Desertplantsfallintotwosortsaccordingtothewaytheydealwiththeproblemofsurvivingdrought.Therearethedrought―avoiders―thosewhichpersist（存留）onlyasseedsreadytospringupwhenitrainstoflowerquicklyandproduceanothercropofseedsandtodieagain.Therearealsothedrought―resisters―thosewhichhaveevolvedvariousmethodsforstrongwaterlocatingundergroundwaterorreducingtheirneedforwaterbysuchdevicesassheddingtheirleaves.Thedrought―resistersareperennials(多年生植物)theymanagetolivefromonerainyseasontoanotherslowlygrowingbiggerandbigger.Ofthesethesucculents(肉质的)isasmallbutinterestingfraction(小部分).Theymaystorewaterintheirleavesintheirstemsorinundergroundcontainers.

IntheAmericandesertsthebest―knownsucculentsarethecacti(仙人掌).Theycomeinawiderangeofsizesfrom50―foottallgiantsaguaros(树形仙人掌)totinyroundcactiaboutthesizeofathumb―nail.Theytakethickcylindrical(圆柱体的)orevensphericalformstherebyexposingaminimumofevaporatingsurfacetotheairandlight.Theyareleaflessexceptinyouthandthentheleavesaresmall.Typicallytheirsurfacesarespinydiscouragingthirstyanimalsandchanneledlikeanaccordion(手风琴)sothefleshystemmayexpandquicklywhentheplantdrinksandcontractslowlyasitusesupthewater.Therootsystemiswidespreadandshallowwithgoodreason;onlyabout3percentoftherainthatfallsonthedesertpenetratestoanysignificantdepthintosoil.

65.Thepassageclassifies（将……分类）thedesertplantsaccordingto___.

A.thewaystheytaketodealwiththetypicalclimaticconditionindesert

B.theirshape

C.howtheyusewater

D.whentheyproducetheirseeds

66.Accordingtothepassagesomedesertplantsdroptheirleaves___.

A.becausethetemperatureistoohigh

B.sothattheplantscanevaporatelesswater

C.becausetheyaredroughtresisters

D.sothattheplantscanstoremorewater

67.Whatintereststheauthormostwhenheorshementionssucculentsisthat___.

A.theytakethickcylindricalorevensphericalshape

B.theyarebest―knowninAmerica

C.theyhavemanywaysofcontainingwater

D.someofthe