In photoperiodism induction occurs if a sufficiently long period of darkness or light is experienced. Rhythmicity can be demonstrated as responses to the timing of interruption of darkness by light (short-day plants, SDP), or additions of far-red light to constant white light (long-day plants, LDP). The resulting photoperiodic response rhythm (PRR) has a period of about 24 h, shows temperature compensation, and is entrained by light. Light also has a direct (‘acute’) response, inhibiting or promoting induction at particular phases, and so giving rise to the external coincidence model for control of induction.