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 s