Fertilization in Flowering Plants

Fertilization is the process of fusion of the female gamete, the ovum or egg and the male gamete produced in the pollen tube by the pollen grain. Fertilization in flowering plants was discovered by Strassburger in 1884.

Flowers are the reproductive structures of angiosperms. They vary greatly physically and are of great diversity in methods of reproduction. The process of fertilization in plants occurs when gametes in haploid conditions meet to create a zygote which is diploid.

The male gametes of the flower are transferred on to the female reproductive organs through pollinators. The final product of this process is the formation of embryo in a seed.

In this lesson you will learn about :
1) What is Fertilization?
2) The Pollen Grain (Male Gamete)
3) The Ovule (Containing Female Gamete)
4) Microsporogenesis and Megasporogenesis
5) Germination of Pollen Grain
6) Double Fertilization
7) Post Fertilization Events

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1. Fertilization in Flowering Plants

2. Introduction
Fertilizationisthefusionofthenucleiofmaleand
femalegametestoformazygote.
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Infloweringplants–
➢Thepollengrainisthemalegamete.
➢Theovuleinsidetheovaryisthefemalegamete.

3. The Pollen Grain
Varyinsizefrom10-200µ.
Eachpollenhastwocoats–
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Exine
Intine
Thisistough,cutinizedouterprotectivecoat.Itis
oftenprovidedwithspinyoutgrowthsorarough
surfaceasanadaptationforpollination.
Itisathindelicatecelluloselayerlyinginternalto
theexine.Itistheintinethatgrowsintothepollen
tube.
Thenucleusofmaturepollengrainhasalreadydivided
intotwonucleiatthetimeofpollination.Thetwonuclei
are–thetubenucleusandthegenerativenucleus.
At this stage, the pollen is transferred to stigma (pollination). Further changes in
pollen grain occur only if it has fallen on the stigma of a plant of the same species.
A Mature Pollen Grain

4. Microsporogenesis
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Theformationofmicrosporesorpollengrainsinside
themicrosporangiumisknownasmicrosporogenesis.
Whentheantherisyoung,alayerofhomogenouscells
occupiesthecenterofeachmicrosporangium,isknown
assporogenoustissue.Itisdiploid(2n)innature.
Thesporogenoustissueundergomeioticdivisionsto
formmicrosporetetrads(fourmicrospores)whichare
haploid(n).
Thesemicrosporesmaturetoformpollengrains,male
gametes.Eachmicrosporangiumcontainsthousandsof
microsporesorpollengrains.
Pollensormicrosporesareverytinyroundstructures.
Aftertheformation,microsporesorpollengrainsdry
upandbecomepowdery.Theantherbecomesadry
structureandpollensareliberatedfromtheantherto
theenvironmentbydehiscenceoftheanther.

5. The Ovule
Ovuleistheinnerpartofovary.
Partsofamatureovule:
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Integuments
Micropyle
Twolayersofprotectivecoating.
Minuteporeoropeningthroughwhichthepollentube
enters.
Nucellus
Embryosac
Nutritivetissueliningtheinterioroftheovule.
Inthecentreofnucellus,itcontainssevencells–
Threecellsatthemicropylarend–oneeggcelland
twosynergids.
Threecellsatoppositeend,calledantipodalcells.
Onelargecentralcell.Thecentralcellisdifferent
containingtwonucleicalledpolarnuclei.
Embryo Sac

6. BiswarupMajumder| Bio-World
Themegasporemothercell(diploidcell)dividesmeiotically
toformfourmegaspores.Outofthesefourmegaspores,
threedegeneratesandoneremainsfunctional.The
functionalmegasporesdividemitoticallytoform2-celled
embryosac.Furthersubsequentmitoticdivisionfinally
forms8-celledembryosac.
Sixoftheeightnucleiaresurroundedbycellwallsand
organizedintocells;theremainingtwonuclei,calledpolar
nucleiaresituatedinthelargecentralcell.Threecellsat
themicropylarendformstheeggapparatus.Theegg
apparatusismadeupoftwosynergidsandoneeggcell.
Threecellsatthechalazalendisknownasantipodals.So,
thematureembryosacis7celledand8nucleated.
Megasporogenesis

7. Ineukaryotes,therearetwodistincttypesofcelldivision:avegetativedivision,
wherebyeachdaughtercellisgeneticallyidenticaltotheparentcell(mitosis),anda
reproductivecelldivision,wherebythenumberofchromosomesinthedaughter
cellsisreducedbyhalftoproducehaploidgametes(meiosis).
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Comparison
Diploid-(acellornucleus)containingtwocompletesetsofchromosomes,onefrom
eachparent.
Haploid-(acellornucleus)havingasinglesetofunpairedchromosomes.

8. BiswarupMajumder| Bio-World
Pollengraingerminatesonlyifitfallsonthestigmaof
thesameplantspecies,otherwise,itdisintegrates.
Thepollengrainonreachingtothestigmaabsorb
nutrientssecretedbythestigmaandthepollentube
beginstogrow.
Thetubenucleusremainsclosetothegrowingtipof
thepollentubeandeventuallydisintegrates.
Meanwhile,thegenerativenucleusdividesintotwomale
gametesornucleiwhichmovetothetipofthetubeand
lieinthemassofcytoplasm.
Thetubethenturnstowardsthemicropyle,passes
inwardsthroughit,andfinallyreachestheembryosac,
closetotheeggcell.
Afterpollentubepenetratesintotheembryosac,the
tipofthetubedissolvesandthemalegametesareset
freeinsidetheovule.
Onemalegametefuseswiththeeggcellandformsthe
fertilizedeggorthezygote.
Thesecondmalegametefuseswiththesecondary
nucleus(definitivenucleus),i.e.,thenucleusformedby
thefusionofthepolarnucleiandformstheendosperm
nucleus.Thefusionofmalegametewiththetwopolar
nucleiisoftentermedastriplefusion.
Thezygotedevelopsintotheembryo,whilethe
endospermnucleusdevelopsintoendospermofthe
seed.
Germination of Pollen Grain

9. BiswarupMajumder| Bio-World
Pollen grain
Tube Nucleus Generative Nucleus
Stimulates
growth of
pollen tube and
eventually
disintegrates.
Sperm Nucleus Sperm Nucleus
Egg Cell Definitive
Nucleus
+ +
Oospore
Endosperm
Nucleus
Embryo Endosperm
Summary

10. Double fertilization is a characteristic feature of angiosperms. It was first
discovered by Sergei NawaschininLiliummartagonandFritillariatenella.
Double Fertilization
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Infloweringplants,twosetsoffertilizationtakes
place.Oneoccursbetweenthespermnucleusandthe
eggcell,whiletheothertakesplacebetweenthe
secondspermcellandthedefinitivenucleus.Thus,
fertilizationinfloweringplantsisreferredtoasdouble
fertilization.
If,forsomereason,theeggcellfailstofertilize,the
synergids(alsocalledhelpcells)assumetheroleofthe
eggcell.Incasefertilizationoccursbetweentheegg
cellandmalenuclei,thesynergidsdisintegratesoon
afterfertilization.Antipodalcellsalsohavenopositive
role.Therealsogetdisorganized,sometimeseven
beforefertilization.

11. The calyx may either fall off or may remain intact in a dried and shrivelledform.
Apple and guava show such dried sepals very clearly, in brinjalit remains.
Post Fertilization
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Afterfertilization,theeggcellsurroundsitselfwitha
cellwallandisknownastheoospore.
Theovaryenlargestoformthefruitandtheovarian
wallformsthefruitwall.Theovulesbecometheseed.
Afruitmaycontainoneormoreseeds.Whenthefruit
isripe,theseedscontainedwithinthefruitisreleased
byoneofthevariousmethodsofdispersalandgrow
intoanewplantundersuitableconditions.