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．
第 51 题