leaf anatomy - university of wisconsin–stevens point：叶片解剖结构-威斯康星大学–史蒂文斯点

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1、18Laboratory7LeafAnatomyModifiedLeavesI.LeafAnatomyIntroductionInstudyingthecharacteristicsofstemsandrootsinpreviouslaboratories,youhaveseenstructuresspecializedforphysicalsupportandforabsorptionandconductionofwaterandminerals.Thislabdealswiththethir

2、dvegetativeplantorgantypicaloflandplants:theleaf.Leaveshaveevolvedasplantorgansthatare(1)obviouslyadaptedforlightinterceptionand(2)subtlyadaptedforcarbondioxideuptakefromtheatmosphereforphotosynthesis.Becauseoftheirlargesurfaceareaandbecausetheyhavep

3、oresforgasexchange,leavesareveryvulnerabletowaterloss.Thisvulnerabilityresultsinafinaladaptationfoundinallterrestrialvascularplantleaves:(3)featuresthatpreventexcesswaterlossbutatthesametimereduceCO2uptake.Whatiscontradictoryaboutaleafpreventingwater

4、loss?Iftheleafwerecompletelywatertight,itwouldalsobecompletelyairtight,andthecarbondioxidesupplywouldbecutoff.Thatwouldbefatal!Anidealleafwouldallowcarbondioxidetoenterwithoutallowinganywatertoescape.Thebestschemethatevolutionhasproduced,asystemofcon

5、trollablepores,stomates,fallsfarshortofthatideal.Stomatecontrolofwaterlosswillbedealtwithextensivelyinalaterlab,butevennowitisvaluabletoknowalittleaboutstomateresponse.Stomatestendtoopenwhenconditionsaregoodforphotosynthesis.Theytendtoclosewhendrying

6、conditionsareextreme.Thebalancebetweenphotosyntheticopportunityanddroughtliabilitydetermines:(1)Whetherstomatesareopenorclosed.(2)Howmuchwaterislostfromtheplant.(3)Howmuchphotosynthesiscanoccur.Anaverageplantislikelytotakeupatleast20timesasmuchwatera

7、sitneedstosupplyitscells,losingtheexcesstotheatmospherewithoutanymajorbenefit[minorbenefitsmightincludecoolingandincreasedmineraluptakewiththewater].ExercisesA.GrossLeafAnatomyandPlantIdentificationOnanindividualplant,leavesrespondintheirsizeandform(

8、seeExerciseD)toprevailingenvironmentalconditions.Thatsomewhatlimitstheirvalueforidentifyingaspecies.Flowers,incontrast,tendtoveryconsistentintheirsizeandstructure.Guidestoplantidentificationtypicallyfocusmostonflowerparts.Still,leavesarelikelytobepre