FEATURED ESSAYS 1. Photoperiodism 2. Seasonal Affective Disorder

Photoperiodism


   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 light.

   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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