Write a short note on: $(1)$ Vernalisation.

(N/A) Vernalisation is the process by which the flowering of a plant is promoted by exposure to a period of low temperature.
It prevents precocious reproductive development late in the growing season and enables the plant to have sufficient time to reach maturity.
Many food plants,such as wheat,barley,and rye,have two kinds of varieties: winter and spring varieties.
The 'spring' variety is normally planted in the spring and comes to flower and produces grain before the end of the growing season.
'Winter' varieties,if planted in spring,would normally fail to flower or produce mature grain within a span of a flowering season.
Hence,they are planted in autumn,germinate,and come out as small seedlings during winter.
They resume growth in the spring and are harvested around mid-summer.
Vernalisation thus refers to the specifically promoted flowering by a period of low temperature.