A visionary attempt to save many of the world's endangered plants is to open in the United Kingdom at the end of August.

It is the Millennium Seed Bank (MSB), one of the largest international conservation projects ever undertaken.

It aims to have saved the seeds of more than 24,000 species by 2010, a tenth of the global seed-bearing flora.

The bank has already collected seeds from almost all the 1,400 or so British species suitable for inclusion.

Extinction threatens more than 300 UK wild plants, and a quarter of the world's plants could be condemned to vanish by 2050.

Cold and dry

The MSB 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 equivalent, Wakehurst Place in southern England.

Seeds arriving at the MSB - it already has more than 250 million, from almost 5,000 species - are first dried in conditions of low humidity.

Hugh Pritchard takes extinction seriously

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 degrees C. 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 to be stored in Scotland.

Securing food

Most of the seeds from abroad are from dryland species. The project leaders say "off-site" conservation like seed banking can contribute more "where land degradation is brought about by climatic effects overlaying unsustainable land use by indigenous people".

They estimate that more than 60,000 sq kms are lost to deserts every year. Dr Hugh Pritchard, head of research at the MSB, told BBC News Online: "The drylands are fundamentally important to the sustenance of something like 20% of the world's population."

Collecting seeds in Burkina Faso

The MSB, which is costing more than �80 m, is planned to be a centre of expertise, collaborating with the countries from which the seeds originate.

The project aims to help other countries to set up their own seed banks, and plans to keep "a substantial proportion" of all the seeds collected in their countries of origin.

Dr Pritchard says the prospect of extinction is real enough for many plants. "I think the threat is probably worse than we imagine.

Untapped potential

"If we think about population expansion and the pressure that will put on natural areas, those areas will decrease. That has to mean we're going to lose species. Most of the models do predict tremendous risk to plant species.

"Many of the species that we currently conserve we perhaps don't have an immediate use for. But something like 30% of the medicines we use currently are based on products or chemicals which have been extracted from plants.

"In the future, if we are to find new remedies, we need to have access to this cornucopia of plants worldwide."