| {{B}}
A Biological
Clock{{/B}} Every living thing has what scientists call a biological clock that controls behavior. The biological clock{{U}} (1) {{/U}}plants when to form flowers and when the flowers should open. It tells insects when to{{U}} (2) {{/U}}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 animals{{U}} (3) {{/U}}the action of some biological clocks. Scientists recently found, for example, that a tiny animal changes the color of its fur because of the{{U}} (4) {{/U}}of hours of daylight. In the short{{U}} (5) {{/U}}of winter, its fur becomes white. The fur becomes gray brown in{{U}} (6) {{/U}}in the longer hours of daylight in summer. Inner signals control other{{U}} (7) {{/U}}clocks. German scientists found that some kind of internal clock seems to order birds to begin their long migration{{U}} (8) {{/U}}twice each year, Birds{{U}} (9) {{/U}}from flying become restless when it is time for the trip,{{U}} (10) {{/U}}they become calm again when the time of the flight has ended. Scientists say they are beginning to learn which{{U}} (11) {{/U}}of the brain contain biological clocks. An American researcher, Martin Moorhead, said a small group of cells near the front of the brain{{U}} (12) {{/U}}to control the timing of some of our actions. These{{U}} (13) {{/U}}tell a person when to wake, when to{{U}} (14) {{/U}}, and when to seek food. Scientists say there probably are other biological clock cells that{{U}} (15) {{/U}}other body activities. |