Photoperiodism is the physiological reaction of organisms to the duration of day or night. It occurs in plants and animals. Photoperiodism can also be defined as the developmental responses of plants to the comparative lengths of light and dark times. Some long-day obligate plants are: Carnation, Henbane, Oat etc. Some long-day facultative plants are: Pea, Barley, Lettuce, Wheat etc.