In general, plant tissue lasts longest when stored at low temperatures.
Cryopreservation is storage of vegetative tissue or seeds in liquid nitrogen for long term storage.
There are several locations around the world where this technique is used for germplasm preservation.
Liquid nitrogen maintains an ultra-low temperature about -196°C. It requires that the tissue is relatively dry, but has been applied very successfully to a number of seed species.
Seeds, dormant buds and pollen can be preserved directly in liquid nitrogen.
Stem pieces can also be frozen in liquid nitrogen, but only after they have been treated with "vitrifying agents" such as sugars.