Nearly half of the world’s plant species could be facing extinction, say two leading botanists today in the journal Science. That finding contrasts sharply with a commonly cited assessment by the World Conservation Union (IUCN), which concluded that only 13 percent of the world’s plant species were at risk of vanishing. The new report argues that the old estimate did not adequately account for the vast number of tropical species in jeopardy; after factoring in those species, it says the figure of species at risk ought to be readjusted to between 22 and 47 percent. The two botanists are calling for the creation of a global database of endangered plants, which would cost about $12 million per year to maintain.