A germplasm center is essentially a bank for genes from plants; these genes are stored within living plant tissue, such as seeds or whole plants. "Seed bank" is another term associated with germplasm centers, but in some cases, seeds of plants cannot be stored and so the entire plant must be conserved. This is common for some trees whose seeds die out very quickly; the National Arboretum in Washington, D.C., for example, has a germplasm center with many trees and shrubs. In the case of tree fruits, the unique combination of traits exhibited by some varieties would be lost if seeds of these plants were used for storage. Therefore, collections of fruit trees must be maintained to preserve this germplasm; such collections are called 'clonal' because clones of individual trees must me conserved.
Seeds are the preferred and most efficient way to store germplasm and much work goes into keeping seeds healthy and alive for as long as possible. Long-term preservation of seeds requires low temperatures (below freezing) and low relative humidity.