Mention the viability of pollen grains.

(N/A) Once the pollen grains are shed,they must land on the stigma before they lose viability to bring about fertilization.
The period for which pollen grains remain viable is highly variable and depends on the prevailing temperature and humidity.
In some cereals such as rice and wheat,pollen grains lose viability within $30$ minutes of their release.
In some members of families like Rosaceae,Leguminosae,and Solanaceae,they maintain viability for months.
It is possible to store pollen grains of a large number of species for years in liquid nitrogen at $-196^{\circ}C$. Such stored pollen can be used in pollen banks,similar to seed banks,for crop breeding programs.