Some plants, especially biennials and perennials, are stimulated to flower by exposing them to low temperatures. This is called vernalisation. Vernalization may be an absolute requirement (as in henbane), or may just hasten flowering (as in winter cereals). The length of chilling varies from four days to three months, temperature around 4 degree Celsius being the most effective. Vernalin is the hormone which has been suggested to be involved in this process of vernalisation. It has also been found that gibberellins level increase during vernalisation and it can also be used to introduce vernalisation in unvernalised plants
