Although many rare plants have seeds that can be stored by conventional methods, not all species have seeds that can live after drying or freezing. Sometimes called "Exceptional plants, " these species have a wide range of variation. Some produce few or no seeds, thus they cannot be seed banked, others have seeds or spores that will die if dried or frozen, while others have seeds that can tolerate drying, but not freezing, and another group have seeds that live less than 10 years at freezing temperatures. Recent studies suggest that there are many rare plant species that need alternative storage for conservation. This video describes a procedure for testing whether rare plants can be stored in liquid nitrogen successfully.