Protocol: 

For succesful cryopreservation the material must be cooled slowly to ca -30 to -60°C, before transfer to liquid nitrogen. This slow cooling gives rise to cellular dehydratation which progressively concentrates the cellular constituents and depresses their freezing point. Once cell solutions have been concentrated by slow cooling, remaining water can be vitrified by freezing the plant material as rapidly as possible. In some cases small plant material can be transferred immediately to minus 196°C without damaging the plant tissue.

Thawing usually is most succesful when it happens rapidly.