British botanists are celebrating 2004 certain they are near their goal of saving the seeds of all UK plants.
BBC News Online environment correspondent
The Millennium Seed Bank project says it must find two more species to reach its target of protecting all the UK's seed-bearing flora from disappearing.
Extinction threatens more than 300 UK wild plants, and a quarter of all the world's plants could vanish by 2050.
The bank, opened in 2000 in southern England, is one of the world's most ambitious conservation projects so far.
It is an initiative of the UK's Royal Botanic Gardens, Kew. But it is housed, not at Kew Gardens in London, but at its rural branch, Wakehurst Place in Sussex.
It already had an extensive seed collection by 1997 - 570 of the UK's 1,400 native species.
That year it received lottery funding to make the collection as complete as possible, and set itself a target of another 800 British species.
EMBLEMS OF A LOST ENGLAND
Modern industrial farming techniques have now pushed many arable "weeds" close to extinction.
Although the bank needs just two species to reach the target, it has more rare plants than that in its sights.
A spokesman explained: "It is not just two specific species we need to hit the target. There is a short 'hit-list' of wanted species to complete the jigsaw."
Plants on the list include the western march orchid (Dactylorhiza majalis), chalk milkwort (Polygala calcarea), and the red-fruited stone bramble (Rubus saxatilis).
Describing the collection of so many species as "a remarkable achievement", the spokesman said: "Some of the species we are trying to save rarely flower in the UK any longer, and we therefore have little chance of collecting their seeds.
"These plants make up part of our rich botanical heritage, and we need them in safe storage to ensure their future. Once banked, seed can survive for hundreds of years."
Cornflowers are an evocative British sight
Seeds from the bank have already been used to reintroduce some species to the wild in the UK, including the strapwort.
The bank aims to have saved the seeds of more than 24,000 species by 2010, a tenth of the global seed-bearing flora.
Seeds arriving at the bank - it already has more than 250 million, from almost 5,000 species - are first dried in conditions of low humidity.
After several weeks their moisture level has fallen to 5%, which means they will last much longer - for about 200 years, the bank's designers hope.
They are then cleaned and checked, with a sample of about 50 of each species being X-rayed for quality.
Each batch is tested for germination in dishes of agar jelly. The test will be repeated every decade.
Finally, they are placed in ordinary glass jars and stored in three underground vaults at temperatures of -20 Celsius. The number of seeds kept in a single jar is as great as the number of people in many modern cities.
As a precaution, a back-up seed collection is being stored in Scotland.