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   Plants and animals in  the temperate zones respond in
different ways to the amount of daylight in 24-hour
periods. This response to day length is called photo
periodism. It controls many activities, among them is the
flowering of plants. The ability to respond to day length
is linked to an inner, light-sensitive circadian rhythm.
   In the temperate zones, day lengths during the natural
24-hour cycle vary with the seasons. In winter and spring,
the period of light lengthens; in summer and autumn, it
shortens. Plants in these zones undergo alternate 12-hour
phases of light sensitivity. During one 12-hour phase,
decreasing exposure to light induces a short-day reaction.
For example, deciduous trees under the influence of the
shorter days of fall drop their leaves. During the other
12-hour phase, more exposure to light creates a long-day
reaction. Deciduous trees grow leaves again during the
lengthening days of spring. This indicates that through
their sensitivity to changes in the duration of light,
plants can measure day length to determine the season and
the time spans within a season.  Florists can often
manipulate greenhouse plants into producing blossoms out
of season by exposing them to  periods of artificial
   Some scientists are not certain that the biological
clock of any organism is completely endogenous. They think
that even under the most constant of laboratory conditions
living things are aware of the Earth's rotation and that
this has an effect on the wheelof their clocks. However,
many scientists believe that such factors are not
essential to the functioning of biological clocks.
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