Megaspore-Structure & Speciality

This PowerPoint presentation intends to explore the beauty of megaspores and some specialities found in ovules.

1. OVULE:
EXCEPTIONAL BEAUTY

2. Welcome to
Ovule-Structure, Types & Specialty
Dr.N. Sannigrahi, Associate Professor,
Department of Botany,
Nistarini College, Purulia, 723101(W.B) India

3. MYSTREYOFBIOLOGY
Plantsaretreasureofallthelifeformsacrosstheglobe.The
beautyoftheplantsanditsreproductionisanawesomeappetite
forthenaturalscienceandscientists.Irrespectiveofthenature
exceptsomecases,theplantslifeiscomprisingofthetwo
generations-Sporophyticgeneration&gametophytegenerations.
Thesporophyticgenerationsstartsfromthesinglecell,zygote.
Thezygotesundergoesdivisionmitoticallytoformembryoand
throughaseriesofbiochemicalandmorphologicalchanges,the
embryoalongwiththeotherrequisitesturnintoseed.Seedisthe
enigmaoflife,undergoesgerminationtopromiseababyplants
enjoyingthedifferentnaturalattributesasrequired.The
sporophyticgenerationssubjecttothecompletionofvegetative
phaseundergoesreproductivestage.

4. Thereproductivestagesstartswithabeautyoftheflower-an
outcomeofthegenesandthedifferentenvironmentalattributes.
Thefloweristhemostattractivepartoftheplantlife
irrespectiveofitsdiversity.Theflowercomprisingofthe
accessoryandessentialwhorls.Thestamen-themalepart&the
carpel-thefemalepartsundergoamutualrelationshiptopromise
thebirthofanotherbabyplant.Themalegametophyte
generationstartswiththehaploidmicrosporeandthefemale
gametophyticgenerationsdevelopfromthemegaspore
popularlyknownasOvule.Ovulebearsanumberofcharacters
andthisisalwaysapleasuretothebiologists.Letusexamine
theovuleanditsdiversityandhowtheovulebringsthe
transformationforanotherzygotetoinitiatetheanother
sporophyticgeneration.

5. OVULE
Ovule,thedevelopmentalprecursorofseedsistheorganofprime
importanceinangiospermflowersanditcanbetracedbackin
evolutionalmost400millionyearsagototheearliestseedplantsas
farasphylogenyisconcerned.Havingdiversepositioninovary,
nucellusthickness,number&thicknessofinteguments,degreeand
directionofcurvatureandhistologicaldifferentiationshavedrawn
theimportanceofplantbiologists.
Theovuledevelopsasmulticellularplacentaloutgrowthincluding
theepidermal&numberofhypodermalcells.Withfurther
development,thisgivestothenucellusandoneortwointeguments
fromitsbasalregion.
Angiospermovulesbasicallyconsistsofanucellusandtwo
integumentsandsessileontheplacentaorattachedbyitsstal

6. WHERE FROM THE OVULE ORIGINATES?

7. Mostcommonlyavascularstrandsextendsfromtheplacenta
throughthefuniculustothechalaza,i.e.theareabelowthe
baseofthenucelluswheretheintegumentsdepart.The
funiculusandthechalazaareintercalarystructureandthus
welldemarcatedthanthenucellusandinteguments.The
nucellusrepresentsthemegasporangium,inwhichthe
meiocytesundergoesmeiosisformingfourmegaspores,
typicallyonlyoneofwhichdevelopsintoembryosac
representingthemegagametophyte.Theembryosaccontains
basicallyfouroreightnuclei,organizedintofourorseven
cells,dependingonwhethertwoorthreeroundmitotic
divisionsinthedevelopingembryo(Maheswari,
1950;Friedman,2006).

8. HOW DOES AN OVULE LOOK?

9. COMPONENTSOFOVULE
Atypicalangiospermovuleconsistsofthefollowings:
Funicle-Stalkoftheovulethatconnectsovuletothe
placenta,short&multicellular.
Hilum-Pointwherefunicleconnectsthemainbodyofthe
ovule.
Integuments-Layersthatsurroundtheovule-outer&inner
integument’mainfunctiontoprotecttheinnertissues.
Micropyle-Openingwhichpresentatthetopofthe
integumentscalledmicropyle.Itallowsentryofthepollen
tubeintotheovule.

10. COMPONENTS OFOVULE
Nucellus-Integumentssurroundsparenchymatoustissues
whichconstitutesthemainbodyoftheovulecallednucellus.
Thenucellusisnutritiveinfunctiontothedevelopingembryos.
Chalaza-Theswollenpartoftheovulepresentoppositeto
micropyleiscalledchalaza.
Embryosac-Itispresentinthenucellusanditcontainsfemale
gametecalledegg.
Nutrientsfromtheplantstravelthroughphloemofthevascular
systemtothefuniculusandtheouterintegumentandfromthere
apoplasticallyandsymplasticallythroughthenucellusinside
theovule.

11. ENDOTHELIUM
Theendotheliumisanadditionalcelllayer,differentiatingfromthe
innerepidermisoftheovuleintegument.Intomato(Solanum
lycopersicumL.),afterfertilization,theendotheliumseparatesfrom
integumentandbecomesanindependenttissuedevelopingnextto
thegrowingembryosac.Intheabsenceoffertilization,the
endotheliummayproliferateandformpseudo-embryo.Endothelial
cellsinnormallydevelopingovuleschangetheirstructuraland
functionalspecializationfrommeristematictosecretaryandbackto
meristematic,andproliferateuntilseedsfullymature.Thesecretary
activityoftheendotheliumisnecessaryforthelysisofdyingcells
oftheintegumentandprovidesthespaceforthegrowthofthenew
sporophyte.

12. OBTURATOR
Anyovularstructurewhichhelpsindirectingthegrowthofpollen
tubetowardsthemicropyleisgenerallyreferredtoasobturator.
Obturatorsexhibitgreatvariationintheirorigin,morphology,
anatomyandextentofdevelopment.Theymayoriginatefrom
placentaorfuniculusorboth.Withintheovary,theobturator
bridgesthepathwayofthepollentubefromthestyletothe
ovule.Inseveralfamiliesofangiospermsacharacteristicplacental
outgrowthispresentneartheovuleandisreferredtoasobturator.
Thisobturatorgrowsintothespacebetweenthenucellusandthe
integumentsandalsobetweentheovaryandovarywall.An
obturatorissaidtobehelpingindirectingthegrowthofpollen
tubetowardsmicropyle.Obturatorexhibitsagreatamountof
diversityinitsorigin,morphology,anatomyandmodeof
development.

14. CARUNCLE
Thecaruncleisastructurepresentinthemicropylarregionof
Euphorbiaceaeseeds.Thisstructurehastheecologicalfunctionof
promotingseeddispersalbyants(myrmecochory),butitis
debatedwhetheritalsohasanagronomicalimportance
influencingseedgermination.Thecaruncleisthespongy
outgrowthoftheintegumentnearmicropyle.Themain
functionofcaruncleistoabsorbwaterwhichisnecessaryfor
germination.Theexampleofseedwhichcontainscaruncleis
castorseeds.Butrecentinvestigationrejectstheroleofcaruncle
intheseedgerminationasfaraswaterabsorptionisconcerned.

16. ARIL
Theterm"aril"issometimesappliedtoanyfleshyappendage
oftheseedinfloweringplants,suchasthemaceofthenutmeg
seed,lichietc.Arilsandarillodesareoftenedibleenticements
thatencourageanimalstotransporttheseed,therebyassistingin
seeddispersal.Theseedsperformimportantfunctionsofthe
plantswhicharenourishmentofembryo,dispersalandtravelsto
newplacesanddormancyduringunfavorableconditions.Seeds
aredefinedastheproductsofsexualreproductionwhicharea
sourceofgeneticvariabilitywhichcanbeinducedintheplants.

18. HYPOSTASE
Hypostasesisoneofthemostunusualfeaturedevelopedinthe
chalazalregionoftheembryosacandsituatedimmediately
belowit.Itrepresentsanirregularlyoutlinedgroupofnucellar
cellshavingpoorcytoplasmiccontentsbuthavetheirwalls
partiallylignifiedorsuberisedduetothethickeningbythe
secondarywallmaterials.ThetermwasfirstusedbyVan
Tieghamtodescribeagroupofunusualcellsinthechalazal
region.AsfarasTiegham,itformsabarrierorboundaryto
preventthefurthergrowthoftheembryosac.Though
hypostasesislimitedtothebasalregionoftheembryosac,
occasionally,theymaycovertheembryosacextendingupto
mycopylarhalfportion.Ithasbeenreportedinthemembersof
thefamiliesofAmaryllidaceae,Viliaceae,Zingiberaceae,
Euphorbiaceae,Theaceae,Apiaceaeetc.,

20. Female gametophyte–megasporogenesis (monosporic, bisporic
and tetrasporic)
Dependingonthenumberofmegasporestakingpartinthe
development,theembryosac(Femalegametophyte)maybe
threetypesasfollows:
MONOSPORIC:Outofthefourmegasporesdevelopedbythe
meioticcelldivisionofthemegasporemothercells,ifonlyone
celltakespartinthedevelopmentofthefemalegametophyte,itis
calledmonosporicdevelopmentofembryosac.
BISPORIC:Ifoutofthefourmegaspores,onlytwomegaspores
nucleitakespartinthedevelopmentoftheembryosac,itis
calledbisporic.
TETRASPORIC:However,allthefourmegasporesnucleitake
partinthedevelopmentoftheembryosac,itiscalledtetrasporic
developmentoffemalegametophyte.

22. MEGAGAMETOGENESIS (POLYGONUM TYPE)
i.Thefunctionalmegaspore,thefirstcellofthefemaledivides
bythreesuccessivedivisionstoformaneight–nucleate
gametophyteorembryosac,
ii.Here,thenucleiofthefunctionalmegasporedividestoform
twonuclei-theprimarymicropylarandprimarychalazalnuclei,
iii.Thenucleiagaindividetoformfournuclei,
iv.Eachofthenucleiagaindividetoformeightcellstagetoform
thecompleteoctateembryosac,
v.Thefemalegametophyteincreasesinsize,onenucleifrom
eachendorpolepassesinwardsandthetwopolarnucleifuse
togethersomewhereinthemiddleoftheembryosacformingthe
secondarynucleus(2N),
vi.Theremainingthreenucleiatthemicropylarend,each
surroundedbyaverythinwall,formeggapparatus

24. vi.Theeggapparatusconsistsoftwosynergidsandaneggcell.
Theotherthreenucleiattheoppositechalazalend,lyingina
group,oftensurroundedbyverythinwalls,formtheantipodal
cells.
vii.Attheendofthedevelopment,theembryosacgettingready
fortheexecutionoftheotherbiologicalprocesstobetakeninto
accountfollowedbytheentryofthepollentubeforthe
fertilizationtolaythefoundationofthefirstsporophyticcell.
Thiskindofmicrosporedevelopmentismonosporicassingle
microsporeoutofthefourderivedtakespartinthedevelopment
oftheembryosac.ThisisalsotreatedasPolygonumtypeof
embryosacdevelopmentasitwasfirstfoundtooccuramongthe
membersofPolygonaceae.

25. TYPESOFOVULE
Theovulesarecharacterizedbyaconsiderablediversityin
shape,degreeofdevelopmentandstructure.Mirbel(1929)for
thefirsttimeclassifiedovulesunderfourgroups-Orthotropus,
Anatropus,Campylotropous&Amphitropous.Butlateron
anothertwotypesaddedmakingtheovulesaltogethersix(6)
typesasfollows:
Orthotropousoratropous
Anatropous
Hemi-anatropousorHemitropous
Campylotropous
Amphitropous
Circinotropous

26. Different types of ovules

27. TYPESOFOVULE
Asstatedearlier,Ovulesmayhavediversity,.Whenonlyone
integumentpresentcalledunitegmic,iftheovulecontainstwo
integumentscalledbitegmicandveryrarelythreeinteguments
calledtritegmicispresentinplantslikeAsphodelus.Insome
plantslikeSantalum,nointegumentcalledategmicisalsofound
tooccur.
Onthebasisofthepositionofmicropyle,withrespectto
funiculus,Ovulesareclassifiedundersixbroadcategoriesas
statedearlier.Thedetailsaregivenbelow.

28. OrthotropousOvule:Itisatropousorstraightwherethe
micropyle,Chalaza&funiculus,allareinthesameline.E.g.
Cycas,membersofPolygonaceaeandPiperaceae.
AnatropousOvule:In80%genera,thisisverycommontype.In
thisovule,thefunicleislong,wholebodyisinvertedthrough
180⁰,Asaresult,micropylecomesclosetothefunicle.Most
commonalmostmonocotsanddicotslikethemembersof
Asteraceae,Solanaceaeetc.
HemianatropousorHemitropousOvule:Thebodyofthe
ovuleisinvertedthrough90⁰.Asaresultthefuniclecomesto
lieatrightangletothenucellus.Micropyleandthechalazalie
inthesameplane.Membersofprimitivefamily
RananunculaceaelikeRanunculusetc.

29. CampylotropousOvule:Whenthebodyoftheovuleisnot
invertedbutitappearslikehorseshoe.Themicropyleandthe
chalazadonotlieonthesameplane,howeverthenucellusor
embryosacremainstraight.Membersofthefamilylike
Capparidaceae,Brassicaceae,CaryophyllaceaeandFabaceae.
AmphitropousOvule:ItisalmostsimilartoCampylotropousbut
thenucellusorembryosacisbentlikehorseshoe.Membersof
Alismataceae.
CircinotropousOvule;Itisveryrareinoccurrence.Here,the
bodyoftheovuleisbentthrough360⁰,sothatittakesa
completeturn.Micropyle,Chalazaandthenucellusareonthe
sameplane.

30. Your Patience deserves appreciation. Thanks a lot

31. REFERENCES:
Encyclopedia Britannica,
An introduction to the Embryology of angiosperms by P.
Maheswari,
A text book of Botany by Hait, Bhattacharya, Ghosh,
Advanced Plant taxonomy by A.K.Mondal,
DifferentwebsitesfromGoogleforimagesandcontents.
DISCLAIMER:
Thispresentationhasbeenmadetoenrichopensourceof
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ofknowledgetodevelopthisPPT.