ARDENTLY, England. -- In the past 400 years, 450 types of plants and trees around the world have become extinct, and the situation is about to get a lot worse.
Because of global warming and pressures on the land caused by population growth, scientists estimate a quarter of the world's remaining 270,000 plant, conifer and fern species will be under threat of extinction by 2050. Deserts are advancing in some regions at a rate of nearly four miles a year, and flooding is destroying plants elsewhere.
To try to head off the loss of such precious resources, Britain is operating what is projected to become the world's biggest seed bank devoted to wild plants. The Millennium Seed Bank is on the grounds of a botanical garden bear Sussex village about 35 miles south of London.
So far nearly 260 million seeds representing 5,118 species of wildflowers and other plants have arrived and by 2010, the number is expected to grow to 24,000, nearly 10 percent of the world's seed-bearing plants.
Among the samples already on hand are the world's biggest seed, the coco-de-mer, or double coconut, which can weigh up to 45 pounds, and the world's smallest, orchid seeds about the size of a dust particle.
The seeds come from arid areas, mostly in Africa and the Middle East but also the United States and Mexico. The bank holds seeds from virtually of the 1,400 plants native to Britain, of which more than 3000 are considered to be endangered. Britain is the only country that has banked its flora in this way.
The seeds are sorted, separated by hand from their pods, cleaned and dried to a moisture content of 5 percent and a humidity level of 15 percent, drier than most deserts. Seeds in each batch are examined by X-rays to make sure they have not been damaged by insects or harmed in other ways that would keep them from germinating.
The the seeds are consigned to underground deep-freeze vaults where they are preserved at a temperature of minus 4 degrees Fahrenheit.
Most seed banks elsewhere concentrate on agricultural and horticultural seeds that are sold commercially. At Wakehurst Place, a 490-acre garden that surrounds a former Elizabethan mansion, the Millennium Seed Bank exists solely for preservation.
A new $18 million, barrel-vaulted building that is open to visitors contains a "seed wall," an exhibit of seeds along with detailed information about their preservation. But the underground cold-storage vaults are open only to the 25 scientists working there.
"One of our challenges is that we are dealing with a broad range of species new to preservation science," said Hugh Pritchard, head of research. "Most seeds readily survive conventional drying and freezing, but about 10 percent of our collection pose problems for germination or conservation."
Pritchard and other scientists are experimenting to determine the optimum temperature for such troublesome species as papaya, oil palm and coffee. Their findings so far suggest that a temperature of minus 94 degrees Fahrenheit may be about right for these seeds.
"But the optimum may vary from species to species," Pritchard said.
One of the rarest seeds in the collection is that of the bottle palm, which produces a fruit. It is found in the Indian Ocean island nation of Mauritius.
The bank also has seeds of the red stinkwood tree from Cameroon. It is used to treat prostate cancer. This tree is rapidly disappearing in its natural habitat.
The Millennium Seed Bank's team of four scientists travels to remote corners of the world to find and collect seeds.
The team trains local scientists, gives them technology, and spends a great deal of time negotiating agreements with Third World governments allowing the seed bank to collect and exports seeds.
"Then comes the Indiana Jones phase," said Michiel van Slageren, a Dutch scientist.
In Kenya, scientists and local workers shook one tree to try to bring down seed-bearing fruit and instead brought down a poisonous snake.
In Yemen, where van Slageren went to collect seeds of myrrh and frankincense, foreigners face the threat of kidnapping by tribesmen who then demand ransoms.
Michael Way works mainly in the American west, on federally owned lands, and said arid lands there are being threatened by expanding human settlement, climate change and intense forest fires.
"Some fires are so hot they are removing the soil seed bank that is naturally present," he said. "If these fires are repeated every few years, the bank can be depleted."
Head scientist Roger Smith said the seed bank concentrates on dry lands. "Most deforestation in wet tropics is caused by humans, and there, governments can keep people from coming in and destroying trees," he said. "The next-largest area under threat is dryland, where one-fifth of the world population lives and where the threat comes from climate change and population growth. The human population is increasing in these areas much faster than food production can keep up."
Smith said scientists have not yet determined how the seed bank will be used to regenerate plants once they have become extinct in their natural habitat.
"But you've got more options if you have seeds, he said. "A seed bank isn't a solution to the problem of evolution. This is a seat belt - you wear it, hoping not to have to use it. This is a way of saying, 'Let's not lose everything'."