The Millennium Seed Bank at Wakehurst Place will begin its mission to save the world's plants from extinction by concentrating on tropical drylands - hot, impoverished countries such as Mozambique or Zambia where plants play a vital role in human life.
They provide fuel, building materials and clothing as well as food, fodder and medicine, and many are threatened with extinction as wild lands become increasingly developed or exhausted by over-use.
The scientists at this outstation of Kew, working with botanists in the countries concerned, hope to safeguard most of these plants over the next decade by collecting their seeds and using their remarkable properties of dormancy.
Research suggests that, carefully dried and frozen, many seeds can germinate after 200, 500 or even 1,000 years. (The accepted record is for a South American canna lily seed, found in an Argentinian tomb and germinated when more than 600 years old.) Roger Smith, the head of Kew's seed conservation department and the man behind the Millennium Seed Bank, can give at least three good reasons for spending pounds 80m on the project.
"The first is direct use. Do we know so much about every plant that if one is lost, we know what has gone in terms of its potential for food or medicine? The second is the web of life. Imagine all the world's species forming a net, with each species a knot in the net. How many knots can you cut out before the net ceases to function?
"The strongest argument is stewardship. What right does the current generation have to take options away from future generations by not handing on the species it inherited?"
The building at Wakehurst Place, due to open next year, will be topped out this week by Sir Ghillean Prance, Kew's director.