65
LESSON 7: PLANT ADAPTATIONS
LEVEL ONE
Lifeisseldomideal.Wealllivewithsituationsthataredifficultforusinsomeway.Somepeoplefacesevereproblemssuchasnotbeingabletofindenoughfoodorcleanwater,livinginextremelyhotorcoldclimates,orhavingtodealwithachronicillness.Othersofuslivewithmilderlimitationshavingtodowithourage,oursize,ourfamilysituation,theneighborhoodwherewelive,theworkourparentsdo,thepersonalitiesofoursiblings,andsoon. Wedon’tthinkofplantsashavingthesameproblemsthatwedo,buttoalargeextent,theyreallydo.Theycanexperiencemalnutrition,extremetemperatures,unfriendlyneighbors,competitionforterritory,“parents”whoovershadowthem,“siblings”whoaremoreorlesssuccessfulthantheyare,chronicandacuteillnesses,andproblemsduetoaging.Obviously,theycan’tthinkabouttheirproblemslikewecan.Alloftheirresponsescomenotfromtheirbrains(whichtheydon’thave)butfromtheirpre-programmedDNAthat allows them to adapt and survive.
Let’stakealookattwoofthemostbasicproblemsaplantmightface.Wecouldintroducetheseproblemsasasilly“WouldYouRather...?”question.Inthe“WouldYouRather...?”game,playersareforcedtochoosebetweentwobadoptions.Forexample,“Wouldyouratherbetrappedinouterspaceoratthebottomoftheocean?”Nomatterwhichyouchoose,theoutcomewillnotbegood.Youmustchoosewhatyoufeeltobe“thelesseroftwoevils,”andthenexplainyourchoice.Thegameisintendedtoencourageinterestingdiscussionsandhopefullybringsomelaughteralongtheway.Ifplantswereplayingthisgame,oneofthemcouldaskthisquestion:“Wouldyouratherhavealmostnowateratall,orsomuchwaterthatyouaredrowninginit?”
Assumingthataplanthadabrainandcouldthinkaboutthisquestion,itwouldpondertheconsequencesofeachoption.Nothavingenoughwaterwouldmeanthatastranspirationoccurred,therewouldbenowaytoreplacethewaterlostthroughtheleaves.Theplantwoulddryoutandprobablydie.Beingimmersedinwaterwouldmeanthatthestomata(holes)intheleaveswouldbefilledwithliquidwater,preventingtheplantfrom“breathing,”causingtheplanttodie.Toomuchwatermightalsoencouragethegrowthofharmfulbacteria.Sowhichoptionisworse?It’slikechoosingbetweendyingofthirstanddrowning.Twobadoptions!
Someplantswoulddefinitelychoosethe“toolittlewater”option.Infact,theyliveoutthatscenarioeverydayandmangetosurvive.Desertplantshavespecialadaptationsthatallowthemtoliveinplacesthatgetlessthan10inches(25cm)ofrainayear.Indeserts,rainisoftenseasonal,andmonthscangobywithnorainatall.Howisitpossibleforaplanttosurvivethatlongwithoutwater?Gardenplantsstartwiltingiftheydon’tgetraineveryweek. Anormalplantgrowinginaplacewithadequaterainfallhasaconstantflowofwatergoinginthroughitsroots,upthroughitsstems,andoutthroughitsleaves.(Remember,thebeginningofthisprocessisactuallythe“outthroughtheleaves”part.Thelossofwaterintheleavescauseswatertoflowupthroughthestems,andfinallycomeinthroughtheroots.)Ifthereisnotaconstantsourceofwateravailable,thismodeoflivingdoesn’twork.Whatchangeswouldhavetobemadetoaplant’sanatomy in order to prepare it for life in a desert?
66
LEAVES:Thisiswherethewaterlossproblemstarts.Thatpeskytranspirationprocessisconstantlyallowingwatermoleculestogoflyingoffintotheatmosphere.Canthisbeprevented?Couldwejustgetridoftheleaves?Maybe,butleavesarewherephotosynthesisoccurs,sotheplantswouldstarvewithoutphotosynthesis.Ifaplantdidnothaveleaves,itwouldhavetohavephotosynthesisoccurringsomewhereelse.Ormaybewecouldjustreducethesizeoftheleavesinsteadofgettingridofthemaltogether.Smallerleaveswouldmeanlesswaterloss.
STEMS:Whatchangescouldbemadetothestems?Couldphotosynthesisoccurhere?Couldthestemsbeadaptedforwaterstorageinsteadofsimplyholdingtubesforwatertransport?Ifso,thestemsmighthavetobecomeverylargeattimes.
ROOTS:Isthereanywaytomakerootsmoreefficientatgatheringwater?Shouldtherootsgodeeper?Orwoulditbebettertokeepthemshallowandnearthesurfacesothattheycanquicklysoakuprainwhenitcomes?Wouldhavingmoretinyroothairshelp?
Themostfamousofalldesertplantsisthecactus,ofcourse.Cactusplantslookliketheyhavenoleaves.Technically,theirsharpspinesareactuallyleaves,buttheseleaves are so thin that they lose almost no water at all. We saw a similar adaptation in pine trees. Pine needles are actuallyverythinleaves.However,sincepineneedlesaregreen,itiseasiertothinkofthemasleaves.Cactusspinesjustdon’tseemlikeleaves,butbotanistsassureusthatspinesareindeedhighlyadaptedleaves. Whatcouldpossiblybethepurposeofaleafthatcanhardly do photosynthesis and doesn’t initiate transpiration? Whatarecactusspinesgoodfor?Themainthingtheydoishurtlikecrazyifyougettheminyourskin.Sharpspineshelptoprotectcactifromthirstydesertanimalswhomightwanttoquenchtheirthirstwithabigbiteofjuicycactus
stem.Somebotaniststhinkthespinesmightalsohelptocollectwateratnight.Littledropletsofdewformonthespines.Thesedropletsrundownthespinestowardthestems,thentheyflowdownthestems,endingupatthebaseoftheplantwheretherootsnearthesurfacecanabsorbthemoisturequicklybeforethesuncomesup.
Thebodyofacactusistechnicallyastem.Mostcactiaregreen(oratleastgreen-ish),whichtellsyouthatthesebulkystemscontainchlorophyll.Whereverthereischlorophyll,photosynthesisisoccurring.Cactimustuseadaptedformsofphotosynthesis,however.Regularphotosynthesisrequiresaconstantsupplyofwaterandcarbondioxidefromtheair.The“downside”ofnothavingregularleavesisthatyoucan’tuseregularphotosynthesis.Thetypesofphotosynthesisdesertplantsusearecalled“C4”and“CAM.”(Regularphotosynthesisisknownas“C3”sinceitproduces3-carbonsugars.C4produces4-carbonsugars.“CAM”isanabbreviationforagruesomelycomplicatedwordthatevenscientistsdon’tliketousebecauseit’stoolong.Theyjustsay“CAM.”) Cactusstemshavetheabilitytostorealotofwater—enoughwatertokeepthecactusaliveduringdryseasonsthatlastformanymonths.Thebodyofacactususuallylookslikeitismadeof“pleats”thatgoinandout.Thisshapeallowsforeasyexpansionandcontractionwithoutanydisruptionoftherootsatthebaseoftheplant.Therootsystemofacactusisoftenveryshallowand
67
extendsoutwardfromtheplantquiteadistance.Whenraincomes,thecactushasthousandsofroothairswaitingandreadyrightnearthesurface.Largecacticantakeinover100gallons(400liters)duringasinglerainstorm. Afewtypesofcactihaveverylongtaprootsinadditiontolotsofsurfaceroots.Thelongcentraltaprootcanbetwiceaslongasthebody(stem)ofthecactus.Thisdeeprootcanstorealotof water. Cactusstemshaveastructurethatotherplantsdonothave:bumpscalledareoles.Botanistsguessthatareolesaremostcloselyrelatedtothebranchesofregularplants.Itisfromtheareolesthatbothspinesandflowersgrow. Onelastimportantcactussurvivalstrategyistogrowslowly.Cactidon’tneedtobeinanyhurry.Growthtakesenergy.Aplant’senergycomesfromsugarsmadeduringphotosynthesis.Desertplantsarealreadyatadisadvantagewhenitcomestophotosynthesisbecausetheyhavetokeeptheirstomatashutalldaytopreventlossofmoisture.Growingslowlyreducestheneedforsugar.
Cactiarenottheonlytypeofplantsthatcansurviveindryclimates.Plantscalledsucculentsaresimilartocactiinmanyways,thoughtheyhaveactualleaves,notspines.Theleavesofsucculentplantsfeelthickand“juicy.”Thestemsareoftenverythickandcanstorewater.Somesucculentshavestemsandleavesthatarefilledwithagooeysubstancecalledmucilage (mew-sell-ahj).Themucilageactslikeaspongeandholdswater.Acommonsucculentplantthatmanypeoplerecognizeisthealoeveraplant.Somepeoplecallitthe“burnplant”becauseyoucanbreakopentheleavesandusethemucilagetosootheburnedskin.Themucilagemayfeelslimyandweird,butitdoesmakeyourskinfeelbetter.
Succulentleavesfeelliketheyarecoveredinawaxysubstance.Thisisbecausetheyare coveredinawaxysubstance.Wemetthiswaxysubstancebackinlessonfivewhensawthecrosssectionofaleaf.Thethintoplayerwascalledthecuticle.Allleaveshaveatleastatinybitofcuticle,butsucculentleaveshavealotofit.Cuticleisfairlywaterproof,soitkeepsmoisturefromgettingoutof(orgoinginto)theleaf. Anothercharacteristicofsucculentleavesisareducednumberofstomata.Thoselittlestomataholesarewhereairandmoisturegetinandoutofaleaf,soifyouhavefewerholesyouhavelessofaproblemwithwatermoleculesescaping.Succulentshavefewerstomatathanregularplants.Also,succulentstendtoclosetheirstomataduringthedaywhenthesunishotandevaporation(andthereforedehydration)willoccurquickly.Theyopentheirstomataatnight,whentheairiscoolerandevaporationislessofaproblem.
Thesestomatatricksareprettyslick,butthere’sadownsidetothem.Thereducednumberofstomataandthedaytimeclosingsalsomeanthatasucculentleafcan’ttakeinasmuchcarbondioxideasanormalleafcan.Likecacti,succulentsneedtousethosealternativeformsofphotosynthesis:“C4”and“CAM.”Intheseformsofphotosynthesis,carbondioxideisbroughtinatnightandthecarbonsaresnippedoffandstoredintheformofanacid.(Youcouldthinkofitlikecanningfood,perhaps?)Thesucculentcellshavetobeveryefficientwiththeircarbonatomsbecausethereisalimitedsupplyofcarbondioxide.
The bumps are areoles.
This beautiful succulent is called aeonium.
This is a photo of a real stoma.
68
Onelastsucculentsurvivaltrickworthmentioningishair.Someplantsgrowhairsontheirleavesorstemsinordertoprovidesomeextraprotectionfromthewindandsun.Thehairsactlikealayerofinsulation,slowingdowntherateofevaporation.Oncolddesertnights,dewcancondenseonthehairsandperhapsasmalltrickleofwaterwillreachtherootsbymorning. Areallythicklayerofhairsprobablyalsofunctionsasawaytodeterinsectsfrombitingintotheplant.Theinsectcan’tgetitsmouthpartscloseenoughtothestembecausethehairsgetintheway.Thisfeaturecanbeseenonplantsintemperate(notsohot)areasoftheworld,notjustinsuperdryclimates.Youmighthavesomeplantswithhairystemsandleavesgrowingrightnearyou.Sunflowers,forinstance,haveveryhairystems.Theplantcalled“Lamb’sEar”hasleavessofuzzytheyfeellikeanimalears,notplantleaves. Plant hairs are not made from the same stuff that animal hairs are.Animalhairismadeof“dead”proteinmolecules.Planthairsaremadeoflivingplantcells.Planthairsareproperlycalledtrichomes,andtheyareextensionsofepidermiscells.(Remembertheupperandlowerepidermiscellsfromtheleafcrosssection?)Thesespecializedepidermiscellscangrowtobeverylongandthin.Somehavespecializedcellsatthetipthatcansecretestrong-smellingsubstances.Thestrongsmellofmostherbs(suchasmint)comefromsmellyoilsproducedbyglandcellsonthetipsoftrichomes. Let’sturnnowtotheotherterribleoptionforplants:drowningin
water.Toomuchwaterwillkillmostplants,buttherearesomeplantsthatareadaptedforlivinginwaterallthetime.Whatadaptationswouldaplantneedtohaveinordertosurvivebeinginwaterallthetime?Wouldthestomatabeaproblem?Wouldwaterstillflowupthexylemtubes?Wouldtherootsneedtobeindirt?
Therearemanytypesofaquaticplantsandtheiradaptationsarejustwhattheyneedfortheirspecificenvironment.Someplantshavetheirrootssubmergedinwaterallthetimewhiletheirleavesstayoutofthewater.Otherplantshaverootsandstemsunderthewaterandleavesthatfloatonthesurface.Themostextremewaterplantslivecompletelysubmergedallthetime. Whatadaptationswouldaplantneedtohaveinordertobeabletolivesubmergedinwaterallthetime?Woulditneedstomataorcuticle?Whataboutroots?Woulditbeabletomakeflowers?
Seagrassesspendtheirentirelifeunderthewater.Theylookandactverymuchlikeregulargrass.Theyevenhavethosepeskyrhizomesthatletthemusevegetativereproductiontospreadveryquickly.Thisisannoyinginyourgardenwhenthegrassstartstakingoveryourstrawberrypatch,butalongseashoresthisabilitytospreadquicklyisaverygoodthing.Seagrassesareaveryimportantpartofshorelineecosystems.Seagrassesreproduceusingflowersandseeds,too.Thepolleniscarriedbywaterinstead of wind.
Seagrassleaveshaveneithercuticlenorstomata.It’spointlesstotrytokeepwaterinoroutwhenyouaresurrounded
byitallthetime!Thecellsintheleavesareokaywithbeingsaturatedwithwaterallthetimeandthecellshavemicro-adaptationsthatletthemtakecarbondioxiderightfromthewater.Likeallleaves,seagrassleavesneedsunlightforphotosynthesis.Thismeansthatseagrassescanonlygrowinrelativelyshallowwater.Ifthewaterbecomescloudybecauseofpollution,thedeeperseagrasseswillstarttodieout.Dyinggrassmightnotsoundlikeabigdeal,butseagrassesoccupyaveryimportantplaceinmanyaquaticecosystems.Seagrassbedsarefilledwithallkindsofanimallife.Ifthegrassesdisappear,sodotheanimals. Seagrasseshaveanotheradaptationthathelpsthemtosurviveinthewater.Theyhave
Some trichome cells are long and skinny. The round tips make smelly oils.
A manatee eating seagrass. (USGS photo)
69
microscopicairsacsintheirleavesandstemsthatactlikelittlefloaties,keepingtheleavesuprightinthewater.(Ifyoutakeseagrassoutofthewater,itdroopsoveryourhandlikeawetnoodle.Theleavesdon’thavestiffcentralveinslikeregulargrassbladesdo.Seagrassesneedtobeabletobendandflexand“gowiththeflow”inthewater.Iftheirleaveswerestiff,they’dgettornapartbythemotionofthewater.)Thetinyairsacsintheleavesandstemsprovidejusttherightamountofbuoyancysothattheleavesstayuprightinthewater,reachingtowardthesunlight. Waterplantsaresurroundedbywater,sotranspirationisnotnecessary.Cellscangetwhatevertheyneedbyosmosis.(Remember,osmosisiswhenmoleculesgorightthroughthecell’smembrane.)Thecarbondioxide,water,andnutrientsareabundantallaroundthecells,soxylemtubesaren’treallynecessary.Inmostwaterplants,thevascularbundlesareverymuchreduced,andinsomecasestheyarealmostabsent.
Aslightlylessdrasticsituationiswhenaplantisonlypartlysubmergedinwater.Thebottomhalfoftheplantstaysinthewaterwhilethetoppartisallowedtotouchtheair.Agreatexampleofaplantadaptedtothissituationisthewaterlily.Let’stakeaclose-uplookatalilythatholdstheworldrecordforthelargestleafintheplantkingdom:theAmazonlily.
FoundintheAmazonRiverinSouthAmerica,thislilycangrowleavesthatareupto8feet(2.5meters)across.Thesegiantleavesarealmostperfectlyroundandlooklikefloatingplates.Theyarequitestableandcansupporttheweightofsmallbirdsandanimals.Theirstrengthisduetoanetworkofstrongveinsontheundersideoftheleaf.Theirabilitytofloatisduetosmallairpocketsinsidetheveins.Inadditiontobeingstrong,theseveinscanalsobedeadly—theyarecoveredinsharpspines.Fishlearnveryquicklythatnibblinglilyveinsisabadidea! Theundersidesofthelilyleaveshavetwootherfeaturesthatareworthmentioning.Likeseagrasses,theundersidesoflilyleaveshavenostomataandnocuticle.Unlikeseagrasses,theseundersideshavelotsofredpigmentinthem.Theredpigmentmoleculeshelptocatchwavelengthsoflightthatthegreenchlorophyllpigmentsontopmightmiss. Thetopsideofalilyleafisnothinglikethebottomside.It’sgreen,iscoveredwithwaxycuticle,andcontainsstomata.Thetopsideisadaptedtolivinginair,thebottomsidetowater.
These three different types of seagrasses all have leaves that fl ex and bend with the water currents.
70
ThelifecycleoftheAmazonwaterlilysoundslikesomethingfromafantasystory.Itsflowersbloomforonlytwodays.Thefirstnightitblooms,theflowerisfemale.It’swhiteandsmellslikefragrantperfume,anodorthatscarabbeetlesfindirresistible.Oncetheflowerhasattractedbeetles,itclosesup,trappingthebeetlesinside.Duringthenight,theflowerchangesitsgenderfromfemaletomale,anditscolorfromwhitetopink.Italsostopsmakingperfume.Whenthefloweropensupattheendofthesecondday,thebeetlesarefreedfromtheirfloweryprisonbutarecoveredwithpollengrains.Theflowernolongersmellsgood,sotheyflyofftofindanotherwhitelilyflower,carryingthepinkpollengrainswiththem. Oncefertilized,theflowerclosesandfallstothebottomoftheriverwhereitformsaseed.Theseeddoesnotbegintogrowuntilthenextyear.LilyseedsusuallygerminaterightastheAmazonRiverisreachingitsannualfloodstage,withthewaterrisingseveralinchesaday.Thenewlysproutedwaterlilystemmustgrowatanunbelievablyfastrateinordertokeepits“head”abovetherisingwater.Thestemiscapableofgrowingupto20feet(7meters)tallinjustafewweeks.Whenthewatersstoprising,thelilystartsgrowinggiantleaves(lilypads). It’sthenabattleoflilyversuslily,witheachplantattemptingtodominateitsarea.(Siblingrivalryhappensevenamongplants!)Onceagiantleafhasspreaditselfoutoverthewater,anyplantunderneathitwilldiefromlackofsunlight.Asidebenefitofthissituationisthatalgaeisalsopreventedfromgrowing.Thisiswhywaterlilies(usuallyvarietiesotherthanAmazon)areplantedinoutdoorwatergardens.Peoplewouldratherlookatlilyleavesthanalgae“scum.”
Oneofthesmallestaquaticplantsonearthiscalled“duckweed”or“waterlentils.”Duckweedlooksliketinygreendotsfloatingonthesurfaceofalakeorpond.Theyarebrightgreenandlookveryclean.Truetoitsname,
ducksloveduckweed.Otherwaterbirdseatittoo,aswellaspeopleinsomepartsoftheworld.It’sagreatsourceofprotein,evenbetterthansoybeans,anditproducesmorestarchperacrethancorn!It’spossiblethatduckweedcouldbeturnedintoethanolfuel,justlikecornis.Somedayourcarsmightrunonfuelmadefromduckweed! Aduckweedplantconsistsofjustatinyleaf,withnovisiblestemsorroots.(Insomespecies,theleaveshaveverysmallroothairsdanglingfromtheunderside.)Ifduckweeddoesn’thavestemsorrootscanitstillbeclassifiedasaplant?Wedecidedwaybackatthebeginningoflessononethatthereareonlytworequirementsforbeingaplant:1)youmustdophotosynthesis,and2)youmustbemadeofmorethanonecell.Duckweedqualifiesonbothcounts.Infact,it’sevenclassifiedasanangiospermsinceitoccasionallyproducesflowers—thetiniestflowersintheplantkingdom,nolargerthanthewidthofapieceofthread!
71
ACTIVITY 1: SIT LIKE A LUMP ON YOUR CHAIR OR COUCH AND WATCH VIDEOS
Thisactivityshouldnotbetoodifficult.Logontowww.YouTube.com/TheBasementWorkshopandenjoythevideosaboutdesertplantsandwaterlilies.
ACTIVITY 2: PLANT PUZZLE
Fillinthemissingwordsintheclues,thentransfertheletterstothenumberedslotsunderneaththepictures.Assumingyou’vefilledinthecluescorrectly,you’llfindoutthenamesofthesucculentplantsinthepictures.(We’recountingcactiassucculents.Sometimesthewordsucculentisusedasamoregeneralterm,withcactibeingasubcategory.)
1)Succulentshavefewer_____________________andmore_____________________.6818941311122654536Theyalsohavegooeystuffcalled________________________intheirstemsandleaves.52549328422)Cactihavebumpscalled_____________________ontheir_______________.5938749553)Desertplantsmustusetwootherformsofphotosynthesis:“___4”and“_________.”9053394)Thewaterplantcalled________________________looksliketinygreendots.Eventhough7128664857itdoesnothavestemsorroots,itisstilla_______________becauseitdoesphotosynthesisandismadeofmorethanonecell.86310
5)Duckweedisverynutritious.Likesoy,itisagoodsourceof_____________________andlikecorn,itisagoodsourceof__________________.47719962067494162566)Likeallplants,whenaduckweedflowerispollinated,a__________________isformed.354670247)Thistypeofanimallovestoeatseagrass:_____________________.7216558)Planthairsarecalled___________________________andoriginatefromthe5087136333___________________________layer.21933478429)The__________________sideofanAmazonlily_________isthecolor_________and371597812587961hassharp__________________ontheveins.88602610)Whenwaterevaporatesoutthroughtheleaves,causinganupwardflowofwaterfromtheroots,thisiscalled_______________________________________.73692740297776641411)Thealoeplantissometimesusedtotreatthistypeofinjury:____________6925812)___________________________plantshavevery_______________stems.Thislets9123438075304451themstorewatersotheycan_____________________indesertclimates.178248913)Thiswordrootmeans“naked”inGreek:_______________839812
Z
BS
S
72
____________________cactus____________plant__________plant123456789101112131415161718192021
______plant____________cactus________plant__________plant222324252627282930313233343536373839
____________________________________404142434445465152535455565758596061____________________47484950626364656667
______________________________________________________686970717273747576777879808182838485909192939495969798 cactus________cactusplant 86 87 88 89
