A new calculation, by leading botanist Dr David Bramwell, suggests there could be about 422,000 species of angiosperm.

We need to make the politicians aware of the size of the problem; to see how much of the world's biodiversity can be saved

Dr David Bramwell

Conservationists say the estimate underlines the urgent need to complete a global inventory of plant diversity. Much of the planet's botanical wealth is concentrated in parts of the world where species-rich tropical and Mediterranean habitats are under greatest pressure.

"By increasing the total number of species, we also increase the number that are threatened," Dr Bramwell, the director of the Jardin Canario "Viera y Clavijo" on Gran Canaria, Spain, told BBC News Online.

He believes more than a fifth of species may now be endangered.

The new estimate is for plants known to science, and does not include speculative numbers of species yet to be discovered.

Wide range

Angiosperms are flowering plants that produce seeds enclosed in fruit. The flowers allow animals, particularly insects, to transport pollen between the plants.

Although they are thought to have evolved relatively recently - about 125 million years ago - they are the dominant plants on the planet.

The angiosperms

The dominant plant division Includes most of the plants we eat Their flowers aid sexual reproduction Seeds are enclosed in a fruit Characteristics have co-evolved with animals, particularly insect species

Dr Bramwell made his estimate (421,968) by adding together the number of plants in each region of the world, concentrating on plants only known from one country or island, and allowing for overlap of floras from one country to another.

"What I have done is take a baseline flora - the largest flora in each region - and then count the number of endemics from the other countries that are definitely not in that baseline. The main point is that my system eliminates a lot of the duplication that occurred in previous estimates."

The figure arrived at is substantially higher than previous estimates that ranged from 231,000 to 320,000 species. However, it fits well with another recent estimate by Dr Raphael Govaerts, from the Royal Botanic Gardens, Kew, UK.

Dr Govaerts used a different method of counting and produced a figure of 422,127.

More resources

The latest estimate has been published in Plant Talk, an international periodical on the conservation of the world's plants.

Those working in the field say the number greatly boosts the argument that more resources should be allocated to the classification and conservation of plant diversity.

Under the international Convention on Biological Diversity, a Global Strategy for Plant Conservation is being worked on. One of its targets is to list all the plants of the world.

"At least now we have an idea of the size of the problem," Dr Bramwell said. "But we need also to get an idea of the conservation status of each plant.

"Conservation isn't only biology, it's politics as well - obviously. We need to make the politicians aware of the size of the problem; to see how much of the world's biodiversity can be saved."