Living things can regulate their
own activities in such a way that they fit into the rhythms (节奏) in nature. For example, plants
consume (大量吸收) sunshine during the day and do most of their breathing at night.
Small animals which live in very hot regions remain deep in their holes during
the heat of the day and only come out at night to obtain food. These rhythmic
patterns of activity in living things can be called biological clocks.
Scientists have discovered some
very interesting facts about biological clocks. Their most important discovery
is that although biological clocks
approximately (近似地) correspond to the rhythms of day and night, light and darkness,
they do not always depend on them. Many biological clocks are independent. How
can this be proved? It can be proved by showing that a biological clock will
continue to operate even when its surroundings are changed so that it can no
longer respond to alternating periods of lights and darkness.
We will take the crab (蟹) as an example. This animal is a
living clock. It indicates (指出) the time by the color of its skin, which is dark during
the day and lighter in color during the night. The daytime darkness of the
crab's skin, which prevents its enemies from noticing it and also prevents the
crab from getting too hot, is repeated every 24 hours. The alternating colors
of the skin appear to follow the alternating rhythm of day and night. How can
we prove the statement that crab is an independent clock which creates its own
rhythm, its own time? Perhaps it only responds to sunlight, by growing darker
when in contact with sunshine and growing lighter in color when the sun sinks
behind the horizon (地平线)?
To answer this question,
biologists did a simple experiment. They kept the crab in a dark room at a
constant temperature for two months. They found that the skin of the crab
continued to change color according to the same 24-hour rhythm, following the
movements of the sun outside. In this way they proved that the crab's
biological clock was independent of events in its natural surroundings. It was
not just a series of responses to external events.
1.
Biological clocks in living things are——.
A. natural
rhythms
B. the
alterations of day and night
C. living
thing's activities corresponding to some natural rhythms
D. the
repetition of activities of living things
2.
The author compares a crab to——.
A. a
biological clock
B. an example of biological clock
C. a living
clock
D. a model of
biological clocks
3.
A crab is an example of biological clocks in living things because it
A. looks
like clocks
B. has dark
skin
C. creates
its own time
D.
alternates color of its skin to follow the alternating rhythm of day and night
4.
The colors of the crab's skin can
A.
indicate the time
B.
prevent its enemies from noticing it
C.
prevent it from getting too hot
D. do all the above