Go down through the woods just past the nursery shop, however, and you'll be in for a big surprise. A 21st-century piece of architecture in the shape of three overground glass tunnels, this is the strategic headquarters of Kew's £80m Millennium Seed Bank Project (MSBP). The £19m complex houses a leading-edge research facility, and an affectingly interactive exhibition that leaves you in no doubt that plants are the flesh, blood and oxygen of our lonely planet. But the most sobering feature is deep beneath your feet: a bomb- and radiation-proof underground bunker, into which the seeds of the world's most threatened plant species are being gathered.

First and foremost, the MSBP is a race against time. On Kew's predictions - extrapolating from the continuing ravages of population increase, urban sprawl, habitat destruction, climate change and desertification - some 25% of the world's plant species are at risk of extinction in the next 50 years. As it is, with the rate of disappearance 1,000 times greater than that indicated by the geological record as 'normal', our planet is already on the back foot and reeling. Hence the urgency of the project's target: concentrating on the dry-land areas that cover a third of the Earth's surface, to collect and store seeds from those 10% of plant species most at risk by the year 2010.

Wakehurst's underground bunker is more than a Noah's Ark for plants, however. Built with a 500-year project life in mind, it's also a beacon against the sort of suicidal short-termism that helped get us into this mess. The seeds, which are stored at -20C, will be good for at least 200 years, and periodical propagation will guarantee replenished stocks ad infinitum. As an insurance policy, it's both cheap and practical. By way of the first strong counteroffensive, however, the MSBP is also developing protocols for all the seeds it collects, with a view to restoring plants to their original, or other, habitats. That will be the end of phase one. Phase two will be the hard part. In this, Kew will endeavour to join conservation of local biodiversity to sustainable economic development, especially in those regions of the world where subsistence survival or headlong development poses the greatest risks of plant extinction.

Given Kew's pre-eminence in seed technology, it's perfectly placed to help get our planet off the critical list. In Kenya, the MSBP is already moving into this therapeutic phase - more of that later.In the UK, the first country in the world to do it, seeds from all 1,442 indigenous plants have been gathered in, following a three-year collection programme and the help of 200 amateur botanists. Vast by comparison, the international project is co-ordinated by just four seed hunters, who barely have time to get their bush wear laundered between sorties out from Wakehurst Place. But in this, their third year, they're well on target in terms of seed numbers collected. No less importantly, given the long-haul nature of the project, they're well ahead in forging partnerships - including training and capacity building - with botanical institutions in a welter of countries including Australia, Egypt, India, Kenya, Mexico, South Africa and the United States. More heartening still is the domino effect. Recently the seed hunter Tim Pearce delivered his first year's progress report on the project's work in Western Australia. As a result, Queensland and New South Wales wish to establish similar partnerships.

Compare this with the rabid opposition that met the emergent ecological movement 30 years ago. As was famously attributed to Ronald Reagan when he was the governor of California, 'When you've seen one redwood tree, you've seen 'em all.' But the alternative consciousness was already blossoming, even at so twee-seeming Kew. 'When I joined in 1974,' said Roger Smith, now head of the MSBP, 'I was aware that I belonged to the first generation of people who would see a doubling of the world's population in their lifetime.' For decades, Kew had been banking the seeds of the main agricultural crops. But by 1972, uniquely among the world's botanical establishments, it had begun to do likewise for under-utilised, wild and threatened species on the future's behalf. Another key change point, according to Smith, was Margaret Thatcher's challenge to ivory towers everywhere. 'It was she, remember, who introduced the principle of accountability, requiring research establishments to assess their effectiveness. For Kew, the assessment was that, yes, we had good seed technology, but that we ought to accelerate our seed-collecting to a rate that would at least match the extinction rate.'

But Thatcher's recipe for raising the necessary funds - via partnerships around the world - wasn't so easy to effect. One complication was the increasing political sensitivity regarding genetic property and bio-piracy. 'Lots of people had become familiar with Kew's overseas activities,' said Smith. 'A Greek farmer seeing us collecting weeds in the corner of his field would just shrug, 'Ah, the English!' But, usually after the fact, you'd get third parties accusing us of theft.'

Noah, of course, got the galvanising word from God. In this instance, it came in the form of the 1992 Convention on Biological Diversity (CBD). Masterstroke of the United Nations Earth summit, no other convention has drawn support from more countries - 186, of which 168 are formal signatories. Although Kew's game plan took a deal of working out, the CBD provided Smith and his colleagues with the ideal instrument to cleave through the paranoid politics and go globally larger. As to scoring essential monies, what did the trick was Smith's numbers-target pitch to the National Lottery's Millennium Commission: 24,000 of the world's most endangered species to be collected by 2010. Heroic and vital, it was also winningly 'bottom line'. Of the main sponsors, the National Lottery put in £30m, the Wellcome Trust £9m, plus £2.5m from the communications company Orange.

Even so, politicians in particular still ask why the MSBP bothers to collect some plant species that are of no known use to man. Smith's answer? 'You tell me which ones to ignore, and let posterity record your decision.'Anyone who has had their eyes stunned wide by South Africa's flora would hardly suspect its precariousness. Take the Western Cape. Of its 8,000 plant species, 6,000 are endemics. That they exist nowhere else makes them especially vulnerable. As a result of the new South Africa's building and farming programmes, more than 60 endemics are extinct. The records are there but the plants are no more. Others are barely clinging on.

Bizarrely, Cape Town's Kenilworth racecourse contains 40 threatened species, more per square metre than anywhere on Earth. Kew's seed hunter for South Africa, Paul Smith (no relation), has them in his sights, but timing is all. 'Optimum collecting means getting there when the seeds are just ripe,' he explained. 'And given the vulnerability of these plants, we never take more than 10% of what's available on the day.'

Nor is the advance of concrete and plough the only threat. Another 58 indigenous Cape species have been lost to so-called invasive aliens. Chiefly acacias and eucalypts, originally imported from Australia, they have no natural competitors or predators. Cunningly evolved, their deep taproots water-starve the indigenous plants and even produce metabolites that inhibit their growth. They're throttling the natural forest.

'In colonial times,' said Smith, 'a great part of the South African economy was based on eucalypts (for building, gum and fodder) and acacias (wattle and tannins) - probably thanks to Kew introducing them through the Durban Botanic Gardens in the first place. Now the government employs thousands of people to pull them up.' As for the threatened Western Cape endemics, on his previous sortie Smith targeted seven and found five, the seeds of which are being grown on at Wakehurst Place.

The rescue that he's proudest of is that of Cylindrophyllum hallii, a tufted perennial with gorgeous yellow flowers. As Smith began to speak of it, we left his office and headed for the Wakehurst glasshouses. Like so many Cape flora, Cylindrophyllum hallii could have become a world favourite. But not only had fashion overlooked it. In 2001, when Smith took up its case, nobody had seen it since 1960, and then only in one place.