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. Events 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______(64) and when
to seek food. Scientists say there probably are other biological clock Cells
that ______(65) other body activities.
