A Special Clock
Every living thing has what scientists call a biological clock that controls behavior．The biological clock______(51)plants when to form．flowers and when the flowers should open．It tells insects when to______(52)the protective cocoon(防护卵袋)and fly away, and it tells animals and human beings when to eat, sleep and wake.
s outside the plant and animal______(53)the actions of some biological clocks．Scientists recently found, for example, that a tiny animal changes the color of its fur because of the______(54)of hours of daylight．In the short______(55)of winter, its fur becomes white．The fur becomes gray brown in______(56)in the longer hours of daylight in summer.
Inner signals control other______(57)clocks．German scientists found that some kind of internal clock seems' to order birds to begin their long migration ______(58)twice each year．Birds______(59)from flying become restless when it is time for the trip,______(60)they become calm again when the time of the flight has ended.
Scientists say they are beginning to learn which______(61)of the brain contain biological clocks．An American researcher, Martin Moorhead, said a small group of cells near the front of the brain______(62)to control the timing of some of our actions．These______(63)tell a person when to wake, when to______()and when to seek food．Scientists say there probably are other biological clock Cells that ______(65)other body activities.