1 Biology 2015 – Evolution and Diversity Lab 5: Seed Plants Introduction Of a total of roughly 300,000 plant species, nearly 90% are seed plants. They form two informal groups, Gymnosperms (naked seed) and the Angiosperms (enclosed seed). The Gymnosperms include four lineages: Cycads, in the class Cycadopsida; Ginkgo biloba, the only species in the class Ginkgoopsida; three genera of Gnetophytes in the class Gnetopsida; and the familiar Conifers of the class Pinopsida. You'll see representatives of all four of these lineages today. The Angiosperms include two major unranked subgroups, the monocots and the dicots. You'll see a representative of each of these groups in today's lab. Seeds are complex propagules that greatly facilitate the dispersal and establishment of new plants. Seeds start out as ovules–megasporangia in which a female gametophyte develops and produces one or more eggs. After fertilization, seeds contain the diploid embryo of a new generation, and nutritive tissue consisting of, or derived from, the female gametophyte. These are enclosed by the sporangium and integuments (the seed coat) produced by the previous sporophyte generation. Therefore, three of the alternating generations of the plant's life cycle contribute to one reproductive structure! But seeds are only one of the evolutionary innovations found in the seed plants. Others include the evolution of pollen and pollination, of secondary growth, and of larger and more efficient xylem and phloem conduits. Pollen grains are mature male gametophytes enclosed within a tough wall of sporopollenin. Pollen may be disseminated great distances in the wind, or be carried by many Figure 1. Phylogeny of land plants.
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Biology 2015 – Evolution and Diversity Lab5:SeedPlants
IntroductionOfatotalofroughly300,000plantspecies,nearly90%areseedplants.Theyformtwoinformalgroups,Gymnosperms(nakedseed)andtheAngiosperms(enclosedseed).TheGymnospermsincludefourlineages:Cycads,intheclassCycadopsida;Ginkgobiloba,theonlyspeciesintheclassGinkgoopsida;threegeneraofGnetophytesintheclassGnetopsida;andthefamiliarConifersoftheclassPinopsida.You'llseerepresentativesofallfouroftheselineagestoday.TheAngiospermsincludetwomajorunrankedsubgroups,themonocotsandthedicots.You'llseearepresentativeofeachofthesegroupsintoday'slab.Seedsarecomplexpropagulesthatgreatlyfacilitatethedispersalandestablishmentofnewplants.Seedsstartoutasovules–megasporangiainwhichafemalegametophytedevelopsandproducesoneormoreeggs.Afterfertilization,seedscontainthediploidembryoofanewgeneration,andnutritivetissueconsistingof,orderivedfrom,thefemalegametophyte.Theseareenclosedbythesporangiumandinteguments(theseedcoat)producedbytheprevioussporophytegeneration.Therefore,threeofthealternatinggenerationsoftheplant'slifecyclecontributetoonereproductivestructure!Butseedsareonlyoneoftheevolutionaryinnovationsfoundintheseedplants.Othersincludetheevolutionofpollenandpollination,ofsecondarygrowth,andoflargerandmoreefficientxylemandphloemconduits.Pollengrainsarematuremalegametophytesenclosedwithinatoughwallofsporopollenin.Pollenmaybedisseminatedgreatdistancesinthewind,orbecarriedbymany
Figure1.Phylogenyoflandplants.
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insectpollinatorsaswellassomebirdsandmammals.Pollinationefficientlydeliversspermtotheegg.It'sagreatimprovementoverthemethodusedbyseedlessplants,wherespermcellsmustswimtheirwaytoanegg.Seedplantsarethelargestofplants,andthesizeofthelargestseedplantsismadepossiblethepresenceofvascularcambium(alateralmeristem)andsecondarygrowth.Secondarygrowthallowsthestemsandrootstoproduceindefinitelylargequantitiesofxylemandphloem.Intemperatezones,atleast,onefamiliarresultofthatsecondarygrowthisthepresenceofannualringsincrosssectionsofwoodystems.Increasedvasculartransport,andthesupportprovidedbymanylignifiedxylemcellsinthickstemsandroots,allowsseedplantstobecomeverylarge.TheAngiospermshaveotherevolutionaryinnovationsthathavecontributedtotheirsuccess(over250,000oftheapproximately300,000plantspeciesareAngiosperms).ThevesselsthattransportwaterinAngiospermxylemarebothlongerandwiderthanthoseofmostGymnospermsandseedlessplants.ButthegreatsuccessofAngiospermsisduelargelytotwooftheirreproductivestructures:flowers,whichcangreatlyincreasetheefficiencyofpollination;andfruits,whichgreatlyaidseeddispersalandestablishment.
GymnospermsPinopsida(Conifers)ThePinopsidsarethelargestgroupwithintheGymnosperms,containingabout550species.Theyincludethepines,junipers,firs,hemlocks,spruces,redwoods,yewsandDouglas-fir.AlthoughwewillusepinestorepresentGymnosperms,wewillalsohavespruceandothermaterialsavailable.ThematuresporophyteObtainoneofthepinebranchesyou'llfindonthelabbenches.First,noticethegreenneedles.Eachoneisaleaf.You'llnoticethattheneedlesarefoundinclusters.Yourbranchescontainneedlesinclustersoftwo,butotherspeciesmayhaveneedlesinclustersofthreeorfive,andinonespecies,theneedlesoccursingly.Thenumberofneedlesinaclusterisoneofthefirstthingsyouneedtoknowinidentifyingapinespecies.Yourbranchhastwokindsofshoots(ashootisabranchanditsattachedleaves).Thebranchitselfisanexampleofalongshoot.However,eachclusterofneedlesisactuallyonashortshoot-averyshortbranchwithshortdistancesbetweenleaves.Inseedplants,eachbrancharisesfromtheaxilofaleaf-thepointonastemrightabovealeaf.Therefore,sinceeachclusterofneedlesisashortbranch,itmustariseintheaxilofaleaf.Lookforthisleaf.Howisitdifferentfromtheneedles?Eachofyourbrancheswillincludecones.Andconescomeintwotypes:male(staminate)cones,inwhichthepollenisproduced,andfemale(ovulate)conesinwhichtheseedsareproduced.Conesaremodifiedshortshoots.
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Thefreshbranchesonthelabbenchesallhavefemalecones.Eachbranchwillhaveoneortwoverysmallfemalecones,locatednearthetipofthebranch.Itwillalsohaveatleastonemorematurefemalecone,locatedfartherbackonthebranch.Thesmallerfemaleconeswereformedduringlastyear'sgrowingseason,andwerepollinatednearlyayearago!Thelargerfemaleconeswereformednearlytwoyearsago.Theseconeswereallclosedwhenthebrancheswerecollected,butinthewarm,drylaboratoryroom,mostwilllikelyhaveopenedbynow.Youwilllikelyfindwingedseedsonthelabbenchunderthebranches,andyoumayfindotherseedsstillrestingontheconescales.Maleconesaresmallerthanmaturefemalecones,andareverydelicate.Themaleconesfoundonfreshbranchesatthistimeofyeararedarkreddish-brownincolor,andoftennoticeablycurved.Ifyoutouchthem,theywilltendtofallapartinyourhands.Wedidn'tcollectmaleconesthisyear,buthavepreservedbranchesondisplaythatincludefreshmaleconescollectedlaterinthespring,severalyearsago.Thesecones,andbranches,arestillveryfragile,sopleasehandlethemgently.Male(staminate)cones.
FindapreparedslidelabeledPineStaminateConec.s.&l.s.
Youshouldidentifythesporophylls(modifiedleaves)thatbearthesporangiaontheirlowersurfaces.Meiosisoccursinsidethesporangia.Thesporesthatresultfrommeiosiseachdevelopintoamulticellulargametophytethatformswithinthesporewall.Thegametophyteismalebecauseitproducesonlysperm.Whenthegametophyteismature,itiscalledapollengrain.Thesporangiaproducedinmaleconesarecalledmicrosporangiabecausetheyproducetherelativelysmallsporesthatdevelopintothemalegametophytes.Thestaminatecone,orstrobilus,isamodifiedshoot.Eachsporophyllontheshootsupportstwomicrosporangia.Thissporangiumcontainsnumerousmaturepollengrains.PollenIntheslideofthestaminatecones,youshouldseepollengrainswithinthesporangia.NoticethateachpollengrainhasMickeyMouseears.Theearsorwingsareoutgrowthsofthepollenwallthatarethoughttohelpinwinddissemination.
Figure2.Pinemicrosporangium.Thesporangiumformsontheundersideofamodifiedleaf,thesporophyll.
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FindapreparedslidelabeledPineMaturePollen.Trytocountthenucleiinmaturepollengrains.Noticethatthougheachpollengrainisamalegametophyte,therearenosperm-producingantheridia.Antheridiahavebeenlostinseedplants.Onenucleusinthepollengrainiscalledthegenerativenucleus.Afterpollinationthisnucleuswilldividetoproducethesperm.Asecondnucleusiscalledthetubenucleus.Female(ovulate)cones.Findthepine,spruce,Douglas-firandhemlockconesondisplay.Noticethatwhiletheyshareanoverallsimilarity,theyarenonethelessquitedistinctive.Examinetheexposedinnersurfacesofthepinecone.Howmanyseedswereproducedoneachscale?Lookforindentationsatthebaseofeachscale(wheretheseedswerelocated)andforimprintsoroutlinesofthewings(extensionsoftheintegumentthataidinwinddissemination)ontheinsidesurfacesoftheconescales.Eachscaleonyourpineconeisactuallyamodifiedbranch.Ifbranchesformintheaxilsofleaves,wherearethoseleaves?Well,theleavesonyourpineconeareactuallyfusedwiththescalesattheirbase,andareverysmall,sotheyarehardtodistinguish.ExamineaDouglas-fircone.Here,youshouldbeabletoseethemodifiedleaves(calledbracts)thatstickoutfrombetweenthescales.
FindapreparedslidelabeledPineOvulateConel.s.
Findtheconescalesandovules.Anovuleisamegasporangiumprotectedbycelllayerscalledanintegument.EachMegasporangiumproducesasinglesurvivingspore,whichthendevelopsintoasinglefemalegametophyteperovule.Theovulehasanopeningatoneend.It'scalledthemicropyle.Isitfacinginwardoroutward?Atfertilization,pollentubeswillentertheovuleatthemicropyle.Youmaybeabletoseepollengrainslocatednearthemicropyle.
Figure3.Pinepollengrain.Whenyouexaminetheslideofmaturepinepollen,you'llfindthattheorientationofeachgraininfluenceswhichnucleiyousee.Takealittletimetofindpollengrainsthatshowboththegenerativeandtubenuclei.
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FindapreparedslidelabeledPineOvulel.s.
Nearthemicropyle,justinsidethegametophyte,amaturefemalegametophytewillhavetwoormorearchegonia.Eachwillcontainasingleegg.Thoughmorethanoneeggmaybefertilized,onlyonezygoteislikelytofullydevelopintoamatureembryo.PineseedsFindthepineseedswehaveondisplayandtakeacoupleofthembacktoyourworkstationtoexamine.TheseedswehavearepinenutsofthePinyonPine,Pinusedulis.Takeaseedapartbyfirstcrackingthehardoutercovering(yourteethwillworkforthis).Thatoutercoveringistheseedcoat–theintegument.Inside,youmayfindathin,paperylayerthatistheremainsofthesporangiawall.Insidethatlayeristhefemalegametophyteandthesporophyteembryoitcontains.Theslightlypointyendofthegametophyteisthemicropylarend.Gentlyopenthefemalegametophytetoexposethesporophyteembryoinside.(Arazorblade,andthenadissectingneedle,mayproveusefulhere.)Theembryowillbeelongated,andwillhavelittleneedleleavesononeend.Theseembryoleavesarecalledcotyledonsorseedleaves.Alltheremainingtissuearoundtheembryoisgametophytetissuethatisconsumedbythedevelopingembryoastheseedgerminates.Becauseofthenutrientsstoredinthegametophytetissue,theseseedsareeatenbyavarietyofmammalsandbirds.Manyoftheseanimalscacheseedsforfutureuse.Seedsincachesthatremainhiddenwilloftenbegintogerminate,thereforeanimalscanplayanimportantroleinseeddispersalforsomeGymnosperms.Rememberthatthescaleisalsoimportantinseeddispersalviawind.
Figure 4. A. Pine ovulate cone. B. Pine Ovule
A
B
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Cycadopsida,GinkgoopsidaandGnetopsida.Findthespecimenswehaveondisplayforeachofthethreeclasseslistedabove.CycadopsidaTodaythereareonlyafewgeneraofcycads.Todaytheyaremostlyfoundinsubtropicalregions.However,cycadswerelargerandmuchmorenumeroussome260mya.Observe the shoot structure, and the large, compound leaves. (A compound leaf is one that isdividedintodistinctleaflets.)Cycadplantsareeithermaleorfemale.Wehavefemaleindividualsthathaveproducedcones.Examinethefemaleconesondisplay.Comparetheconestructuretowhatyou'veseeninpine,spruceandDouglas-fir.Cycadsarewindandinsect-pollinated.Thespermmoveonlyashortdistancedownapollentube,butcycadspermstillhaveflagella.GinkgoopsidaGinkgobilobaistheonlylivingmemberofGinkgoopsida,andisnotknowntosurviveinthewild.ExaminethebranchesandleavesofGinkgothatareondisplay.NoticethatGinkgohasbothlongshootsandshortshoots(orspurshoots).TheleavesofmanyGinkgotreesarebi-lobed,butfewofourleavesshowthischaracteristic.YoushouldnoticethatGinkgoleaveshaveveinscharacterizedbydichotomous,orbifurcate,branching.
Figure5.Ginkgo.Atthetopofthebranchinleftpanelisashort,spurshootwithleafs.Therightpanelshowsaclose-upofaGinkgoleaf.SeveraloftheveinsaremarkedwitharrowsatpointswheretheyshowthedichotomousbranchingcharacteristicofGinkgo.
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WehavepreservedseedsofGinkgoondisplay.Atleastoneoftheseseedswillbeopenedtoshowthattheouterseedcoatisfleshy,buttheinnerseedcoatisquitehard.Thepreservedseedswillshowthatalthoughtheovulesareproducedinpairs,itisoftentruethatonlyoneoftheovuleswilldevelopintoaseed.Ginkgotreesareeithermaleorfemale.Mosttreesplantedaroundwalkwaysorinyardsaremales,becausetheseedshaveaquiteunpleasantodor.WhenyouleaveSouthBiologytoday,headtowardtheChemistrybuilding.Alongthewalkwaybetweenthetwobuildings,youshouldbeabletofindaGinkgotree.You'llrecognizeitbyitsshort,spurshoots.GnetopsidaThereareonlythreegenerainthisclass.ThegenusEphedra(MormonTea)isfoundinUtah.YoushouldexaminetheEphedraplantsondisplay.Noticethattheyhaveveryreducedleaves,andsoresembleEquisetum.(Howwouldyoutellthemapartifnoreproductivestructureswerepresent?)TheconesofEphedraarefoundonseparatemaleandfemaleplants.WehaveafemaleEphedraplantinthegreenhousethatisproducingcones,andwillhaveitinlabtoday.
AngiospermsMagnoliopsidaYoushouldconsultthefloweringplantlifecycleandtheflowerstructurediagramasyouexaminetheavailablespecimens(seefigures10and11).TheFlowerWehavetwoflowersavailabletoday.YoushouldstartwithAlstroemeria,thelargerofthetwo;andthenwhenyou'refinished,examineKalanchoe.Theessentialfunctionofafloweristoincreasetheefficiencyofpollination.Whilesomeangiospermsarewind-pollinated,asaremostgymnosperms,manymoreareanimal-pollinated.Thiscanbeaveryeffectivemeansofinsuringpollination-ifananimalcanbeinducedtocooperate.Theapproachthatmanyflowershavetakenistoattractananimal,rewardit,andhavepollentravelontheanimaltoanotherflowerofthesamespecies.Someflowershavenectaryglandswherethestamensareattached.Thesugarsecretedbytheseglandsiswhatattractsmanyinsectstotheseflowers.Inotherflowers,theanimalsareattractedtothepollenitself.
Figure6.Ginkgoseeds.
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Manyflowersproducebothpollenandovules.Self-fertilization,withinflowersofasingleplantorevenwithinasingleflower,israre.Plantshaveseveralmechanismsthatreduceself-fertilization,suchasgeneticself-incompatibility,andstructuralordevelopmentalmechanismsthatreducethechancesofself-pollination.TheAngiospermsaredividedintotwounrankedgroupsbasedontheirembryo,flowerandleafstructures.InMonocots,theembryoshaveasingleseedleaf,theplantshaveleaveswithparallelveins(parallelvenation),andthepetals,sepals,andstamensareinthreesorinmultiplesofthree.InDicots,theembryoshavetwoseedleaves,theplantshaveleaveswithreticulate(netlike)venation,andthepetals,sepals,andstamensareofteninfoursorfives,orinmultiplesoffourorfive.Asyouexaminetheflowersintoday'slab,keepinmindwhetheryou'reexaminingamonocotoradicot.Andrememberthataflowerisashortshootwithmodifiedleavesthatproducespollenand/orovules.Inatypicalflower,themodifiedleavesarearrangedinfourwhorls.SeealsoFigure9.
Alstroemeria
First,beforeyoureturntoyourbenchwithaflower,takealookattheveinsontheleaves.Aretheyparallelorreticulate?Nowyoushouldbegintodissecttheflower.Youmayfindthatadissectingneedleisausefultool.Startwiththelowermost(oroutermost)whorlofmodifiedleaves–thesepals.Intypicalflowers,thesepalsaremodifiedleavesthatprotecttheflowerinitsbud.Oftentheyaregreen,butsometimes,(asinAlstroemeria)theyarecoloredtohelpattractinsects.Countthesepalsasyouremovethemfromtheflower.Thenextwhorlformsthepetals.Thepetalsaremodifiedleavesthatservetoattractpollinators.Theyareoftenbrightlycolored.Sometimestheydisplaypatternsthatarevisibleonlybyultravioletlightandthatservetoguidepollinatorstothecenteroftheflower.Countthepetalsasyouremovethemfromtheflower.
Figure7.FlowersofAlstroemeria(left)andKalanchoe(right).
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Thenextwhorlformsthestamens.Thestamensaremodifiedleavesthatincludetheanthers(sporangia)thatproducepollen.Trytoplacesomeofthepollenontoamicroscopeslidebyshakingastamenovertheslide,orbytappinganantheronthesurfaceoftheslide.Addadropofwaterandacoverslip,andobservethepollengrainsunderyourmicroscope.Noticethesizeandshapeofthepollen,andlookforanygroovesoropeningsonitssurface.TheonlypartofyourAlstroemeriaflowerlefttoexamineisitssingleovary.Thelowerpartoftheovaryisenlarged,andenclosestheovules.Abovetheenlargedpartoftheovaryisaslenderstalkcalledthestyle.Atthetopofthestyleisasmallpadoftissuecalledthestigma.Pollenlandsonthestigma.Thenthepollentubesgrowdownthroughthetissueofthestyle,andreachtheovulesinthelowerportionoftheovary.Theovaryevolvedfromoneormoreovulebearingleavesthatfoldedoverandfused.Eachofthemodifiedleavesthatmakeupanovaryiscalledacarpel.Thecarpelsaretheinnermostwhorloftheflower.YoushouldmakeathickcrosssectionthroughtheenlargedpartofyourAlstroemeriaovary.Youshouldseethreecompartments,eachcontainingglobularovules.Eachcompartmentisformedbyacarpel,sothisovaryismadeupofthreefusedcarpels-eachwithitsownovules.IsAlstroemeriaamonocotoradicot?KalanchoeOnceagain,beforeyoureturntoyourbench,lookattheleavesoftheplant.Noticethattheyarethickandsucculent.KalanchoeareCAMplants(forCrassulaceanAcidMetabolism)andareusedtostudythisvariationonphotosynthesis.Nowyoushoulddissecttheflower,muchasyoudissectedtheAlstroemeriaflower,comparingthetwoflowersateachstepoftheprocess.BecausetheKalanchoeissomuchsmallerthanAlstroemeria,youmaywanttouseadissectingmicroscopetohelpyouseewhatyou'redoing.Onceagain,adissectingneedlemayprovetobeausefultool.IsKalanchoeamonocotoradicot?Anthers
FindapreparedslidelabeledLillium:MatureAnthersx.s..Whenyouexamineyourslide,youshouldseematurepollengrainsinsidethepollensacs.Examinethesepollengrainscarefully.Howmanynucleidoyouseeineachpollengrain?Howmanykindsofnucleidoyousee?
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Atoraboutthetimethepollengraingerminatestoproduceapollentube,thegenerativenucleuswilldividetoproducethetwospermcellsneededforthedoublefertilizationcharacteristicofAngiosperms.FemaleGametophyte
FindapreparedslidelabeledLillium:ovary;matureembryosacorLillium:ovaryx.s..Theslidewillhaveseveralcrosssectionsthroughanovaryandaboutsixofitsovules.Thesesectionsareconsecutive,andsoyoumayneedtoexamineseveralofthesesectionstoobserveallofthenucleiwithinthefemalegametophyteofanyoneovule.(And,forsomeofthegametophytes,notallofthenucleiwillbeobservable.)Thefemalegametophytemayalsobecalledanembryosac,ormegagametophyte.Atmaturity,readyforfertilization,atypicalfemalegametophytewillconsistofsevencells.Exceptforthecentralcell,thesecellshavethehaploidnucleiyouwouldexpect.Butthecentralcellcontainsadiploidnucleusthatresultsfromthefusionoftwopolarnuclei.DoublefertilizationischaracteristicofAngiosperms.Oneofthespermcellsfromthesuccessfulpollengrainwillfertilizetheeggcellofthefemalegametophytetoproducethezygote.Thesecondspermcellwillfertilizethecentralcelltoproduceatriploidcellthatbeginstodividetoproduceendosperm.Theendospermprovidesnourishmentforthedevelopingembryo.
Besuretoexaminetheoverallstructureoftheovule,andtomakenoteofwherethemicropyleislocated-andwhereitislocatedinrelationtotheeggcellnucleus.
Figure8.PollenofLillium.Onekindofnucleusiscalledatubenucleus.Itgenerallyappearsroughlyroundorspherical.Anotherkindiscalledagenerativenucleus.Thisnucleuswilloftenappearelongatedandcurved,ormayappeartohaveunstained,hook-likeextensions.
Figure9.OvuleandfemalegametophyteofLillium.
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FruitsandSeedsThereisatremendousvarietyoffruitsproducedbytheAngiosperms.Somearesweetandfleshy,andareconsumedbyhumans.(Orbyanimals,whothentransporttheseedstosomenewlocation.)Somefruitsareverydryandlightweight,andmayhaveappendagesthatactlikewingsorlittleparachutes,andmaybecarriedbythewindtosomeplacefarfromtheparentplant.Othersmayhavehooksthatattachthem,temporarily,tosomepassinganimal.Coconutsmayfloatawayfromoneislandtoarrivesomedayonanewbeach.Wewon'tevenattempttocoverthisgreatvarietyoffruits.Peanuts(Arachishypogaea)Theflowerofapeanutplantisproducedaboveground.Afterpollination,however,theflowerstalkbendsdownandgrowsintothesoil.Thefruitdevelopsonorbelowthegroundsurface.(Thiscouldbeastrategytopromotedispersalbygroundforaginganimals.)Takeapeanutortwotoexamineinsomedetail.Theshellofapeanutistheovarywall.It'sverydry-notatallsweetandfleshy.Whenyoucrackopenapeanut,you'llnoticehoweasilyitsplitsalongitstwoseams.Theseseamsrepresenttheedgesandthemidveinofthecarpel-foldedoverandfusedtosealtheovulesinside.Thepeanutovarydevelopsfromasinglecarpel.Thepaperycoveringoneachofthenutsinsidethepeanutfruitistheseedcoat,sobothofthenutsinatypicalpeanutareseeds.Whenyouhaveremovedtheseedcoat,whatisleftistheembryo.Thetwohalvesoftheembryousuallyseparatequiteeasily.Thesetwohalvesarethetwocotyledons,orseedleaves,oftheembryo.Noticethatatthebaseofoneofthesetwohalvesistheroot-shootaxisoftheembryo.Atthebottomofthisaxisistherootapicalmeristem.Atthetopend,betweenthecotyledons,istheshootapicalmeristemthatwillproducenewstem,andleaves,andbranches.Youwillprobablynoticeacoupleofverysmallleavesalreadyformed.Thematureseedcontainsnoendosperm;theendospermhasbeenabsorbedbythecotyledons-that'swhytheyaresolargeandthick.It'scommonnottofindendosperminthematureseedsofdicots.
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Figure10.FlowerStructure.
Figure11.Thelifecycleofafloweringplant.
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