650 Gene Banks broader plant diversity collections where uses include a wide array of pure and applied research in addition to field trials During 1994–1996, there was a 50% request rate for seeds offered through an extensive list offered by the Kew Seed Bank Several elements need to be considered concerning the distribution of seed to users The recipient should be provided with accurate information about the collection and how to germinate the seeds It should also be remembered that certain species require a symbiont for effective growth (e.g., legumes and Rhizobium), and that the user may need to draw on germplasm for both plant and symbiont The seed sample must be dispatched having fulfilled all necessary plant health and, where appropriate, CITES requirements Finally, to meet the needs of the CBD and to clarify the conditions under which the material can be used, all germplasm samples are increasingly dispatched under material supply agreements Regeneration Seed bank accessions are grown out for the purposes of regeneration of seed stock (either when seed numbers are low or when viability has reduced), for characterization and for evaluation Many banks have a regeneration standard below which the germination of a seed lot should not fall This is usually set at 85% This high value limits the risk of accumulated genetic damage that is associated with seed aging Even though falling levels of seed germination are correlated with falling levels of field establishment, many banks have adopted lower standards This may in part be due to the backlog of regeneration work that in some national facilities highlighted by FAO (1996) is nearly 100% of the collection By collecting high-quality seed lots in good quantity, other banks have reduced the necessity for regeneration that can be time and labor consuming and that can have adverse effects on the genetics of the collection Samples regenerated under conditions different from where they originated can experience selection If too small a sample is regenerated, genetic drift may occur in which rarer alleles are lost through chance Under some circumstances, recollection, if possible, may the more desirable option Seed Bank Design Having considered the aspects of seed bank management, a brief consideration of seed bank design is appropriate (also see Cromarty et al., 1985) The location of the bank is important from political, practical, and security aspects Potential risks have to be considered be they earthquake, flooding, or radiation fallout Some facilities are placed underground such as the seed bank at Krasnodar in Russia and the Millennium Seed Bank in the United Kingdom Others such as the NSSL are located on the first floor to limit possible impact from structures above resultant from seismic activity The size of most banks should be dictated by peak annual intake (seed drying and cleaning facilities), projected capacity before a rebuild is practical (seed storage), and annual collection maintenance (germination, field, and greenhouse facilities) Cold storage facilities vary from a few domestic deep-freezers up to large rooms such as one of 140 m2 (with capacity for 150,000 samples) at NIAR in Japan Other Types of Gene Bank This section provides summaries of the current status of nonseed gene banks, starting with dry propagules (pollen and spores), which can be stored under conditions similar to those used for seeds, and covering normally hydrated tissues that can also be preserved under a different set of controlled conditions Finally, field gene banks are covered and the role of botanical and zoological gardens is briefly mentioned Pollen There are many practical reasons for storing pollen: to support work on allergenic responses, plant hybridization, and fertility; haploid plant production; and genetic transformation systems with isolated pollen (or gametes) Optimal storage conditions for pollen are similar to those for seeds (i.e., at low moisture content and subzero temperatures) In addition, there is some evidence (e.g., in maize and Impatiens) that longevity in dry storage is enhanced in anoxic atmospheres Moreover, pollen has also been stored in a vacuum-dried state The ease of storage though relates in part to the cellular and physiological nature of the pollen Bicellular pollen, as found in Liliaceae, Orchidaceae, Solanaceae, and Rosaceae, generally tolerates desiccation to about 10% moisture content and is relatively long-lived By comparison, tricellular pollen, as found in Graminae and Compositae, is relatively short-lived and is much more sensitive to desiccation Most work on the long-term storage of pollen has focused on fruit tree or forestry species, for which 10 years storage at conventional gene bank temperatures ( À 20 1C) is easily attainable Pollen of at least 30 species are known to survive liquid nitrogen temperatures Although pollen banking is evidently possible for many species, there does not appear to be any large-scale gene bank operation using such material Spores Spores of many species of both pteridophytes and bryophytes are stable for months or years when dried, and this time can be extended with storage at cold or freezing temperatures The longevity of short-lived (chlorophyllous) spores of some species can be extended significantly by drying and freezing in liquid nitrogen (Pence, 2000) Although there are data on fungal spore storage (e.g., work by Hong et al in 1998), the majority of fungal germplasm appears to be conserved in culture or through cryopreservation of hyphae (see later) Somatic and Zygotic Embryos of Plants with Nonbankable Seeds Nonbankable seeds can nonetheless be stored using alternative approaches Usually, these revolve around the use of rapid, partial desiccation of embryos or embryonic axes to about 20% moisture content and subsequent transferal to liquid nitrogen temperature or the use of other subzero storage temperatures Recovery levels may be improved by pretreatment of embryos with cryoprotectants, encapsulation of the material in alginate beads, or careful manipulation of the