Download Biology 2015 – Evolution and Diversity

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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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?
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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.
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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.
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Figure10.FlowerStructure.
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Figure11.Thelifecycleofafloweringplant.