Activity like breeding and migration in animals; flowering, seed germination in plants are regulated by the length of daily period of light and darkness. This behavioural phenomenon is known as photoperiodism. For example, plants such as radish, potatoes and spinach bloom when the light duration is more than 12 hours/day. Such plants are called long day plants. Cereal, tobacco, dahelia and many other plants bloom when light duration is less than 12 hrs/day. These are called short day plants. Such responses show that plants have built-in mechanisms for measuring the duration of illumination and darkness and hence flower in a specific seasons.
Similar photoperiodic responses are observed in animals. These may be diurnal, lunar or annual. Reproduction and migration in birds are such annual photoperiodic responses. From such responses it seems that distribution of some plants and animals may be restricted because the necessary photoperiodic stimulus is available only at certain latitudes.