细菌(Bacteria)

bacteriaareextremelysmalllivingthings.whilewemeasureourownsizesininchesorcentimeters,bacterialsizeismeasuredinmicrons.onemicronisathousandthofamillimeter:apinheadisaboutamillimeteracross.

rod-shapedbacteriaareusuallyfromtwotofourmicronslong,whileroundedonesaregenerallyonemicronindiameter.thusifyouenlargedaroundedbacteriumathousandtimes,itwouldbejustaboutthesizeofapinhead.anadulthumanmagnifiedbythesameamountwouldbeoveramile(1.6kilometer)tall.

evenwithanordinarymicroscope,youmustlookcloselytoseebacteria.usingamagnificationof100times,onefindsthatbacteriaarebarelyvisibleastinyrodsordots.onecannotmakeoutanythingoftheirstructure.

usingspecialstains,onecanseethatsomebacteriahaveattachedtothemwavy-looking"hairs"calledflagella.othershaveonlyoneflagellum.theflagellarotate,pushingthebacteriathroughthewater.manybacterialackflagellaandcannotmoveaboutbytheirownpower,whileotherscanglidealongoversurfacesbysomelittle-understoodmechanism.

fromthebacteriapointofview,theworldisaverydifferentplacefromwhatitistohumans.toabacteriumwaterisasthickasmolassesistous.bacteriaaresosmallthattheyareinfluencedbythemovementsofthechemicalmoleculesaroundthem.bacteriaunderthemicroscope,eventhosewithnoflagella,oftenbounceaboutinthewater.

thisisbecausetheycollidewiththewaterymoleculesandarepushedthiswayandthat.moleculesmovesorapidlythatwithinatenthofasecondthemoleculesaroundabacteriahaveallbeenreplacedbynewones;evenbacteriawithoutflagellaarethusconstantlyexposedtoachangingenvironment.