A Biological Clock Every living thing has whatscientists call a biological clock that controls behavior. The biological clock tells__ (51) when to form. flowers and when the flowers should open. It tells____(52) when to leave the protective cocoons and fly away, and it tells animalsand human beings when to eat, sleep and wake. Events outside the plant andanimal_________(53) the actions of some biological clocks. Scientists recentlyfound, for example, that a tiny animal changes the color of its fur _ (54) the number of hours of daylight. In theshort _(55) of winter, its furbecomes white. The fur becomes gray brown in color in the longer hours ofdaylight in summer. Inner signals control otherbiological clocks. German scientists found that some kind of internal clockseems to order birds to begin their long migration _(56) twice each year. Birds (57) flying become restlesswhen it is time for the trip, _ (58) they become calm again when the time ofthe flight has ended. Scientists say they arebeginning to learn which _(59)of the brain contain biological clocks. An American researcher, MartinMoorhead, said a small group of cells near the front of the brain _(60) tocontrol the timing of some of our actions. These _______(61) tell a person whento _ (62) , when to sleep andwhen to seek food. Scientists say there probably are other bio-logical clockcells that control other body activities. Dr. Moorhead is studying _(63) our biological clocks affect theway we do our work. For example, most of us have great-difficulty if we mustoften change to different work hours. _(64) can take many days for a human bodyto accept the major change in work hours. Dr. Moorhead said industrialofficials should have a better understanding of biological clocks and how theyaffect workers. He said __ (65)understanding could cut sickness and accidents at work andwould help increase afactory's production.