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1、第一部分动物、植物、生物学ClassificationofPlantsThenewerclassificationsystemlistsallofthemorethan300,000knownplantsinjusttwophyla,theBryophytesandtheTracheophytes.Bryophytes,themossesandliverworts,areusuallysoftandnonwoodyinstructure,takeinwaterthroughshortroot-likefilamentscalledr
2、hizoids,andmayhavestemsandsimpleleavesbut,unlikethemorecomplexTracheophytes,donothavetruerootsorvasculartissuewhosefunctionitistocirculatewater,food,andessentialmineralsthroughouttheorganism.Tracheophytesaredividedintofoursub-phyla:lycopsids,whichnumbersome900livingspe
3、cies;sphenopsids,whosefossilspeciescontributedtocoalformationintheCarboniferousperiod,butwhichhavefewlivingspecies;psilopsids,anextinctgroupofrelativelysimpleplants,whichfossilstudiesshowtohavebeenmoreadvancedthananyofthemosses;andpterosids,subdividedintothreeclasses,t
4、heferns,thegymnosperms,andtheangiosperms.Theangiosperms,themosthighlydevelopedandcomplexclassofplants,reproducebymeansofsingleanddoubleseedleavescalledcotyledons.Monocots,suchascorn,wheat,lilies,andorchids,haveleaveswithparallelveins,whiledicots,whichincludeoaks,maples
5、,rosesandthistles,amongothers,havenet-veinedleavesandstemswithannualgrowthrings.===============================================================AdaptationsofDesertPlantsDesertplantpopulationshaveevolvedsophisticatedphysiologicalandbehavioraltraitsthataidsurvivalinaridco
6、nditions.Somesendoutlong,unusuallydeeptaproots;othersutilizeshallowbutwidespreadroots,whichallowthemtoabsorblargeintermittentflowsofwater.Certainplantsprotecttheiraccesstowater.Thecreosotebushproducesapotentroottoxinwhichinhibitsthegrowthofcompetingrootsystems.Daytimec
7、losureofstomataexemplifiesafurthergeneticadaptation;guardcellsworktominimizedaytimewaterloss,laterallowingthestomatatoopenwhenconditionsaremorefavorabletogasexchangewiththeenvironment.Certainadaptationsreflecttheprinciplethatalargesurfaceareafacilitateswaterandgasexcha
8、nge.Moseplantshavesmallleaves,modifiedleaves(spines),ornoleavesatall.Themainfood-producingoragnisnottheleafbutthestem
