Q23 to 26:
Recently biologists have been
interested in a tide-associated
periodic behavior displayed by
Line the diatom Hantzschia virgata, a
(5) microscopic golden-brown alga that
inhabits that portion of a shoreline
washed by tides (the intertidal zone).
Diatoms of this species, sometimes
called “commuter” diatoms, remain
(10) burrowed in the sand during high
tide, and emerge on the sand sur-
face during the daytime low tide.
Just before the sand is inundated by
the rising tide, the diatoms burrow
(15) again. Some scientists hypothesize
that commuter diatoms know that it
is low tide because they sense an
environmental change, such as an
alteration in temperature or a change
(20) in pressure caused by tidal move-
ment. However, when diatoms are
observed under constant conditions
in a laboratory, they still display
periodic behavior, continuing to bur-
(25) row on schedule for several weeks.
This indicates that commuter diatoms,
rather than relying on environmental
cues to keep time, possess an inter-
nal pacemaker or biological clock
(30) that enables them to anticipate peri-
odic changes in the environment.
A commuter diatom has an unusu-
ally accurate biological clock, a
consequence of the unrelenting
(35) environmental pressures to which
it is subjected; any diatoms that do
not burrow before the tide arrives
are washed away.
This is not to suggest that the
(40) period of this biological clock is
immutably fixed. Biologists have
concluded that even though a
diatom does not rely on the envi-
ronment to keep time, environmental
(45) factors—including changes in the
tide’s hydrostatic pressure, salin-
ity, mechanical agitation, and
temperature—can alter the period
of its biological clock according to
(50) changes in the tidal cycle. In short,
the relation between an organism’s
biological clock and its environment
is similar to that between a wristwatch
and its owner: the owner cannot
(55) make the watch run faster or slower,
but can reset the hands. However,
this relation is complicated in intertidal
dwellers such as commuter diatoms
by the fact that these organisms are
(60) exposed to the solar-day cycle as
well as to the tidal cycle, and some-
times display both solar-day and
tidal periods in a single behavior.
Commuter diatoms, for example,
(65) emerge only during those low tides
that occur during the day.
Q24:
The passage suggests which of the following about the accuracy of the commuter diatom’s biological clock?
The accuracy of the commuter diatom’s biological clock varies according to changes in the tidal cycle.
The unusual accuracy that characterizes the commuter diatom’s biological clock is rare among intertidal species.
The commuter diatom’s biological clock is likely to be more accurate than the biological clock of a species that is subject to less intense environmental pressures.
The commuter diatom’s biological clock tends to be more accurate than the biological clocks of most other species because of the consistency of the tidal cycle.
The accuracy of the commuter diatom’s biological clock tends to fluctuate when the diatom is observed under variable laboratory conditions.
答案C，我选E。
C错的原因：文中只说diatom准确的生物钟是严酷复杂的外界环境造成的。但并没说生活环境不严酷的生物生物钟就不准啊。
选E的原因：试验表明，把diatom的外界环境改变，他们还是会慢慢改变生物钟的。 | 
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发表于 2005-11-15 18:31:00
