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Biology 2015 – Evolution and Diversity Lab6:SeedPlants Introduction Ofatotalofroughly300,000 plantspecies,nearly90%are seedplants.Theyformtwo informalgroups, Gymnosperms(nakedseed) andtheAngiosperms (enclosedseed). TheGymnospermsinclude fourlineages:Cycads,inthe classCycadopsida;Ginkgo biloba,theonlyspeciesinthe classGinkgoopsida;three generaofGnetophytesinthe classGnetopsida;andthe familiarConifersoftheclass Pinopsida.You'llsee representativesofallfourof theselineagestoday. TheAngiospermsincludetwo majorunrankedsubgroups, themonocotsandthedicots. Figure1.Phylogenyoflandplants. You'llseearepresentativeof eachofthesegroupsintoday'slab. Seedsarecomplexpropagulesthatgreatlyfacilitatethedispersalandestablishmentofnewplants. Seedsstartoutasovules–megasporangiainwhichafemalegametophytedevelopsandproduces oneormoreeggs.Afterfertilization,seedscontainthediploidembryoofanewgeneration,and nutritivetissueconsistingof,orderivedfrom,thefemalegametophyte.Theseareenclosedbythe sporangiumandinteguments(theseedcoat)producedbytheprevioussporophytegeneration. Therefore,threeofthealternatinggenerationsoftheplant'slifecyclecontributetoone reproductivestructure! Butseedsareonlyoneoftheevolutionaryinnovationsfoundintheseedplants.Othersincludethe evolutionofpollenandpollination,ofsecondarygrowth,andoflargerandmoreefficientxylem andphloemconduits.Pollengrainsarematuremalegametophytesenclosedwithinatoughwallof sporopollenin.Pollenmaybedisseminatedgreatdistancesinthewind,orbecarriedbymany 1 insectpollinatorsaswellassomebirdsandmammals.Pollinationefficientlydeliversspermtothe egg.It'sagreatimprovementoverthemethodusedbyseedlessplants,wherespermcellsmust swimtheirwaytoanegg. Seedplantsarethelargestofplants,andthesizeofthelargestseedplantsismadepossiblethe presenceofvascularcambium(alateralmeristem)andsecondarygrowth.Secondarygrowth allowsthestemsandrootstoproduceindefinitelylargequantitiesofxylemandphloem.In temperatezones,atleast,onefamiliarresultofthatsecondarygrowthisthepresenceofannual ringsincrosssectionsofwoodystems.Increasedvasculartransport,andthesupportprovidedby manylignifiedxylemcellsinthickstemsandroots,allowsseedplantstobecomeverylarge. TheAngiospermshaveotherevolutionaryinnovationsthathavecontributedtotheirsuccess(over 250,000oftheapproximately300,000plantspeciesareAngiosperms).Thevesselsthattransport waterinAngiospermxylemarebothlongerandwiderthanthoseofmostGymnospermsand seedlessplants.ButthegreatsuccessofAngiospermsisduelargelytotwooftheirreproductive structures:flowers,whichcangreatlyincreasetheefficiencyofpollination;andfruits,which greatlyaidseeddispersalandestablishment. Gymnosperms Pinopsida(Conifers) ThePinopsidsarethelargestgroupwithintheGymnosperms,containingabout550species.They includethepines,junipers,firs,hemlocks,spruces,redwoods,yewsandDouglas-fir.Althoughwe willusepinestorepresentGymnosperms,wewillalsohavespruceandothermaterialsavailable. Thematuresporophyte Obtainoneofthepinebranchesyou'llfindonthelabbenches.First,noticethegreenneedles.Each oneisaleaf.You'llnoticethattheneedlesarefoundinclusters.Yourbranchescontainneedlesin clustersoftwo,butotherspeciesmayhaveneedlesinclustersofthreeorfive,andinonespecies, theneedlesoccursingly.Thenumberofneedlesinaclusterisoneofthefirstthingsyouneedto knowinidentifyingapinespecies. Yourbranchhastwokindsofshoots(ashootisabranchanditsattachedleaves).Thebranchitself isanexampleofalongshoot.However,eachclusterofneedlesisactuallyonashortshoot-avery shortbranchwithshortdistancesbetweenleaves.Inseedplants,eachbrancharisesfromtheaxil ofaleaf-thepointonastemrightabovealeaf.Therefore,sinceeachclusterofneedlesisashort branch,itmustariseintheaxilofaleaf.Lookforthisleaf.Howisitdifferentfromtheneedles? Eachofyourbrancheswillincludecones.Andconescomeintwotypes:male(staminate)cones,in whichthepollenisproduced,andfemale(ovulate)conesinwhichtheseedsareproduced.Cones aremodifiedshortshoots. 2 Thefreshbranchesonthelabbenchesallhavefemalecones.Eachbranchwillhaveoneortwo verysmallfemalecones,locatednearthetipofthebranch.Itwillalsohaveatleastonemore maturefemalecone,locatedfartherbackonthebranch.Thesmallerfemaleconeswereformed duringlastyear'sgrowingseason,andwerepollinatednearlyayearago! Thelargerfemaleconeswereformednearlytwoyearsago.Theseconeswereallclosedwhenthe brancheswerecollected,butinthewarm,drylaboratoryroom,mostwilllikelyhaveopenedby now.Youwilllikelyfindwingedseedsonthelabbenchunderthebranches,andyoumayfindother seedsstillrestingontheconescales. Maleconesaresmallerthanmaturefemalecones,andareverydelicate.Themaleconesfoundon freshbranchesatthistimeofyeararedarkreddish-brownincolor,andoftennoticeablycurved.If youtouchthem,theywilltendtofallapartinyourhands.Wedidn'tcollectmaleconesthisyear, buthavepreservedbranchesondisplaythatincludefreshmaleconescollectedlaterinthespring, severalyearsago.Thesecones,andbranches,arestillveryfragile,sopleasehandlethemgently. Male(staminate)cones. FindapreparedslidelabeledPine StaminateConec.s.&l.s. Youshouldidentifythesporophylls (modifiedleaves)thatbearthesporangia ontheirlowersurfaces.Meiosisoccurs insidethesporangia.Thesporesthat resultfrommeiosiseachdevelopintoa multicellulargametophytethatforms withinthesporewall.Thegametophyteis malebecauseitproducesonlysperm. Whenthegametophyteismature,itis Figure2.Pinemicrosporangium.Thesporangiumformson calledapollengrain.Thesporangia theundersideofamodifiedleaf,thesporophyll. producedinmaleconesarecalled microsporangiabecausetheyproducetherelativelysmallsporesthatdevelopintothemale gametophytes. Thestaminatecone,orstrobilus,isamodifiedshoot.Eachsporophyllontheshootsupportstwo microsporangia.Thissporangiumcontainsnumerousmaturepollengrains. Pollen Intheslideofthestaminatecones,youshouldseepollengrainswithinthesporangia.Noticethat eachpollengrainhasMickeyMouseears.Theearsorwingsareoutgrowthsofthepollenwallthat arethoughttohelpinwinddissemination. 3 FindapreparedslidelabeledPineMaturePollen. Trytocountthenucleiinmaturepollen grains.Noticethatthougheachpollen grainisamalegametophyte,thereareno sperm-producingantheridia.Antheridia havebeenlostinseedplants.Onenucleus inthepollengrainiscalledthegenerative nucleus.Afterpollinationthisnucleuswill dividetoproducethesperm.Asecond nucleusiscalledthetubenucleus. Female(ovulate)cones. Findthepine,spruce,Douglas-firand Figure3.Pinepollengrain.Whenyouexaminetheslide hemlockconesondisplay.Noticethat ofmaturepinepollen,you'llfindthattheorientationof whiletheyshareanoverallsimilarity,they eachgraininfluenceswhichnucleiyousee.Takealittle arenonethelessquitedistinctive. timetofindpollengrainsthatshowboththegenerative Examinetheexposedinnersurfacesofthe andtubenuclei. pinecone.Howmanyseedswereproduced oneachscale?Lookforindentationsatthebaseofeachscale(wheretheseedswerelocated)and forimprintsoroutlinesofthewings(extensionsoftheintegumentthataidinwinddissemination) ontheinsidesurfacesoftheconescales. Eachscaleonyourpineconeisactuallyamodifiedbranch.Ifbranchesformintheaxilsofleaves, wherearethoseleaves?Well,theleavesonyourpineconeareactuallyfusedwiththescalesat theirbase,andareverysmall,sotheyarehardtodistinguish.ExamineaDouglas-fircone.Here, youshouldbeabletoseethemodifiedleaves(calledbracts)thatstickoutfrombetweenthe scales. FindapreparedslidelabeledPineOvulateConel.s. Findtheconescalesandovules.Anovuleisamegasporangiumprotectedbycelllayerscalledan integument.EachMegasporangiumproducesasinglesurvivingspore,whichthendevelopsintoa singlefemalegametophyteperovule. Theovulehasanopeningatoneend.It'scalledthemicropyle.Isitfacinginwardoroutward?At fertilization,pollentubeswillentertheovuleatthemicropyle.Youmaybeabletoseepollen grainslocatednearthemicropyle. 4 FindapreparedslidelabeledPineOvulel.s. A Nearthemicropyle,justinsidethegametophyte,a maturefemalegametophytewillhavetwoormore archegonia.Eachwillcontainasingleegg.Though morethanoneeggmaybefertilized,onlyonezygote islikelytofullydevelopintoamatureembryo. Pineseeds Findthepineseedswehaveondisplayandtakea coupleofthembacktoyourworkstationtoexamine. TheseedswehavearepinenutsofthePinyonPine, Pinusedulis.Takeaseedapartbyfirstcrackingthe hardoutercovering(yourteethwillworkforthis). Thatoutercoveringistheseedcoat–theintegument. Inside,youmayfindathin,paperylayerthatisthe remainsofthesporangiawall.Insidethatlayeristhe femalegametophyteandthesporophyteembryoit contains. Theslightlypointyendofthegametophyteisthe micropylarend.Gentlyopenthefemalegametophyte toexposethesporophyteembryoinside.(Arazor B blade,andthenadissectingneedle,mayproveuseful here.)Theembryowillbeelongated,andwillhave littleneedleleavesononeend.Theseembryoleaves arecalledcotyledonsorseedleaves.Allthe remainingtissuearoundtheembryoisgametophyte tissuethatisconsumedbythedevelopingembryoas theseedgerminates. Becauseofthenutrientsstoredinthegametophyte tissue,theseseedsareeatenbyavarietyofmammals Figure 4. A. Pine ovulate cone. B. Pine Ovule andbirds.Manyoftheseanimalscacheseedsfor futureuse.Seedsincachesthatremainhiddenwilloftenbegintogerminate,thereforeanimals canplayanimportantroleinseeddispersalforsomeGymnosperms.Rememberthatthescaleis alsoimportantinseeddispersalviawind. 5 Cycadopsida,GinkgoopsidaandGnetopsida. Findthespecimenswehaveondisplayforeachofthethreeclasseslistedabove. Cycadopsida Todaythereareonlyafewgeneraofcycads.Todaytheyaremostlyfoundinsubtropicalregions. However,cycadswerelargerandmuchmorenumeroussome260mya. Observe the shoot structure, and the large, compound leaves. (A compound leaf is one that is dividedintodistinctleaflets.)Cycadplantsareeithermaleorfemale.Wehavefemaleindividuals thathaveproducedcones.Examinethefemaleconesondisplay.Comparetheconestructureto whatyou'veseeninpine,spruceandDouglas-fir. Cycadsarewindandinsect-pollinated.Thespermmoveonlyashortdistancedownapollentube, butcycadspermstillhaveflagella. Ginkgoopsida GinkgobilobaistheonlylivingmemberofGinkgoopsida,andisnotknowntosurviveinthewild. ExaminethebranchesandleavesofGinkgothatareondisplay.NoticethatGinkgohasbothlong shootsandshortshoots(orspurshoots).TheleavesofmanyGinkgotreesarebi-lobed,butfewof ourleavesshowthischaracteristic.YoushouldnoticethatGinkgoleaveshaveveinscharacterized bydichotomous,orbifurcate,branching. Figure5.Ginkgo.Atthetopofthebranchinleftpanelisashort,spurshootwithleafs.Therightpanel showsaclose-upofaGinkgoleaf.Severaloftheveinsaremarkedwitharrowsatpointswheretheyshow thedichotomousbranchingcharacteristicofGinkgo. 6 WehavepreservedseedsofGinkgoondisplay.Atleastoneoftheseseedswillbeopenedtoshow thattheouterseedcoatisfleshy,buttheinnerseed coatisquitehard.Thepreservedseedswillshow thatalthoughtheovulesareproducedinpairs,itis oftentruethatonlyoneoftheovuleswilldevelop intoaseed.Ginkgotreesareeithermaleorfemale. Mosttreesplantedaroundwalkwaysorinyardsare males,becausetheseedshaveaquiteunpleasant odor. WhenyouleaveSouthBiologytoday,headtoward theChemistrybuilding.Alongthewalkwaybetween thetwobuildings,youshouldbeabletofinda Ginkgotree.You'llrecognizeitbyitsshort,spur shoots. Figure6.Ginkgoseeds. Gnetopsida Thereareonlythreegenerainthisclass.Thegenus Ephedra(MormonTea)isfoundinUtah.YoushouldexaminetheEphedraplantsondisplay.Notice thattheyhaveveryreducedleaves,andsoresembleEquisetum.(Howwouldyoutellthemapartif noreproductivestructureswerepresent?)TheconesofEphedraarefoundonseparatemaleand femaleplants.WehaveafemaleEphedraplantinthegreenhousethatisproducingcones,andwill haveitinlabtoday. Angiosperms Magnoliopsida Youshouldconsultthefloweringplantlifecycleandtheflowerstructurediagramasyouexamine theavailablespecimens(seefigures10and11). TheFlower Wehavetwoflowersavailabletoday.YoushouldstartwithAlstroemeria,thelargerofthetwo; andthenwhenyou'refinished,examineKalanchoe. Theessentialfunctionofafloweristoincreasetheefficiencyofpollination.Whilesome angiospermsarewind-pollinated,asaremostgymnosperms,manymoreareanimal-pollinated. Thiscanbeaveryeffectivemeansofinsuringpollination-ifananimalcanbeinducedto cooperate.Theapproachthatmanyflowershavetakenistoattractananimal,rewardit,andhave pollentravelontheanimaltoanotherflowerofthesamespecies.Someflowershavenectary glandswherethestamensareattached.Thesugarsecretedbytheseglandsiswhatattractsmany insectstotheseflowers.Inotherflowers,theanimalsareattractedtothepollenitself. 7 Figure7.FlowersofAlstroemeria(left)andKalanchoe(right). Manyflowersproducebothpollenandovules.Self-fertilization,withinflowersofasingleplantor evenwithinasingleflower,israre.Plantshaveseveralmechanismsthatreduceself-fertilization, suchasgeneticself-incompatibility,andstructuralordevelopmentalmechanismsthatreducethe chancesofself-pollination. TheAngiospermsaredividedintotwounrankedgroupsbasedontheirembryo,flowerandleaf structures.InMonocots,theembryoshaveasingleseedleaf,theplantshaveleaveswithparallel veins(parallelvenation),andthepetals,sepals,andstamensareinthreesorinmultiplesofthree. InDicots,theembryoshavetwoseedleaves,theplantshaveleaveswithreticulate(netlike) venation,andthepetals,sepals,andstamensareofteninfoursorfives,orinmultiplesoffouror five.Asyouexaminetheflowersintoday'slab,keepinmindwhetheryou'reexaminingamonocot oradicot.Andrememberthataflowerisashortshootwithmodifiedleavesthatproducespollen and/orovules.Inatypicalflower,themodifiedleavesarearrangedinfourwhorls.SeealsoFigure 9. Alstroemeria First,beforeyoureturntoyourbenchwithaflower,takealookattheveinsontheleaves.Are theyparallelorreticulate? Nowyoushouldbegintodissecttheflower.Youmayfindthatadissectingneedleisausefultool. Startwiththelowermost(oroutermost)whorlofmodifiedleaves–thesepals.Intypicalflowers, thesepalsaremodifiedleavesthatprotecttheflowerinitsbud.Oftentheyaregreen,but sometimes,(asinAlstroemeria)theyarecoloredtohelpattractinsects.Countthesepalsasyou removethemfromtheflower. Thenextwhorlformsthepetals.Thepetalsaremodifiedleavesthatservetoattractpollinators. Theyareoftenbrightlycolored.Sometimestheydisplaypatternsthatarevisibleonlybyultraviolet lightandthatservetoguidepollinatorstothecenteroftheflower.Countthepetalsasyou removethemfromtheflower. 8 Thenextwhorlformsthestamens.Thestamensaremodifiedleavesthatincludetheanthers (sporangia)thatproducepollen.Trytoplacesomeofthepollenontoamicroscopeslidebyshaking astamenovertheslide,orbytappinganantheronthesurfaceoftheslide.Addadropofwater andacoverslip,andobservethepollengrainsunderyourmicroscope.Noticethesizeandshapeof thepollen,andlookforanygroovesoropeningsonitssurface. TheonlypartofyourAlstroemeriaflowerlefttoexamineisitssingleovary.Thelowerpartofthe ovaryisenlarged,andenclosestheovules.Abovetheenlargedpartoftheovaryisaslenderstalk calledthestyle.Atthetopofthestyleisasmallpadoftissuecalledthestigma.Pollenlandsonthe stigma.Thenthepollentubesgrowdownthroughthetissueofthestyle,andreachtheovulesin thelowerportionoftheovary. Theovaryevolvedfromoneormoreovulebearingleavesthatfoldedoverandfused.Eachofthe modifiedleavesthatmakeupanovaryiscalledacarpel.Thecarpelsaretheinnermostwhorlof theflower. YoushouldmakeathickcrosssectionthroughtheenlargedpartofyourAlstroemeriaovary.You shouldseethreecompartments,eachcontainingglobularovules.Eachcompartmentisformedby acarpel,sothisovaryismadeupofthreefusedcarpels-eachwithitsownovules. IsAlstroemeriaamonocotoradicot? Kalanchoe Onceagain,beforeyoureturntoyourbench,lookattheleavesoftheplant.Noticethattheyare thickandsucculent.KalanchoeareCAMplants(forCrassulaceanAcidMetabolism)andareusedto studythisvariationonphotosynthesis. Nowyoushoulddissecttheflower,muchasyoudissectedtheAlstroemeriaflower,comparingthe twoflowersateachstepoftheprocess.BecausetheKalanchoeissomuchsmallerthan Alstroemeria,youmaywanttouseadissectingmicroscopetohelpyouseewhatyou'redoing. Onceagain,adissectingneedlemayprovetobeausefultool. IsKalanchoeamonocotoradicot? Anthers FindapreparedslidelabeledLillium:MatureAnthersx.s.. Whenyouexamineyourslide,youshouldseematurepollengrainsinsidethepollensacs.Examine thesepollengrainscarefully.Howmanynucleidoyouseeineachpollengrain?Howmanykindsof nucleidoyousee? 9 Figure8.PollenofLillium.Onekindofnucleusis calledatubenucleus.Itgenerallyappears roughlyroundorspherical.Anotherkindiscalled agenerativenucleus.Thisnucleuswilloften appearelongatedandcurved,ormayappearto haveunstained,hook-likeextensions. Figure9.OvuleandfemalegametophyteofLillium. Atoraboutthetimethepollengraingerminatestoproduceapollentube,thegenerativenucleus willdividetoproducethetwospermcellsneededforthedoublefertilizationcharacteristicof Angiosperms. FemaleGametophyte FindapreparedslidelabeledLillium:ovary;matureembryosacorLillium:ovaryx.s.. Theslidewillhaveseveralcrosssectionsthroughanovaryandaboutsixofitsovules.These sectionsareconsecutive,andsoyoumayneedtoexamineseveralofthesesectionstoobserveall ofthenucleiwithinthefemalegametophyteofanyoneovule.(And,forsomeofthe gametophytes,notallofthenucleiwillbeobservable.) Thefemalegametophytemayalsobecalledanembryosac,ormegagametophyte.Atmaturity, readyforfertilization,atypicalfemalegametophytewillconsistofsevencells.Exceptforthe centralcell,thesecellshavethehaploidnucleiyouwouldexpect.Butthecentralcellcontainsa diploidnucleusthatresultsfromthefusionoftwopolarnuclei. DoublefertilizationischaracteristicofAngiosperms.Oneofthespermcellsfromthesuccessful pollengrainwillfertilizetheeggcellofthefemalegametophytetoproducethezygote.Thesecond spermcellwillfertilizethecentralcelltoproduceatriploidcellthatbeginstodividetoproduce endosperm.Theendospermprovidesnourishmentforthedevelopingembryo. Besuretoexaminetheoverallstructureoftheovule,andtomakenoteofwherethemicropyleis located-andwhereitislocatedinrelationtotheeggcellnucleus. 10 FruitsandSeeds ThereisatremendousvarietyoffruitsproducedbytheAngiosperms.Somearesweetandfleshy, andareconsumedbyhumans.(Orbyanimals,whothentransporttheseedstosomenew location.)Somefruitsareverydryandlightweight,andmayhaveappendagesthatactlikewingsor littleparachutes,andmaybecarriedbythewindtosomeplacefarfromtheparentplant.Others mayhavehooksthatattachthem,temporarily,tosomepassinganimal.Coconutsmayfloataway fromoneislandtoarrivesomedayonanewbeach.Wewon'tevenattempttocoverthisgreat varietyoffruits. Peanuts(Arachishypogaea) Theflowerofapeanutplantisproducedaboveground.Afterpollination,however,theflower stalkbendsdownandgrowsintothesoil.Thefruitdevelopsonorbelowthegroundsurface.(This couldbeastrategytopromotedispersalbygroundforaginganimals.)Takeapeanutortwoto examineinsomedetail. Theshellofapeanutistheovarywall.It'sverydry-notatallsweetandfleshy.Whenyoucrack openapeanut,you'llnoticehoweasilyitsplitsalongitstwoseams.Theseseamsrepresentthe edgesandthemidveinofthecarpel-foldedoverandfusedtosealtheovulesinside.Thepeanut ovarydevelopsfromasinglecarpel. Thepaperycoveringoneachofthenutsinsidethepeanutfruitistheseedcoat,sobothofthe nutsinatypicalpeanutareseeds.Whenyouhaveremovedtheseedcoat,whatisleftisthe embryo. Thetwohalvesoftheembryousuallyseparatequiteeasily.Thesetwohalvesarethetwo cotyledons,orseedleaves,oftheembryo.Noticethatatthebaseofoneofthesetwohalvesisthe root-shootaxisoftheembryo.Atthebottomofthisaxisistherootapicalmeristem.Atthe topend,betweenthecotyledons,istheshootapicalmeristemthatwillproducenewstem,and leaves,andbranches.Youwillprobablynoticeacoupleofverysmallleavesalreadyformed. Thematureseedcontainsnoendosperm;theendospermhasbeenabsorbedbythecotyledonsthat'swhytheyaresolargeandthick.It'scommonnottofindendosperminthematureseedsof dicots. 11 Figure10.FlowerStructure. 12 Figure11.Thelifecycleofafloweringplant.