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.