1

Biology 2015 – Evolution and Diversity Lab Lab 4: Seedless Plants

IntroductionLastweekyoustudiedalgae,thephotosyntheticprotiststhatincludethegreenalgalCharophytesthatarethesistergrouptothelandplants.Youwillstudylandplantsboththisweekandnext.Plantsareafascinatinggroupaswewillbeabletoobservemanyoftheadaptationstolivingonlandthatoccurredthroughouttheirevolutionaryhistory.Theplantsaredistinguishedfromthegreenalgaebythreecharacteristics:thecuticle,thediploidembryo,andalternationofgenerations.Thecuticleisawaxy,waterproofcoatingfoundontheaerialpartsofaplant,whichminimizestheplant'slossofwatertothesurroundingair.Thecuticleisnotfoundonthebelow-groundpartsofaplantbecauseitwouldinhibittheuptakeofwaterandnutrients.Forthesamereasons,cuticlesarenotfoundinthealgae.Theembryoisadeveloping,multicellular'baby'plantthatisnurturedandprotectedbytheparentplant.Theevolutionofthecuticleandoftheembryofacilitatedthecolonizationoflandbyplants.Theseedlessplantsreproducebothsexuallyandasexually,but,asthenamesuggests,theseplantsdonotproduceseeds.Thoughseedsprovideimportantadvantagesinbothdispersalandestablishmentofoffspring,seedlessplantscontinuetobecommoninavarietyofhumidterrestrialenvironments.

Figure1.Phylogenyoflandplantsandrelatedgreenalgae.

2

Thehaplodiplonticlifecycleischaracteristicofplants.Thislifecyclemayalsobecalled"diplohaplontic",orbereferredtoas"alternationofgenerations".Inahaplodiplonticlifecycle,bothmulticellularhaploidindividualsandmulticellulardiploidindividualsareproduced.Thereareseveralexamplesofhaplodiplonticlifecyclesinthishandout.Itwillhelptofurtherintroducesomenewterms,aswewillbeusingseveralnewonesinthetwocominglabs.A“gametophyte”isthehaploidmulticellularorganismthatproducesgametes(throughmitosis);twogametesfusetoproduceazygote.Thezygote,insteadofgoingthroughmeiosis,asinCharophitealgae,formsamulticellular“embryo”.Theembryolaterdevelopsintothe“sporophyte”,whichisthediploidmulticellularorganismthatproducesspores(throughmeiosis).Sporesarehaploidandtheydon’tfusewithothercells;rather,theydevelopintoamulticellulargametophytes.Itisconvenienttoconsider'seedlessplants'asagroupdistinguishablefromtheseedplants,buttheseedlessplantsdonotformamonophyleticgroup.Andthoughit'salsoconvenienttodividetheseedlessplantsintotwosubgroupsbasedonthepresenceorabsenceofvasculartissues,thesesubgroupsarenotmonophyleticeither.Thefirstsubgroupoftheseedlessplantsconsistsofthenon-vascularplants,or"bryophytes".Therearethreecladeswithinthebryophytes:Hepatophyta(theliverworts);Bryophyta(themosses);andAnthocerophyta(thehornworts).You'llseeexamplesofeachcladeintoday'slab.Thereareabout24,000knownspeciesofbryophytes.Theyareallsmallplantsthatmostlylackspecializedvasculartissues.Theyhave“dominant”gametophytesand“dependent”sporophytes.Thesecondsubgroupcontainstheseedlessvascularplants.Vascularplantsproducetwoimportanttissues:xylemtransportswaterandsolutesthroughouttheplantandprovidesstructuralsupport;andphloemtransportsorganiccompounds.Thesetissuesallowthevascularplantstogrowmuchlargerthanthenon-vascularbryophytes.Theseedlessvascularplantsincludetwophyla:Lycophyta(the"clubmosses")andPteridophyta(theferns).You'llseeexamplesofbothintoday'slab.Togetherthesephyla

Figure2.Haplodiplonticlifecycleofferns.

3

includeabout14,000species,andthegreatmajorityofthemareferns.SeedlessvascularplantsweredominantduringtheCarboniferous,andmuchofourfossilfuelcomesfromtheirremains.Fromanevolutionaryperspective,thedominanceofthegametophyteandthesporophytehaveswappedinimportance,goingfromadominantgametophyteanddependentsporophyte,toadominantsporophyteandareducedanddependentgametophyte.Asyouwillseeinthisandthefollowingweek,thegametophyteisthedominantstageinthe“bryophytes”,whilethesporophyteisshort-livedanddependentonthegametophytefornutrition.Invascularplantsthere’sbeenanimportanttransition:thesporophyteislargeandlong-livedand,bycontrast,gametophytesaresmallandshort-lived.MARCHANTIOPHYTA(Liverworts)Thereareseverallivingliverwortsforyoutostudytoday.TheyincludeexamplesfromthegeneraMarchantia.InFigure3wehavethelife-cycleofliverworts.

Figure3.Liverwortlife-cycle.

4

LiverwortGametophyteStructure

Therearetwomajorliverwortgroupingsbasedongametophytemorphology.TheMarchantiaspecimenswehavetodayaregoodexamplesof"thalloid"gametophytes(Figure4).Thalloidplantsareessentiallyflatlayersoftissue.Buttherearealso"leafy"liverwortgametophytesthatappeartohavea"stemandleaves"morphology.YoushouldexamineBazzaniaforanexampleofthisform.Liverwortsalwaysgrowinverymoistplaces.ExaminetheMarchantiaspecimens,andplantouseamagnifyingglassforobservingthesmallerstructures.UseamagnifyingglasstolookforopeningsonthesurfaceoftheMarchantiathallus.Rememberthattheaerialportionsoflandplantsarecoveredwithcuticle,andthatopeningsthroughthecuticleareneededforgasexchange.• Findandexamineprepared

slideslabeled"MarchantiaArchegoniophorel.s."and"MarchantiaAntheridiophorel.s."

BesuretoseetheArchegoniaandAntheridiaontheseslides(Figure5).Knowingthatliverwortsliveinwet,rainyplaces,andthatliverwortspermaremotile,howdoyousupposethestructuresoftheantheridiophoresandarchegoniophoresfunctioninfertilizationoftheeggcells?

Figure4.Examplesof"thalloid"liverwortsofthegenusMarchantia.

Figure5.Archegonia(left)andantheridia(right)ofMarchantia.

5

Youshouldbeabletofindthestructurescalled"gemmaecups"onthelivingspecimens.Gemmaecupscontainasexualpropagulescalled"gemmae"(singular,"gemma").Whenraindropshitoneofthecups,thegemmaecanbedispersedinthesplash.Thegemmaearethenabletogrowintonewgametophytethalli.Ontheundersideofthethallusthereareroot-likerhizoids.Youmaybeabletoseethemifyougentlyliftanedgeofthethallus.• Findapreparedslidelabeled

"MarchantiaGemmaCup.Sec.".Observethestructureofthegemmaecupsandofthegemmaefoundinside.

LiverwortSporophyteStructure

Liverwortsporophytesarequitesmall.Theyformwithinthearchegoniaontheundersideofthearchegoniophores.Theyconsistofarelativelylargesporangium(or"capsule"),afairlyshort,stoutseta,andasmall"foot"asshowninthelifecyclediagramofFig.10.

• Findapreparedslidelabeled"MarchantiaSporophytel.s."Findthesporophytes,andcomparethemtotheoneshowninFigure7.

HowdotheMarchantiasporophytecomparetothesporophyteofthebryophytes?

Figure6:Gemmaecup.Thegemmae(arrows)areroughlydisk-shapedinfaceview.

Figure7.SporophyteofMarchantia.

6

ANTHOCEROTOPHYTA(Hornworts)Forcompleteness,we'dliketoshowyouanexampleofahornwortbutwehavehaddifficultygrowinghornwortsinourgreenhouse-theytendtodryoutveryeasily.Fortunatelywehavepreparedslidesthatshowimportantstructures.Findandexaminepreparedslideslabeled"Anthoceros,."BRYOPHYTA(Mosses)Wehaveseveralspeciesoflivingmossforyoutoexaminetoday,includingexamplesfromthegeneraHypnum,Mnium,Dicranum,Polytrichum,andSphagnum.Wealsohavelivingmossexamplesthatwerecollected"wild"intheDepartmentgreenhouses.And,inadditiontothelivingspecimens,wehaveslidepreparationsofPolytrichumandMnium.Youshoulduseamagnifyingglasstoobservetheliving,'wild',mossexamplestobesureyourecognizethegametophytesandsporophytes,andcanrelatethemtothelifecyclediagram.Pleasetreatthespecimensgently.

Figure9.Life-cycleofatypicalmoss(Bryophyta).

Figure8.Apictureofahornwort,withsporophytes(lightgreenstalks)growingfromthegametophytes.

7

MossGametophyteStructure

You'llnoticethatthegametophytesaregreen,andappeartohavea"stemandleaves"structure-likewhatyou'dexpectofaplant.Buttrueleavesandtruestemsareabsentinnon-vascularplants.Andmostmossestrulyare"non-vascular".Afew,however,andPolytrichumisone,dohavexylem-likeandphloem-liketissuesintheir"stems".Thesemosstissueslikelyaroseindependentlyofthetruexylemandphloemfoundinthevascularplants,sothisisanexampleofconvergentevolution.Thegametophytesalsohavestructurescalled"rhizoids".Theseareunicellular,filamentousstructuresthatservetoanchorthegametophytetothesoil,andthatalsofunctioninwaterandnutrientuptake.

• Findapreparedslidelabeled"MossAntheridiaandArchegoniaw.m."

Mostmosseshaveseparatemaleandfemalegametophytes.Thegametangiaarefoundclusteredneartheapexoftheirstalks.Youprobablywon'tobservegametangiaonthelivingmossspecimens,butthepreparedslideswillshowthegametangiawell.Thespermreleasedfromtheantheridiamustreachanarchegonium,andthenswimdowntheneckcanaltoreachtheeggcell.Sowaterisnecessaryforsexualreproductiontooccurinthemosses(andforotherseedlessplants).Rainfallmayprovideacontinuousfilmofwaterforthespermcellstoswimin,butthesplashofraindropsprobablyalsohelpsmanyspermcellstoreachthefemalegametangia.

Figure10.Mossantheridia(left,arrows)andarchegonia(right).Lookcarefullyatthethickerportionsofthearchegonia,indicatedbyarrows.Youmaybeabletoseetheeggcellsthatarelocatedthere.

8

MossSporophyteStructure

• Examinethelivingmosses,withsporophytes,ondisplay.• Examinethedried/preservedexamplesofPolytrichumsporophytesondisplay.• Findapreparedslidelabeled"Mnium:capsule,medianl.s."

Thesporophytesofthemossesconsistmainlyofa"capsule"supportedandelevatedbyastalkcalleda"seta".You'veprobablyalreadynoticedthatthesporophytegrowsrightoutofthearchegoniuminwhichitbegandevelopmentasanembryo.Thecapsulesarethesporangiainwhichmeiosisoccursandthesporesform.Capsulesthataregreenareimmature;browncapsulesmaybesheddingspores,ormayalreadybeempty.YoushouldexaminethepreparedslidesofMniumcapsules,andthedried/preservedsporophytesofPolytrichumthatwehaveavailable.AsexualReproduction

• Findapreparedslidelabeled"Mossprotonema:withbulbils,w.m."Manyplantsarecapableofasexualreproduction,andsomeofthemproducestructuresthathavethisspecificfunction.Whenmosssporesgerminate,aloosefilamentousmassofcellscalleda"protonema"formsbeforedevelopmentofanorganizedgametophytebegins.Insomespecies,smallglobularstructurescalled"bulbils"mayformonshortstalksthatbranchfromthefilamentsoftheprotonema.Thesebulbilsfunctionasvegetativepropagules.Youshouldobservethepreparedslidesofmossprotonematathathaveproducedbulbils.

Figure11.Mossbulbils.

9

TRACHEOPHYTA(Ferns)

Wewillstartbydescribingthe"typical"ferns,thencoverthehorsetails.Fernsarethemostdiverseoftheseedlessvascularplants.Wehavesomeofthatdiversityondisplayinthelabtoday.Fernsdonothaveabigrootsystem.Instead,theyhaveashallowrhizoidalsystemthroughwhichtheyobtainthematerialstheyrequireforphotosynthesis.Someferngenera,likeAzolla,canevenliveonthesurfaceofwater.Thisisoneofthefernsondisplaytoday.Azollaformsamutualisticrelationshipwithanorganismyouhavealreadylearned.Removealeaflet,crushitwitharazorblade,andmixitwithadroportwoofwater.Whatisthenameofthesymbiont?FernSporophyteStructure

Fernsoftenhaveverylargeleaves-theyaremegaphyllsthathavemultiplevascularstrands.Theleavesareoften"compound"leaves,inthattheyaredividedintoseveral(ormany)leaflets.Inmanyferns,thereisnoabovegroundstem;theleavesarisefromundergroundrhizomes,fromwhichtherootsalsobranch.• Findapreparedslidelabeled"FernSporangiaMaturec.s."Inmany ferns, includingalmostallof thosewehaveondisplay,sporangiaareborneonthe

Figure12.Lifecycleofferns(Pterydophyta).

10

undersides of the leaves, generally arranged in circular or elongated clusters (called "sori",singular"sorus")thatareoftentantodarkbrownincolor.Inmanycasestherewillbealayeroftissuethatpartlycoverstheclusterofsporangia.It'scalledan"indusium".Examinethepreparedslidesoffernsporangia,anddetermineifanindusiumispresent.Examinethevariousfernsondisplay.Seeifyoucandeterminewhetheranysoriarepresent,andifso,whethertheyarecoveredwithanindusium.

• Findthefernleafwithsoriondisplayatadissectionscope.Examinethesari.Isanindusiumpresent?Examinethesporangia.

Onthe'spine'ofeachsporangiumisarowofthick-walled"annular"cellsthatcontractastheydryout.Thiscontractioncausesthesporangiumtosplitopen,andthetopofthesporangium-containingthespores-pullsback,andthensuddenlysnapsforwardagain.Thismovement'catapults'thesporesintotheair.Ifyoudryoutsomesporangiaunderthelightofthedissectionscope,youmaybeabletowitnessthisdispersalaction.

FernGametophyteStructure

• Findapreparedslidelabeled"Fern:youngprothalliaw.m."Whenthesporesgerminate,newgametophytes(alsocalled"prothallia")areformed.Examinethepreparedslidesofyoungfernprothallia.Youshouldbeabletoobserveexamplesofgerminatedsporesandthesmallgametophytesthathavebeguntodevelop.Noticethateventhesmallestofthesenewgametophyteshasatleastonerhizoid.

• Findapreparedslidelabeled"Fernprothallium:antheridia&archegonia,w.m."• Observethelivingfernprothallia(gametophytes)ondisplay.

Matureferngametophytesaresmall,oftenheart-shaped,haploidplants.Ontheirundersidestherearerhizoids,andalsoantheridiaandarchegonia.Youshouldexaminethepreparedslidesoftheseprothallia.Youshouldobservearchegoniaclusterednearthe'cleft'oftheheart-shapedthallus.Youshould

Indusium

Figure13.Sectionthroughasorusanditsindusium.Sporangiashowingannularcellsarelabeled,andseveralsporangiacontainspores(smallarrows).

Figure14.Fernantheridia(left)andarchegonia(right).The'best'ofthearchegoniaarelabeledwitharrows.

11

observeantheridianeartheclusteredarchegonia,andespeciallyneartherhizoids.

• Findapreparedslidelabeled"Fernprothalliumyoungsporophytew.m."Fertilizationoccurswhenspermcellsreleasedfromtheantheridiaswimtoanarchegonium,anddowntheneckcanaltotheeggcell.Asporophyteembryothenformsinthearchegonium,andeventuallygrowsintoamaturesporophyte.Examinethepreparedslidesthatshowyoungsporophytesdeveloping-stillattachedtoagametophyte.

Horsetails

Thereisonlyonegenusofhorsetails:Equisetum.Wehavealivingexampleondisplay.ThegenusEquisetumismorphologicallyidenticaltothefossilgenusEquisetitesfromtheCarboniferous,some300millionyearsago.Ifthesegeneraarethesame,thenEquisetummaybetheoldestsurvivinggenusofplantsonearth-trulya"livingfossil".

HorsetailSporophyteStructure

Horsetailsappearatfirsttobe"allstem".But,ifyoulookclosely,thestemhas'segments'.Wherethesegmentsjoin,ateach"node",you'llfindacircleofleaves.Theseleavesaresuppliedbymorethanonevascularstrand,andsoarecalled"megaphylls"-despitetheirsmallsize.Theaerialstemsbranchfromundergroundstemscalled"rhizomes".Andtherootsalsobranchfromtheserhizomes.

• Findapreparedslidelabeled"EquisetumMatureStrobilusl.s."

Ourplantsdonothavereproductivestructures.Thesestrobiliaremadeupofmodifiedbranches,calledsporangiophores,whichbearthesporangia.Youshouldexaminethepreparedslidesofhorsetailstrobili.ThesearefromadifferentspeciesofEquisetum,soarelargerthanwhatyoumightexpectonourlivingplants.Youmaybeabletoseethe"elators"oneachspore.Thesestructurescoiluparoundthesporeunderhumidconditions,butextendoutfromthesporeastheydry.Elatershelpwithdispersalofthesporesinthewind.WehavepreparedwholemountslidesshowingEquisetumsporesandtheirelators.• Findapreparedslidelabeled"EquisetumSpores

w.m."SomeofourslidesofEquisetumhavegonemissing-wemayadjustbysettingupdemonstrationslidesinstead.

Figure15.SporesofEquisetum.Therearetwoelatersoneachspore.Noticethattheendsoftheelatersareshapedlikepaddles.It'scommontofindelatersontheseslidesthathaveseparatedfromtheirspores.

12

Lycopodiopsida(Lycopods)TherearethreefamiliesofLycopods:Lycopodiaceae,Selaginaceaeandlsoetaceae.WewillonlybelookingatarepresentativeofSelaginaceae,Selaginellasp.Moleculardatasuggestthatthelycopodsareprobablythebasalgroupofthevascularplants.

SelaginellaSporophyteStructure

Selaginellahastruestemsandleaves,andcomparedwithbryophytes,theplantscanbequitelarge.Lycopodleavesarecalled"microphylls"becausetheyaresuppliedbyonlyasinglestrandofvasculartissue(xylemandphloem).Thestemscontainacentral,lobedcylinderofvasculartissue;andtherearetrue,vascularizedrootsaswell.YoushouldexaminetheSelaginellaplantswehaveondisplay.Youmaybeabletofindstrobili,locatedattheendsofsomeofthebranches.Theremayalsobebulbilsonsomeoftheplants.Bulbilswillalsobeterminal.

Microsporangium

Figure17.Selaginellastrobilusandsporangia.

Figure16.Life-cycleoftheLycopodSelaginella.

13

• Findapreparedslidelabeled"SelaginellaStrobilusl.s."Whenyoustudythisslide,besuretodistinguishbetweentheMegasporangiaandtheMegasporestheycontain,andtheMicrosporangiaandtheirMicrospores.Megasporesgrowintothefemalegametophyte,andmicrosporesbecomethemalegametophyte.

LycopodGametophyteStructure

Thegametophytesoflycopodsaresmallandinconspicuous.Somegametophytesdevelopentirelywithintheconfinesofthesporewall,evenbeforethesporeisshed(Selaginella,isoetes).InLycopodium,thesmallgametophytedevelopson,orjustbelow,thesoilsurface.Inallthelycopodgenera,fertilizationoccurswhenspermcellsswimtotheeggcellcontainedinanarchegonium.