Abstract:
Many organisms use an internal calendar. It allows to recognize the season. This calendar is based either on an internal annual clock or on an internal daily clock. Annual clocks control, for instance, the eclosion of dinoflagellates out of the cyst and the germination of seeds, body weight, fur color, reproduction and behavior of the Dsungarian hamster, molt and migration of birds. Photoperiodic reactions measure the day length and induce beyond a critical length (longday organisms) or below a critical length (shortday organisms) certain processes. In this way flowers are induced in shortday plants such as Kalanchoe blossfeldiana and Pharbitis nil or in longday plants such as Arabidopsis thaliana. Potato tubers are formed under shortday conditions, bulbs under longday conditions. Some proposals for experimenting are made.