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Many plants open their flowers each morning, only to
close them again ___1___
dusk. Others, such as night-blooming cacti, open only at
night. Some plants also fold or drop their leaves into a
sleeping position each night. Take any of these plants
into a windowless room, and they will continue their
opening and closing like clockwork, regardless of
darkness or light. Such regular daily ___2___
are called circadian. They are controlled by a mechanism
that scientists often call a biological clock. The
nature and location of this clock in plants remains a
great mystery. The usefulness of a biological clock is
more obvious : night-blooming plants, for example, tend
to be pollinated by moths and other ___3___
of the darkness, while flowers that open in the morning
have pollinators that work by day. Similarly, plants
that fold their leaves at night may benefit from ___4___
heat and moisture loss.
Plants can also measure changes in the length of day
and night, and use this ___5___
to time the season in which they flower. The effect of
day length on plants, discovered by scientists in the
1920s, is called photoperiodism. |
