In the garden we would think it strange to chuck a plant on the compost heap before we had seen what it could do for us by way of leaf, flower, fruit or scent. But that is what we are allowing to happen to plants in the wild. They are disappearing before we have even had a chance to record them or their potential.
Put the 25,000 in context like this: in Britain there are about 1,700 endemic species of plant, and even if you take in the plants of the whole of Europe you will not find more than 11,000. The island of Madagascar has as many as that.
There are also 1,500 botanic gardens in the world. On paper, you can do a comforting sum: if each botanic garden looked after 17 endangered plants, the problem of their survival would be solved. But of course, it is not that simple. In the countries where strategies for conservation are most urgently needed, funds are hardest to find. Madagascar, with its 11,000 species of endemic plants, has only one botanic garden.
Until recently there was no
central database recording who was trying to do what where. That difficulty was met when the Botanic Garden Conservation Secretariat (BGCS) was set up five years ago in lodgings borrowed from Kew. Its primary aim was to forge botanic gardens into a worldwide network acting in the most effective way to save endangered species.
It provides a clearing house for information and expertise, and helps botanic gardens to set up conservation programmes to suit local needs - and purses.
As prospective adoptees, some plants have more pulling power than others. Cycads, trees of the palm family, are well protected in botanic gardens because they are showy, look good in avenues and were much planted when gardens were first set up, often in the wake of 19th-century colonisation. Almost all known cycads are well conserved.
Cacti are also well represented: 85 per cent of the world's endangered cacti are in botanic garden collections.
That is the good news. There is plenty of bad, as Peter Wyse-Jackson, programme director at the BGCS, is quick to point out. Zaire has the largest area of intact rainforest in the world outside Brazil, and one of the largest concentrations of endangered plants. But the director of the botanic garden at Kisanfu has difficulty getting to see any of them. Funds are too tight for a car.
And before Zaire can start to conserve its plants, it needs up-to- date information on the best way to do it. Yet the director cannot afford to attend any of the conferences where these things are thrashed out. Nor is it any good sending books, says Mr Wyse- Jackson: they will be stolen before they ever reach Kisanfu.
He has got round this problem by finding a Swiss aid agency to fund a trip to Kew for the director. He can then take back with him the books that have been bought for the botanic garden by the World Wide Fund for Nature.
Mr Wyse-Jackson has recently advised on restoration plans for the Hanoi botanic garden in Vietnam, which by the end of the war had been reduced to a derelict open space. Its director, Professor Thin of the State University, spent the war years as a botanist with the army in the jungles of North Vietnam. His job was to identify plants that an army on the move could use to feed itself. Now some of those same plants are endangered species. He is planning to propagate them in the Hanoi botanic garden to ensure their survival.
Peradeniya in Sri Lanka is typical of the many botanic gardens that were established by English colonists to test plants of possible commercial value. Now it plays a valuable role in conserving the island's medicinal plants. These are so heavily used they are in danger of disappearing altogether.
Staff at the garden collected all the known medicinal plants on the island and documented their uses. Then they bulked up stocks of the plants and distributed them to local herbalists. This proved to be the most practical way to relieve pressure on wild sources.
Mr Wyse-Jackson has also been advising on strategy at the Kingstown botanic garden on St Vincent in the West Indies. This was set up in 1765 and is one of the oldest botanic gardens in the western hemisphere. It still has an important role to play with regard to the island's economy.
Most of the 15 kinds of citrus grown by the island's fruit farmers were propagated in the garden. Every breadfruit tree there derives from the original brought in by Captain Bligh in 1793.
I was there at about this time last year. It is an enchanting place, laid out in exactly the style of the borough gardens of a seaside town in England, but of course planted with magnificently exotic tropical species.
To get in, you had to run the gauntlet of unofficial guides at the entrance. Ours was extremely knowledgeable and very good company, but whenever I expressed admiration for something, perhaps a sweet-smelling tropical blue water-lily or a pale spray of begonia, he picked it for me. I ended the tour with lips uncharacteristically sealed, nodding dumbly at the wonders he was showing me.
Unfortunately, as Mr Wyse- Jackson pointed out, none of the money that visitors pay to the guides swells the coffers of the botanic garden. He is recommending that the Kingstown garden sets up a germ-plasm bank to protect plants of economic importance on the island: fruit, ornamentals and medicinal plants.
Politics have much to do with the survival of plants, though of course they shouldn't. Under the old Communist regime of the Soviet Union, botanic gardens did rather well. The Soviet Academy of Sciences supported pure research. The importance of their work was well understood by the governments of the day. Since Communism was banished, things have not been so easy.
The director of a botanic garden in Latvia does not want to be told what to do by one in Moscow. The co-operation that used to exist between botanic gardens has largely broken down and the BGCS hopes this year to help in its repair. 'But the point is not to have the network for its own sake,' says Mr Wyse-Jackson. 'There has to be a reason for it, an ultimate benefit to the plants.'
A plant growing in a botanic garden, however, is like an animal in a zoo. Once the rate of decline has been halted and the species is reproducing well in captivity, there is a strong case for returning populations to the wild. With plants, as with animals, the difficulty lies in finding areas suitable to receive them.
Staff at the Bok Tower Gardens in Florida recently carried out a successful pilot project for the conservation of an endangered species endemic to Florida: Conradina glabra, a woody sub-shrub belonging to the mint family. The foliage is similar to rosemary and the plant's flowers are a pale lavender shade.
In the wild it existed on only
two sites, both owned by a paper company which harvests pine pulp-wood on them. There was no legal requirement for the owner to protect the plant in its native habitat. Population growth, land development and agriculture have had a devastating effect on Florida's wild flowers and many rare species now hang on in isolated groups in parking lots and urban wastelands.
Staff from the botanic garden took tip cuttings from the few specimens of conradina left among the chewed-up pines, and grew them on at the garden. Then, taking cuttings from cuttings, they built up a collection of 1,300 plants to reintroduce to the wild.
The conradina was lucky, in that adjoining the paper company's land was the Nature Conservancy's Apalachicola Bluffs and Ravines Preserve. The plants that had originally provided the cuttings grew only two miles away. Soil, drainage, rainfall, neighbouring plants and visiting animals would be pretty much what they were used to. Winter freezes, summer fires - often the keys that unlock dormancy in seeds - would occur with much the same frequency.
Blackberry and species of vine were the conradina's chief problem. Both grew quicker than it did. But there was rain soon after they were planted out, which helped them, and one year after the mass transplant 95 per cent of the plants were still alive. Although the real victory will not be won until the plants start self- seeding in their new home, this sounds like a success story. Only another 24,999 to go.