If all flesh is grass, then it makes sense to take care of the lawn. And all flesh truly is grass. If you could weigh every human and all their domestic livestock, the scales would tip at 1.1bn tons - and every ounce of this living flesh depends on plant life. Humans are not just leaders in the field, they and their livestock consume most of the field.
Consume is the word. There could be about 270,000 species of flowering plants already named, and perhaps another 50,000 to be identified. And, says Peter Crane, director of the Royal Botanic Gardens at Kew in London, around 30% of them are threatened. A quarter of them could vanish within 50 years.
Which is why he is about to formally open a new kind of world bank to store the seeds of plants at risk for up to 200 years and push for yet more research into the green things that bankroll all animal life. Plants produce the oxygen that humans breathe, and the food they eat, the medicines they use, the clothes they wear and the shelters in which they live.
The odd thing is that after millennia of co-existence, most humans know very little about them. Only about 7,000 species of plant have been gathered or cultivated, a mere 200 have been domesticated and around a dozen of them provide 75% of the world's calories. Two of them, rice and wheat, produce - directly - 45% of the world's protein. Indirectly, of course, plants provide all of it.
These brutal economic facts make research institutions such as Kew the keepers of the planet's future. It is not the only important botanical gardens - there is a network of them, co-operating in research - but it is the tallest poppy in the meadow. Its £80m millennium seed bank at Wakehurst Place in Sussex is about to open to the public.
It already holds seeds from most of Britain's flora and for the next nine years Kew's scientists will be out in the field collecting tens of thousands of specimens to be dried, and kept at sub-zero temperatures, to be taken out again and grown and gathered and frozen again - a kind of vegetable Noah's Ark, to save species otherwise doomed by human expansion into the wilderness.
Many of them will be from the dry lands, the regions of the earth where plants are already most at risk and humans struggle every day for survival. "Why should we care?" says Crane. "Someone like me who sees what we have today as the product of tens of millions of years of evolution asks: is it really rational to allow, in a space of a couple of generations, such a catastrophic change to occur? I think there is an ethical, moral issue there that has to do with our place in nature and the way we view ourselves and our general stewardship of the planet, these sorts of things. For me that is a pretty powerful argument. For some people that wouldn't cut a lot of ice. But for me there is a justification for pausing a moment and saying: is this rational? Do we want to wreak these kinds of irreversible changes?"
He has seen such changes already. His first business was palaeobotany. He worked in the Field Museum in Chicago on fossil plants. Flowering plants began to bloom on the planet only about 130m years ago, and soon trees related to magnolias, and precursors of the walnut and the oak were co-existing with the dinosaurs. There were palms, dogwoods, gingers and a diversity of other things. There were no grasses - the precursors of the wheats and ryes and cocksfoots that feed the world turned up only 30m years ago - but the shales and coal measures and chalk deposits kept producing evidence of an enduring and slowly altering kingdom, flowering into greater and greater diversity.
From Chicago, Crane - born in Britain, educated at Reading, 45, and married with two children - moved to take over Kew and his view switched from the past to the future. What he saw - what any biologist would have seen - was a massacre of living things, a huge narrowing of green diversity.
"In the western world we are very insulated from nature, or at least we tend to think we are. Our day to day reliance on plant diversity is not the same as it is in other places," he says. "But they reckon that 80% of the world is still reliant on plant resources for its primary health care."
He has just come back from a visit to a co-operative project in the dry lands of north-east Brazil. "They don't have the money to pay the bus to go to the hospital. If they go to the hospital they haven't the money to pay for the drugs they need. So they grow medicinal plants in their gardens and those medicinal plants help them through the less serious parts of their health care needs. They are very, very important. Intestinal parasites, for instance: there are lots of plants they use as purgatives. Curiously enough we are seeing the same trend here, with Chinese herbalists."
Diversity pays dividends in all sorts of ways. An enthusiastic British gardener might have a dozen species of useful plants on his plot. An African or Latin American family might have more than 100: when the game is uncertain, it pays to spread your bets. The botanists went to Brazil to help, but they also went to learn. One farmer in the Brazilian drylands had found a new way with a neglected resource, Crane says.
"He was growing cattle, but they basically don't have enough water. So periodically he has nothing to feed these things, so he was very interested in how you get through the bad times. One of the things they had been working on in these groups was the use of cacti for the boundary fences. He was using three or four different species of cacti for keeping animals out of growing areas but what he had also found out was he could harvest these cacti as food for his animals. In the ordinary course of events he wouldn't be feeding his animals cacti, but when things get really rough he can almost coppice them, get the spines off them and have good, short-term animal fodder. It's pretty nutritious."
The tropical wet forests are disappearing because of ruthless commercial exploitation of hardwoods and colonisation by desperate farmers. In the drylands, plants are perishing because people are struggling to survive: their cattle clear the landscape and the farmers collect what is left as fuel because they have no other choice. Greed and poverty are one set of enemies, human movement is another, as lusty foreign species take over in precarious ecosystems. The only way ahead is to work with the people who depend on their local flora - and collect like mad for the millennium seed bank.
"It's a fairly last-ditch approach," says Crane. "It's better to conserve the plants in the field in which they grow, and they might support the eco-system they support, or on which they depend. But in a lot of cases it's a matter of saying: let's get these things into a safe haven, that we can then use in the future for reintroduction, before they diminish to such a level that either the genetic diversity has been stripped out, or they have gone more or less completely."
A seed bank exhibition opens to the public at Wakehurst Place, Ardingley, Sussex (01444 894 000) on Saturday. The bank itself opens for business in October. Link: www.kew.org.
