The most important influence of temperature on flowering time is the vernalisation response. Many plant species, such as winter cereals, must go through a prolonged period of cold that mimic winter conditions before flowering occurs. This ensures that they undergo reproductive development and produce seeds in the environmentally favourable conditions of spring and summer. This process of responding to prolonged exposure to very low temperatures is called vernalisation. The SAM must be vernalised in order to promote flowering (Wellensiek, 1962; Levy and Dean, 1998) .
Vernalisation of the meristem appears to confer competence to respond to floral inductive signals on the meristem. A vernalised meristem retains competence for as long as 300 days in the absence of an inductive signal (Taiz and Zeiger, 2002) . It is possible to de-vernalise a plant by exposure to high temperatures subsequent to vernalisation (Lang, 1965; Taiz and Zeiger, 2002).
Winter and Summer Annuals
There are two distinct classes of Arabidopsis accessions with respect to flowering-time control. These are referred to as winter and summer annuals (Napp-Zinn, 1979; Karlsson, 1993) . Winter annuals flower very late unless given a prolonged exposure to low temperature, whereas summer annual can flower early without a vernalisation treatment. Winter annuals, such as Santa Fe, will eventually flower even if they are not subjected to a vernalisation process (Levy and Dean, 1998) . The response to vernalisation depends upon many factors, including the age of the plant, and the duration and temperature of the treatment (Bernier et al., 1981; Napp-Zinn, 1985) .
Endogenous Control of Flowering
Many plants show a facultative response to environmental conditions that induce flowering. Therefore, they flower even if not exposed to inductive conditions. For example, summer annuals will flower eventually under short days, even if not exposed to inductive long-day conditions. Similar, in genetic terms, no single gene mutation prevents transition to flowering of Arabidopsis.
This suggests that parallel pathways, which include endogenous control developmental mechanisms, ensure flowering occurs even in the absence of a promotive environmental condition. In Arabidopsis, these involve the autonomous flowering pathway, growth regulators, and probably other so far undefined pathways.
The autonomous pathway includes genes that promote the floral transition under both short and long days. The growth regulators gibberellins have a strong effect on flowering time in Arabidopsis and other rosette plants (Lang, 1965) . In Arabidopsis gibberellins have an inductive effect, especially when plants are grown in short-day conditions (Koornneef and van der Veen, 1980; Koornneef et al., 1985).