A Biological Clock

　　Every living thing has what scientists call a biological clock that controls behavior. The biological clock tells   1   when to form flowers and when the flowers should open¹. It tells   2   when to leave the protective cocoons and fly away，and it tells animals and human beings when to eat, sleep and wake.
　　Events outside the plant and animal   3   the actions of some biological clocks. Scientists recently found, for example, that a tiny animal changes the color of its fur   4 _ the number of hours of daylight. In the short   5   of winter, its fur becomes white. The fur becomes gray brown in color in the longer hours of daylight in summer.
　　Inner signals control other biological clocks. German scientists found that some kind of internal clock seems to order birds to begin their long migration   6   twice each year. Birds   7   flying become restless when it is time for the trip,   8   they become calm again when the time of the flight has ended.
　　Scientists say they are beginning to learn which   9   of the brain contain biological clocks. An American researcher, Martin Moorhead, said a small group of cells near the front of the brain   10   to control the timing of some of our actions. These   11   tell a person when to   12   ，when to sleep and when to seek food . Scientists say there probably are other biological clock cells that control other body activities.
　　Dr. Moorhead is studying   13   our biological clocks affect the way we do our work². For example, most of us have great difficulty if we must often change to different work hours.

 　14   can take³ many days for a human body to accept the major change in work hours. Dr. Moorhead said industrial officials should have a better understanding of biological clocks and how they affect workers. He said   15   understanding could cut sickness and accidents at work and would help increase a factory’s production,