细菌(Bacteria )

bacteriaareextremelysmalllivingthings.whilewemeasureourownsizesininchesorcentimeters,bacterialsizeismeasuredinmicrons.onemicronisathousandthofamillimeter:apinheadisaboutamillimeteracross.rod-shapedbacteriaareusuallyfromtwotofourmicronslong,whileroundedonesaregenerallyonemicronindiameter.thusifyouenlargedaroundedbacteriumathousandtimes,itwouldbejustaboutthesizeofapinhead.anadulthumanmagnifiedbythesameamountwouldbeoveramile(1.6kilometer)tall.evenwithanordinarymicroscope,youmustlookcloselytoseebacteria.usingamagnificationof100times,onefindsthatbacteriaarebarelyvisibleastinyrodsordots.onecannotmakeoutanythingoftheirstructure.usingspecialstains,onecanseethatsomebacteriahaveattachedtothemwavy-lookinghairscalledflagella.othershaveonlyoneflagellum.theflagellarotate,pushingthebacteriathroughthewater.manybacterialackflagellaandcannotmoveaboutbytheirownpower,whileotherscanglidealongoversurfacesbysomelittle-understoodmechanism.

fromthebacteriapointofview,theworldisaverydifferentplacefromwhatitistohumans.toabacteriumwaterisasthickasmolassesistous.bacteriaaresosmallthattheyareinfluencedbythemovementsofthechemicalmoleculesaroundthem.bacteriaunderthemicroscope,eventhosewithnoflagella,oftenbounceaboutinthewater.thisisbecausetheycollidewiththewaterymoleculesandarepushedthiswayandthat.moleculesmovesorapidlythatwithinatenthofasecondthemoleculesaroundabacteriahaveallbeenreplacedbynewones;evenbacteriawithoutflagellaarethusconstantlyexposedtoachangingenvironment.