Mention about the viability of pollen grain

Explanation

Once the pollen grains are shed, they have to land on the stigma before they lose viability if they have to bring about fertilisation. The period for which pollen grains remain viable is highly variable and to some extent depends on the prevailing termperature and humidity.
In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes of their release and in some members of Rosacease, Leguminoseae, and Solanaceae, they maintain viability for month. It is possible to store pollen grains of a large number of species for years in liquid nitrogen `(-196^(@)C)`. Such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programmes.