty acids for the growing axis. Radicle emerges first as root, then plumule as shoot. Epigeal growth of hypocotyl lifts cotyledons to form first leaves (castor, neem, bean); hypogeal weak hypocotyl leaves cotyledons buried while plumule pushes up (maize, rice). Dormancy is adaptive: avoids germination in drought or cold and times seedling establishment to favourable windows.

Section 10: Vegetative reproduction (19.5–19.7)

New plants from vegetative parts; clones of parent.

Natural: underground stems — rhizome (ginger), tuber (potato), bulb (onion), corm (zamikand); subaerial — offset (Pistia), sucker (Chrysanthemum); tuberous roots (sweet potato); leaf buds (Bryophyllum); bulbils (Agave, Oxalis).

Artificial: cuttings (rose, sugarcane); layering (jasmine — bent branch soil-covered); gootee/aerial layering; grafting (stock + scion — mango, rose; dicots mainly). Grafting: scion of desired variety on disease-resistant stock; vascular union.

Advantages: rapid; true-to-type; stores food for survival; cheap multiplication of improved varieties. Disadvantages: overcrowding; no new genetic varieties (except mutation); diseases spread fast.

Section 11: Micropropagation (19.8)

Tissue culture: small sterile explant on nutrient medium → callus (unorganised mass) → hormones induce plantlets → harden into soil. Advantages: unlimited identical plants from tiny parent tissue. Used in orchids, carnations, Chrysanthemum, Asparagus in India.

Graft question: Dasehri scion on desi stock → grafted branch bears Dasehri type; other stock branches remain desi — scion genotype decides fruit of that shoot.

Section 12: Exam map and closed-book drill

Define reproduction; asexual vs sexual; apomixis; Chlamy zoospore/isogamy/meiosis in zygote; Spirogyra fragmentation/scalariform; annual/biennial/perennial; flower essential whorls; tapetum; pollen exine/intine; embryo sac structure; self/cross pollination; anemophily vs entomophily; double fertilisation; seed vs fruit; parthenocarpy; epigeal/hypogeal; vegetative organs; cutting/layering/grafting; pros/cons vegetative; callus/micropropagation.

One-line keys: pollen = male gametophyte; embryo sac = female gametophyte; syngamy + triple fusion; 7 cells 8 nuclei; cleistogamy closed selfing; dichogamy different maturity times; scion = graft shoot; stock = rooted base.

Use Formula Sheet; drill 10 MCQs and 20 flashcards. Prioritise double fertilisation diagram, embryo-sac labels, pollination tables, and vegetative method examples.

Section 13: Extra depth for full coverage

Why sexual reproduction matters: meiosis and fertilisation reshuffle genes — raw material for selection and variety. Vegetative keeps elite fruit genotypes true (mango varieties) because no recombination. Apomixis is a natural “seed clone” path plant breeders study for hybrid seed fixation.

Double fertilisation efficiency: endosperm forms only if fertilisation succeeds, matching food store to embryo need. Nuclear endosperm in cereals becomes the bulk of grain food for humans. Persistent endosperm vs non-endospermic dicot seeds (pea stores food in cotyledons) is a standard comparison.

Pollination ecology: wind is wasteful but works for open grasslands; insect mutualism trades nectar for precision transfer. Devices against selfing promote outbreeding and heterozygosity. Cleistogamy guarantees seed set when pollinators fail.

Grafting compatibility is mostly among dicots with cambium for vascular union. Scion determines aerial traits (fruit quality); stock can influence vigour and disease resistance of the root system — board answers should name both roles.

Germination needs water, oxygen and suitable temperature; dormancy is an evolutionary pause. Epigeal cotyledons become first photosynthetic leaves; hypogeal keep reserves underground protected.

Micropropagation equals industrial vegetative cloning: disease-free starts, rare orchids multiplied, year-round production. Callus is undifferentiated — hormone ratios decide root vs shoot induction (exam need only “hormones induce differentiation”).

Closed-book drill: (1) three modes + apomixis; (2) Chlamy asexual + sexual types; (3) Spirogyra conjugations; (4) annual/biennial/perennial + mono/polycarpic; (5) flower whorls; (6) microsporogenesis outline; (7) embryo sac cells; (8) pollination types + two agencies; (9) double fertilisation equations; (10) dicot vs monocot seed; (11) fruit vs parthenocarpy; (12) epigeal vs hypogeal; (13) five natural vegetative examples; (14) three artificial methods; (15) vegetative pros/cons; (16) micropropagation steps. Completing these covers Lesson 19 terminals.

Section 14: Seed, fruit and vegetative detail bank

Seed importance: contains embryo; coat protects against drying and damage; storable and transportable for dispersal. Gram seed: pod enclosure, conical shape, hilum attachment, testa fused with tegmen, micropyle pore, two fleshy cotyledons around axis. Maize grain: broader; seed coats fused with pericarp; massive endosperm with protein-rich aleurone; one cotyledon scutellum; coleoptile protects plumule, coleorhiza protects radicle. Fruit significance: seed protection, soil enrichment on decay, dispersal. Ripening chemistry: starch to sugar; esters for aroma; chlorophyll loss changes colour.

Parthenocarpy commercial points: seedless eating quality; abortive seeds that will not grow; high growth hormones in tissue. Adventive polyembryony = embryo from non-egg cell of embryo sac; cleavage polyembryony = multiple embryos from one zygote.

Natural vegetative table memory: Asparagus/sweet potato roots; runner grass; sucker mint; bulb onion; tuber potato; rhizome ginger; Bryophyllum leaf notches; bulbil pineapple/oxalis. Artificial: rose/money plant cuttings; jasmine layering; citrus/mango grafting; orchids/chrysanthemum tissue culture. Layering rings bark before soil burial so roots form above the ring; gootee uses moss and polythene on high woody branches. Graft: wedge scion into slit stock, tape until vascular continuity.

Vegetative advantages expanded: speed, genetic fidelity for horticulture, perennation organs store food through bad seasons, low cost. Disadvantages: overcrowding competition, no breeding progress except mutation, monoculture disease risk. Micropropagation steps: explant → sterile medium → callus multiplication → hormone medium → plantlets → gradual soil transfer. Advantage: unlimited identical plants from tiny tissue.

Quick formula strip: zoospore/aplanospore/hypnospore; scalariform vs lateral; double fertilisation 2n+3n; 7-cell embryo sac; anemophily vs entomophily; seed=ovule fruit=ovary; epigeal/hypogeal; stock/scion; callus. Say each with one example plant. Module 3 continues into animal reproduction and genetics — keep meiosis and gamete vocabulary sharp.

Terminal-style memory: if Dasehri mango scion is grafted on desi stock, the grafted branch fruits Dasehri while other branches of the stock tree remain desi — scion genotype rules that shoot. Define corm, scion, callus, micropropagation and vegetative reproduction in one line each before the paper. List wind- and water-pollinated traits side by side. Sketch mature ovule and mature pollen from memory with five labels each. That closes the sexual and vegetative halves of Lesson 19. You are exam-ready when double fertilisation, embryo-sac labels, and three vegetative methods each with an example all come without opening the notes again on the day of the full Module 3 NIOS biology examination paper for this academic year of senior secondary biology study workbook notes.