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.