The length of time that pollen grains remain viable is highly variable and, to some extent, is determined by the temperature and humidity conditions that are now in effect. After being released into the air, the pollen grains of some cereals, such as rice, and wheat, lose their vitality after just 30 minutes. It is possible for pollen grains to retain their vitality for several months in certain Rosaceae, Leguminosae, and Solanaceae species. It is feasible to preserve pollen grains from a wide variety of species for an extended period of time in liquid nitrogen, which has a temperature of -196 degrees Celsius.