Animalscanmovefromplacetoplacebutplantscannot.Whenananimalisunderattackitcanrunawayorfightback.Plantscertainlycannotrunawayandtheylackteethandclaws.Butplantscandefendthemselvesbyusingbothphysicalandchemicalmeans.

Someplantshavetheirownwaystokeepanimalsaway.Forexampletheleavesofthehollyplanthavesharpspines(刺)thatdiscouragegrass-eatinganimals.Hollyleavesonlowerbrancheshavemorespinesthanleavesonupperbranches.Thisisbecausethelowerleavesareeasierformostanimalstoreach.

Someplantssuchastheoaktreehavethickandhardleavesthataredifficultforanimalstoeat.Somegrassesmaycontainasandymaterial;eatingsuchgrasseswearsdowntheanimal’steeth.

Manyplantsalsohavechemicaldefenses.Someplantsproducechemicalsthattastebitterorcauseanunpleasantreaction.Someplantsmayfightagainstanattackbyincreasingtheproductionofthesechemicals.Whenacaterpillar(毛虫)bitesatobaccoleaftheleafproducesachemicalmessenger.Thismessengersendstotherootstheinformationtoproducemorenicotine.Thehigherlevelsofnicotinediscouragethecaterpillar.

Manyplantsdependonbothphysicalandchemicaldefenses.AcertainplantinChinaforinstancehasprickly(多刺的)leavesandeachpricklecontainspoisonousvenom(毒液).Asingleexperiencewiththiskindofplantwillteachananimaltostayawayfromitinthefuture.

67．Thehollyplanthasmorespinesonthelowerleavesbecausemostanimals________.

A．arenottallenough

B．likethelowerleavesonly

C．arenotcleverenough

D．cangetthelowerleaveseasily

68．Todefendthemselvesoaktreesuse________.

A．chemicalmeans B．physicalmeans

C．bitterchemicals D．sandymaterials

69．Howdoestobaccoprotectitselfagainstanattackfromacaterpillar?

A．Itsleavesfightagainsttheattackbyphysicalmeans.

B．Itsrootssendamessengertodiscouragethecaterpillar.

C．Itsrootsincreasetheproductionofnicotinewhenitisattacked.

D．Itsleavesproducepoisonoussandtodrivet