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At least 1 of every 8 plant species in the world -- and nearly 1 of 3 in the United States -- is under threat of extinction, according to the first comprehensive worldwide assessment of plant endangerment.
The assessment, which required more than 20 years of work by botanists and conservationists around the globe, added nearly 34,000 plant species to the World Conservation Union's growing Red List of imperiled organisms.
The survey, made public yesterday in Washington, found that among the plants most at risk were 14 percent of rose species, 32 percent of lilies, 32 percent of irises, 14 percent of cherry species and 29 percent of palms. Many species of coniferous trees, and many plant species found in island nations, were also judged especially vulnerable.
While endangered mammals and birds have commanded more public attention, it is plants, scientists say, that are more fundamental to nature's functioning. They undergird most of the rest of life, including human life, by converting sunlight into food. They provide the raw material for many medicines and the genetic stock from which agricultural strains of plants are developed. And they constitute the very warp and woof of the natural landscape, the framework within which everything else happens.
The census of imperiled plants should be taken not as an exact measure of the situation, leaders of the survey said, but rather as a first, rough approximation.
And some acknowledged that the majority of species were ''secure and widespread,'' in the words of Dr. Bruce Stein, a botanist who is a senior scientist with the Nature Conservancy, one of nine scientific and conservation organizations that participated in drawing up the list. Furthermore, Dr. Stein said, some plants were placed on the list simply because they are rare, not because their numbers are declining or their habitat is threatened.
Nevertheless, of the world's 270,000 known species of plants, the 12.5 percent found to be at risk is a huge proportion, said David Brackett of Ottawa, chairman of the World Conservation Union's Species Survival Commission. Moreover, he said, the figure is probably an underestimate, since data from most places in the world -- including some species-rich tropical nations where the countryside is being rapidly cleared -- are fragmentary.
The list of imperiled plants fills more than 750 pages of a large red-bound book. Nine of every 10 plants on the list are native to only one country, making them especially vulnerable to national or local economic and social conditions. Many species are found only on a few islands, and countries like Mauritius, the Seychelles and Jamaica consequently have disproportionately high numbers of threatened plants.
Scientists generally cite two main reasons plants become endangered: destruction of large swaths of wild countryside by agriculture, logging or development, and invasions of plants from one part of the world that run riot and crowd out native species in another part.
The new listing of threatened plants is one more piece of evidence that ''a whole chunk of creation is at risk,'' said Dr. Stuart Pimm, an ecologist at the University of Tennessee, who was not involved in producing the report.
While 1 plant in 8 may not seem like much, Dr. Pimm said, ''that's what's threatened now, as a consequence of what we've done so far; but all the evidence is that the destruction is continuing at an accelerating pace.''
With 4,669 of its species judged to be threatened to one degree or another, the United States ranked first, by far, among all nations in total number of plants at risk. That is 29 percent of the country's 16,108 plant species.
But Dr. Stein said the United States' situation looked comparatively grim only because plants were probably better surveyed here than elsewhere.
''I don't believe the U.S. is worse off than other countries,'' he said. ''If anything, I think the U.S. has taken a more active interest in plant conservation.''
Dr. Stein's group, the Nature Conservancy, maintains what is widely regarded as one of North America's most comprehensive databases on endangered plants. Other major American participants in drawing up the Red List were the New York Botanical Garden and the Smithsonian Institution's National Museum of Natural History.
The conservation union, also called the International Union for the Conservation of Nature, is based in Gland, Switzerland. Many governments and scientific organizations are among its members. Since 1960, it has been maintaining and adding to its Red List of threatened species. The list has no official effect but is widely regarded as an influential guide for conservation policy makers.
Two years ago, the union placed nearly a quarter of all known mammal species and 11 percent of birds on the list. It also added a number of marine species for the first time.
The Red List establishes five categories of organisms: species not seen in the wild in 50 years and presumed extinct; species suspected of having recently become extinct; endangered species, those likely to become extinct if the causes of endangerment continue; vulnerable species, those likely to become endangered if the causes of vulnerability continue, and rare species, those with small worldwide populations not yet endangered or vulnerable. Of the total number of plants on the Red List, 43 percent are classified as rare, 24 percent as vulnerable and 20 percent as endangered.