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.