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.