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      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.

文章第一句是中心句，本文的中心就是研究这个藻类的periodic behavior。

第一段讲biological clock对periodic behavior的决定性。

第二段讲environmental factors对biological clock的调整作用，从而间接影响periodic behavior。

两者是文章同等重要的组成部分。

所以main idea一定要既讲biological又讲environmental。

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.

如何理解这句话呢？

顶

明了！