By Richard Black
Environment correspondent, BBC News website
Britain's "Noah's Ark" for plants has just collected its billionth seed.
The Millennium Seed Bank already contains 18,000 species
The Millennium Seed Bank will present the seed, from an African bamboo, to Chancellor Gordon Brown, as it seeks funds to continue operating after 2010.
Part of the Royal Botanic Gardens (RBG) at Kew, the bank already stores material from 18,000 species, some of which have become extinct in the wild.
Seed banks are seen as an essential part of plans to curb the rapid loss of biodiversity, in Britain and worldwide.
By 2010, Kew plans to have amassed seeds from 30,000 species, representing 10% of the world's plants.
"Now we're starting to think about where we go beyond 2010," the project's head, Paul Smith, told the BBC News website.
"And we want to get to 25% of species stored away by 2020. If policymakers are serious about funding adaptation to climate change, seed banks are a key part of that."
Seeds are collected by Kew's partner organisations around the world and sent to the RBG site at Wakehurst Place in Sussex.
They come from all over the globe, although British varieties are particularly well represented, with seeds from 88% of its native flora sequestered away.
Seeds for the bank are collected all over the world
Most of the seeds can be preserved by careful drying, after which they are stored at minus 20C. A few need more specialised, tailor-made treatment.
Some will last like this for millennia, others for decades; these will be planted and germinated before their expiry date comes up, and the seed of their offspring collected and stored anew.
But the idea is not to hide them away for ever. Where species have gone extinct, or are teetering on the edge, Kew's stores are used to replenish wild populations.
One British example is strapwort (Corrigiola litoralis), a critically endangered native of southwest England now found on only one nature reserve, which Kew's stocks are helping to keep alive.
The billionth seed comes from the African bamboo species Oxytenanthera abyssinica, a plant used in Mali and other West African countries for building, furniture, and wine-making.
Its presentation to Mr Brown is aimed at persuading the Chancellor and prospective Prime Minister to continue funding the Millennium Seed Bank after 2010.
It is a key year in conservation, marking the target date by which, under the UN biodiversity convention, the world's governments are pledged to have halted and begun to reverse the seemingly inexorable biodiversity decline.
"Scientists are always asking for money," conceded Dr Smith. "But what makes us different is that we have a proven methodology here, we have the network and we know how to do what we do.
"This costs about £2,000 ($4,000) per species; so to collect a quarter of what's out there would cost about £100m ($200m).
"With threats not only from climate change but also deforestation, changes in land use and so on, seed-banking is the bare minimum."