There are environmental factors regulating flowering other than night length. Temperature is also important. Some plants require exposure to a low temperature, 0 to 10℃, for a few days to a few weeks for flowering. Such an induction of flowering by a low temperature is called vernalization. Vernalization is observed in many long-day plants, but it is not correlated with photoperiodism. Winter cereals respond to low temperature at the imbibed seed stage. Biannual varieties of Hyoscyamus niger respond to low temperature at rosette stage. The organ which responds to a low temperature is the apical meristem.