Flexi Says: Although you might detect seasonal changes by the change in temperature, this is not the way in which plants know the seasons are changing. Plants determine the time of year by the length of daylight, known as the photoperiod. With a light-sensitive chemical, plants can sense the differences in day length. For example, in the fall, when the days start to get shorter, the trees sense that there is less sunlight. The plant is stimulated, and it sends messages telling the leaves to change colors and fall. This is an example of photoperiodism, the reaction of organisms, such as plants, to the length of day or night. Photoperiodism is also the reaction of plants to the length of light and dark periods. Many flowering plants sense the length of night, a dark period, as a signal to flower. Each plant has a different photoperiod, or night length. When the plant senses the appropriate length of darkness, resulting in an appropriate length of daylight, it flowers. Flowering plants are classified as long-day plants or short-day plants.