Photoperiodism

1 Pages 240 Words

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....